2918-25 STORMWATER final 4_1_25.doc.pdf

26 regarding the methodology is available from the Natural Resources Conservation Service website at: https://directives.sc.egov.usda.gov/viewerFS.aspx?hid=21422 or at United States Department of Agriculture Natural Resources Conservation Service, New Jersey State Office. 2. For calculating curve numbers and groundwater recharge, there is a presumption that the pre-construction condition of a site or portion thereof is a wooded land use with good hydrologic condition. The term “curve number” applies to the NRCS methodology above at § 25-29.5A1(a). A curve number or a groundwater recharge land cover for an existing condition may be used on all or a portion of the site if the design engineer verifies that the hydrologic condition has existed on the site or portion of the site for at least five years without interruption prior to the time of application. If more than one land cover has existed on the site during the five years immediately prior to the time of application, the land cover with the lowest runoff potential shall be used for the computations. In addition, there is the presumption that the site is in good hydrologic condition (if the land use type is pasture, lawn, or park), with good cover (if the land use type is woods), or with good hydrologic condition and conservation treatment (if the land use type is cultivation). To calculate runoff and groundwater recharge, the preconstruction land cover of a site or portion thereof shall be woods, unless the applicant provides sufficient evidence that another land cover (e.g., farmland) had existed for at least five years without interruption prior to development of the site. Furthermore, the pre-construction condition of the site or portion thereof shall be considered to have been in good hydrologic condition. If more than one land use existed on the site during the five years prior to initial development of the site, the land cover with the lowest runoff potential shall be used for the computations. 3. In computing pre-construction stormwater runoff, the design engineer shall account for all significant land features and structures, such as ponds, wetlands, depressions, hedgerows, or culverts, that may reduce pre-construction stormwater runoff rates and volumes. 4. In computing stormwater runoff from all design storms, the design engineer shall consider the relative stormwater runoff rates and/or volumes

e pre-construction stormwater runoff rates and volumes. 4. In computing stormwater runoff from all design storms, the design engineer shall consider the relative stormwater runoff rates and/or volumes of pervious and impervious surfaces separately to accurately compute the rates and volume of stormwater runoff from the site. To calculate runoff from unconnected impervious cover, urban impervious area modifications as described in the NRCS Technical Release 55 – Urban Hydrology for Small Watersheds or other methods may be employed. 5. If the invert of the outlet structure of a stormwater management measure is below the flood hazard design flood elevation as defined at N.J.A.C. 7:13, the design engineer shall consider the effects of tailwater in the design of structural stormwater management measures.