HomeMy WebLinkAboutNHC Flood Study Final ReportNew Hanover County Northern
Watersheds Flood Mapping Project
Hydrologic and Hydraulic Analysis
New Hanover County, North Carolina
Prepared for:
New Hanover County
230 Government Center Drive, Suite 110
Wilmington, North Carolina 28403
Prepared by:
ESP Associates, Inc.
2200 Gateway Center Blvd, Suite 216
Morrisville, North Carolina 27560
August 6, 2025
ii
Table of Contents
List of Figures ............................................................................................................................................... iii
List of Tables ................................................................................................................................................ iii
1 Overview ............................................................................................................................................... 4
1.1 Study Introduction ........................................................................................................................ 4
1.2 Study Area ..................................................................................................................................... 4
2 Previous North Carolina Floodplain Mapping Program Studies ........................................................... 6
3 Hydrologic Analysis ............................................................................................................................... 6
3.1 Terrain Development .................................................................................................................... 8
3.2 Watershed / Drainage Area Development ................................................................................... 8
3.3 Impervious Area Development ..................................................................................................... 8
3.4 2D Model Hydrology ..................................................................................................................... 8
3.5 Regression Analysis ....................................................................................................................... 9
3.6 USGS Gage Analysis ..................................................................................................................... 10
3.7 Calibration ................................................................................................................................... 10
3.8 Comparison to Effective FEMA Discharges ................................................................................. 10
4 Hydraulic Analysis ............................................................................................................................... 10
4.1 HEC-RAS Modeling ...................................................................................................................... 10
4.2 Peak Discharges .......................................................................................................................... 11
4.3 Detailed Study ............................................................................................................................. 11
4.4 Model Validation ......................................................................................................................... 15
4.5 Revised Effective Models ............................................................................................................ 15
4.6 Tie-ins .......................................................................................................................................... 15
4.7 Comparison to Effective FEMA Study ......................................................................................... 15
References .................................................................................................................................................. 17
ENGINEER CERTIFICATION .......................................................................................................................... 18
iii
List of Figures
Figure 1: Study Areas .................................................................................................................................... 5
Figure 2: NRCS Regional Rainfall Distribution Map....................................................................................... 7
Figure 3: NRCS Regional Rainfall Distributions ............................................................................................. 7
Figure 4: BFE Comparisons Using New and Effective Water Surface Elevations ........................................ 16
List of Tables
Table 1: Study Areas for Figure 1 .................................................................................................................. 5
Table 2: Previous NCFMP Studies ................................................................................................................. 6
Table 3: Curve Numbers for Associated Land Cover and Hydrologic Soil Group (AMC-II) ........................... 9
Table 4: Comparison to Effective FEMA Discharges ................................................................................... 10
Table 5: Detailed Study Stream Extents ...................................................................................................... 12
Table 6: 2D Manning's n-value Summary ................................................................................................... 13
Table 7: BFE Comparison Summary ............................................................................................................ 16
Attachments
Digital Model Support Data - Hydrology
Digital Model Support Data - Hydraulics
Digital Model Support Data – Field Data
Digital Model Support Data – FIS Documents
Digital Model Support Data – GIS Mapping
New Hanover County Northern Watersheds Hydrologic & Hydraulic Analysis Report
August 6, 2025 Page 4
1 Overview
1.1 Study Introduction
This report documents the hydrologic and hydraulic modeling efforts of ESP Associates, Inc. (ESP) for the
New Hanover County, NC Northen Watersheds. ESP performed this work under contract #23-0496 with
New Hanover County.
New Hanover County is located in the southeastern portion of the State within the Coastal Plain
physiographical region and is bordered by Brunswick County to the west and Pender County to the
north.
The Northern watersheds of New Hanover County are experiencing rapid development into areas that
are not included in the current FEMA Flood insurance Study (FIS). The intent of this study was to
produce additional flood study data at targeted locations. The results of this project will be used by the
county to improve awareness of potential flood hazards where data was not previously available. While
the study was not conducted with the purpose to pursue a Physical Map Revision (PMR), it is in keeping
with methodology that would meet FEMA’s parameters for a PMR to the currently effective FIS.
This study produced one-dimensional (1D) detailed study hydraulic modeling for Acorn Branch, Acorn
Branch Tributary, Island Creek, Kings Grant Tributary, Kings Grant Tributary 2, Kings Grant Tributary 2A,
Ness Creek Tributary 1, Prince George Creek, Prince George Creek Tributary 4, Pumpkin Creek, Pumpkin
Creek Tributary 1, Pumpkin Creek Tributary 2, Spring Branch, and Wildcat Branch. Additionally,
approximate hydraulic modeling was performed using rain-grid, two-dimensional (2D) model techniques
for the areas around Murrayville Tributary, Ness Creek and Ness Creek Tributary 2, and Prince George
Tributary 3. The 2D models are approximate and therefore provide advisory level flood information.
