104 - Hydrology and Hydraulics
The primary objective of the hydraulic design of a highway stream crossing is to convey storm event flows under roadways and embankment without causing interruption of the traffic, changes in the behavior of the stream. Other objectives of a hydraulic design are to determine the backwater and hydraulic capacity of the bridge or culvert; to identify the stream forces that may cause damage to the bridge, culvert or roadway system; and to provide a safe level of service acceptable to the traveling public without causing unreasonable effects on adjacent property or the environment.
ATON – Aids to Navigation
CBF – Channel bed fill
HEC-HMS – U.S. Army Corps of Engineers (USACE) Hydrologic Engineering Center (HEC) Hydrologic Modeling System
HEC-RAS – USACE HEC River Analysis System
HFAWG – Hydrologic Frequency Analysis Work Group
HY-8 – FHWA Culvert Hydraulics Computer Program
LiDAR – Light Detection and Ranging remote sensing method
NAVD 88 – North American Vertical Datum of 1988
PDM – DelDOT’s Project Development Manual (PDM; 2015)
PeakFQ – U. S. Geological Survey (USGS) computer program to estimate magnitude and frequency of floods
StreamStats – USGS web-based geographic information system (GIS) that provides analytical tools that are useful for engineering design applications, such as the design of bridges
TR-20 – Natural Resources Conservation Service’s (NRCS’s) hydrologic computer program
TR-55 and WinTR-55 – NRCS’s hydrologic method and computer program, respectively
UDC – New Castle County Unified Development Code
WATSTORE – National Water Data Storage and Retrieval System of the USGS
Consideration of the effects of constructing a bridge or culvert across a waterway is key to ensuring the long-term stability of the structure. Confining the floodwater may cause excessive backwater or overtopping of the roadway, may impact structural stability when the water is impacting the superstructure of the bridge (i.e., causing a pressure flow situation), or may induce excessive scour. These effects may result in damage to upstream land and improvements or endanger the bridge. Conversely, an excessively long bridge does not create a backwater or any attenuation and may cost far more than can be justified by the benefits obtained. Somewhere between these extremes is the design that will be the most economical to the public over a long period of time, yet remain safe and stable during large storm events.
Standard DelDOT QA/QC procedures will be followed for development and review of hydrology and hydraulics submittals.
104.1.3 Design Responsibilities
Responsibilities for hydraulic design are divided between the Bridge Design Section and the Project Development Sections based primarily on the size of the drainage area. Bridge Design is responsible for all watersheds equal to or over 300 acres and existing structures with openings (bridge, culvert, pipes) that exceed 20 square feet. The Project Development Section is responsible for watersheds smaller than 300 acres. The Bridge Design Section is responsible for “bridge-only” projects where support from the Project Development Groups is not required. In those cases, the Bridge Design Section designs any pipe culverts, closed drainage and roadside ditches, and stormwater management systems affiliated with the bridge project. Typical projects include bridge replacement or rehabilitation projects.
When the Bridge Design Section collaborates on a project with the Project Development Section, the Project Development Section will develop the closed drainage and roadside ditches. A new alignment bridge is a typical project in which this type of coordination takes place: the Bridge Design Section designs the structure, while the Project Development Section designs the ramps, profiles, alignment, drainage, and all other aspects of the project.
Refer to Section 6.2 of the DelDOT Road Design Manual for the design and construction of adjacent drainage ditches, pipe culverts (less than 20 square feet), closed drainage systems, and erosion control near stream crossings.
104.1.4 Field Data Collection
One of the first and most important aspects of any hydraulic analysis is a field evaluation. This involves an in-depth inspection of the proposed bridge site and completion of the Field Hydraulic Assessment Checklist in Appendix 104-1. The designer is responsible for completing the checklist.
The purpose of field inspecting the proposed bridge site is to evaluate the stream characteristics and hydraulic properties, the performance of the existing bridge (if applicable), the channel and floodplain topography, and the adequacy and accuracy of the survey data. Any man-made dams located in the reach that will affect the bridge should also be investigated. Additionally an estimate of streambed particle size, including D50, can be made by visual inspection using field tools such as a sand gage card, gravelometer, or wire screen.
The designer should walk along the channel both upstream and downstream at a distance at least equal to the floodplain width, if possible. Any natural hydraulic controls such as rock shoals, or beaver dams as well as man-made controls such as bridges, dams, sewer or water lines suspended across the channel, or other constrictions that have taken place in the floodplain should be evaluated. If these controls have any effect on the high-water profile, they should be taken into account in the modeling. The stream alignment and relation to structure (e.g., outside of bend, bad angle of attack) should also be noted. Coordination is recommended with the Environmental Studies Section to determine if current environmental study, wetland delineation, and/or biological stream section forms are available that have any of the required information described above.
104.1.5 Topographic Survey and Extent of Hydraulic Study
For most projects, topographic data will be developed through obtaining new survey; however, available survey data and USGS, LiDAR, or other topographic mapping should first be assessed to determine if additional survey data is needed. The channel and hydraulic controls should be surveyed so that their effects on the high-water profile can be defined. NAVD 88 is the required datum for hydraulic surveys and studies. Elevation contours at 1-foot intervals were produced for the State of Delaware (based on the 2014 LIDAR.) Data is available in line shapefile format and ESRI file-base geodatabase format (GDB). LiDAR data is typically useful for overbank elevation data; however, LiDAR data do not provide elevation data in the stream channel, so a survey is required. The LiDAR data and specifications with respect to the data may be accessed from the Delaware Geological Survey.
Data that will need to be gathered from a field survey include data on stream banks and the channel, any required dam data, and bridge/culvert data. If LiDAR data are available for data in the overbanks the survey of the channel and structures can be merged with the LiDAR data. If LiDAR data is not going to be used, the survey should include the overbank area with the lateral extents of the topographic data to contain the width of the floodplain within the hydraulic cross sections. A survey is required for all projects that require an H&H analysis, and it is the designer’s responsibility to request the survey. Any specific information needed for the Hydraulic Checklist or information in addition to that normally required must be included in the survey request. See Appendix 104-2 for further guidance on filling out a survey request.
For hydraulic studies, the downstream and upstream limits vary based on a number of factors, including tidal influences, other structures within the reach, backwater from other streams/rivers, and the slope of the channel. Streams with flatter slopes or with backwater conditions from a downstream river typically require a longer study reach to be able to balance energies and get an accurate analysis at the bridge.
The limits of the profile computation should be extended downstream to the point where a flow is not affected by the structure (i.e., the flow has fully expanded). This downstream limit can be determined by computing a sensitivity analysis. The HEC-RAS model can be executed starting at normal depth, and then subsequent runs can be started 1 foot below and above normal depth to see if the model converges before the location of the proposed bridge, as shown on Figure 104‑1. The expansion reach length is defined as the distance from the cross section placed immediately downstream of the bridge to the cross section where the flow is assumed to be fully expanded. Chapter 5 of the HEC-RAS River Analysis System Hydraulic Reference Manual (USACE HEC, 2010) provides additional guidance on determining the distance to the downstream end of the expansion reach.
The upstream limit should extend to where any increase from the new bridge or proposed modifications merges into the existing conditions profile (e.g., where the flow lines are approximately parallel and the cross section is fully effective). If the proposed conditions water surface elevation (WSE) is lower than the existing conditions profile, then the minimum distance upstream to be modeled shall be 500 feet. The model should be calibrated using known flood data if sufficient reliable data is available.
Note that for small in-kind pipe or culvert replacements with minimal changes to the hydraulic opening, width, and roadway profile, the upstream and downstream hydraulic limits may be shortened as appropriate. Also, for small projects that use HY-8 or a similar culvert modeling methodology and that do not require backwater calculations, a limited survey is required to define the downstream tailwater condition and the existing structure and roadway data.
Hydrologic analysis is used to determine the rate of flow, runoff, or discharge that the drainage facility will be required to accommodate. The designer must evaluate existing upstream conditions in sizing a structure. If warranted, the designer may evaluate the potential effects of future land cover conditions on calculated flows by using procedures outlined in USGS Scientific Investigations Report (USGS SIR 2022-5005), Peak-Flow and Low-Flow at Defined Frequencies and Durations for Nontidal Streams in Delaware (2022) or by exercising engineering judgment.
The design of highway facilities should be adequately documented. It is frequently necessary to refer to plans, specifications, and hydrologic analyses long after the actual construction has been completed. One of the primary reasons for documentation is to evaluate the hydraulic performance of structures after large floods to determine whether the structures performed as anticipated or to establish the cause of unexpected behavior. In the event of a failure, it is essential that contributing factors be identified to avoid recurring damage and help improve future hydraulic designs.
The documentation of a hydrologic analysis is the compilation and preservation of all pertinent information on which the hydrologic decision was based. This might include drainage areas and other maps, field survey information, source references, photographs, hydrologic calculations, flood-frequency analyses, stage-discharge data, and flood history, including narratives from highway maintenance personnel and local residents who witnessed or had knowledge of an unusual event.
Hydrologic data shown on project plans ensure a permanent record, serve as a reference in developing plan reviews, and aid field engineers during construction. Required plan and H&H Report presentation data are provided on Figure 104‑2 and in Section 104.6 – Hydrologic and Hydraulic Report.
Several hydrologic methods that can be used to estimate flows will require precipitation data in the form of total precipitation or as rainfall intensity as part of the hydrologic input for the method. As precipitation data are regional, the National Oceanic and Atmospheric Administration (NOAA) Atlas 14 has published rainfall intensity-duration data for Delaware’s 12 rainfall gages located throughout the state. Refer to the Delaware Rainfall IDF Curves to find precipitation intensity values for use in the Rational Method. Figure 104‑3 provides 24-hour rainfall totals for use in methods requiring a 24-hour duration, such as the NRCS Curve Number method.
220.127.116.11 The Rational Method
The rational method is an empirical formula relating rainfall to runoff. It is the method used almost universally for computing urban runoff. It is also used to estimate bridge deck drainage for the design of scuppers.
Discharge, as computed by this method, is related to frequency by assuming the discharge has the same frequency as the rainfall used. The storm duration is set equal to the time of concentration of the drainage area. Because of the assumption that the rainfall is of equal intensity over the entire watershed, it is recommended that this formula should be used only for estimating runoff from small areas. Although the rational method is typically only applied to a maximum watershed size of 200 acres, with caution and consideration for watershed characteristics, larger watersheds up to 326 acres (the lower limit of the regression method) may be applicable. The rational method is most frequently used for estimating small, homogenous, or highly impervious drainage areas.
FHWA's HEC-22,Urban Drainage Design Manual (2009) provides more specifics on use of the rational method, including the procedure, time-of-concentration (Tc) calculations, acceptable “C” value sources, and determination of rainfall intensity. The rational “C” values should be obtained from Table 3-1 of HEC-22.
18.104.22.168 Delaware Regression Method (SIR 2022-5005)
DelDOT uses the equations in the current version of the SIR 2022-5005 to estimate flood runoff. These equations are based on specific studies of the nontidal watersheds in Delaware and adjacent states. This method relies on data from streamflow gaging station records combined statistically within a hydrologically homogenous region to produce flood-frequency relationships applicable throughout the region. If the designer is using gaging station records and wishes to evaluate these values for upstream or downstream sites, the procedures in the USGS publication should be followed.
From the study, it was concluded that reasonable estimates of flood runoff can be made by dividing the state into two hydrologic regions, which correspond to the Coastal Plain and Piedmont physiographic regions as shown on Figure 104‑4. In the Coastal Plain region, the design event is determined using the drainage area, the mean basin slope (in percent) determined from a 3-meter digital elevation model (DEM), and the percent of hydrologic soil group A. In the Piedmont region, the size of the drainage area, percent of impervious area, percent of storage from the National Hydrography Dataset (NHD) are considered in the equations for each flood event. Another variable changes between mean basin slope or percent of hydrologic soil group D based on the design event. Each of these parameters is discussed in the SIR 2022-5005 publication. Land use is not considered in the runoff equations for the Coastal Plain region.
In areas where land use may change, the empirical methods using lump parameters or models such as WinTR-55, HEC-HMS, or HEC-1 are recommended. If the Delaware regression method is used, based on engineering judgment, the designer may consider the effects of possible changes in land use.
The SIR 2022-5005 method is incorporated into the USGS online StreamStats program. StreamStats is a web-based GIS that provides users with access to an assortment of analytical tools that are useful for water-resources planning and management and for engineering design applications, such as the design of bridges. StreamStats allows users to easily obtain streamflow statistics, drainage-basin characteristics, and other information for user-selected sites on streams.
The best estimates of flood frequencies for a site are often obtained through a weighted combination of estimates produced from the regression results and the results from a statistical analysis of stream gage data. The U.S. Department of the Interior, Interagency Advisory Committee on Water Data (1982) recommends, and Tasker (1975) demonstrated, that if two independent estimates of a streamflow statistic are available, a weighted average will provide an estimate that is more accurate than either of the independent estimates. Improved flood-frequency estimates can be determined for Delaware stream gaging stations by weighting the systematic peak-flow record estimates at the station with the regression peak flow estimates. SIR 2022-5005 provides guidelines for the weighting process as well as procedures and equations to estimate flows for a site upstream or downstream of a gaged location and for sites between gaged locations.
