HomeMy WebLinkAboutRM_Fill in flood zone_____________________________________________________________________________________________________________________________________
3805 Cherry Avenue ‐ Wilmington, NC 28403 ‐ 910 791 4441 ‐ www.csd‐engineering.com – License # C‐2710
February 4, 2022
Ron Meredith
Current Planner
New Hanover County Planning
230 Government Center Dr.
Suite 110
Wilmington, NC 28403
Re: Letter to address fill within FEMA Zones
Sunset Towns Plan
Dear Ron:
In addressing item 2.b of your 1‐14‐22 TRC comments concerning meeting the requirements of Sections 9.8.2.E or
9.8.6.I, CSD Engineering has performed a volume calculation to determine the potential impact of proposed fill in the
Sunset Towns development within the FEMA floodplain. The site is currently within Zone AE (EL 9) as shown on FEMA
National Flood Insurance Program Map Number 372032100k, latest revision dated 8/28/2018. For the purposes of
these calculations, this analysis studied an area of Zone AE (EL 9) along the Cape Fear River ½ mile upstream and ½ mile
downstream of the Sunset Towns site, and was further limited to a portion of Zone AE (EL 9) that is onsite and outside of
the 75 ft CAMA buffer where development would be constructed. LIDAR topographic data was utilized to calculate the
total available flood plain volume from elevation 4 to elevation 9 within the subject flood zone area. The contours
below elevation 4 were not very defined in the LIDAR file and therefore were not utilized in the volume calculation.
Total proposed fill volume was calculated comparing existing ground elevations to a proposed elevation of 9.5 which
would put the area above the flood elevation of 9.0. The results were as follows:
Total Flood Zone AE (EL 9) Available Flood Volume = 122,360,419 cubic feet
Proposed Volume of Fill Within the flood plain = 4,568 cubic feet
Based upon this analysis, the total proposed onsite fill volume equates to roughly 0.004% of total available flood water
capacities within the analyzed area of Zone AE (EL 9). Additionally, the total displaced water from the proposed fill
translates to approximately a 0.0021 inch of increased flood elevation within the analyzed area of Zone AE (EL 9). Since
the actual size of Zone AE (EL 9) is exponentially larger than the areas analyzed and considering the limited fill placement
proposed within the subject site, the calculated effect of the proposed fill on available flood waters is extremely small.
As such, it is my opinion that placement of the proposed fill on the Sunset Towns site will not adversely impact adjacent
properties. Attached for your reference please find the Fill/Flood Plain Volume Displacement Calculations and Flood
Plain/Base Flood Impact Overall Map.
Sunset Towns Site
Fill / Flood Plain Volume Displacement Calculations
Flood Zone AE (EL 9) Volume Determination (contours taken from Q2 Lidar)
INCREMENTAL
ELEVATION AREA VOLUME STORAGE
(FT) (SF) (cf) (cf)
4 23,619,409 -
5 24,024,457 23,821,933 23,821,933
6 24,220,848 24,122,653 47,944,586
7 24,439,489 24,330,169 72,274,755
8 25,041,763 24,740,626 97,015,381
"Base flood EL" 9 25,648,312 25,345,038 122,360,419
Total Basin Flood Volume as Analyzed = 122,360,419 cf
(0.5 Mile Upstream/Downstream
FloodWay Volume to be effected)
Proposed Fill Volume / Flood Zone Displacement = 4,568 cf
(Amount of Fill within Flood Zone Line to reach Elev. 9)
Percent Volume Impacted = 0.004 %
((Proposed Fill / Total Basin Volume) x 100)
(( 4,568 CF / 122,360,419 SF) 100)
Flood Impact above Base Flood EL = 0.0021 Inches
((Fill Amount / Base Flood EL SA) x 12)
(( 4,568 CF / 25,648,312 SF) 12)
CSD ENGINEERING
PO BOX 4041
WILMINGTON, NC 28406