Simulation of flood hydrographs for Georgia streams

Flood hydrographs are needed for the design of many highway drainage structures and embankments. A method for simulating these flood hydrographs at ungaged sites in Georgia is presented in this report.

The O'Donnell method was used to compute unit hydrographs and lagtimes for 355 floods at 80 gaging stations. An average unit hydrograph and an average lagtime were computed for each station. These average unit hydrographs were transformed to unit hydrographs having durations of one-fourth, one-third, one-half, and three-fourths lagtime, then reduced to dimensionless terms by dividing the time by lagtime and the discharge by peak discharge. Hydrographs were simulated for these 355 floods and their widths were compared with the widths of the observed hydrographs at 50 and 75 percent of peak flow. The dimensionless hydrograph based on one-half lagtime duration provided the best fit of the observed data.

Multiple-regression analysis was then used to define relations between lagtime and certain physical basin characteristics, of which drainage area and slope were found to be significant for the rural-stream equations, and drainage area, slope, and impervious area were found to be significant for the Atlanta urban-stream equation.

A hydrograph can be simulated from the dimensionless hydrograph, the peak discharge of a specific recurrence interval, and the lagtime obtained from regression equations for any site with less than a 500 square mile drainage area in Georgia.

For simulating hydrographs at sites with basins larger than 500 square miles, the U.S. Geological Survey computer model CONROUT can be used. This model routes streamflow from an upstream channel location to a user-defined location downstream. The product of CONROUT is a simulated discharge hydrograph for the downstream site, which has a peak discharge of a specific recurrence interval. The diffusion-analogy routing method with single linearization was used in this study.