Techniques for estimating regional flood characteristics of small rural watersheds in the plains region of eastern Colorado

Recorded and synthetic flood data for 52 watersheds (35 in Colorado and 17 in adjoining States) were analyzed to develop regional techniques for estimating the magnitude, frequency, volume, and hydrograph shape of floods that typically occur on small rural watersheds in the plains region of eastern Colorado. The analysis of flood magnitude and frequency included 21 floodfrequency relations that were based on recorded annual peak discharges, 2 flood-frequency relations that were based on synthetic annual peak discharges, and 28 flood-frequency relations that were based on recorded and synthetic annual peak discharges (a relation could not be determined for 1 watershed). Similarly, the analysis of flood volumes included volumes for 103 recorded floods and 4,391 synthetic floods. Synthetic flood data were generated from long-term rainfall data from National Weather Service stations and a rainfallrunoff model calibrated for each watershed. The 5-, 10-, 25-, 50-, and 100-year peak discharges were regionalized using ordinary least-squares and generalized least-squares regressions. The smallest errors of prediction were obtained using the generalized least-squares regressions, and the relations developed included the independent variables of effective drainage area, relief factor, and 24-hour, 100-year rainfall intensity; standard errors of prediction ranged from 35 to 50 percent. A relation was developed to estimate flood volume from peak discharge; the standard error of prediction was 78 percent. To develop a flood hydrograph from estimates of peak discharge and flood volume, a dimensionless-hydrograph technique is presented that produces synthetic flood hydrographs very similar in shape to recorded flood hydrographs. INTRODUCTION Flood characteristics, such as magnitude of peak discharges, frequency of occurrence, and volumes are major considerations in the design of highway bridges and culverts. Extensive discharge data available for large perennial streams generally have provided flood information necessary for the design of major drainage structures. Previous reports on the estimation of flood characteristics of Colorado streams include Patterson (1964, 1965), Patterson and Somers (1966), Matthai (1968), Livingston (1970), Hedman and others (1972), McCain and Jarrett (1976), U.S. Soil Conservation Service (1975, 1977) and Kircher and others (1985). However, except for the reports by McCain and Jarrett (1976) and the U.S. Soil Conservation Service (1975, 1977), the methods described by these reports generally do not apply to very small watersheds, particularly to watersheds that have ephemeral streams. McCain and Jarrett (1976) presented regression equations applicable to watersheds that have drainage areas greater than 1 mi 2 in the plains region of eastern Colorado (fig. 1). Their equations, however, were based only on limited data for small watersheds; only 2 of 36 watersheds studied had drainage areas less than 30 mi 2 . Procedures for estimating flood characteristics described by the U.S. Soil Conservation Service (1975, 1977) apply to small watersheds, but primarily are based on empirical rainfall-runoff relations developed for regions encompassing many States, rather than local hydrologic areas within any particular State such as Colorado. In 1968, the U.S. Geological Survey, in cooperation with the Colorado Department of Highways and the Federal Highway Administration, began a study to: (1) Collect data during thunderstorm-caused floods in small (generally