22.214.171.124 Published Reports
Published reports may be used for comparison with the calculated runoff. Delaware Department of Natural Resources and Environmental Control (DNREC) has developed or is in various stages of developing watershed stormwater management plans for the Appoquinimink, Upper Nanticoke, and Murderkill watersheds. These plans include a detailed hydrologic model that has been calibrated against stream gage data, data obtained by regression methods, or other reliable hydrologic data. The watersheds were divided into subwatersheds; therefore, flow values at road crossings may be available that are not available from any other source. For projects located in these watersheds, these reports should be reviewed and flows used as appropriate. The calibrated HEC-HMS files may be available from DNREC.
The Federal Emergency Management Agency (FEMA) Flood Insurance Study (FIS) contains runoff information for many streams in Delaware. The report documents the methods used to determine runoff for each stream. The FIS reports were prepared by a variety of sources (e.g., the USACE, private consultants, the Delaware River Basin Commission). These reports contain floodplain information for many streams in Delaware. The reports include historical runoff data as well as calculated runoff data. However, due to the variety of preparers, the flows should be checked against other reliable methods. The flow values reported in these other reports should be verified as to consistency with the standards presented herein, and checked for validity of data utilized, and methodology. Published dam reports should also be referenced as contained in Section 104.3.3 – Hydraulics for Dam Safety Projects.
The USACE and FEMA have developed and applied a state-of-the-art storm surge risk assessment capability for Region III, which includes the Delaware Bay, the Delaware-Maryland-Virginia Eastern Shore, and all the waterways connected to these systems. This information, some of which is contained in ERDC/CHL TR-11-1 Coastal Storm Surge Analysis: Modeling System Validation Report 4 (USACE ERDC, 2013), would be helpful for any tidal bridges.
126.96.36.199 Flood-Frequency Analysis of Recorded Stream Gage Data
The method of analyzing flood-frequency relationships from actual streamflow data for a single gaging station enables the use of records of past events to predict future occurrences. The procedures described in Guidelines for Determining Flood Flow Frequency, Bulletin 17B (U.S. Department of the Interior, Interagency Advisory Committee on Water Data, 1982) should be followed. This method is often referred to as the Bulletin 17B method, and uses the Log Pearson Type III distribution. The Log-Pearson Type III distribution is a statistical technique for fitting frequency distribution data to predict the design flood for a river at some site and is performed on records of annual maximum instantaneous peak discharges collected systematically at streamflow gaging stations.
188.8.131.52.1 Flood-Frequency Analysis Guidelines
The HFWAG, consisting of representatives from Federal agencies, private consultants, academia, and water management agencies, has recommended procedures to increase the usefulness of the current guidelines for hydrologic frequency analysis and to evaluate other procedures for hydrologic frequency analysis (HFAWG, 2013). The HFAWG will be incorporating their findings into Bulletin 17C. When Bulletin 17C is adopted by the FHWA, the procedures in that publication should supersede those in Bulletin 17B.
The computer programs PeakFQ, developed by the USGS, and Hydrologic Engineering Center Statistical Software Package (HEC-SSP), developed by the USACE HEC, provide estimates of instantaneous annual-maximum peak flows for a range of recurrence intervals. The Pearson Type III frequency distribution is fitted to the logarithms of instantaneous annual peak flows following the Bulletin 17B guidelines of the U.S. Department of the Interior Interagency Advisory Committee on Water Data. The parameters of the Pearson Type III frequency curve are estimated by the logarithmic sample moments (mean, standard deviation, and coefficient of skewness) with adjustments for low outliers, high outliers, historic peaks, and generalized skew.
PeakFQ reads annual peaks in the WATSTORE standard format and in the Watershed Data Management (WDM) format. Annual peak flows are available from NWISWeb (http://nwis.waterdata.usgs.gov/usa/nwis/peak). (Data should be retrieved in the WATSTORE standard format, not the tab-separated format.)
This method assumes that there are no changes during the period of record in the nature of the factors causing the peak magnitudes. The ramifications of this assumption can be minimized by making every effort to determine the past conditions of the drainage area and, if possible, making allowances for changes. The most common changes are man-made and consist of such modifications as storage and land development. The user of hydrologic data must be acquainted with the procedures for evaluating streamflow data, the techniques for preparing a flood-frequency curve, and the proper interpretation of the curve.
Since most of the stream records in Delaware are sufficiently long to give good flood-frequency relationships, considerable weight should be given to the stream record in estimating design floods. When a gage record is of short duration or poor quality, or when the results are judged to be inconsistent with field observations or sound engineering judgment, the analysis of the gage record should be supplemented with other methods. The validity of a gage record should be demonstrated and documented. Gage records should contain at least 10 years of consecutive peak flow data, and they should span at least one wet year and one dry year. If the runoff characteristics of a watershed are changing (e.g., from urbanization), then a portion of the record may not be valid.
Where there is a stream gage at a bridge or culvert, the USGS has developed the flood flow frequency for the 50- , 20-, 10-, 4-, 2-, 1-, 0.5-, and 0.2-percent-chance of occurrence (2-, 5-, 10-, 25-, 50-, 100-, 200-, and 500-year) events; these frequencies are reported in Table 6 of USGS SIR 2022-5005. In addition, the publication reports calculated SIR 2022-5005 values as well as a weighted value based on the statistical analysis value and the regression method in Table 8. As time elapses and more annual rainfall events are recorded, the data in these tables will become outdated.
There will be times when estimates made from a flood-frequency analysis of a gaging station on the stream being studied will not agree with a regional analysis, such as the SIR 2022-5005 method. Various factors such as length of runoff records, storm distribution, and parameters used in the regional analysis could account for some of the discrepancies. When gaging station records are used, the designer should consult SIR 2022-5005 and current USGS data.
184.108.40.206.2 Transposition of Flows
When a project site falls between 0.5 and 1.5 times the drainage area of a stream gaging station on the same stream, the flow may be transposed to the project site using the methodology presented in SIR 2022-5005 (equation 36, page 24).
220.127.116.11 Other Methods/Models
18.104.22.168.1 NRCS TR-55 Curve Number Method (WinTR-55 Program)
Technical Release 55 (TR-55) Urban Hydrology for Small Watersheds is an NRCS, formerly the Soil Conservation Service (SCS), curve number method and is applicable to small urban watersheds (NRCS, 1986). The required input data are drainage area, curve number (which is a function of land cover and hydrologic soil group), and a Tc. TR-55 uses a segmental method to compute Tc (i.e., flow time is computed by adding the times for the overland, shallow concentrated, and channel segments).
This method must also meet the following conditions:
- Assumes that rainfall is uniformly distributed over the entire basin.
- Basin is drained by a single main channel or by multiple channels with times of concentration (Tc) within 10 percent of each other.
- Tc is between 0.1 and 10 hours.
- Storage in the drainage area is ≤ 5 percent and does not affect the time of concentration.
- Watershed can be accurately represented by a single composite curve number.
The TR-55 method greatly overestimates runoff for very flat watersheds in the Delaware coastal plain. If TR-55 is used, the Curve Number must be calibrated by comparing the flows generated by TR-55 against results from another method in this section, such as the Delaware regression method. Curve Numbers must be adjusted to match the desired peak of the design event within the 90-percent confidence interval, upper limit.
Limitations of TR-55 are described on page 5-3 of the Small Watershed Hydrology: WinTR-55 User Guide (USDA NRCS, 2009). The tabular method is used to determine peak flows and hydrographs within a watershed. However, its accuracy decreases as the complexity of the watershed increases. NRCS recommends that the computer program TR-20 be used instead of the tabular method if any of the following conditions apply:
- Tt is greater than 3 hours.
- Tc is greater than 2 hours.
- Drainage areas of individual subareas differ by a factor of 5 or more.
- The entire flood hydrograph is needed for flood routing.
- The time to peak discharge needs to be more accurate than that obtained by the tabular method.
WinTR-55 is a single-event, rainfall-runoff, small-watershed hydrologic model based on the TR-55 methodology. The WinTR-55 program can generate and plot hydrographs, compute peak discharges, and perform detention pond storage estimates. It can account for hydrograph shift and attenuation due to reach routing. WinTR-55 has limitations that assume a less complex watershed (e.g., 10 subwatersheds or less, 25-square-mile drainage area maximum, trapezoidal-shaped channel, and 2-point stage-storage curve for a reservoir). Refer to the TR-55 manual and WinTR-55 User Guide for additional limitations.
The WinTR-20 computer program, developed by the NRCS, computes flood hydrographs from runoff and routes the flow through stream channels and reservoirs; WinTR-20 is preferred over TR-55 and the DOS version of TR-20.
In the WinTR-20 program, routed hydrographs are combined with those of tributaries. The program provides procedures for hydrograph separation by branching or diversion of flow and for adding baseflow. Peak discharges, their times of occurrence, WSEs and duration of flows can be computed at any desired cross section or structure. Complete discharge hydrographs as well as discharge hydrograph elevations can be obtained if requested. The program provides for the analysis of up to nine different rainstorm distributions over a watershed under various combinations of land treatment, floodwater retarding structures, diversions, and channel modifications. Such analyses can be performed on as many as 200 subwatersheds or reaches and 99 structures in any one continuous run.
22.214.171.124.3 HEC-HMS and HEC-1
HEC-HMS is a program that is a generalized modeling system capable of representing many different watersheds. It is designed to simulate the precipitation-runoff processes of dendritic watershed systems and is applicable in a wide range of geographic areas for solving the widest- possible range of problems. HEC-HMS, like its predecessor, HEC-1, is extremely flexible in that its hydrologic elements include subbasins, reaches, junctions, reservoirs, and diversions. Hydrograph computations should be performed using the Delmarva Unit Hydrograph (UH) for all projects south of the Chesapeake and Delaware Canal, and the NRCS standard UH or Snyder UH north of the Chesapeake and Delaware Canal. User-specified s-graphs and UHs are allowed if more specific data are available. The Snyder method allows for variable peak flow rate factor, and calibrating the Snyder method makes it more versatile. HEC-HMS is preferred over HEC-1, which is Fortran based; HEC-HMS is Windows-compatible with a graphical user interface.
126.96.36.199.4 GIS Preprocessing Models
There are various GIS software packages and/or extensions to GIS software that allow preprocessing of digital terrain (DEMs, triangulated irregular networks [TINs]), land use, and soil data to develop the parameters (time-of-concentration values, rational “C” values, NRCS CN, etc.) required for hydrologic methods or models. These packages can save valuable time and provide accurate data and parameters that can be modified for various scenarios. Two such packages are discussed below.
188.8.131.52.4.1 The Watershed Modeling System
The Watershed Modeling System (WMS) is a comprehensive GIS/modeling environment for hydrologic analysis. It was developed by the Environmental Modeling Research Laboratory of Brigham Young University in cooperation with the USACE Waterways Experiment Station and FHWA, and is currently being developed by Aquaveo LLC. WMS offers state-of-the-art tools to perform automated basin delineation and to compute important basin parameters such as area, slope, and runoff distances for input into the H&H models discussed in this section. WMS supports the HEC-1, HEC-HMS, TR-20, and SWMM models, and the TR-55, rational, and NFF methods.
The Geospatial Hydrologic Modeling Extension (HEC-GeoHMS) has been developed as a geospatial hydrology toolkit for engineers and hydrologists with limited GIS experience. HEC-GeoHMS uses ArcGIS and the Spatial Analyst extension to develop a number of hydrologic modeling inputs for the Hydrologic Engineering Center's Hydrologic Modeling System, HEC-HMS, and is useful when hydrologic modeling is required (e.g., for a bridge crossing below a dam). It can be downloaded from the USACE HEC website. ArcGIS and its Spatial Analyst extension are available from the Environmental Systems Research Institute, Inc. (ESRI). Analyzing digital terrain data, HEC-GeoHMS transforms the drainage paths and watershed boundaries into a hydrologic data structure that represents the drainage network. The program allows users to visualize spatial information, document watershed characteristics, perform spatial analysis, and delineate subbasins and streams. Working with HEC-GeoHMS through its interfaces, menus, tools, buttons, and context-sensitive online help allows the user to quickly create hydrologic inputs for HEC-HMS.
104.2.4 Methodology Selection Guidance
The criteria below provide general guidelines and identify which method to use for particular circumstances. However, the final decisions regarding the suitability of a particular method or model for a particular project must be determined by engineering judgment on a case-by-case basis. Even though a methodology or model is recommended for various circumstances below, those methods or models should still be compared against other methods and models, field observations, local testimony, and any additional maintenance or site history.
- For drainage areas less than 200 acres, the rational method is recommended.
- For drainage areas greater than 200 acres, but less than the lower limits of the regression methods, consider TR-55, or TR-20 as applicable.
- For larger drainage areas:
- For project locations at a stream gage, perform the flood frequency analysis of recorded stream gage data and consider the weighted method described above and in the Delaware regression method (USGS SIR 2022-5005).
- For ungaged site locations with a drainage area that is between 0.5 and 1.5 times the drainage area of a stream gaging station that is on the same stream, use the transposition method described above.
- For site locations downstream of a dam, lake, or reservoir that will attenuate flows and impact the flows at the site, the results of Delaware’s Dam Safety Program should be used. If these data are not available for a particular site, the procedure outlined in Section 104.3.3 – Hydraulics for Dam Safety Projects must be used.
