N.J. Admin. Code § 7:13-3.4

Current through Register Vol. 56, No. 21, November 4, 2024

Section 7:13-3.4 - Flood hazard area and floodway based on FEMA flood mapping (Methods 2 through 4)

(a) This section sets forth the procedure for determining a flood hazard area design flood elevation and floodway limit from FEMA flood mapping.

(b) The methods set forth in this section for determining the flood hazard area and floodway along a regulated water may be used only if FEMA flood mapping exists for the section of regulated water in question, which includes the information needed for the Method that is being used. For example, Methods 2 and 3 below require that the FEMA flood mapping includes the regulated water's 100-year flood elevation, and Method 4 below requires that the FEMA flood mapping includes the regulated water's 100-year flow rate.

(c) There are three methods by which FEMA flood mapping may be used to determine the flood hazard area and floodway limit along a regulated water as follows:

1. Method 2 (FEMA tidal method), set forth at (d) below, which applies to a tidal flood hazard area for which a flood profile exists for the section of regulated water in question. If FEMA flood mapping does not provide a 100-year flood elevation, it is not possible to use this method;

2. Method 3 (FEMA fluvial method), set forth at (e) below, which applies to a fluvial flood hazard area for which a flood profile exists for the section of regulated water in question. If FEMA flood mapping does not provide a 100-year flood elevation, it is not possible to use this method; and

3. Method 4 (FEMA hydraulic method), set forth at (f) below. This method may be used only if the following requirements are satisfied:

i. FEMA flood mapping provides a 100-year flow rate for the regulated water. In most tidal flood hazard areas a large area is inundated due to flooding from the Atlantic Ocean, and therefore FEMA does not provide a 100-year flow rate for the regulated water itself. In such a case, it is not possible to use this method; and

ii. The applicant proposes a regulated activity in the flood hazard area and applies for an individual permit under this chapter, for which the Department requires hydraulic calculations comparing pre-construction and post-construction water surface elevations within the regulated water, in order to demonstrate that the regulated activity complies with this chapter. Examples of activities that require such an analysis are detailed at 7:13-12.1(f) , 12.1(g), and 12.7.

(d) Under Method 2 (FEMA tidal method):

1. The flood hazard area design flood elevation shall be equal to the FEMA 100-year flood elevation; and

2. The floodway limit shall be determined as follows:

i. Where FEMA flood mapping provides a floodway limit for the section of regulated water in question, the floodway limit shall be equal to the floodway limit depicted by FEMA; or

ii. Where FEMA flood mapping does not provide a floodway limit for the section of regulated water in question, the floodway limit shall be equal to the top of bank along the regulated water or channel. The Atlantic Ocean and other non-linear tidal waters such as bays and inlets do not have a floodway.

(e) Under Method 3 (FEMA fluvial method):

1. The flood hazard area design flood elevation shall be equal to three feet above the FEMA 100-year flood elevation; and

2. The floodway limit shall be determined as follows:

i. Where FEMA flood mapping provides a floodway limit for the section of regulated water in question, the floodway limit shall be equal to the floodway limit depicted by FEMA; or

ii. Where FEMA flood mapping does not provide a floodway limit for the section of regulated water in question, the floodway limit cannot be determined using this method. The applicant shall instead calculate the floodway limit using Method 4 as described in (f) below. In such a case, applicants are encouraged to first contact the Department to discuss whether it is necessary to determine the floodway limit on a site for a given project.

(f) Under Method 4 (FEMA hydraulic method):

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

i. A hydraulic analysis, such as a standard step backwater analysis, shall be performed to determine the flood elevation using 125 percent of the 100-year flow rate reported by FEMA flood mapping for the regulated water (see (c)3i above). The flood hazard area design flood elevation pursuant to this method shall be equal to two feet above the flood elevation determined in accordance with this subchapter; and

ii. A hydraulic analysis, such as a standard step backwater analysis, shall be performed to determine the floodway limit using the 100-year flow rate reported by FEMA flood mapping for the regulated water. The floodway limits shall be determined in accordance with this subchapter, assuming a maximum rise of 0.2 feet in the 100-year flood elevation, as follows:

(1) 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

(2) 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 hydraulic analysis, such as a standard step backwater analysis, shall be performed to determine the flood hazard area design flood elevation using the 100-year flow rate reported by FEMA flood mapping for the regulated water (see (c)3i above); and

ii. The floodway limits along a linear tidal water shall be calculated in accordance with (f)1ii above. Non-linear tidal waters do not possess a floodway, pursuant to 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 7:13-12.1(f) , 12.1(g), and/or 12.7 may necessitate undertaking a fluvial analysis under (f)1 above in addition to relying upon and/or recalculating tidal flood elevations under (f)2 above; and

4. Demonstrating compliance with the flood storage displacement limitations of 7:13-11.4 and/or the offsite impact limitations at 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.

N.J. Admin. Code § 7:13-3.4

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

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