Methods are presented to estimate peak-frequency relations, flood hydrographs, and volume-duration-frequency relations of urban streams in Ohio with drainage areas less than 6.5 square miles. The methods were developed to assist planners in the design of hydraulic structures for which hydrograph routing is required or where the temporary storage of water is an important element of the design criteria. Examples of how to use the methods also are presented.
The data base for the analyses consisted of 5-minute rainfall-runoff data collected for a period of 5 to 8 years at 62 small drainage basins distributed throughout Ohio. The U.S. Geological Survey rainfall-runoff model A634 was used and was calibrated for each site. The calibrayed models were used in conjunction with long-term (66-87 years) rainfall and evaporation records to synthesize a long-term series of flood-hydrograph records at each site. A method was developed and used to increase the variance of the synthetic flood characterictics in order to make them more representative of observed flood characteristics.
Multiple-regression equations were developed to estimate peak discharges having recurrence intervals of 2, 5, 10, 25, 50, and 100 years. The explanatory variables in the peak-discharge equations are drainage area, average annual precipitation, and basin development factor. Average standard errors of prediction for the peak-frequency equations range from ? 34 to ? 40 percent.
A method is presented to estimate flood hydrographs by applying a specific peak discharge and basin lagtime to a dimensionless hydrograph. An equation was developed to estimate basin lagtime in which main-channel length divided by the square root of the main-channel slope (L/SL) and basin-development factor are the explanatory variables and the average standard error of prediction is ? 53 percent. A dimensional hydrograph originally developed by the U.S. Geological Survey for use in Georgia was verified for use in urban areas of Ohio.
Multiple-regression equations were developed to estimate maximum flood volumes of d-hour duration and T-year recurrence interval (dVT). Annual maximum flood-volume data for all combinations of six durations (1, 2, 4, 8, 16, and 32 hours) and six recurrence intervals (2, 5, 10, 25, 50, and 100 years) were analyzed. The explanatory variables in the resulting 36 volume-duration-frequency equations are drainage area, average annual precipitation, and basin-development-factor. Average standard errors of prediction for the 36 dVT equations range from ? 28 percent to ? percent.
Step-by-step examples show how to estimate (1) peak discharges for selected recurrence intervals, (2) flood hydrographs and compute their volumes, and (3) volume-duration-frequency relations of small ungaged urban streams in Ohio. Volumes estimated by use of the volume-duration-frequency equations were compared with volumes estimated by integrating under an estimated under an estimated hydrograph. Both methods yield similar results for volume estimates of short duration, which are applicable to convective-type storm runoff. The volume-duration-frequency equations can be used to compute volume estimates of long and short duration because to equations are based on maximum-annual-volume data of long and short duration. The dimensionless-hydrograph method is based on flood hydrographs of average duration and cannot be used to compute volume estimates of long duration. Volume estimates of long duration may be considerably greater than volume estimates of short duration and are applicable to runoff from frontal-type storms.