Memo_ABCC_Comp_2021-02-262950 Halcyon Lane, Suite 501
Jacksonville, Florida 32223
T 904.217.7485 www.geminiengineering.com
Memorandum
Date: February 26, 2021
To: Mr. Steve Swann, City of Atlantic Beach
From: Ki Hong Pak, PE
Subject: Floodplain Compensation Calculations for the Atlantic Beach Country Club
in the City of Atlantic Beach, Florida
Distribution: Mr. Robert Stone; Ms. Jaime Eaton; Mr. Alan Howard;
Victoria Domogala; Jamie Monohan, PE; Project File
This technical memorandum details the floodplain compensation calculations performed in association
with the Atlantic Beach Country Club in the City of Atlantic Beach, Florida (Figure 1). This project
involved the re-development of the existing Selva Marina Golf Course and Country Club, which included
the addition of approximately two hundred single-family lots and reconfiguration of the golf course and
clubhouse. The project was completed circa 2015. The analysis presented here is intended to provide
greater transparency in the floodplain compensation volume calculations that have previously been
developed for the site.
The intent of the City’s requirement to compensate, or to offset, “cup-for-cup,” for fill in the 1-percent-
annual-chance (100-year) floodplain is to maintain no net loss in the floodplain storage capacity. An
equivalent (or greater) floodplain volume which was displaced by the project fill will need to have been
created/excavated in the same hydraulically connected floodplain. The calculation process to address this
requirement is to first quantify the displaced pre-project conditions floodplain storage volume, followed by
a calculation of the post-project conditions floodplain compensation volume, with the intent of the latter
being equal to or greater than the former. A 100-year flood elevation ranging from 6.2 feet NAVD to
6.5 feet NAVD was utilized for these volume calculations, which is described in more detail below. Data
provided to Gemini were assumed to reference NAVD, therefore no additional vertical datum shifts were
applied.
1) DETERMINATION OF 100-YEAR FLOOD ELEVATION
A. The project site is shown on FEMA map panels 12031C0406J, 12031C0407J, 12031C0408J,
and 12031C0409J, with effective date of November 2, 2018. The property is affected by
flooding from Sherman Creek and Sherman Creek Canal. These Flood Insurance Rate Maps
(FIRMS) present the Base Flood Elevations (BFEs) which have been rounded to a whole
foot, which in this case is 7.0 feet NAVD.
B. The Floodway Data Table for Sherman Creek and Sherman Creek Canal indicates that
combined backwater effects from the Atlantic Ocean comprise a portion of the regulatory
BFE. Because floodplain volume associated with hurricane storm surge should not be
factored into rainfall-runoff flooding for floodplain volume compensation purposes, BFEs
ranging from 6.2 to 6.5 feet NAVD were used. These “riverine” BFEs are presented in the
effective Floodway Data Table in the effective Flood Insurance Study Report for Duval
County.
2) ESTIMATION OF SEASONAL HIGH GROUNDWATER TABLE
Based on available data, the seasonal high groundwater table ranges between 0-1 foot below the
natural ground elevation for the majority of the site. In lieu of extensive geotechnical surveys
spanning an extended sampling period, a conservative estimate of 3.5 feet NAVD was used
across the site to represent the seasonal high groundwater elevation. This elevation represents
the lower elevation limit of potential floodplain compensation which could be credited.
3) DEVELOPMENT OF PRE-PROJECT CONDITIONS TERRAIN
A. A digital terrain model was developed to represent the land surface on the property prior to
construction activities. Potential data sources were evaluated and examined against pre-
project aerial photography. This terrain model is comprised mostly of surveyed topography
provided by Melrose Surveying and Mapping, Inc.
B. The 2007 Duval County lidar data was sparingly used to fill any remaining gaps in the
surveyed topography. The pre-project conditions terrain model is depicted in Figure 2.
