Technical Bulletin : 2004-2.0: revisions to the NRCS stormwater modeling runoff procedures utilizing the the DelMarVa unit hydrograph

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NEW JERSEY DEPARTMENT OF AGRICULTURE STATE SOIL CONSERVATION COMMITTEE Chapter 251, PL 1975 as amended, Engineering Policies- Technical Bulletin Technical Bulletin: 2004-2.0 Subject: NRCS change in stormwater modeling for the NJ Coastal Plain 1.01 Adoption Date: July 12, 2004 From: Hunter Birckhead, P.E., Section Chief PURPOSE To distribute revisions to the NRCS stormwater modeling runoff procedures utilizing the the DelMarVa unit hydrograph. 1.02 SUMMARY NRCS runoff modeling procedures utilize a dimensionless unit hydrograph in the computation of runoff rate and volume. Until recently, the hydrograph officially used by NRCS in New Jersey was the “Standard” unit hydrograph, which represents “average” conditions. This hydrograph is characterized by a unit peak discharge factor of 484, which is more representative of the Piedmont areas of the State. Due to recent studies by NJDA as well as an accumulation of other data, NRCS-NJ has instructed its field staff to utilize the DelMarVa unit hydrograph for runoff estimation on agricultural lands which are located within the coastal zones identified on the attached map and NRCS bulletin. This hydrograph is characterized by a peak rate factor of 285, and will predict a substantially lower peak discharge than that of the Standard hydrograph. Volume of runoff will not be affected by this change. DelMarVa hydrograph usage is primarily restricted to areas that have slopes less than 5%, permeable soils and are characterized by “ponded” topography capable of capturing and holding some degree of precipitation prior to runoff occurring. It should not be utilized for design in areas where the predeveloped conditions are heavily urbanized and the drainage area is characterized by significant areas of impervious cover. The Standard hydrograph should continue to be used in these areas. Districts located within the specified zones are to use and require usage of the DelMarVa unit hydrograph in the preparation of Chpt. 251 applications where it is necessary to compute runoff rate and volume with NRCS modeling methods. This requirement is effective 60 days from the date of this bulletin. L:\SSCC\Engineering Policies\Delmarva Unit Hydrograph 2004-2.0.doc