Saturday, November 5, 2022

Delmarva Unit Hydrograph

Delmarva Unit Hydrograph

As previously described, the NRCS dimensionless unit hydrograph and the NRCS triangular unit hydrograph are extensively used to develop storm Hydrographs for hydrologic evaluation and design of soil and water resources management practices throughout the United States. However, in areas such as the Delmarva Peninsula where the local topography is flat and where considerable surface storage is available, shape of observed storm Hydrographs significantly differ from those generated using the NRCS unit Hydrographs. As a result, a unit hydrograph that is similar with the NRCS dimensionless unit hydrograph, but with modifications to better represent the runoff characteristics of the Delmarva Peninsula has been used by utilities in the states of Delaware, Maryland, Virginia, and some parts of New Jersey. This unit hydrograph is known as the Delmarva unit hydrograph.

The Delmarva unit hydrograph uses the following equation to estimate peak flow rate.

Qp = 284 * A / Tp


Qp = peak flow rate in cfs.

A = area of the watershed, in square miles, draining to the location of the unit hydrograph.

Tp = time to peak of the unit hydrograph in hour

Time to peak, and lag time are calculated according to Equations 97 and 98, respectively. When compared with the NRCS methods, the Delmarva unit hydrograph produces lower peak flow rate but yields the same flow volume.

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