Hydrologic modeling involved calculating peak discharges for a range of existing conditions return
periods (10%, 4%, 2%, 1%, 0.2% annual chance events) and future conditions (assuming 10%, 20%, and
35% impervious area) consistent with the effective FIS study. Discharges were developed using USGS
regression equations for all streams. Calculated discharges were then inserted into hydraulic models to
develop water surface elevation profiles for the range of return periods.
The 2D hydrologic modeling utilizes rain-on-grid techniques and spatially distributed Atlas 14 rainfall
grids to model return period and future condition events consistent with the North Carolina Floodplain
Mapping Program, Rain-on-Grid 2D Base Level Engineering Analysis and Mapping requirements. These
models provide awareness of potential flooding within the study area.
1.2 Study Area
Figure 1 and Table 1 identify the stream reaches and 2D model areas that were studied.
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Figure 1: Study Areas
Table 1: Study Areas for Figure 1
Number Name
1 Acorn Branch
2 Acorn Branch Tributary
3 Island Creek
4 Kings Grant Tributary
5 Kings Grant Tributary 2
6 Kings Grant Tributary 2A
7 Ness Creek Tributary 1
8 Prince George Creek
9 Prince George Creek Tributary 4
10 Pumpkin Creek
11 Pumpkin Creek Tributary 1
12 Pumpkin Creek Tributary 2
13 Spring Branch
14 Wildcat Branch
15 Murrayville Tributary - 2D Advisory
New Hanover County Northern Watersheds Hydrologic & Hydraulic Analysis Report
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Number Name
16 Ness Creek (and Ness Creek Tributary
2) – 2D Advisory
17 Prince George Tributary 3 – 2D
Advisory
2 Previous North Carolina Floodplain Mapping Program Studies
Previous studies in the area were reviewed to identify and compare hydrologic and hydraulic
methodologies to this study.
Table 2: Previous NCFMP Studies
Study Name Date Methods
Hydrologic and Hydraulic Analysis – New Hanover
County
February
2003
USGS Regression Hydrology
HEC-RAS Profile Hydraulics
New Hanover County: Hydraulic and Hydrologic
Analyses for Detailed Studied Streams in New Hanover
County
June 2010 USGS Regression Hydrology
HEC-RAS Profile Hydraulics
The previous NCFMP hydrologic analysis was performed with the effective US Geologic Survey (USGS)
regression equations. None of the previously studied streams used gage analysis.
Previous study hydraulic analyses were performed with the U.S. Army Corps of Engineers (USACE)
Hydraulic Engineering Centers River Analysis System (HEC-RAS). Topographic data for the floodplain
models was developed using NCFMP lidar elevation data along with field surveys and measurements of
hydraulic structure information.
3 Hydrologic Analysis
For 1D stream modeling, Peak flood discharges for the 10%, 4%, 2%, 1%, and 0.2% annual chance events
and three future conditions – 10%, 20%, and 35% impervious percentages, were determined for studied
streams using the current USGS regression equations. Per NCFMP Guidance, the USGS equations used
for this study are found in multiple USGS reports: Scientific Investigation Report (SIR) 2023-5006,
Magnitude and Frequency of Floods for Rural Streams in Georgia, South Carolina, and North Carolina,
2017-Results; and (SIR) 2014-5030, Methods for Estimating the Magnitude and Frequency of Floods for
Urban and Small, Rural Streams in Georgia, South Carolina, and North Carolina, 2011.
Since there are no USGS gages in the study area, no gage adjustments were made to the regression
calculated discharges.
2D models use rainfall data from NOAA Atlas 14 Volume 2: Ohio River Basin and Surrounding States
(orb) grid files (https://hdsc.nws.noaa.gov/hdsc/pfds/pfds_gis.html), processed into spatially distributed
rainfall grids in HEC.DSS format for use in HEC-RAS. Atlas 14 gridded data is provided in rainfall inches
times 1000, therefore, a ratio of 0.001 was applied in the HEC-RAS Meteorological Variables to yield
rainfall in inches.
The Natural Resource Conservation Service (NRCS) Type C rainfall distribution was applied to the 20%,
10%, 4%, 2%, 1%, 0.5%, 0.2% and 0.1% annual-chance storm events over a 24-hour period. The Type C
distribution is appropriate for New Hanover County as shown in Figure 2 and Figure 3.