- For ungaged sites with natural flow conditions on nontidal streams, the Delaware regression method (USGS SIR 2022-5005) is recommended. Apply confidence intervals as per Section 104.2.6 – Confidence Intervals. With engineering judgment, HEC-HMS, TR-55, or TR-20, if applicable, can be considered for use at a particular bridge site.
- Account for urbanization, if warranted based on engineering judgment.
104.2.5 Design Flood Frequency
The design frequencies for bridges and pipe culverts for each highway functional classification are shown on Figure 104‑5. If a design frequency less than that shown on Figure 104‑5 is used, the design must be based on a risk analysis and must be approved by the Bridge Design Engineer. The requirements for a risk analysis are documented in Section 104.8.4 – Risk Assessment or Analysis. Evacuation routes should be evaluated to determine if a larger design event is applicable. As a minimum criteria, evacuation routes should not overtop or have water encroach upon the travel lanes at Q100, though it is not necessary to maintain any freeboard. For bridges located immediately downstream of a dam, coordination with DNREC’s Dam Safety Program is required.
(Click here for the URL in Figure 104-5 - https://deldot.maps.arcgis.com/apps/webappviewer/index.html?id=dfa83d46ba27493bbebb642ff0733c40)
104.2.6 Confidence Intervals
Due to the standard error inherent in the regression equations, it is possible that the design flow value does not capture the actual flow value. A confidence interval is applied to increase the flow values obtained from the regression equations in order for the designer to have a greater assurance that the design flow envelopes the actual flow value. Consequently, the resulting structure, designed to pass the design flow, is hydraulically sufficient for the location.
While the discussion in SIR 2022-5005 focuses on 90-percent confidence intervals, DelDOT has developed tiers of confidence intervals to be applied according to the functional classification of the roadway and the structure type as presented in Table 104‑1. Use these flows for both existing and proposed hydraulic analyses.
|Table 104-1. Design Frequency and Confidence Interval|
|Functional Classification||Design Frequency (Years: Confidence Interval)|
|Bridges Under 20 feet, Pipes and|
|Interstates and Freeways
Principal Arterials and Minor Arterials
Major Collectors and Minor Collectors
Local Roads and Streets and Subdivision Streets
However, with engineering judgment, use of confidence intervals can be waived or reduced to a different tier, with approval of the Bridge Design Engineer, if the results do not agree with other sources of information such as previous models or studies, field observations, local testimony, and any additional maintenance or site history. Document any such adjustments to design flows in the Hydrology section of the H&H Report.
104.2.7 Frequency Mixing (Probability of Coincidental Occurrence)
The designer is often faced with a situation in which the hydraulic characteristics of the subject facility are influenced by a flood condition of a separate and independent drainage course. For example, a small stream may outfall into a major river that itself is an outfall for a large and independently active watershed. It can reasonably be expected that these two waterways would seldom peak at the same time. Consequently, there are two independent events: one, a storm event occurring on the small stream; the other, a storm event applicable to the larger watershed.
In ordinary hydrologic circumstances, flood events on different watersheds are not usually entirely independent. Therefore, guidelines have been developed by the NCHRP Transportation Research Board to provide acceptable mixing criteria for independent waterways affected by separate storm events. NCHRP Web-Only Document 199, Estimating Joint Probabilities of Design Coincident Flows at Stream Confluences (2013) is a scientific approach to this issue that may be used for bridges, riverine structures, and culverts.
The effects of tidal flows must be considered when the designer is evaluating the frequency mixing relationships. For more information, see Section 104.3.4 – Tidal Hydraulics – Bridges and Culverts.
Hydraulic analysis is used to evaluate the effect of proposed highway structures on water surface profiles, flow and velocity distributions, lateral and vertical stability of channels, flood risk, and the potential reaction of streams to changes in variables such as structure type, shape, location, and scour control measures. Various hydraulic considerations and models for culverts and bridges are described below.
A culvert is a structure that is usually a closed conduit or waterway that may be designed hydraulically to take advantage of submergence to increase hydraulic capacity. A culvert conveys surface water through a roadway embankment or away from the highway right-of-way. In addition to this hydraulic function, it also must carry construction traffic, highway traffic, and earth loads; therefore, culvert design involves both hydraulic and structural design. The hydraulic and structural designs must be such that risks to traffic, property damage, and failure from floods are consistent with good engineering practice and economics.
Hydraulic design of culverts should be in conformance with DelDOT Road Design Manual, FHWA’s HDS-5, Hydraulic Design of Highway Culverts (2012a), and other support documents such as FHWA’s HEC-14, Hydraulic Design of Energy Dissipators for Culverts and Channels (2006). In most cases, frames should be designed as culverts. Computer programs such as HEC-RAS and HY-8 are recommended for the hydraulic analysis.
Where debris accumulation may be a problem, single-barrel culvert designs are preferred. In many instances, three culvert installations could be a single box, but the three pipes are more economical to install. No more than three barrels should be constructed at a single location. Allow at least 2 feet between pipe culverts on multi-pipe installations to allow room for compaction equipment.
Culverts, as distinguished from bridges, are usually covered with embankment and are composed of structural materials around the entire perimeter, although some are supported on spread footings with the streambed or riprap channel serving as the bottom. For economic and hydraulic efficiency, culverts should be designed to operate with the inlet submerged during design flows if conditions permit. Bridges, on the other hand, are not designed to take advantage of submergence to increase hydraulic capacity, even though some are designed to be inundated under flood conditions. The designer must consider analysis of the following items before starting the culvert design process:
- Site and roadway data
- Design parameters, including shape, material and orientation
- Hydrology (flood magnitude versus frequency relation)
- Channel analysis (stage versus discharge relation)
The maximum allowable headwater (HW) is the depth of water, measured from the entrance invert, that can be ponded during the design flood. Freeboard is an additional depth regarded as a safety factor, above the peak design water elevation. The minimum freeboard for culverts is 1 foot below the edge of pavement or top of curb in town sections. The peak design water elevation in this case will be based on the design event displayed on Figure 104‑5. Consideration should be given to the impact on the upstream properties. The headwater should be checked for the design flood, based on roadway classification, and for the 100-year flood to ensure compliance with floodplain management criteria and safety.
The culvert must be designed according to the appropriate design frequency in conformance with Section 104.2.6 – Confidence Intervals.
184.108.40.206 Site Conditions and Skew
The performance, capacity, and required culvert size of a culvert are functions of several parameters, including the culvert geometric configuration and stream characteristics. Roadway profile, terrain, foundation condition, aquatic organism passage requirements, shape of the existing channel, allowable headwater, channel characteristics, flood damage evaluations, construction and maintenance costs, and service life are some of the factors that influence culvert type selection.
Where the stream approach is skewed, all waterway areas should be measured normal to the stream flow, i.e., corrected by the bridge length times the cosine of the skew angle. Adjustment for skew should be made for projects with a skew between 20 and 35 degrees. In Hydraulics for Bridge Waterways (FHWA HDS1, 1978) model testing of the effect of skew on low-flow skewed crossings shows angles less than 20 degrees provide acceptable flow conditions without adjusting for skew. For increasing angles, flow efficiency decreased. The results indicate that using the projected opening width is adequate for angles up to 30 degrees for small flow contractions. A skew angle greater than 30 to 35 degrees requires closer examination, as the skew adjustment may be underestimating the true effective flow width. The projected area of the piers should likewise be corrected. The plans should indicate that the waterway areas are normal to stream flow when corrected for a skewed approach.
220.127.116.11.1 Channel Characteristics
The design of the culvert should consider the physical characteristics of the existing stream channel. For purposes of documentation and design analysis, sufficient channel cross sections (at least four), a streambed profile, and the horizontal alignment should be obtained to provide an accurate representation of the channel, including the floodplain area. These cross sections can be used to obtain the natural streambed width, side slopes, and floodplain width. Often, the proposed culvert is positioned at the same longitudinal slope as the streambed. The channel profile should extend far enough beyond the proposed culvert location to define the slope and location of any large streambed irregularities, such as headcutting. The designer must also use this preconstruction data to predict the consequences of constricting the natural floodplain by installing an embankment across a floodplain.
General characteristics helpful in making design decisions should be noted. These include channel roughness, Manning’s n values, the type of soil or rock in the streambed, the bank conditions, type and extent of vegetal cover, permanent or intermittent wetlands, amount of drift and debris, ice conditions, and any other factors that could affect the sizing of the culvert and the durability of culvert materials. Photographs of the channel and the adjoining area can be valuable aids to the designer and serve as documentation of existing conditions.
18.104.22.168.2 High-Water Information
High-water marks can be can be used to check results of flood estimating procedures, establish highway grade lines, and locate hydraulic controls. Often the high-water mark represents the energy of the stream and not the water surface. Even if the high-water marks are available, it often is difficult to determine the flood discharge that created them.
When high-water information is obtained, the individuals contacted should be identified and the length of their familiarity with the site should be noted. In addition, the designer should ascertain whether irregularities such as channel blockage or downstream backwater altered the expected high water. Other sources for such data might include commercial and school bus drivers, mail carriers, law enforcement officers, and highway and railroad maintenance personnel.
22.214.171.124.3 Inlet/Outlet Conditions
Culverts exhibit a wide range of flow patterns under varying discharges and tailwater elevations. To simplify the design process, two broad flow types are defined—inlet control and outlet control. A culvert operates with inlet control when the flow capacity is controlled at the entrance by the depth of headwater and the entrance geometry, including the barrel shape, the cross-sectional area, and the inlet edge. With inlet control, the roughness and length of the culvert barrel and outlet conditions are not factors in determining culvert hydraulic performance. Special entrance designs can improve hydraulic performance and result in a more efficient and economical structure. Entrance geometry and wingwall configuration are factors where improvement in performance can be achieved by modifications to the culvert inlet, particularly between projecting inlets and beveled edge inlets.
With outlet control, factors that may appreciably affect performance for a given culvert size and headwater are barrel length and roughness, culvert slope, and tailwater depth.
For each type of control, the headwater elevation is computed using applicable hydraulic principles and coefficients, and the greater headwater elevation is adopted for the design.
The maximum acceptable outlet velocity should be identified. High headwater can produce unacceptable velocities; therefore, the headwater should be set to produce acceptable velocities. Otherwise, stabilization or energy dissipation should be provided where acceptable velocities are exceeded. For streams with debris issues, trash racks should be considered.
Refer to Section 350.01 - Pipe Culvert Details for the required pipe cover. Refer to DelDOT’s Standard Construction Details, Standard Specifications, AASHTO LRFD, and manufacturer’s recommendations for proper bedding and cover requirements under roadway pavements.
Culvert shape and material are discussed in Section 107 – Final Design Considerations – Substructure.
126.96.36.199 Environmental Considerations
Culverts must be designed with environmental considerations such as fish, reptile and amphibian migration, habitat, riparian buffers, channel erosion, and sedimentation based on the recommendations of the Environmental Studies Section. In most cases, a natural bottom in culverts is required to facilitate the passage of aquatic organisms and endangered species such as the bog turtle, and for stream continuity.
Many resource agencies have established design criteria for the passage of aquatic organisms through culverts. These include maximum allowable velocity, minimum water depth, maximum culvert length and gradient, type of structure, and construction scheduling. For culvert locations on streams with a continuous flow, the ability to accommodate migrating and resident aquatic organisms is an important design consideration. Excessive velocity, inadequate water depth, and high outlet elevations are the most frequent causes of passage problems for aquatic organisms. Culverts should be designed to simulate the natural stream bottom conditions by maintaining desirable flow depths and velocities.
Constructing depressed culverts will help to simulate natural conditions by promoting the deposition and retention of streambed material inside the culvert. The streambed material will increase the roughness coefficient of the culvert bottom, which helps to maintain the minimum flow depth and reduce velocities. Baffles or weir plates may be added for this purpose. Baffles should be used to retain channel-bed fill (CBF) in culverts placed on stream slopes greater than 2 percent.
In addition, low-flow channels and correction of grades for better stream continuity should be applied where recommended by the Environmental Studies Section. DelDOT’s Environmental Studies Section has developed guidelines for pipes, boxes, and covering riprap. General guidelines to address environmental concerns are summarized below:
- Only one barrel of a multiple-barrel pipe or box culvert installation needs to be lowered.
- Pipes are depressed 6 inches to allow siltation to provide a natural bottom. If there is a series of pipes, the center pipe is to be lowered 6 inches below the streambed and the side pipes are to be raised 6 inches above unless cover is a problem. If cover is a problem, coordinate with the Environmental Studies Section for a variance. (See Section 350.01 – Pipe Culvert Details).
- Box culverts are depressed 12 inches. Depressed boxes should be filled with channel-bed fill material. Additionally, riprap should be depressed 12 inches below the streambed, choked with borrow (type B), and covered with channel-bed fill material or the gradation material specified for the county. (See Section 355.01 – Precast Concrete Box Culvert Details).
- Pipe and culvert outlet inverts should not be above the stream invert to avoid a hanging culvert situation. The designer should work with the Environmental Studies Section and reference the biological stream forms.
