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