4) DEVELOPMENT OF POST-PROJECT CONDITIONS TERRAIN
A. A digital terrain model was developed to represent the land surface on the property following
completion of construction activities, generally representing topographic conditions after
2015. Similar to pre-project conditions, potential data sources were evaluated and examined
against recent aerial photography. This terrain model is comprised of the following data
sources:
a. Designed depression areas on the golf course, engineered by Taylor & White, Inc.
b. Stormwater pond top-of-bank and normal water levels, and depression top-of bank
elevations, based on project plans by Taylor & White, Inc.
c. Lot grading taken from “As-Built” surveys by Harold Eiland & Associates.
B. Additional topographic breaklines were developed, as necessary, to generate a reasonably
seamless terrain. The post-project conditions terrain model is depicted in Figure 3.
5) FLOODPLAIN NET AVAILABLE STORAGE VOLUME CALCULATIONS
Using ArcGIS tools, the pre- and post-project conditions terrains were compared in executing a
cut/fill algorithm. Terrain elevations and extent of the comparison were limited based on the
extent and elevations of the BFEs, as well as the estimation of the seasonal high groundwater
table. Also, we understand the City allows for floodplain compensation volume to be accounted
for within stormwater ponds, however, the approach presented here does not specifically
differentiate stormwater pond compensation volumes compared with the remainder of the
development site.
Based on our methodology outlined above, the calculated net floodplain volume available on the subject
property, encompassing the stormwater ponds and golf course, is 5,007 CY (Figure 4). Figures 5 and 6
provide conceptualized graphics of the calculations performed.
If there are questions regarding the floodplain compensation calculations presented here, please feel free
to contact me directly at 904.217.7485.
$2950 Halcyon Lane, Suite 501Jacksonville, Florida 32223Tel: 904.217.7485
February 2021
0 1,500750
Feet1 inch = 1,500 feet
Notes:ATLANTIC BEACH COUNTRY CLUBFIGURE 1 - SITE LOCATION
$2950 Halcyon Lane, Suite 501Jacksonville, Florida 32223Tel: 904.217.7485
February 2021
0 350175
Feet1 inch = 500 feet
Notes:ATLANTIC BEACH COUNTRY CLUBFIGURE 2 - PRE-FILL TERRAIN
$2950 Halcyon Lane, Suite 501Jacksonville, Florida 32223Tel: 904.217.7485
February 2021
0 350175
Feet1 inch = 500 feet
Notes:ATLANTIC BEACH COUNTRY CLUBFIGURE 3 - POST-FILL TERRAIN
$2950 Halcyon Lane, Suite 501Jacksonville, Florida 32223Tel: 904.217.7485
February 2021
0 400200
Feet1 inch = 400 feet
Notes:ATLANTIC BEACH COUNTRY CLUB
FIGURE 4 - FLOODPLAIN COMPENSATION CALCULATION SUMMARY
FLOODPLAIN VOLUME DISPLACED = 33,517 CUBIC YARDS EXCAVATED FLOODPLAIN COMPENSATION VOLUME = 38,524 CUBIC YARDS
100-YEAR BFE = 6.2-6.5 FT NAVD88
SEASONAL HIGH GROUNDWATER ELEVATION = 3.5 FT NAVD88
2950 Halcyon Lane, Suite 501Jacksonville, Florida 32223Tel: 904.217.7485
February 2021
ATLANTIC BEACH COUNTRY CLUB
FIGURE 5 - FLOODPLAIN VOLUME DISPLACEMENT CALCULATION APPROACH
100-YEAR BFE = 6.2 - 6.5 FT NAVD88
POST-FILL TERRAIN
PRE-FILL TERRAIN
DISPLACED FLOODPLAIN VOLUME = 33,517 CUBIC YARDS
2950 Halcyon Lane, Suite 501Jacksonville, Florida 32223
Tel: 904.217.7485
February 2021
ATLANTIC BEACH COUNTRY CLUB
FIGURE 6 - EXCAVATED FLOODPLAIN COMPENSATION CALCULATION APPROACH
100-YEAR BFE = 6.2 -6.5 FT NAVD88
EXCAVATION
PRE-FILL TERRAINEXCAVATED FLOODPLAIN COMPENSATION VOLUME = 38,524 CUBIC YARDS
SEASONAL HIGH GROUNDWATER
ELEVATION = 3.5 FT NAVD88