- In wide, shallow streams, one barrel of a multiple-barrel culvert should be depressed to carry low flow, or weirs can be installed at the upstream end of some barrels to provide for passage of aquatic organisms through other barrels at low flow. The weir option is particularly useful for cover-challenged pipes.
- For low-flow channels in rigid frames and bridges, stream bottoms should have riprap depressed 12 inches and should follow the shape of the proposed low-flow channel to help with its long-term stability. Locations with sufficient depth of water in all seasons do not require low-flow channels in most cases. Low-flow channels should be used as recommended by the Environmental Studies Section.
- Side slopes where riprap is used should be backfilled with #57 stone and cover with soil and seed from roughly the ordinary high water to the top of bank as appropriate.
- Riprap design at smaller structures should be based on scour countermeasure calculations. Riprap should be choked with Delaware #57 stone or channel-bed material unless conditions warrant otherwise.
- The designer is directed to Section 300 – Typical Bridge Design Detail for typical pipe, culvert, or rigid frame details.
The H&H report must contain information as to whether the stream flow is continuous or intermittent. The report must contain all necessary information to support the decision to provide or not provide passage for aquatic organisms through the culvert.
The proposed arrangements for passage of aquatic organisms must be indicated on the plans for the proposed culvert.
H&H analyses are required for all bridge projects over waterways. Typically, these analyses should include an estimate of peak discharge (sometimes complete runoff hydrographs), comparisons of water surface profiles for existing and proposed conditions, consideration of potential stream stability problems, and consideration of scour potential.
Bridges are important and expensive highway-hydraulic structures and are vulnerable to failure from flood-related causes. To minimize the risk of failure, the hydraulic requirements of stream crossings must be recognized and considered carefully.
The hydraulic analysis of a bridge for a particular flood frequency involves the following general considerations related to the hydraulic analyses for the location and design of the bridge:
- Backwater associated with each alternative vertical profile and waterway opening should not significantly increase flood damage to property upstream of the crossing.
- Effects on flow distribution and velocities – the velocities through the structure(s) should not damage either the highway facility or increase damages to adjacent property.
- Existing flow distribution should be maintained to the extent practicable.
- Pier spacing and orientation, and abutment should be designed to minimize flow disruption and potential scour.
- Foundation design and/or scour countermeasures should be considered to avoid failure by scour.
- Freeboard at structure(s) should be designed to pass anticipated debris and ice.
- Risks of damage should be considered.
- Stream instability countermeasures.
- Ways to achieve minimal disruption of ecosystems and values unique to the floodplain and stream should be considered.
- Highway level of service should be compatible with that commonly expected for the class of highway.
- Design choices should support costs for construction, maintenance, and operation, including probable repair and reconstruction and potential liability that are affordable.
The bridge routines in HEC-RAS allow for three different methods to model flow through bridges; low flow, high flow and combination flow. Low flow occurs when the water only flows through the bridge opening without coming into contact with the low chord and is considered open channel flow. The energy equations (standard step backwater) would be applied in this instance. If piers are present, then the momentum and/or Yarnell equations should be applied. Although HEC-RAS allows computations of all three methods simultaneously, the results based on the highest energy answer should be used.
High flow occurs when the water surface encounters the highest point in the low chord on the upstream side of the bridge. Orifice or sluice gate flow will occur though the waterway opening and if the road is overtopped, weir flow will occur over the roadway. The pressure and/or weir high-flow method should be toggled on in HEC-RAS if this situation occurs. Combination flow occurs when both low flow and pressure flow occur simultaneously with flow over the bridge.
FHWA’s HDS-7, Design of Safe Bridges (2012b) is a document that provides technical information and guidance on the hydraulic analysis and design of bridges. The goal is to provide information such that bridges can be designed as safely as possible while optimizing costs and limiting impacts to property and the environment. Many significant aspects of bridge hydraulic design are discussed, including regulatory topics, specific approaches for bridge hydraulic modeling, hydraulic model selection, bridge design impacts on scour and stream instability, and sediment transport.
Freeboard for a bridge is defined as the clear vertical distance between the water surface and the low point of the superstructure. The minimum freeboard is 1 foot for the design event. In no case should the bearings be submerged during the design event.
188.8.131.52 Site Conditions and Skew
Hydraulic considerations in site selection are numerous because of the many possible flow conditions that may be encountered at the crossing and because of the many water-related environmental factors. Flow may be in an incised stream channel, or the stream may have floodplains that are several miles wide. Floodplains may be clear or heavily vegetated, symmetrical about the stream channel or highly eccentric, clearly defined by natural topography or man-made levees, or indeterminate. Flow may be uniformly distributed across the floodplains or concentrated in swales in the overbank areas. Flow direction often varies with the return period of the flow, so that a bridge substructure oriented for one flow would be incorrectly oriented for another. Flow direction in overbank areas is often unrelated to that in the main or low-flow channel. In some instances, the floodplains convey a large proportion of the total flow during extreme floods and the stream channel conveys only a small proportion.
Not all of the above will apply to each stream crossing or bridge location, but many of the most important site considerations are hydraulic or water related. Crossing location alternatives often do not include the most desirable site from the hydraulic design viewpoint, but the difficulties involved often can be reduced by careful hydraulic analysis.
Features that are important to the hydraulic performance of a bridge include the approach fill alignment, skew, and profile; bridge location and length; span lengths; bent and pier location and design; and foundation and superstructure configuration and elevations. These features of a highway stream crossing are usually the responsibility of location, design, and bridge engineers; however, the integrity and safety of the facility are often as dependent upon competent hydraulic design as on competent structural and geometric design.
The same principles that apply to culvert skew as discussed in Section 184.108.40.206 – Site Conditions and Skew apply to bridge skew.
Incorporation of roadway approaches that will accommodate overflow may be necessary for some configurations. Such overflow reduces the threat to the bridge structure itself. Of course, the flow of traffic is interrupted, and the potential costs associated with such interruption and potential damage to the roadway embankment and bridge integrity should be considered by the designer.
Some of the factors to consider in the selection and orientation of bridge alignments are as follows:
- The safety of the highway user
- Vertical profile and horizontal alignment
- Hydraulic performance
- Construction and maintenance costs
- Foundation conditions
- Highway capacity
- Navigation requirements
- Stream regime
Bridge shape and material are discussed in Section 107 – Final Design Considerations – Substructure.
104.3.3 Hydraulics for Dam Safety Projects
Dams in Delaware are regulated by Section 5103 Dam Safety Regulations of Title 7 of the Delaware Code. It is the purpose of these Regulations to provide for the proper design, construction, operation, maintenance, and inspection of dams in the interest of public health, safety, and welfare in order to reduce the risk of failure of dams and to prevent death or injuries to persons; damage to downstream property, infrastructure, and lifeline facilities; and loss of reservoir storage. The Delaware Dam Safety Program is administered by the Delaware Department of Natural Resources and Environmental Control.
The owner of any new or existing dam that is regulated under these Regulations and is classified as a Class I High Hazard Potential, or Class II Significant Hazard Potential, in accordance with Section 5.0 of the Regulations, must prepare an Emergency Action Plan (EAP) in accordance with the requirements of the Regulations.
All bridge and culvert projects should consider any H&H studies of nearby dams. Studies of all state-regulated dams have been completed. These studies typically include a hydrologic and dam break analyses and inundation mapping with flows computed to several bridge sites. Data and results of these studies should be referenced to see if any information is applicable to the bridge site.
The designer must also consider how dams might impact sediment transport conditions in downstream reaches (possibly affecting stream stability and scour at infrastructure) and tailwater design conditions in upstream reaches.
Occasionally bridges impact or are impacted by dams. Spillway design must take into consideration field survey data, drainage areas, reservoir capacity (from elevation and storage data), tidal influences, magnitude of peak in-flows for the design storm (considering frequency mixing), Spillway Design Flood (SDF), the Probable Maximum Flood (PMF), required freeboard below the top of the reservoir, detention or retention structures, water surface profiles, anticipated future development, and breach damage potentials.
The significant range and nature of the influences that apply to normal H&H analyses also apply to spillway design. The designer is referred to Title 7 Natural Resources & Environmental Control of the Delaware Administrative Code, 5000 Division of Soil and Water Conservation, 5103 Delaware Dam Safety Regulations for the dam hazard classification, SDF, and other requirements. The designer is also referred to various publications of the USACE and to the U.S. Bureau of Reclamation (USBR) Design of Small Dams (1987) publication concerning design requirements.
The design storm for spillway design should be based on a risk evaluation as described in Section 104.8.4 – Risk Assessment or Analysis. The design storm must be approved by the Bridge Design Engineer. The minimum design storm for spillway design is the 100-year storm. Provisions should be made for drainage of the pond.
Typically, HEC-RAS (River Analysis System), HEC-HMS (Dam Breach Routine), or HEC-1 (Flood Hydrograph Package) software is used by the designer for dam/reservoir analysis. Critical to any spillway design are the breach analysis, inundation area mapping, and flood damage estimates, including estimates for economic losses and loss of life. Care must be taken not to affect existing water levels in the new design. Changes could have detrimental effects on adjacent properties.
220.127.116.11 Site Conditions and Bridges Near Non-regulated Dams
Dams attenuate flow, reducing the inflow to a reduced outflow, and cause backwater behind the impoundment. Bridges near nonregulated dams need to have considered the effects of the dam and the storage area behind it. For bridges or culverts below nonregulated dams, a determination should be made as to whether the dam will attenuate the flows. If the dam does, a storage-indication routing must be performed using a program such as HEC-HMS or HEC-1, and the attenuated outflow from the dam or reservoir should be used to evaluate the waterway opening. For bridges above nonregulated dams or reservoirs, the backwater or ponding area should be evaluated to see if it affects the capacity of the waterway opening.
Bridge shape and material would be the same as the shape and material for bridges not affected by dams and are discussed in Section 107 – Final Design Considerations – Substructure.
18.104.22.168 Dam Safety Regulations
Any work with a dam or spillway should be coordinated with DNREC’s Division of Watershed Stewardship, Dam Safety Program.
104.3.4 Tidal Hydraulics – Bridges and Culverts
Tidally affected river crossings are characterized by both river flow and tidal fluctuations. From a hydraulic standpoint, the flow in the river is influenced by tidal fluctuations that result in a cyclic variation in the downstream control of the tailwater in the river estuary. The degree to which tidal fluctuations influence the discharge at the river crossing depends on such factors as the relative distance from the ocean to the crossing, riverbed slope, cross-sectional area, storage volume, and hydraulic resistance. Although other factors are involved, relative distance of the river crossing from the ocean can be used as a qualitative indicator of tidal influence. At one extreme, where the crossing is located far upstream, the flow in the river may only be affected to a minor degree by changes in tailwater control due to tidal fluctuations. As such, the tidal fluctuation downstream will result in only minor fluctuations in the depth, velocity, and discharge through the bridge crossing. Therefore, an analysis of bridges or culverts in tidal areas needs to consider these processes.
There are several circumstances in which the potential for tidal impacts is significant.
Channel migration of tidal streams is a particular problem. Tidal hydraulics are produced by astronomical tides and storm surges and are sometimes combined with riverine flows. Storm surges are produced by wind action and rapid changes in barometric pressure. The driving force in riverine hydraulics is the gravitational force down the topographic slope of the stream. In tidal hydraulics, the driving force is the rapidly changing elevation of the tide and wind setup. For sites located near the coast there are three potential hydraulic conditions:
- Structure hydraulics is riverine controlled and not impacted by tide/storm surge;
- Structure hydraulics is tidally influenced in that the tailwater condition is influenced by the tide/storm surge, but there is no flow reversal through the structure; and
- Structure hydraulics is tidally controlled in that flow reverses through the structure during tide/storm surge.
Tidal gages, current FEMA mapping, and historic data can be used to evaluate whether the structure is riverine, tidally influenced or tidally controlled. The size of the bridge opening may be controlled in a case of incoming (flood) tidal flows and peak storm discharge. Another consideration that may control the size of the opening is the storm surge at peak flood tidal flows. In the same manner, scour of the stream bottom is a concern on outgoing (ebb) tidal flows. These and other combinations of tidal and storm flows must be considered in the sizing and design of a structure. Historic aerial photographs dating back as early as possible should be studied to determine the direction and speed of channel migration in the vicinity of the proposed bridge.
In tidally controlled areas, bridge lengths are generally controlled by wetland considerations rather than hydraulics. The primary purpose of hydraulic analyses for bridges in tidal areas is typically to establish the grade of the bridge and to determine the scour depths around the substructure. Exceptions to this rule are where an opening is being created or increased in an existing causeway or where a culvert is used. In these cases, the opening must be sized so that the velocities through the opening will not create scour problems. A significant head difference can develop across a causeway due to either the tide or wind setup. A sufficient opening should be provided to relieve this difference. A detailed analysis should be conducted to correctly size the opening.
Where the stream is influenced or controlled by tidal fluctuations at the structure location, the most critical of the following three hydraulic scenarios should be used to analyze backwater elevations and/or scour conditions. The most critical scenario for the waterway opening design (backwater elevations) may not be the most critical scenario for scour analysis (velocity analysis).
Scenario 1: A steady-flow scenario with design upland flow (from the stream or river) for the hydraulic design event and the scour design event. The overtopping event and 100-year event may be required for projects in New Castle County. The downstream boundary is set to the MHW elevation of the tidal receiving water daily astronomical tide. Note that the downstream MHW elevation may be higher than the roadway overtopping elevation, in which case no overtopping flood profile will result from Scenario 1.
