Section 4: Hydrologic Method Selection
Anchor: #i1026021Method Selection
In general, follow these guidelines.
- Compare results from several methods.
- Use the discharge that appears to best reflect local project conditions. Averaging of results of several methods is not recommended.
- Document reasons supporting the selection of the results.
The peak discharge is adequate for design of conveyance systems such as storm drains, open channels, culverts, and bridges. However, if the design necessitates flood routing through areas such as storage basins, complex conveyance networks, and pump stations, a flood hydrograph is required.
Anchor: #i1026051Hydrologic Methods
Countless hydrologic methods are available for estimating peak discharges and runoff hydrographs. The omission of other methods from this manual does not necessarily preclude their use. Determine which method seems to be the most reasonable for the specific situation. Here are some of the most widely used methods:
- Rational Method
- NRCS Runoff Curve Number Methods
- Statistical analysis of stream data
- Regional regression equations.
Rational Method. The Rational Method provides estimates of peak runoff rates for small urban and rural watersheds of less than 200 acres (80 hectares) and in which natural or man-made storage is small. It is best suited to the design of urban storm drain systems, small side ditches and median ditches, and driveway pipes. See Section 6 for more information on The Rational Method.
NRCS Runoff Curve Number Methods. The Natural Resources Conservation Service (formerly Soil Conservation Service) developed the runoff curve number method as a means of estimating the amount of rainfall appearing as runoff. Technical Release 20 (TR 20) employs the Runoff Curve Number Method and a dimensionless unit hydrograph to provide estimation of peak discharges and runoff hydrographs from complex watersheds. The procedure allows you to estimate the effect of urbanization, channel storage, flood control storage, and multiple tributaries. Apply TR 20 to the design of culverts, bridges, detention ponds, channel modification, and analysis of flood control reservoirs. Technical Release 55 (TR 55) is a simplified form of TR 20 for use estimating peak discharges for small watersheds (urban and rural) whose time of concentration does not exceed 10 hours. TR 55 includes a hydrograph development procedure; however, where hydrograph determination is necessary, use TR 20 or another hydrograph procedure. See Section 7 for more information on the NRCS Runoff Curve Number Methods.
Statistical Analysis of Stream Gauge Data. Statistical analysis of stream gauge data provides peak discharge estimates using annual peak stream flow data. The method is particularly useful where long records (in excess of 25 years) of stream gauge data are available at or near to and on the same stream as the structure site. See Section 10 for more information on statistical analysis of stream gauge data.
Regional Regression Equations. Regional regression equations provide estimates of peak discharge for watersheds in specific geographic regions. See Section 11 for more information on regional regressional methods and equations.