Hydraulics and Hydrology
Hydrology calculations should be completed by a hydrologist familiar with the local conditions and streamflows. These calculations should include at least the Q2 and Q100 flows. Streamflow in the United States usually is calculated using regression equations or modeling programs, such as the U.S. Geological Survey National Streamflow Statistics Program or the Hydrologic Engineering Center–Hydrologic Modeling System (HEC–HMS). The results of several models should be compared because discharge calculation is not an exact science.
Other methods for calculating streamflow compare the watershed being crossed to an adjacent watershed with similar physical characteristics that already has hydrologic data. A gauged stream in the adjacent watershed can be used to compare your results and calibrate the modeled streamflow. Discharge measurements (Harrelson and others 1994) are a great way to calibrate the flow model for your site. Try to get at least one discharge measurement above a baseflow (the streamflow during the drier portions of the year). In addition, a hydrologist should conduct a pebble count and gather substrate information to estimate the channel roughness value and scour potential. The channel roughness values, as well as substrate and streamflow information, will be used to calculate the hydraulics (characteristics such as depth, velocity, and slope) for the site.
Hydraulic calculations can be performed using many different computer programs. Two of the most common programs used in the United States are the Hydrologic Engineering Center–River Analysis System (HEC–RAS, http://www.hec.usace.army.mil/, (figure 20) and WinXSPRO http://www.stream.fs.fed.us/publications/winxspro.html, a computer model that analyzes a stream channel cross section to estimate the elevation of the water surface for a given flow, slope, and streambed roughness. Both of these programs are public domain (not copyrighted) and can be downloaded for free.
Figure 20—Example water elevation profile produced by the Hydrologic Engineering Center–River
Analysis System.
After calculations are completed, verify results with field observations, such as bankfull stage indicators, highwater marks' streambed strata, stream velocity, and information from local residents.
A scour analysis should be completed for every streamcrossing project. This analysis considers the stream velocities and the materials on the bottom and banks of the channel to determine how much the bottom and banks might erode at different flows.
Figure 21—This bridge is set high to allow rafters to pass underneath
The seasons of the peak flows and their causes should be taken into account during bridge design so the bridge can be constructed when flows are low. The timing of peak flows varies from region to region. For instance, the peak flows may be caused by spring runoff from mountain snowpack or by hurricanes or monsoons. Construction also should be scheduled during periods when fish are not migrating or spawning.
Navigational clearance is required in many streams and must be provided at high water. Minimum clearance for navigation varies, depending on the type of boat traffic. Floating trees or debris present another problem during floods. The minimum clearance (freeboard) for floating trees can be estimated as half of the root wad's longest dimension, plus 1 meter added for safety (figures 21 and 22). This estimate is conservative and should be based on the size of the root wads of the largest trees that the stream can transport during design floods.