Scenario 2: A steady-flow scenario with design upland flow (from the stream or river) for the hydraulic design event and the scour design event. The overtopping event and 100-year event may be required for projects in New Castle County. The downstream boundary is set to the MLW elevation of the tidal receiving water daily astronomical tide. Note that the overtopping flood may be higher than the 100-year flood event, in which case the overtopping flood is not considered under Scenario 2.
Scenario 3: An unsteady-flow scenario with the source of flooding being the ebb and flood tides from the tidal receiving water (no upland flow from the stream or river). Downstream boundary conditions are the design, 100-year, and 200-year storm surge hydrographs from the tidal receiving water as well as the daily astronomical tide hydrograph, which generates the overtopping flood event.
The astronomical high- and low-tide elevations (MHW and MLW) and the design storm surge hydrographs should be calculated based on the approach described in the FHWA’s HEC-25, Highways in the Coastal Environment (2020). The unsteady HEC-RAS model under Scenario 3, “no upland flow,” should be simulated for a total period of 60 hours, which comprises the entire surge period in Delaware. Stillwater elevations of the tidal receiving water can be obtained from FEMA’s FISs.
22.214.171.124 Use of Qualified Coastal Engineers
If coastal hydraulics are significant to the bridge or culvert design, a qualified coastal engineer should review the complexity of the tidal conditions to determine the appropriate level of coastal engineering expertise needed in the design. Conditions that typically require direct attention by a coastal engineer are as follows:
- Hydraulic analysis of interconnected inlet systems
- Analysis of inlet or channel instability, either vertically or horizontally
- Determination of design wave parameters
- Prediction of overwash and channel cutting
- Design of countermeasures for inlet instability, wave attack, or channel cutting
- Prediction of sediment transport or design of countermeasures to control sediment transport
- Assessment of wave loading on bridges and other structures
126.96.36.199 Tidal Hydraulic and Scour Analysis
FHWA’s HEC-18, Evaluating Scour at Bridges (2012c), Chapter 9 contains three levels for tidal hydraulic analysis and scour. Level 1 includes a qualitative evaluation of the stability of the inlet or estuary, estimating the magnitude of the tides, storm surges, and flow in the tidal waterway and attempting to determine whether the hydraulic analysis depends on tidal or river conditions, or both. Level 2 represents the engineering analysis necessary to obtain the velocity, depths, and discharge for tidal waterways to be used in determining long-term aggradation, degradation, contraction scour, and local scour. In Level 2 analyses, unsteady one-dimensional (1-D) or quasi two-dimensional (2-D) computer models may be used to obtain the hydraulic variables needed for the scour equations. For complex tidal situations, Level 3 analysis using physical and 2-D computer models may be required. The Level 1, 2, and 3 approaches are described in more detail in HEC-18.
For additional information to support the analysis and modeling of scour for bridges crossing tidal waterways, refer to the third edition of FHWA’s HEC-25, Highways in the Coastal Environment (2020; see Chapter 12). For additional information on scour, see Section 104.4 – Scour Evaluation and Protection.
188.8.131.52 Tidal Modeling
Two-dimensional hydrodynamic modeling is an important tool for design water levels, flows, and scour depths at tidally influenced bridge crossings. This tool is particularly useful when examining coastal bridges, since the design flows are often influenced by the incoming tide. For estuaries with large or vegetated floodplains, where the simple tidal prism method is overly conservative due to high-flow resistance, dynamic modeling is most appropriate. Dynamic modeling is also most appropriate in the case of large bays where an assumed level water surface is overly conservative or where wind effects are significant. If conditions warrant, DelDOT may require a tidal analysis.
184.108.40.206 Freeboard for Tidal Bridges
Bridges on tidal streams will be designed to protect the bridge structure itself. Often, much of the surrounding land and the approach roadways will be inundated by relatively frequent (10- to 25-year) tidal storm surges. The recommended design freeboard for bridges in these areas is 2.0 feet above the 10-year high-water elevation, including wave height), or the results of the analysis in scenario 3 in Section 220.127.116.11 – General, whichever is greater. It is also recommended that the bottom of all interior bent cap elevations be above the extreme high tide. The finished grade of the bridge will be set based on this recommendation, navigation clearances, the approach roadways, topography, and practical engineering judgment. If these conditions are not currently met with an existing structure, improvements to the proposed structure should be considered and evaluated.
The selection of a design water level can be one of the most critical coastal engineering decisions for the design of tidal bridges and structures. For example, the design water level often controls the design wave height, stone size, and extent of armoring on coastal revetments. Also, wave loads on elevated bridge decks are extremely sensitive to water level. Essentially, the water level dictates where waves can reach and attack.
It should be noted, that final freeboard should be chosen to be site specific and the choice should be based on practical judgment. Design water level decisions should be addressed using the traditional risk-based approach of a “design return period,” which is common in hydraulic engineering. For example, the 100-year storm surge level is the surge elevation with a 1 percent annual risk of exceedance. Each year, there is a 1 percent chance that a storm surge of this magnitude (or greater) will occur. Some coastal designs may justify a lower return period (e.g., a 25-year or 50-year return period) in certain areas, balancing the greater risks affiliated with such design with engineering and economic considerations. The selection of the design storm surge SWL (still-water-level) can be based on an analysis of historic storm surge elevations at the specific site or on an analysis that incorporates site-specific modeling of historical (hindcast) storm surges. Evacuation routes should be evaluated for access during events that require evacuation. HEC-25 provides more detail on the design of bridges and culverts in tidal areas.
18.104.22.168 Sea Level Rise
In accordance with Executive Order 41, all state agencies must incorporate measures for adapting to increased flood heights and sea level rise in the siting and design of projects for construction of new structures and reconstruction of substantially damaged structures and infrastructure. Such projects must be sited to avoid and minimize flood risks that would unnecessarily increase state liability and decrease public safety.
Construction projects should also incorporate measures to improve resiliency to flood heights, erosion, and sea level rise using natural systems or green infrastructure to improve resiliency wherever practical and effective; if the structures are within an area mapped by DNREC as vulnerable to sea level rise inundation, the projects must be designed and constructed to account for sea level changes anticipated during the lifespan of the structure in addition to FEMA flood levels; and all state agencies must consider and incorporate the sea level rise scenarios set forth by the DNREC Sea Level Rise Technical Committee into appropriate long-range plans for infrastructure, facilities, land management, land use, and capital spending.
22.214.171.124 Tidal Hydraulics References
The following models, studies, and reports should be referred to as appropriate:
- HEC-18, Evaluating Scour at Bridges, Fifth Edition (FHWA, 2012) (tidal prism method);
- HEC-25, Highways in the Coastal Environment, Third Edition (FHWA, 2020);
- UNET, RMA-2, or ADCIRC models;
- Any of the various tidal models for Chesapeake and Delaware Bays in combination with the nontidal flow calculated above to produce the maximum flood, which does not overtop the roadway or structure;
- Existing FEMA studies; or
- Existing Coastal Engineering Research Center reports.
104.3.5 Hydraulics Methodologies and Software
Listed and described below are hydraulic models typically used in the design of culverts and bridges. For a hydraulic analysis that would involve revisions to FEMA’s Flood Insurance Rate Maps, selection of the hydraulic model should be coordinated with FEMA.
HEC-RAS is a Windows-based program that performs 1-D open channel analysis for steady or unsteady flow, gradually varied flow, sediment transport-mobile bed modeling, and water temperature analysis in both natural and man-made river channels. It is the preferred program for analysis of DelDOT bridges. Information from this program is used to make WSE and freeboard calculations. Some HEC-RAS capabilities include the modeling of water surface profiles in both subcritical and supercritical flows around various obstructions, such as bridges, culverts, weirs, and structures in the floodplain.
HEC-RAS is the recommended model for performing hydraulic analysis of steady, gradually varied (longitudinal), 1-D open channel flow. HEC-RAS includes a culvert module that is consistent with HDS-5 and HY-8. HEC-RAS applies conservation of momentum as well as energy and mass in its hydraulic analysis. HEC-RAS includes all the features inherent to HEC-2 and WSPRO plus several friction slope methods, mixed flow regime support, ice cover, quasi 2-D velocity distribution, bank erosion, riprap design, stable channel design, sediment transport calculations, and scour at bridges. HEC-RAS, HEC-2, and HY-8 do not produce identical results. For detailed information on a comparison of HEC-RAS to HEC-2, see Appendix C of the HEC-RAS River Analysis System Hydraulic Reference Manual.
The bridge scour routines in the hydraulic design module of HEC-RAS should not be used for bridge scour computations or to compute scour depths. However, HEC-RAS allows the user to input nondefault parameters into the scour computations, which can be a useful check. The designer should exercise caution when using HEC-RAS output parameters other than velocities in scour computations. The designer should request that the appropriate cross sections be surveyed to provide for scour considerations.
Culvert hydraulic computations should follow the standard FHWA procedures for conventional culverts described in HDS-5. The HY-8 computer program applies the theories and principles of HDS-5 and HEC-14. HY-8 automates culvert hydraulic computations and includes a routine for analysis and design of culverts with improved inlets and energy dissipators. HY-8 can perform computations associated with tailwater elevations, road overtopping, hydrographs, simple flood routing, and multiple independent barrels. HY-8's most convenient features are its well-designed reports and plots, especially the culvert performance curves and the tailwater rating curves. Caution should be used when using HY-8 if a significant backwater is present at the outlet due to downstream conditions; if that is the case, a rating curve may be more appropriate to represent the downstream backwater.
126.96.36.199 Two-Dimensional Hydraulic Models
In certain complex situations, 1-D models may not be able to adequately model the situation. In this case, 2-D models are typically employed. Examples of acceptable 2-D hydraulic modeling programs are the TUFLOW Program and the USBR SRH-2D hydraulic model; both programs interface with Aquaveo’s SMS (Surface-Water Modeling System) software. The FHWA Hydraulics Team has adopted the USBR SRH-2D hydraulic model, which includes the development of several new modeling features. SRH-2D uses a hybrid irregular mesh that accommodates arbitrarily shaped cells. A combination of quadrilateral and triangular elements may be used with varying densities to obtain the desired detail and solution accuracy in specific areas of interest. In other words, the entire model mesh does not need to have a high density throughout the entire model to get a high resolution of results at a bridge or other structure. This flexibility allows for greater detail in specified areas without compromising computing time. Second, SRH-2D uses a numerical solution scheme that is impressively robust and stable. The element wetting and drying issues that plagued many FST2DH (FESWMS) models are no longer a problem. Together, the improved SRH-2D model and custom SMS interface provide a powerful tool for transportation hydraulics.
The TUFLOW model was developed by BMT WBM Pty Ltd in Australia. TUFLOW offers 1-D and 2-D flood and tide simulation software. TUFLOW is a finite difference model that can handle a wide range of hydraulic situations, including mixed flow regimes, weir flow, bridge decks, box culverts, and robust wetting and drying. 2-D models are useful in situations of flows with significant horizontal velocity components other than in the downstream direction (i.e. 2-D flow patterns) as well as situations with time-variant flow patterns such as those in tidal environments.
Examples of hydraulic conditions where a 2-D model may be needed are outlined below; however, this list is not all-inclusive (for further information, refer to HDS-7):
- The stream slope is very flat, and bridge piers cause localized effects on WSEs. The 1-D model will average these localized increases in WSE across the entire cross section and apply the calculated WSE increase across the entire floodplain width, which is not realistic. The 1-D model may also overestimate the magnitude and upstream extent of the pier-induced WSE increase.
- Hydraulics at the project site are affected by a confluence that changes location for different flood events and cause 2-D characteristics in the floodplain.
- Flow is split between multiple structures across a wide floodplain.
- A structure is on a severe channel bend (making the velocity vary between the inside and outside of the bend), and scour is a major concern.
- A project is anticipated to cause WSE increases in a highly developed area, and flooding impacts need to be more accurately defined.
- Tidal areas.
104.4 Scour Evaluation and Protection
Scour is the result of the erosive action of running water excavating and carrying away material from the bed and banks of streams. Every bridge over a waterway should be evaluated as to its vulnerability to scour in order to determine the appropriate protective measures. Most waterways can be expected to experience scour over a bridge’s service life. The need to ensure public safety and to minimize the adverse effects stemming from bridge closures requires the best effort to improve the state-of-practice of designing and maintaining bridge foundations to resist the effects of scour.
The reference for scour investigation is HEC-18. The intent of HEC-18 is to establish methods for estimating the various scour components for use in conjunction with engineering judgment to determine the total potential depth of scour. In addition, FHWA’s HEC-23, Bridge Scour and Stream Instability Countermeasures (1997), contains useful information on the selection and design of measures to minimize the potential damage to bridges and other highway components at stream crossings. For bridges that are tidally impacted, the FHWA’s HEC-25, Highways in the Coastal Environment (2020) is the primary reference, as discussed in Section 104.3.4 – Tidal Hydraulics – Bridges and Culverts.
Incipient motion is where hydrodynamic forces acting on a grain of sediment reach a value that, if increased slightly, will move the grain.
