104 - Hydrology and Hydraulics

104.1 Introduction

The primary objective in the design of a highway stream crossing is to avoid causing interruption of the traffic using the bridge or crossing and 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.

104.1.1 Terms

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

104.1.2 Coordination

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 Chapter 6 of the DelDOT Road Design Manual (2004) 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

Data for the project will be developed from available survey data and USGS, LiDAR, or other topographic mapping. If sufficient data are not available, additional survey data will have to be obtained. 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 2-foot intervals for the State of Delaware were produced for New Castle and Kent Counties (based on the 2007 LIDAR) and for Sussex County (based on the 2005 LIDAR.) Data are in line shapefile format. 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 100-year event 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 a sample 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.

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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.

104.2 Hydrology

104.2.1 Introduction

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) 2006-5146, Magnitude and Frequency of Floods on Nontidal Streams in Delaware (2006) or by exercising engineering judgment.

104.2.2 Documentation

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.

104.2.3 Precipitation

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. Precipitation values in Figures 6-5 through 6-7 of the Road Design Manual correlate well with the rainfall data in NOAA Atlas 14. Precipitation intensity values for use in the Rational Method may be obtained from Figures 6-5 through 6-7. Figure 104‑3 provides 24-hour rainfall totals for use in methods requiring a 24-hour duration, such as the NRCS Curve Number method. Figure 6.11 in the Road Design Manual provides these values.

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104.2.3.1 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.

Section 6.6.3.1 of the Road Design Manual 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. It should be noted that the Road Design Manual provides equations to calculate Tc for the rational method (Section 6.6.3.1) that are different from those that it provides for the NRCS curve number method (Section 6.6.3.2). The rational “C” values should be obtained from Figure 6-8 of the Road Design Manual.

104.2.3.2 Delaware Regression Method (SIR 2006-5146)

DelDOT uses the equations in the current version of the SIR 2006-5146 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 Piedmont region, the size of the drainage area, percent of forest, percent of hydrologic soil group “A” and percent of storage from the National Hydrography Dataset (NHD) are considered in the equations. The variables used vary based on the design event. In the Coastal Plain region, the mean basin slope (in percent) determined from a 10-meter digital elevation model (DEM) must be considered in addition to the drainage area and percent hydrologic soil group A. Each of these parameters is discussed in the SIR 2006-5146 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 2006-5146 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 2006-5146 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.

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104.2.3.3 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.

104.2.3.4 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.

104.2.3.4.1 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 2006-5146. In addition, the publication reports calculated SIR 2006-5146 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 2006-5146 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 2006-5146 and current USGS data.

104.2.3.4.2 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 2006-5146 (equation 22, page 31).

104.2.3.5 Other Methods/Models

104.2.3.5.1 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 report provides a graphical and a tabular method for computing peak discharges of drainage basins with areas ranging from 10 acres up to 2,000 acres (3.1 square miles); however, the Road Design Manual states that TR-55 can be used for complex watersheds up to 300 acres. The required input data are drainage area, curve number (which is a function of land cover and hydrologic soil group), and a Tc. The Delaware soils and their assigned hydrologic soil group are shown on Figure 6-10 of the Road Design Manual.

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). Chapter 6 of the Road Design Manual provides the methodology and equations to compute Tc. Travel time (Tt) is the ratio of flow length to flow velocity.

This method must also meet the following conditions:

1. Assumes that rainfall is uniformly distributed over the entire basin.
2. Basin is drained by a single main channel or by multiple channels with times of concentration (Tc) within 10 percent of each other.
3. Tc is between 0.1 and 10 hours.
4. Storage in the drainage area is ≤ 5 percent and does not affect the time of concentration.
5. 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:

1. Tt is greater than 3 hours.
2. Tc is greater than 2 hours.
3. Drainage areas of individual subareas differ by a factor of 5 or more.
4. The entire flood hydrograph is needed for flood routing.
5. 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.

104.2.3.5.2 WinTR-20

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.

104.2.3.5.3 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.

104.2.3.5.4 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.

104.2.3.5.4.1	The Watershed Modeling System - need another sub-section

104.2.3.5.4.2	GeoHMS - need another sub-section

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.

