Changes in Rainfall Data and
the Use of the Delmarva Unit Hydrograph

Based upon recent updates to the rainfall amounts for specific storm events, the National Weather Service has provided updates of the rainfall frequency data available at http://hdsc.nws.noaa.gov/hdsc/pfds/index.html. The Natural Resources Conservation Service (NRCS) has provided revised rainfall frequency data for New Jersey available by county at http://www.nj.nrcs.usda.gov/technical/engineering/rainfall.html (pdf).

The NRCS, in the supplement attached to the New Jersey Bulletin No. NJ210-3-1, dated September 8, 2003, states that the use of the Delmarva Unit Hydrograph may be used in modeling watersheds in the Coastal Plain Region of New Jersey “that are characterized by flat topography (average watershed slope less than 5 percent), low relief, and significant surface storage in swales and depressions.” Use of the Delmarva Unit Hydrograph will not affect the determination of runoff volume, but should result in lower peak discharges when compared to the Standard Unit Hydrograph. For developed sites or heavily urbanized areas in the Coastal Plain of New Jersey, care should be taken to determine whether the use of the Delmarva Unit Hydrograph is consistent with the conditions above.

All applications requiring runoff computations, under the Stormwater Management Rules at N.J.A.C. 7:8-5.6(a) and the Flood Hazard Area Control Act Rules at 7:13-2.4, are required to utilize the new rainfall amounts and the Delmarva Unit Hydrograph, where appropriate, effective September 1, 2005. For applications that have been deemed complete by the Department’s Land Use Regulation Program or the municipality under a municipal review, prior to September 1, 2005, the applicant has the choice of rainfall amounts or unit hydrograph method, provided there is consistency of use throughout the application.