Clear-water scour occurs when the bed material sediment transported in the uncontracted approach flow is negligible or the material being transported in the upstream reach is transported through the downstream reach at less than the capacity of the flow. In this case, the scour hole reaches equilibrium when the average bed shear stress is less than that required for incipient motion of the bed material.
Live-bed scour occurs when there is streambed sediment being transported into the contracted section from upstream. In this case, the scour hole reaches equilibrium when the transport of bed material out of the scour hole is equal to that transported into the scour hole from upstream.
104.4.1 Scour Investigation
Scour investigations must be completed for all structures crossing waterways. This investigation should be included with the foundation submission and H&H Report. The investigation should contain scour calculations per Section 104.4.4 – Design Considerations. The investigation should also include site inspections, including inspections of nearby structures as necessary and interviews with DelDOT maintenance personnel in charge of post-event inspections. Scour investigations must be developed using a multidisciplinary approach involving the hydraulics engineer, the geotechnical engineer, structural engineer, and coastal engineer (if needed per Section 188.8.131.52 – Use of Qualified Coastal Engineers). The investigation is required to evaluate and design bridge foundations and scour countermeasures. For bridge replacement projects, a determination of historical scour at the existing structure is important. The evaluation of historical scour can be based on previous bridge inspection reports and/or geotechnical assessments of the streambed materials. For most new bridges, pier scour will be accommodated by adjusting the pier design in cooperation with the geotechnical and structural engineers, and abutment scour will be mitigated with countermeasures. However, modifying the size of the opening to reduce the total scour or minimize countermeasures may be a consideration depending on the bridge site. For existing bridges, pier and abutment scour are mitigated with hydraulic or structural countermeasures or monitoring. NCHRP Web-Only Document 181, Evaluation of Bridge Scour Research – Abutment and Contraction Scour – Processes and Prediction (2013) is an additional resource for abutment and contraction scour abatement.
104.4.2 Scour Components
The current published guidelines provide that bridge scour be evaluated as interrelated components, including:
- Long-term scour (aggradation or degradation of the stream channel)
- Contraction scour, including vertical pressure scour if applicable
- Local scour (pier and abutment including constrictions caused by ice or debris)
In addition, lateral migration of the channel must be assessed when evaluating total scour at bridge piers and abutments. The summation of each scour component depth is defined as the total scour depth. Design considerations and applications related to the various scour components are covered in Section 104.4.4 – Design Considerations.
The FHWA hydraulic toolbox has scour calculators that follow the procedures presented in HEC-18. They can be found online at: http://www.fhwa.dot.gov/engineering/hydraulics/software.cfm.
184.108.40.206 Long-Term Scour
Aggradation and degradation are long-term changes in stream channel elevation. Degradation is the scouring of bed material due to increased stream sediment transport capacity, while aggradation is the deposition of bedload. The effects of aggradation or degradation changes are not the same as local scour or erosion because they extend greater distances along the streambed and are not localized to the structure of interest. Vertical stream morphology changes take place slowly but well within the service life of a bridge. It is necessary to look at where the river or channel bed has been and where it is now, and to anticipate its position in the future. Channel alteration, changes in upstream land use, streambed mining, and the construction of dams and control structures are the major causes of degradation problems. Long-term profile changes can result from streambed profile changes that occur from aggradation and/or degradation. Forms of degradation and aggradation should be considered as imposing a permanent future change for the streambed elevation at a bridge site where they can be identified.
220.127.116.11 Contraction Scour
Contraction scour equations are based on the principle of conservation of sediment transport (continuity). As scour develops, the shear stress in the contracted section decreases as a result of a larger flow area and decreasing average velocity. For live-bed scour, maximum scour occurs when the shear stress reduces to the point that the sediment transported in equals the bed sediment transported out and the conditions for sediment continuity are in balance. For clear-water scour, the transport into the contracted section is essentially zero, and maximum scour occurs when the shear stress reduces to the critical shear stress of the bed material in the bridge cross section.
Contraction scour occurs when the flow area of a stream at flood stage is reduced, either by a natural contraction of the stream channel or by a bridge. It also occurs when overbank flow is forced back to the channel by roadway embankments at the approaches to a bridge. Contraction scour depths should be calculated using the live-bed and/or clear-water equations. Pressure flow scour (vertical contraction scour) should be calculated for all structures under pressure flow, according to HEC-18 Section 6.10.
18.104.22.168 Local Scour
At piers or abutments, local scour is caused by the formation of vortices at their base. The horseshoe vortex at a bridge pier results from the pileup of water on the upstream surface of the obstruction and subsequent acceleration of the flow around the nose of the pier. The action of the vortex removes bed material from around the base of the pier. The transport rate of sediment away from the base region is greater than the transport rate into the region; consequently, a scour hole develops.
Local scour depths for piers and unprotected abutments should be calculated using equations that apply to the sites and design conditions. Because the local scour equations tend to overestimate the magnitude of scour at abutments, they are generally used only to gain insight into the scour potential at an abutment. The NCHRP 24-20 Abutment Scour Approach presented in HEC-18, Section 8.6.3, calculates the total abutment scour, including contraction scour. The NCHRP 24-20 method may provide more reasonable estimates of abutment scour, as it does not require the effective embankment length, which can be difficult to determine. The equations are more physically representative of the abutment scour process, and the equations predict total scour at the abutment rather than the abutment scour component that is then added to the contraction scour.
Local pier scour depth should be calculated using the HEC-18 pier scour equation (Chapter 7.2, HEC-18) for live-bed and clear-water conditions when the pier footing is not exposed to the flow. The pier width in the HEC-18 equation should be the pier width perpendicular to the flow direction for the frequency event being considered. When there is a history of debris accumulation on bridge piers, scour from debris on piers should be calculated with Equation 7.32 of HEC-18; engineering judgment, bridge inspection records (including underwater inspection reports), and maintenance records are required to estimate several variables. Scour for complex pier foundations should be calculated in accordance with the procedures described in Section 7.5 of HEC-18. Local pier scour for wide piers in fine bed material should be calculated with the Florida Department of Transportation pier scour methodology (Chapter 7.3, HEC-18).
104.4.3 Scour Flood Magnitude
The scour design flood and the scour design check flood should be evaluated in the scour design for new bridges and existing bridges that have a scour plan of action (POA) or where emergency maintenance countermeasures are required. For the scour design flood, the stability of the bridge foundation should be investigated using the service and strength limit states. The scour check flood should be used as the scour design flood. The scour design flood and check flood are determined from Table 104‑2.
|Table 104-2. DelDOT Scour Design Flood and Check Flood|
|Hydraulic Design Flood Frequency from Figure 104-5||Scour Design (QS) and Check Flood Frequency (QC)|
Note a pressure-flow event of a lesser recurrence interval may cause the worst-case scour condition and should be considered at sites with pressure-flow conditions. Both tidal and nontidal bridges over waterways with scourable beds should withstand the effects of scour from the scour design check flood without failing. For the check flood for scour, the stability of a bridge foundation must be investigated for scour conditions resulting from a designated flood storm surge, tide, or mixed population flood, and must be designed to be stable for the extreme event limit state.
If the site conditions due to ice or debris jams and low tailwater conditions near stream confluences dictate the use of a more severe flood event for either the design or check flood for scour, the designer may use the more severe flood event.
104.4.4 Design Considerations
Bridge foundations must be designed to withstand the effects of scour for the worst conditions resulting from floods. The geotechnical analysis of bridge foundations should be performed on the basis that all streambed material in the scour prism above the total scour line for the scour design flood has been removed. In general, foundations are designed based on the total scour depth obtained from the scour design flood and to be stable without relying on scour countermeasures.
The only exception to this is when designing for local scour at abutments (see Section 104.2.4 Local Scour). Use engineering judgment when considering the results of local abutment scour calculations. If the full scour depth is not used to set the abutment foundation, then both the abutment foundation and the scour countermeasure must be designed to be stable after the effects of the estimated long-term degradation and contraction scour.
No scour analysis for a pipe or box culvert is required. For rigid frames with a 25-foot span or less, scour analysis may be omitted if properly designed scour countermeasures are installed.
22.214.171.124 Scour Due to Lateral Movement
Pier and abutment foundations must be designed for the maximum total scour to account for channel and thalweg shifting. The scour due to lateral movement or shifting of the stream should be evaluated for bridges on floodplains with a history of lateral movements of the stream from one side of the floodplain to the other through geologic time. FHWA’s HEC-20, Stream Stability at Highway Structures (2012d) and HEC-23, Bridge Scour and Stream Instability Countermeasures (1997) should be referenced for lateral stream movement and instability issues. For multi-span bridges, a scour prism plot (Chapter 8, HEC-18), which illustrates the total scour depth at any location in the bridge opening, and a site evaluation should be included with the scour analysis in the H&H and Foundation Reports. Refer to HEC-18, Appendix D, for an example of a total scour prism plot.
126.96.36.199 Spread Footings
Spread footings on erodible material should be considered only if scour calculations are completed and can be corroborated by a site inspection and by the performance of spread footings in nearby structures that have survived major floods. Set the top of footing elevation of spread footings on erodible material below scour depth with consideration of local scour. See Section 107.3.2 Spread Footing Foundations and Section 107.3.5 Additional Foundation Details for further information. Otherwise, the bridge foundation should be extended to sound bedrock or supported on piles.
Rigid frames with a 25-foot span or less do not have to be designed with the footing elevation below scour depth when properly designed scour countermeasures are provided. Footing elevations should be placed below the bottom of countermeasure elevation.
188.8.131.52 Dams and Backwater
Where the maximum high-water elevation at the structure is due to a backwater condition resulting from the stage of a downstream waterbody, the scour investigation should consider the calculations based on a 100-year flood resulting from the watershed upstream from the structure, assuming no backwater from a downstream confluence.
Where dams exist upstream from the structure, the design flood for the dam and its spillway should be considered in the scour investigation. In addition, if the road is expected to be in service in an emergency event according to the dam’s EAP, then the sunny-day dam break flow should also be considered in the scour investigation.
184.108.40.206 Streambed Material
The D50 value is important in scour equations. The D50 is taken as an average of the streambed material size in the reach of the stream just upstream of the bridge. It is a characteristic size of the material that will be transported by the stream. Normally, this would be the bed material size in the upper 1 foot of the streambed, which may capture the armor layer (i.e., larger, more uniform particles) of the stream, if present. Significantly underestimating the D50 value may result in overly conservative scour depths. Therefore, acceptable means to estimate D50 include:
- Visual inspection – Appropriate for all types of bed materials. Field tools (e.g., sand gage card, gravelometer, wire screen) are readily available to assist the hydraulic engineer in streambed particle size determination.
- Sieve analysis from volume/bulk samples.
The D50 should typically not be estimated from soil surveys or soil borings only. When a boring is taken within the channel area, it will sample a small-diameter core (2 to 4 inches) through the bed material and soil layers, typically down to bedrock. The boring diameter may limit the D50 measurement because any sediment size greater than the boring diameter will not be captured. If the D100 particle size is less than the core diameter and the sample is taken in the stream channel, the soil borings may provide a reasonable D50. Additionally, the soil boring locations are determined based on the substructure unit's (e.g., a pier) location and are not representative of the streambed material within the entire channel section.
However, soil borings are a critical component of a site investigation to determine critical soil parameters for scour estimates. Soil borings help determine soil layer stratification (differential erosion rates) and can be used for grain size analysis for finer-grained soils. Poor scour estimates can often be due, in part, to poor soil classification and the use of surficial samples only for soil properties.
220.127.116.11 Scour in Cohesive Soils
The clay content in soil increases cohesion, and relatively large forces are required to erode the riverbed. Higher pulsating drag and lift forces increase dynamic action on aggregates until the bonds between aggregates are gradually destroyed. Aggregates are carried away by the flow. Dr. Jean-Louis Briaud at Texas A&M University has proposed the SRICOS-EFA (Scour Rate in Cohesive Soil – Erosion Function Apparatus) method of scour measurement in cohesive soils (NCHRP, 2003).
- In cohesive soils such as clay, both local scour and contraction scour magnitudes may be similar. However, scour takes place considerably later than in the noncohesive sand.
- Scour analysis methods are different for cohesive and noncohesive soils.
- Bridge foundations supported by cohesive soils resist erosion for a much longer period than usually calculated, and may result in a longer life of bridge.
The bed material may be comprised of sediments (alluvial deposits) or other erodible materials. If bed materials are stratified, a conservative approach needs to be adopted regarding the risks of the scour breaking through the more resistant layer into the less resistant layer. Scour analysis of bridge piers and abutments in cohesive soils can be carried out on the basis of the NCHRP 516 report, Pier and Contraction Scour in Cohesive Soils (2004) and the procedures for scour in cohesive soil in HEC-18.
18.104.22.168 Scourability of Rock
The scour potential of rock may be evaluated by following the latest information on procedures for scour in rock in HEC-18, Sections 4.2.3, 4.6 and 4.7. Section 6.8 describes how to compute contraction scour in erodible rock, while Section 7.13 discusses pier scour in erodible rock and provides calculation examples. The designer should also reference NCHRP Report 717: Scour at Bridge Foundations on Rock (2012). Section 3.4 of NCHRP Report 717, Modes of Rock Scour, identifies four erosion processes in natural rock-bed channels: dissolution of soluble rocks, cavitation, quarrying and plucking of fractured rocks, and abrasion of degradable rocks.
