West Orange City Zoning Code

The design engineer shall calculate runoff using the following method:

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.

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.

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.

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.

The applicant shall utilize the National Oceanographic and Atmospheric Administration (NOAA), National Weather Service's Atlas 14 Point Precipitation Frequency Estimates: NJ, in accordance with the location(s) of the drainage area(s) of the site. This data is available at https://hdsc.nws.noaa.gov/hdsc/pfds/pfds_map_cont.html?bkmrk=nj; and

The applicant shall utilize Table 5: Current Precipitation Adjustment Factors, below, which sets forth the applicable multiplier for the drainage area(s) of the site, in accordance with the county or counties where the drainage area(s) of the site is located. Where the major development lies in more than one county, the precipitation values shall be adjusted according to the percentage of the drainage area in each county. Alternately, separate rainfall totals can be developed for each county using the values in the table below.

NOTE: The municipality may instead wish to abbreviate this table along with the text in subsection c2 above to reflect only the relevant information, depending on the location of the municipality. The current precipitation adjustment factors added to the ordinance shall be those found in N.J.A.C. 7:8-5.7(c) as Table 5-5.