N.J. Admin. Code § Tit. 7, ch. 13, app 1
APPROXIMATING THE FLOOD HAZARD AREA DESIGN FLOOD ELEVATION
As described in detail at N.J.A.C. 7:13-3, the Department and FEMA have adopted flood mapping along many of the State's waters. In absence of a Department delineation1, or FEMA flood mapping that meets the requirements at N.J.A.C. 7:13-3.4(b), an applicant may use the approximation method described at N.J.A.C. 7:13-3.5 in conjunction with this appendix.
Note that this method approximates only the flood hazard area design flood elevation. This method does not approximate the floodway limit. Many activities are restricted within floodways and some calculations cannot be performed if the floodway limit is unknown. Therefore, the Department shall issue an individual permit for a regulated activity within an approximated flood hazard area only if the regulated activity meets the requirements at N.J.A.C. 7:13-3.5(f).
HOW TO USE METHOD 5 (APPROXIMATION METHOD)
1. Determine which Watershed Management Area (WMA) the project is located within based on Figure 5 below. The Department can help in this determination at the applicant's request.2
2. Determine the contributory drainage area (CDA) of the water in question. USGS provides topographical mapping that can be used to make this determination. The Department can also help in this determination at the applicant's request.
3. Find the approximate depth of flooding from Table 1 below based on the WMA and CDA.
4. Find the low point elevation of each roadway crossing or other water control structure within one mile downstream of the site.3 Figure 1 illustrates a typical roadway profile with a low point.
5. The approximate flood hazard area design flood elevation will be the higher of the following (see Figures 1 through 4):
The depth from Table 1, measured above the average streambed.4
The depth from Table 2, measured above the highest roadway low point described in 4 above.
NOTES
1. See Appendix 2 for a complete list of delineations and N.J.A.C. 7:13-3.3 for more detail.
2. If a project spans more than one WMA, the approximate flood hazard area shall be determined separately within each WMA.
3. Some roadway or railroad crossings over very large bridges need not be included if the Department determines that such crossings will not affect flooding on the site. Contact the Department for further information.
4. The average streambed is the general "smooth" grade of the bottom of the channel, and does not include small pockets of erosion, individual boulders, or other minor irregularities. The average streambed always has a positive slope toward downstream.
EXAMPLE: Going from left to right in any row, each number represents the upper drainage area limit for the flood depth shown at the bottom of the column. For example, in the row for WMA 10, a water with a drainage area of 70 acres or less has a flood depth of six feet. Similarly, any water draining between 70 and 110 acres has a flood depth of seven feet. In the example illustrated with arrows above, any water with a drainage area of between 19.4 and 30.0 square miles in WMA 10 has a flood depth of 19 feet.
NOTES
1. The numbers in this column denote the Watershed Management Areas shown in Figure 5.
2. Flood depths shall be measured above the average streambed elevation as described elsewhere in this Appendix and as shown in Figure 3 below.
N.J. Admin. Code Tit. 7, ch. 13, app 1