The following criteria represent the values to define rock quality and scourability of rock:
- The RQD value is a modified computation of the percent of rock core recovery that reflects the relative frequency of discontinuities and the compressibility of the rock mass and may indirectly be used as a measure of scourability. The RQD is determined by measuring and summing all the pieces of sound rock 6 inches (150 millimeters) and longer in a core run and dividing this by the total core run length. The RQD should be computed using NX diameter cores or larger and on samples from double tube core barrels. Scourability potential will increase as the quality of rock becomes poorer. Rock with an RQD value of less than 50 percent should be assumed to be soil-like with regard to scour potential.
- The primary intact rock property for foundation design is unconfined compressive strength (ASTM Test D7012). Although the strength of jointed rocks is generally less than individual units of the rock mass, the unconfined compressive strength provides an upper limit of the rock mass bearing capacity and an index value for rock classification. In general, samples with unconfined compressive strength below 250 pounds per square inch are not considered to behave as rock. There is only a generalized correlation between unconfined compressive strength and scourability.
- The slake durability index (SDI as defined by the International Society of Rock Mechanics) is a test used on metamorphic and sedimentary rocks such as slate and shale. An SDI value of less than 90 indicates poor rock quality. The lower the value, the more scourable and less durable the rock.
- AASHTO Test T104 is a laboratory test for soundness of rock. A soaking procedure in a magnesium and sodium sulfate solution is used. Generally the less sound the rock, the more scourable it will be. Threshold loss rates of 12 (sodium) and 18 (magnesium) can be used as an indirect measure of scour potential.
- The Los Angeles abrasion test (AASHTO T96) is an empirical test to assess abrasion of aggregates. In general, the less a material abrades during this test, the less it will scour. Loss percentages greater than 40 percent indicate scourable rock.
The other methods described in that memorandum should be used if required. For other soil types, existing surface borings and tests of soil samples should be interpreted.
104.4.5 Scour Countermeasures
In most cases, a scour countermeasure, properly designed and installed in accordance with the procedures outlined in HEC-23, is provided to protect piers and to resist local scour at abutments.
Substructure layout must be designed and balanced with other bridge design concerns to minimize flow disruption and potential scour, subject to navigation requirements. Countermeasures to alleviate local scour at abutments consist of measures that improve flow orientation at the bridge face and move local scour away from the abutment, such as guide banks, as well as revetments and riprap placed on spill slopes.
For rigid frames with a 25-foot span or less, refer to HEC-23 for scour countermeasure design procedures. In cases where an existing structure does not exhibit signs of scour and a proposed frame is anticipated to reduce the effects of scour, the frame may be considered as a culvert for the purpose of designing scour countermeasures. See Section 104.5.3 - Scour Protection at Culverts.
22.214.171.124 Riprap Protection
The use of a minimum of R-4 riprap is allowed where countermeasure calculations show it is adequate, as long as the riprap is covered by topsoil or CBF as specified in the Standard Specifications. If the riprap is exposed, a minimum of R-5 riprap should be used. Larger riprap, R-7 maximum, may be specified, if it is needed to meet design requirements. Where R-6 riprap is insufficient, consider use of other stabilization options such as articulated concrete block (preferred next choice), grouted riprap or roller compacted concrete. The scour countermeasure in the channel should be covered with a minimum of 1 foot of CBF. Riprap, despite its efficiency, is not recommended as an adequate substitute for foundations or piling located below expected scour depths for the new or replacement bridge.
Slopes in front of stub abutments should be adequately protected, and/or sheeting should be provided to prevent undermining of the abutment and loss of fill. Riprap must always be used to protect abutments from erosion for maintenance purposes, even if it is not required to resist the effects of local scour. The use of concrete slope paving is prohibited; concrete slope paving must be replaced with riprap on any rehab projects where it exists.
Refer to Section 355.01 – Precast Concrete Box Culvert Details for scour protection details for box culverts, to Section 350.01 – Pipe Culvert Details for scour protection for pipes, and to Section 360.01 – Precast Concrete Rigid Frame Details for scour protection for rigid frames.
Also, refer to NCHRP Report 587, Countermeasures to Protect Bridge Abutments from Scour (2007); HEC-23; and NCHRP Project 24-23, Riprap Design Criteria, Specifications, and Quality Control (2006) for additional information.
126.96.36.199 Guide Banks
A guide bank is a dike extending upstream from the approach embankment at either or both sides of the bridge opening to direct the flow through the opening. Some guide banks extend downstream from the bridge (also referred to as a spur dike). Guide banks are quite useful where a stream makes a turn into a structure and have been applied successfully for abutment protection in braided, meandering, and straight streams. Flow disturbances, such as eddies and cross-flow, will be eliminated where a properly designed and constructed guide bank is placed at a bridge abutment.
188.8.131.52 Scour Protection at Culverts
HEC-14, Chapter 4 of the AASHTO Highway Drainage Guidelines (2007), and HEC-23 provide design procedures for the hydraulic design of highway culverts. Included therein are design examples, tables, and charts that provide a basis for determining the selection of a culvert opening.
- Footings for any flared wingwalls, provided at the entry and the exit of culverts, will be protected by riprap or alternate armoring countermeasures.
- For velocities exceeding 12 feet per second, a less constrictive opening should be considered to reduce velocities. Regular monitoring will be required if riprap has been installed at the entry and exit of culverts.
- Skew of a culvert should be matched to the angle of attack of the stream as much as possible to help alleviate local scour.
- Wingwall orientation chosen should eliminate sharp corners at entrances that may cause eddies.
See also Section 184.108.40.206 – Scour Aprons for additional information on scour protection at culverts.
104.4.6 Scour Evaluation Documentation
The scour evaluation documentation must be included as part of the H&H Report and Foundation Report and should contain the following information:
- Bridge description — bridge number, type, size, location, and NBI Record Item 113, Scour coding;
- Executive summary of scour results, conclusions, and any countermeasure recommendations required, with plan and profile views showing scour depths and limits;
- Scour computations (including computer input and output) that should include scour depths and plotted depths on cross sections and profiles; and
- Bridge drawings, cross sections, soils information, test results, other miscellaneous data, and references.
The report must contain a scour summary table in accordance with Table 104‑3.
|Table 104-3. Scour Summary|
|Computed Scour Depths (feet)||Proposed|
|Discharge Frequency||Long- term Scour||Contraction Scour||Local Scour||Total Scour||Top of Footing||Bottom of Footing|
104.4.7 Scour Plan Presentation
The calculated scour depth elevations are shown on the cross sections and profiles, and the overtopping flood discharge and elevation must be shown on the bridge profile sheets per the Title 23 of the Code of Federal Regulations (CFR) Part 650 and FHWA policy and technical guidance.
The following information will be provided in the Project Notes on the plans:
- Note stating that the structure has been analyzed for the effects of scour in accordance with the procedures described in HEC-18;
- Scour design flood flow, frequency, bridge opening velocity, and WSE immediately upstream from the bridge; and
- Calculated design scour depth, including a plot in cross section and profile.
A sample scour project note is provided in Section 301.01 - Bridge Project Notes.
The natural or altered condition of stream channels affects the flow characteristics. Any work being performed, proposed, or completed that modifies a stream channel changes the hydraulic efficiency of the stream and must be studied to determine its effect on the stream both upstream and downstream. The effect on WSEs at the structure site due to modification of a stream’s hydraulic characteristics must be determined. The designer should be aware of plans for channel modifications that might affect the stream hydraulics. Similarly, the effects of storm drainage systems and other water-related projects should be investigated. Any modifications that affect stream alignment should be kept to a minimum, particularly for the straightening of meandering streams.
104.5.1 Stream Stability Analysis
Streams upstream and downstream of the bridge or culvert must be stable, and if they are not, stabilization measures must be applied. Erosion is considered to be the loss of material on side slopes and stream banks. Types of stream erosion, which are all interrelated to some degree, include:
- The natural tendency of streams to meander within the floodplain
- Bank erosion
- Aggradation and degradation
The computed velocity is a measure of the potential erosion and scour. Exit velocity from culverts will be computed on the assumptions shown in HDS-5. (HY-8, Culvert Analysis, software based on HDS-5 for the computations should be used.) Average velocity computed on the gross waterway will be the representative velocity for open-span structures, furnished by computer analysis for WSEs.
Examples of highly erodible soil can be found in all areas of the state. Areas of loamy deposits, which are highly sensitive to erosion, are prevalent in Delaware. County NRCS soil maps and field investigations may aid in judging the in-situ material.
The designer must consider the downstream erosion potential in evaluating and sizing the structure. Under some conditions, any additional erosion would be intolerable. Thus, risk considerations should be included in the site study. Stream banks erode regardless of the presence of a highway crossing. Any alteration of erosion potential by a structure must be closely evaluated in judging the adequacy of a design. Designs should consider the angle of attack to the inlet and the direction of discharge of high-velocity flow (i.e., direction should not be into the opposite stream bank).
Streams naturally tend to seek their own gradient through either degradation or aggradation. Degradation is the erosion of streambed material, which lowers the streambed. Aggradation is the transport and deposition of the eroded material to change the streambed at another location. The effect of the structure on degradation or aggradation of a stream must be evaluated in bridge-crossing design.
The designer should evaluate the stability of the bed and banks of the waterway channel, including lateral movement, aggradation, and degradation, using HEC-20. When designing a replacement structure, an evaluation using HEC-20 is not required if existing conditions appear stable and proposed conditions are similar.
104.5.2 Bank Protection
The most common method of bank protection is the use of rock riprap. Factors to consider in the design of rock riprap protection include:
- Stream velocity
- Angle of the side slopes
- Size of the rock
Filter blankets of smaller gradation bedding stone or geotextiles are used under riprap to stabilize the subsoil and prevent piping damage. Riprap bank protection should terminate with a flexible cutoff wall.
The designer should specify a minimum riprap thickness of 18 inches for embankment protection and 24 inches for slope protection along stream banks and for streambeds, or the thickness of the riprap, whichever is greater. Refer to FHWA’s HDS-6, River Engineering for Highway Encroachments (2001), and FHWA’s HEC-23, Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Design Guidance-Third Edition (2009) - Design Guideline 4. See Section 300 – Typical Bridge Design Detail for typical riprap details and an example of a riprap installation. Typical slope and bank protection and channel lining are shown on Figure 104‑7.
104.5.3 Channel Modifications
A channel modification is the physical relocation of the streambed channel. Channel modifications are to be avoided in general, as it difficult to get approval from permitting agencies. However, a channel modification is sometimes the best solution and must be evaluated.
The preferred procedure for dealing with channel changes is as follows:
- Establish the nature of the existing stream (slope, section, meander pattern [sinuosity], stage-discharge relationship).
- Determine limits for changes in the various stream parameters.
- Duplicate existing conditions where possible, within established change tolerances, while protecting roadway and bridge infrastructure.
- Evaluate constructability, considering water table elevations, streambed materials, and site conditions.
For more guidance, refer to AASHTO’s Highway Drainage Guidelines and HEC-23.
104.5.4 Stream Diversions
When choosing a stream diversion method, multiple factors should be considered: volume of stream flow, structure type, site conditions, sequence of construction, and construction duration or duration needed for diversion. Incorporating the existing structure into the diversion plan should be considered as it often reduces time, impact, and cost. The sequence of construction should minimize the length of time a stream diversion is in place.
The construction sequence plans should show a complete plan for stream diversion and construction sequence for the convenience of contractors who do not have the experience necessary to design their own system. The plans should show a diversion method that maximizes the work area within the easements without adversely affecting the movement of manpower and equipment. The proposed plan should be simple to construct and made from common materials available to every contractor. Item sizes and locations must accurately be shown given consideration for site conditions and environmental impacts.
The contractor may submit alternate plans for stream diversion. The alternate plans must be approved by the Bridge Design Engineer, the Stormwater Engineer, and Environmental Studies group. The submission must clearly state any alternate plans and must demonstrate the capacity to pass the design diversion flow stated on the plans. Alterations may be rejected due to permit restrictions.
Temporary stream diversions should be sized based on Table 104‑4. Confidence Intervals should not be used and the quantity of flow designed for the diversion should be clearly stated in the plans.
|Table 104-4. Design Storm for Various Construction Periods|
|Construction Time||Design Storm|
|1-30 days||Estimate of base flow using surface velocity.1|
|31-90 days||25% of the 2-year storm|
|91-150 days||50% of the 2-year storm|
|151 days or more||100% of the 2-year storm|
In general, pump diversions are preferred over pipe diversions when the construction period design storm is 35 cubic feet per second or less. Pumping can be considered for higher flows if necessary. A typical 12-inch pump has a capacity of 9 cubic feet per second or less. Higher capacity 12-inch pumps have a capacity up to 20 cubic feet per second.
The size of diversion pipes should be specified in the plans. If pumps are used, pump sizes will not be designated in the plans. Weir dimensions and elevations should be noted for sheeting and sandbag dikes. The method for diverting clean water and stabilizing the outfall should be specified. Payment for these items will be included in the lump sum cost of the stream diversion item.
