Unit Hydrograph
“In the 1930’s, L.K. Sherman (Sherman 1932, 1940) advanced the theory of the unit hydrograph, or unit graph. The unit hydrograph procedure assumes that discharge at any time is proportional to the volume of runoff and that time factors affecting hydrograph shape are constant”.[1]
“Field data and laboratory tests have shown that the assumption of a linear relationship among watershed components is not strictly true. The nonlinear relationships have not been investigated sufficiently to ascertain their effects on a synthetic hydrograph.”[1]
“Many variables are integrated into the shape of a unit hydrograph. Since a dimensionless unit hydrograph is used and the only parameters readily available from field data are drainage area and time of concentration, consideration should be given to dividing the watershed into hydrologic units of uniformly shaped areas. These subareas, it at all possible, should have a homogeneous land use and approximately the same size. To assure that all contributing subareas are adequately represented, it is suggested that no subarea exceed 20 square miles in area and that the ratio of the largest to smallest drainage area not exceed 10”.[1]
Best Practices Resources
Engineering Handbook: Chapter 4 - Storm Rainfall Depth and Distribution, NRCS, 2019
National Engineering Handbook: Chapter 21- Design Hydrographs, NRCS, 2019
National Engineering Handbook: Chapter 15- Time of Concentration, NRCS, 2010
National Engineering Handbook: Chapter 16- Hydrographs, NRCS, 2007
Flood Hydrology Manual, USBR, 1989
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