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.