1. For drainage areas less than 326 acres, the rational method is recommended.
2. 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 2006-5146).
3. 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.
4. 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.
5. For unregulated, ungaged site locations on nontidal streams, the Delaware regression method (USGS SIR 2006-5146), HEC-HMS, TR-55, or TR-20 should be considered.
6. For ungaged site locations with a drainage area that is not between 0.5 and 1.5 times the drainage area of a stream gaging station that is on the same stream, use the most appropriate method from the guidance above.
7. Account for urbanization, if warranted based on engineering judgment, according to the guidelines provided in USGS SIR 2006-5146.

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. For bridges located immediately downstream of a dam, coordination with DNREC’s Dam Safety Program is required.

104.2.6 Confidence Intervals

Confidence limits are used to estimate the uncertainties associated with the determination of floods of specified return periods from frequency distributions, as shown on Figure 104‑6. Since a given frequency distribution is only an estimated determinant from a sample of a population, it is probable that another sample from the same stream but taken at a different time would yield a different frequency curve. Confidence limits, or more correctly, confidence intervals, define the range within which these frequency curves could be expected to fall with specified confidence or levels of significance.

It should be left to engineer’s judgment and their confidence in the calculated results whether or not confident limits need to be explored. Bulletin 17B outlines a method for developing upper and lower confidence intervals. If confidence limits are employed, they should follow Table 104‑1, which provides the confidence interval for each road design classification.

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The designer is given leeway for adjusting the design frequency and/or confidence interval to account for special circumstances as warranted for individual projects based on risk/failure analyses.

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.

104.3 Hydraulics

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.

104.3.1 Culverts

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 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.

104.3.1.1 Sizing

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:

1. Site and roadway data
2. Design parameters, including shape, material and orientation
3. Hydrology (flood magnitude versus frequency relation)
4. 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.

104.3.1.2 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.

104.3.1.2.1 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.

104.3.1.2.2 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.

104.3.1.2.3 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.

In outlet control, the culvert hydraulic performance is determined by the factors governing inlet control plus the controlling WSE at the outlet and the slope, length, and roughness of the culvert barrel. 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 the Road Design Manual, Figure 6.3, 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.

104.3.1.3 Shape/Material

Culvert shape and material are discussed in Section 107 – Final Design Considerations – Substructure.

104.3.1.4 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:

1. Only one barrel of a multiple-barrel pipe or box culvert installation needs to be lowered.
2. 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).
3. 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).
4. Pipe and culvert outlets 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.
5. 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.
6. 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 and channels should be used as recommended by the Environmental Studies Section.
7. 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.
8. Riprap at smaller structures should be based on scour calculations. Riprap should be choked with Delaware #57 stone or channel-bed material unless conditions warrant otherwise.
9. 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.

104.3.2 Bridges

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.

104.3.2.1 Sizing

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:

1. Backwater associated with each alternative vertical profile and waterway opening should not significantly increase flood damage to property upstream of the crossing.
2. 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.
3. Existing flow distribution should be maintained to the extent practicable.
4. Pier spacing and orientation, and abutment should be designed to minimize flow disruption and potential scour.
5. Foundation design and/or scour countermeasures should be considered to avoid failure by scour.
6. Freeboard at structure(s) should be designed to pass anticipated debris and ice.
7. Risks of damage should be considered.
8. Stream instability countermeasures.
9. Ways to achieve minimal disruption of ecosystems and values unique to the floodplain and stream should be considered.
10. Highway level of service should be compatible with that commonly expected for the class of highway.
11. 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.

104.3.2.2 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 104.3.1.2 – 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:

1. The safety of the highway user
2. Vertical profile and horizontal alignment
3. Hydraulic performance
4. Construction and maintenance costs
5. Foundation conditions
6. Highway capacity
7. Navigation requirements
8. Stream regime

104.3.2.3 Shape/Material

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 are to be completed by the year 2020. 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.

104.3.3.1 Sizing

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.

104.3.3.2 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.

104.3.3.3 Shape/Material

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.

104.3.3.4 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.

104.3.4.1 General

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:

1. Structure hydraulics is riverine controlled and not impacted by tide/storm surge;
2. 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
3. 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 (2008). 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.