104.5.5 Ice and Debris
The quantity and size of ice and debris carried by a stream should be investigated and recorded for use in the design of drainage structures. The times of occurrence of ice or debris in relation to the occurrence of flood peaks should be determined, and the effect of backwater from ice or debris jams or recorded flood heights should be considered in using stream-flow records.
The location of the constriction or other obstacle-causing jams, whether at the site or structure under study or downstream, should be investigated, and the feasibility of correcting the problem should be considered. Maintenance personnel shall be consulted if ice and/or debris problems are expected.
Under normal circumstances, 1 foot of freeboard is sufficient to permit passage of ice flow and debris. When the drainage area produces unusually large amounts of debris, additional freeboard to protect the structure is desirable. At locations where large pieces or quantities of debris are anticipated, the designer should consider increasing the freeboard. Multiple pipe installations, multi-cell boxes, or in-stream piers should be avoided at locations with debris issues.
104.6 Hydrologic and Hydraulic Report
The intent of the H&H report is to document the H&H investigations and recommendations for a new alignment structure or a structure replacement or rehabilitation. The H&H report should be sealed by a registered Professional Engineer.
A recommended table of contents for the report and supporting information (Appendices) are provided in Appendix 104-4.
104.6.1 Hydraulic Summary Data Sheet and Definitions
The Hydraulic Summary Data Sheet found in Appendix 104-3 must be included with all H&H Reports. The summary data sheet is intended to provide a quick overview of the project site, channel and watershed, and existing and proposed structure information and hydraulics.
Descriptions of terms used in the checklist and to represent hydraulic data on plans follow. See Figure 104‑2 for a graphical depiction of the definitions.
- Documentation of Historic High Water includes year(s) of occurrence and source of information.
- Ordinary High Water is required information for the “404” permit. From instructions and definitions furnished by the USACE for “404” permit applications, the Ordinary High Water mark as defined by the USACE means the line on the shore established by the fluctuation of water and indicated by physical characteristics (such as a clear, natural line impressed on the bank, shelving, changes in the character of the soil, destruction of terrestrial vegetation, or the presence of litter and debris) or established by other appropriate means that consider the characteristics of the surrounding areas. Ordinary High Water will usually be established by the environmental studies. Where Ordinary High Water is not determined by a survey of the physical characteristics or a visual field inspection, it may be estimated by computation of the normal WSEs at the 50 percent chance rainfall (2-year) frequency (Q2).
- Design Discharge (Qdes) should be computed by the methods noted in this Manual. When other methods are applicable and are used to compute the Design Discharge, it should be noted in the hydraulic report.
- Design Headwater: As a conservative estimate of the headwater for design, the elevation of the water surface under unrestricted conditions at the upstream face of the bridge or culvert is used to compute clearance. It is the assumed condition where the water surface profile is computed at the design discharge (Qdes) with gradually varied flow. This computed high-water elevation should always be compared to the high-water elevation of record furnished by the field survey to determine whether an additional grade adjustment should be made for the extreme condition.
- Average Velocity is computed from the gross area at the bridge opening below the design flow depth, i.e., Q/An, where An is the gross waterway area in the constriction at Design High Water depth. Design Waterway Provided is the net flow area below the Design High Water elevation. Total Waterway Provided is the net flow area below the bridge. Total Waterway and Design Waterway will be the net flow area (i.e., they are deducted from the pier area).
- Design Backwater Elevation: For convenience, the amount of design backwater is measured as shown on the profile section on Figure 104‑1, for the computed design discharge (Qdes). Although this may not be the exact location of the maximum high water, it is accurate enough to provide a reasonable estimate. For critical locations where the exact backwater computation might affect the design (e.g., where a FEMA floodway exists), the designer should refer to the methods in HDS-7.
- The location of the Overtopping Elevation for the bridge and approaches may be referred by stationing (e.g., Station 6+95.7) or by distance from the bridge (e.g., 375 feet south of bridge abutment No. 1). The location of the overtopping may occur on the bridge or on an approach. The overtopping roadway elevation may be either the centerline elevation or the high shoulder elevation in a superelevated section.
- Freeboard, as applied to bridge hydraulics, is the vertical distance from the design headwater elevation to the low point of the superstructure. This distance is recorded on the Hydraulic Field Assessment Checklist (Appendix 104-1). Where the design headwater elevation is higher than the low point of the superstructure, there is no freeboard. For culverts, the design headwater elevation is 1 foot below the top of the slope to prevent overtopping. For bridges, freeboard is defined as the clear vertical distance between the water surface and the low point of the superstructure. The preferred minimum freeboard is 1 foot. The designer should increase freeboard above the routinely applied 1-foot criterion in areas where debris and ice could potentially diminish flow conveyance. Coordination with the bridge unit is required if the bridge structure cannot meet the preferred minimum freeboard.
104.7 Plan Presentation
The following hydrological and hydraulic information is required on the plans of structures over streams and should be included in the Project Notes on the General Notes sheet. A sample Hydraulic Data Note is provided in Section 301.01 - Bridge Project Notes.
- Drainage Area (square miles)
- Design Frequency (years)
- Design Discharge and Q100 (cubic feet per second)
- Existing and Proposed Design Flood Elevation (feet) (cross section just upstream of the structure)
- Existing and Proposed 100-Year Flood Elevation (feet)
- Existing and Proposed Waterway Opening (square feet)
- For tidal areas, the following information should be included:
- Mean High Water Elevation (feet)
- Mean Low Water Elevation (feet)
- Vertical Under Clearance (feet)
Refer to Section 104.4.7 – Scour Plan Presentation for plan presentation of scour analysis data. Additional site-specific information, such as the data described in Section 104.1.4 – Field Data Collection, may be required and noted on the plans as determined by the Bridge Design Engineer.
104.8 Laws, Policy, Regulations and Permits
The PDM has a summary of DelDOT’s policy and an extensive list of environmental laws, regulations, and policies in Appendix B. It also describes streamlining for cooperatively obtaining timely approval for transportation projects. The designer should be familiar with Appendix B before design begins.
104.8.1 FEMA Compliance
Floodplain management regulations are based on Executive Order 11988. A new Executive Order is being developed that will establish Federal flood risk management standards and consider climate change. As these guidelines are developed, there may be changes related to the FEMA considerations, flood heights, and sea level rise.
All projects affecting waterways within National Flood Insurance Program (NFIP) study areas will follow the standard procedures for compliance with floodway regulations (such as, but not limited to, 44 CFR 65.3, 44 CFR 65.12, and 23 CFR 650). FEMA floodway maps should be used to determine whether the proposed activity encroaches on the “Regulatory Floodway.” Any encroachment on a regulatory floodway should be avoided, where practicable. If this encroachment cannot be practicably avoided and results in an increase in the 100-year flood elevation, a revision of the floodway data and/or maps should be made. On an individual project basis, approval or concurrence will be required from FEMA and the applicable county for providing the corrective measure and revising the floodway information.
Where appropriate and applicable, the procedures as established between FEMA and the FHWA should be used for coordinating or adopting FEMA regulatory requirements on highway encroachments. Two such procedures are the letter of map revision (LOMR) and conditional letter of map revision (CLOMR). The CLOMR and LOMR are required if the DelDOT project impacts a designated floodway and causes an increase in the 100-year base flood elevation (BFE). Additionally, for projects located in a FEMA floodplain but not within the FEMA floodway, increases to the BFE above 1 foot will require a CLOMR and LOMR. These procedures are discussed in an FHWA memorandum Attachment 2 – Procedures for Coordinating Highway Encroachments of Floodplains with Federal Emergency Management Agency (FEMA) (1992). Additional regulations on this topic are found in 23 CFR Part 650, Subpart A, Location and Hydraulic Design of Encroachments on Flood Plains.
As a result of continuous FEMA floodplain map updates, all communities in Delaware that participate in the NFIP will be required to adopt updated floodplain regulatory language to comply with NFIP requirements.
104.8.2 New Castle County Requirements
Chapter 40, Article 10 of the UDC establishes criteria for structures in or near floodplains and floodways. All projects in New Castle County are subject to this ordinance. Any structure to be located, relocated, constructed, reconstructed, extended, enlarged, or structurally altered within a designated floodplain is subject to the UDC. The major items that must be included in the application procedures and plan that affect structure designers are as follows:
- Site location and tax parcel number;
- Brief description of the proposed work;
- Plan of the site showing the exact size and location of the proposed construction as well as any existing structures;
- Engineering analysis of the impact on the floodplain using HEC-RAS or another acceptable backwater analysis model;
- An accurate delineation of the floodplain area, including the location of any adjacent floodplain development or structures and the location of any existing or proposed subdivision and land development;
- Delineation of existing and proposed contours;
- Information concerning the 1-percent chance of occurrence (100-year) flood elevations and other applicable information, such as the size of structures, location and elevation of streets, water supply and sanitary sewer facilities, soil types, and flood-proofing measures; and
- An H&H report, certified by a registered Professional Engineer, that states that any proposed construction has been adequately designed to withstand the 100-year flood pressures, velocities, impact and uplift forces, and other hydrostatic, hydrodynamic, and buoyancy factors associated with the 100-year flood.
Refer to Appendix 1 of the New Castle County Unified Development Code for the specific requirements.
Projects in New Castle, Kent, and Sussex Counties are also subject to the regulations administered by FEMA. However, the UDC contains more stringent requirements concerning increases in water surface profiles that must be followed within the county. When water surface profiles are increased greater than permitted by the FEMA regulations, a CLOMR is required. Refer to FHWA memorandum Attachment 2: Procedures for Coordinating Highway Encroachments on Floodplains with Federal Emergency Management Agency (FEMA) and 23 CFR 650, Bridges, Structures, and Hydraulics.
104.8.3 Tax Ditches
Tax ditches are private organizations formed by adjacent property owners to construct and maintain a drainage system. These organizations are managed by officers elected by the owners and maintained by the County Conservation District (see Title 7, Chapter 41 of the Delaware Code). While there are existing tax ditch easements (solely for construction and maintenance of the ditches), DelDOT cannot use these easements without proper coordination. Alternately, DelDOT can secure separate easements for bridge and roadway construction and maintenance.
When designing structures over waterways that may be tax ditches, one must research the right-of-way and property owners in order to determine the existence and extents of the tax ditch. The Team Support Section can provide assistance in this research. Tax ditches are subject to an H&H design just like any other waterway.
Tax ditch easements need to be submitted through the Team Support Section, which will prepare and submit tax ditch agreements for approval. (Note that for projects designed by a consultant, the consultant will write the agreement and submit it to the Team Support Section for review and distribution.) Section 107 – Final Design Considerations – Substructure covers the position of footing when adjacent to tax ditches.
104.8.4 Risk Assessment or Analysis
A risk assessment or analysis with consideration given to capital costs and risks, and to other economic, engineering, social, and environmental concerns should be included for the applicable design alternative(s) of any waterway structure. Refer to 23 CFR 650 Subpart A, Section 650.105 for an explanation and definition of "risk analysis." Generally, the risk analysis involves monetary figures in the calculation of the risk and other factors, whereas the risk assessment only involves narrative description of the relevant factors.
According to 23 CFR 650 Subpart A, Section 650.115, the design selected for an encroachment must be supported by analyses of design alternatives, with consideration given to capital costs and risks and to other economic, engineering, social, and environmental concerns.
- Consideration of capital costs and risks should include, as appropriate, a risk analysis or assessment that includes:
- The overtopping flood or the base flood, whichever is greater, or
- The greatest flood that must flow through the highway drainage structure(s), where overtopping is not practicable. The greatest flood used in the analysis is subject to state-of-the-art capability to estimate the exceedance probability.
- The design flood for encroachments by through lanes of Interstate highways should not be less than the flood with a 2 percent chance of being exceeded in any given year. No minimum design flood is specified for Interstate highway ramps and frontage roads or for other highways.
Risk analysis must be performed and included for the following types of waterway structures or impacts:
- Encroachments at sensitive urban areas associated with new locations.
- Any encroachment determined to be a "significant encroachment" as defined in 23 CFR 650 Subpart A, Section 650.105.
If the design flood frequency for the structure is less than that shown on Figure 104‑5, a risk analysis may be required for submission to the Bridge Design Engineer.
Grossly undersized bridges can be impractical to change drastically, could cause downstream flooding, or can be difficult to get permitted. The flexibility to choose a lesser or appropriate design storm based on engineering judgment will be allowed for these types of structures.
Where a risk analysis is needed, a complete hydraulic report should be prepared that gives consideration to each alternative under study. The risk analysis, based on the least total expected cost (LTEC) design process, should be performed in accordance with the procedure as specified in FHWA's HEC-17, 2nd Edition, Highways in the River Environment - Floodplains, Extreme Events, Risk, and Resilience (2016).
Risk assessment should be performed and included for all other waterway structures not specified in items (1), (2), and (3) above. The lower level of study or risk assessment should always be considered as the first course of action. The risk assessment should include a comparison of existing versus proposed WSEs and floodplain boundaries for the design and 100-year event, and for affected structures and their first floor.
Many of the Department's bridge replacement projects require ATON, which warn waterway users of the changing conditions ahead as well as help guide these users through or around the project area. Projects on navigable waters within Coast Guard jurisdiction should coordinate with the Coast Guard. Place a standard note on plans that references DelDOT’s detailed Coast Guard Specific Conditions specification number 763522.
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