Hydraulic Reference Manual

[Vinnie Frevert]

Theprimary 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.

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.

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.

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 Hydraulic Field Assessment Checklist. 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.

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.

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 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.

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.

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.

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.

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