DES 606 : Watershed Modeling with HEC-HMS

1. DES 606 : Watershed Modeling with HEC-HMS Module 9: S-Hydrographs Theodore G. Cleveland, Ph.D., P.E, M. ASCE, F. EWRI 26-28 August 2015 Module 9

2. Outline for Module 9 • Define an S-hydrograph • Advantages and Uses • How to enter in HEC-HMS Module 9

3. S-Hydrographs • What is an S-hydrograph? • An S-hydrograph (summation) is the hydrograph that results from continuous rainfall excess at a constant rate for an indefinite (long) time. • S-hydrographs can be constructed from existing unit hydrographs by accumulating the flows Module 9

4. S-Graph • T-duration UH • Add back-to-back precipitation • Convolved result is an S-hydrograph Module 9

5. S-Hydrographs • Why are they useful • S-hydrographs are useful for changing time base in unit hydrographs (common use) • Also useful in HMS because if a watershed is sub-divided, a new unit hydrograph would be required for each sub-basin, but the original S-hydrograph is unchanged Module 9

6. S-Hydrographs • How to make one from observed data? • Develop a unit hydrograph. • Accumulate the unit values – the result is an S-hydrograph, for a X-hour storm. • Then non-dimensionalize into %-Flow Module 9

7. S-Hydrographs • How to make one from observed data? • Accumulate total time • The lag time in the HMS table, is the total time less ½ the X-hour duration • Non-dimensionalize into %-time Module 9

8. S-Hydrographs • How to enter an S-hydrograph into HEC-HMS • Paired data manager. • Percent-curve. • Cut/paste the non-dimensional time and flow into the curve. Module 9

9. S-Hydrographs • Example 9 • Illustrate a subdivided watershed and non-subdivided with lag routing • Same resulting net hydrograph with S-hydrograph. Module 9

10. Summary • S-hydrographs are an alternate way to provide user-developed transformation functions to HMS • Provides a way to input Gamma unit hydrographs (or other distributions) • Flexible when time-base may be altered • Flexible when subdivision may be used Module 9