N.H. Admin. Code § Env-Wq 1503.09

Current through Register No. 45, November 7, 2024

Section Env-Wq 1503.09 - Information Required for Projects Within the 100-year Floodplain

The applicant shall submit a supplementary report for all projects proposed within the 100-year floodplain that includes the following:

(a) The size of the river's watershed above the project's furthest downstream boundary;

(b) Photographs of the right bank and left bank;

(c) A delineation of the 100-year floodplain located within the site's boundaries, using one of the following methods:

(1) In Zone AE, where FEMA has performed detailed studies, the 100-year floodplain boundary shall be determined using FEMA's 100-year floodplain elevations in combination with detailed topography for the site; or

(2) In Zone A, where FEMA has mapped the floodplain by approximate methods, a study to determine 100-year floodplain elevations and boundaries shall be performed in accordance with (e)(1), below;

(d) The total volume of fill in acre-feet that is proposed to be placed within the 100-year floodplain between existing grades and the 100-year flood elevations;

(e) For any project where the total fill volume is greater than 0.5 acre-feet or where a bridge or culvert crossing is proposed, a hydraulic model and analysis check as follows:

(1) A HEC-RAS analysis, stamped by a qualified engineer, that includes cross-sections and profiles for the pre- and post-development conditions, wherein the cross-sectional surveys and flow data are completed according to the following:

a. A minimum of 6 cross-sections that span the entire river and 100-year floodplain shall be surveyed to analyze the project's effect on flood elevations through the affected reach, where:

1. At least 2 cross-sections are surveyed at the project site in those areas where floodplain encroachment are greatest;

2. At least 2 cross-sections are surveyed upstream of the proposed floodplain fill area with one being located at the upstream property line; and

3. At least 2 cross-sections are surveyed downstream of the proposed floodplain fill area with one being located at the downstream property line;

b. For projects involving bridges or culverts, or both, a minimum of 6 cross-sections shall be required as follows:

1. At least 3 cross-sections shall be surveyed above each proposed bridge or culvert; and

2. At least 3 cross-sections shall be surveyed below each proposed bridge or culvert; and

c. Flood discharges shall be estimated using the first available method listed below:

1. Flood discharges from a FEMA Flood Insurance Studies (FIS) covering the proposed project site, if available;

2. Stream gauge data collected by the USGS, if available; or

3. The USGS National Flood Frequency (NFF) program for the State of New Hampshire, which uses a regression equation to determine flow; and

(2) A cHECK-RAS analysis to verify the parameters used in the HEC-RAS hydraulic model;

(f) The GPS coordinates of the beginning and end points of each of the cross-sections required by (e)(1)a. and b., above, in units of degrees, minutes, and seconds of latitude and longitude, with at least 3 decimal places of precision (DDMMSS.sss) and referenced to the North American Datum of 1983 (NAD 83) or its successor;

(g) If the hydraulic model results indicate that the proposed project will raise flood stages on abutting properties or alter flow and sediment transport characteristics in a manner that could adversely affect channel stability and surface water quality, a proposal for compensatory flood storage or conveyance, or both, that is designed to ensure that:

(1) There is no increase in flood stages on abutting properties; and

(2) Flow and sediment transport characteristics will not be affected in a manner which could adversely affect channel stability; and

(h) For any project where the total fill volume is less than 0.5 acre-feet and no bridge or culvert crossing is proposed:

(1) An on-site cut and fill balance such that there is no net decrease in the 100-year flood storage volume; or

(2) A hydraulic model per the methods described in (e), above, which demonstrates that there is no increase in flood stages on abutting properties and that flow and sediment transport characteristics will not be affected in a manner that could adversely affect channel stability and surface water quality.

N.H. Admin. Code § Env-Wq 1503.09

(See Revision Note #1 at chapter heading for Env-Wq 1500) #9343, eff 1-1-09

Amended by Volume XXXVII Number 32, Filed August 10, 2017, Proposed by #12342, Effective 8/15/2017, Expires 8/15/2027.