N.J. Admin. Code § 7:13-3.6

Current through Register Vol. 56, No. 24, December 18, 2024

Section 7:13-3.6 - Flood hazard area and floodway determined by calculation (Method 6)

(a) This section sets forth the procedure for determining a flood hazard area design flood elevation and floodway limit via hydrologic and hydraulic calculations.

(b) If the following conditions exist, Method 6 is the only method by which an applicant may determine the flood hazard area and floodway along a regulated water:

1. Method 1 (Department delineation method) set forth at 7:13-3.3 cannot be used because no Department delineation exists for the section of regulated water in question;

2. Methods 2 through 4 (FEMA fluvial, FEMA tidal, and FEMA hydraulic methods) set forth at 7:13-3.4 cannot be used because no qualifying FEMA flood mapping exists for the section of regulated water in question; and

3. Method 5 (approximation method) set forth at 7:13-3.5 cannot be used for one of the following reasons:

i. The requirements for using the approximate method at 7:13-3.5(b) are not satisfied;

ii. The Department determines that the approximate method will significantly underestimate the depth of flooding on the site in question, pursuant to 7:13-3.5(e); or

iii. The applicant is proposing a regulated activity for which the requirements at N.J.A.C. 7:13-5.4 are not satisfied.

(c) Under Method 6:

1. For a regulated water in a fluvial flood hazard area:

i. Except as provided at (c)1ii below, the flood hazard area design flood elevation shall be determined, as follows:

(1) A hydrologic analysis shall be performed to determine the peak flow rate for the anticipated future 100-year flood for the regulated water using the adjustment factors listed at Table 3.6B below in accordance with (c)6 below. The hydrologic analysis shall assume existing land use coverage in the drainage area, as of the date of the verification application to the Department; and

(2) A hydraulic analysis, such as a standard step backwater analysis, shall be performed to determine the flood hazard area design flood elevation using 125 percent of the anticipated future 100-year peak flow rate determined pursuant to (c)1i(1) above;

ii. The use of calculations not described at (c)1i above to determine the flood hazard area design flood elevation is conditionally acceptable provided:

(1) A hydrologic analysis shall be performed to determine the peak flow rate for the 100-year flood for the regulated water. The hydrologic analysis shall assume existing land use coverage in the drainage area, as of the date of the verification application to the Department; and

(2) The flood hazard area design flood elevation is determined using a flood that is no less than 125 percent of the peak flow and volume of the anticipated future 100-year flood determined pursuant to (c)1i(1) above; and

iii. The floodway limits shall be determined, as follows:

(1) A hydrologic analysis shall be performed to determine the peak flow rate for the current 100-year flood for the regulated water using the adjustment factors listed at Table 3.6A below in accordance with (c)5 below. The hydrologic analysis shall assume existing land use coverage in the drainage area, as of the date of the verification application to the Department; and

(2) A hydraulic analysis, such as a standard step backwater analysis, shall be performed to determine the floodway limit using the 100-year flow rate determined pursuant to (c)1iii(1) above, assuming a maximum rise of 0.2 feet in the 100-year flood elevation, as follows:

(A) The floodway limits shall be calculated assuming equal conveyance reduction, unless the applicant demonstrates (prior to the submission of an application for a verification to the Department) that due to the topography of the area, the proximity of structures to the channel and/or other physical characteristics of the watershed or flood hazard area, use of another method will more optimally calculate the floodway limits at a given location; and

(B) In cases where floodway calculations indicate a decrease in water surface elevations, the entire flood hazard area in the area of the calculated decrease shall be considered to be a floodway;

2. For a regulated water in a tidal flood hazard area:

i. A flood hazard area analysis shall be developed, which is based on the data and methodologies utilized by FEMA to determine its most recent 100-year flood elevation, but which includes new topographic or other relevant data for the region and/or site being analyzed, in order to determine the flood hazard area design flood elevation; and

ii. The floodway limits along a linear tidal water shall be determined as set forth in (c)1iii above. Non-linear tidal waters do not possess a floodway, pursuant to N.J.A.C. 7:13-2.3(b)1;

3. In cases where both fluvial and tidal flooding occurs, such as along a linear regulated water which is inundated by tidal backwater during the 100-year flood but is fluvial for more frequent flood events, compliance with the offsite impact limitations of N.J.A.C. 7:13-12.1(f), 12.1(g), and/or 12.7 may necessitate undertaking a fluvial analysis under (c)1 above in addition to relying upon and/or recalculating FEMA tidal flood elevations under (c)2 above; and

4. Demonstrating compliance with the flood storage displacement limitations of N.J.A.C. 7:13-11.4 and /or the offsite impact limitations at N.J.A.C. 7:13-12.1(f), 12.1(g), and/or 12.7 may require the computation of additional peak flow rates for more frequent flood events. In such a case, the same methodologies detailed in this subsection shall be used, as appropriate for the site conditions, to determine the flow rates and water surface elevations for the desired flood events.

5. Table 3.6A below sets forth the adjustment factors necessary to determine the current 100-year precipitation depths for use in this chapter. The precipitation depth of the current 100-year storm event shall be determined by multiplying the values determined in accordance with (c)5i and ii below:

i. 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: http://hdsc.nws.noaa.gov/hdsc/pfds/pfds_map_cont.html?bkmrk=nj ; and

ii. The applicant shall utilize Table 3.6A 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 drainage area 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 Table 3.6A below.

Table 3.6A: Current Precipitation Adjustment Factors

County	Current Precipitation Adjustment Factors
Atlantic	1.03
Bergen	1.06
Burlington	1.04
Camden	1.05
Cape May	1.04
Cumberland	1.01
Essex	1.06
Gloucester	1.06
Hudson	1.09
Hunterdon	1.13
Mercer	1.04
Middlesex	1.03
Monmouth	1.02
Morris	1.06
Ocean	1.03
Passaic	1.05
Salem	1.03
Somerset	1.09
Sussex	1.07
Union	1.06
Warren	1.15

6. Table 3.6B below sets forth the change factors to be used in determining the projected 100-year storm event for use in this chapter, which are organized alphabetically by county. The precipitation depth of the projected 100-year storm event of a site shall be determined by multiplying the precipitation depth of the 100-year storm event determined from the National Weather Service's Atlas 14 Point Precipitation Frequency Estimates pursuant to (c)5i above, by the change factor in Table 3.6B below, in accordance with the county or counties where the drainage area(s) of the site is located. Where the project and/or its drainage area 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.

Table 3.6B: Future Precipitation Change Factors

County	Future Precipitation Change Factors
Atlantic	1.39
Bergen	1.37
Burlington	1.32
Camden	1.39
Cape May	1.32
Cumberland	1.39
Essex	1.33
Gloucester	1.41
Hudson	1.23
Hunterdon	1.42
Mercer	1.36
Middlesex	1.33
Monmouth	1.26
Morris	1.46
Ocean	1.24
Passaic	1.50
Salem	1.32
Somerset	1.48
Sussex	1.50
Union	1.35
Warren	1.37

N.J. Admin. Code § 7:13-3.6

Amended by 48 N.J.R. 1067(a), effective 6/20/2016

Amended by 55 N.J.R. 1385(b), effective 7/17/2023