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Figure 2: NRCS Regional Rainfall Distribution Map
Figure 3: NRCS Regional Rainfall Distributions
2D models include return period and future condition events consistent with the North Carolina
Floodplain Mapping Program, Rain-on-Grid 2D Base Level Engineering Analysis and Mapping
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New Hanover County Northern Watersheds Hydrologic & Hydraulic Analysis Report
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requirements. The 1%-minus, 1%-plus, 1%-CL10, 1%-CL20, and 1%-CL30 events use the 1% rainfall
distribution with an appropriate scaling factor to decrease or increase the total rainfall amount.
3.1 Terrain Development
For the hydrologic and hydraulic analysis, North Carolina Digital Elevation Models (DEM’s) with 5-foot
cell size (developed by NCEM from QL2 lidar data) were downloaded via the North Carolina Spatial Data
Download site in the NAD 1983 (2011) State Plane North Carolina FIPS 3200 (US Feet) coordinate system
with a vertical datum of NAVD88 in feet. DEM data were combined, projected to NAD 1983 State Plance
FIPS 3200 (US Feet) for watershed basin delineation and hydraulic modeling.
3.2 Watershed / Drainage Area Development
The U.S. Army Corp. of Engineers, Hydrologic Engineering Center, HEC-HMS version 4.11 was used to
delineate watershed boundaries at locations of interest along all 1D studied streams. Watersheds were
developed at key points in the study area to provide discharge changes of approximately 10% or less
along the studied stream. Watershed basin boundaries, “NewHanover_watersheds.shp” are attached as
ESRI shapefiles. All watersheds for this study are located within USGS Hydrologic Region 4.
3.3 Impervious Area Development
Watershed impervious area is expressed in percentage of the subject drainage area. The 2021 National
Land Cover Dataset (NLCD) was used as a base impervious layer. The NLCD was improved in areas where
recent development was observed in the imagery but was not in the NLCD data such as large roadways
and large residential or commercial development. Minor differences between impervious coverage used
for this study and impervious areas visible in the photo imagery were found to be inconsequential since
the regression equation threshold is greater than or equal to 10% impervious area before Urban
Equations are used.
Each watershed’s overall impervious area was calculated by performing a GIS intersect analysis of the
impervious areas and watershed boundaries.
3.4 2D Model Hydrology
2D models use the Soil Conservation Service (SCS) Curve Number Method for runoff hydrology. Curve
numbers are used to describe the amount of rainfall that makes it to the stream as opposed to being
intercepted by vegetation, absorbed into the soil, or otherwise prevented from contributing to riverine
flooding. The two components required to determine the Curve Number are:
1. Landcover – National Land Cover Database (NLCD) 2021 ( https://www.mrlc.gov/data/nlcd-
2021-land-cover-conus)
2. Soils – Natural Resources Conservation Service (NRCS) Soil Survey Geographic Database
(SSURGO) (https://websoilsurvey.nrcs.usda.gov/app/WebSoilSurvey.aspx)
The NCLD landcover was improved by adding classifications for buildings, roadways, other impervious
areas, and ponds using GIS data sets provided by New Hanover County.
SSURGO soil data as a shapefile was prepared in Arcmap and projected to the NC State Plane coordinate
system. The projected soil data was then imported in Rasmapper as a shapefile using the full extent
option to create a raster consistent with the Landcover raster cell size. Rasmapper was then used to
combine soil and landcover layers to generate the Infiltration file. Distributed Antecedent Moisture
Condition II (AMC-II) curve numbers were then assigned to the Infiltration file in Rasmapper assuming
unsaturated soil conditions. Table 3 shows the curve number matrix used for the modeling. The values
shown are based on AMC II, which implies an average moisture condition for the soil.
New Hanover County Northern Watersheds Hydrologic & Hydraulic Analysis Report
August 6, 2025 Page 9
The SSURGO data includes combined soil types such as A/D. Since soil data is used in conjunction with
landcover to generate categories for curve number assignment, for this study combined soil types were
assigned a curve number that is the average of the A and D soil types for a respective landcover type.
The infiltration layer is created by HEC-RAS as a raster layer that is populated with appropriate Curve
Number and Initial Abstraction Ratio.
Table 3: Curve Numbers for Associated Land Cover and Hydrologic Soil Group (AMC-II)
Description
Hydrologic Soil Group
A B D NoData AD BD CD Water
Barren Land Rock-Sand-Clay 39 61 80 71 59.5 70.5 77 98
Cultivated Crops 51 67 80 71 65.5 73.5 78 98
Developed, High Intensity 89 92 95 71 92 93.5 94.5 98
Developed, Low Intensity 61 75 87 71 74 81 85 98
Developed, Medium
Intensity 81 88 93 71 87 90.5 92 98
Developed, Open Space 49 69 84 71 66.5 76.5 81.5 98
Emergent Herbaceous
Wetlands 72 80 93 71 82.5 86.5 90 98
Evergreen Forest 30 55 77 71 53.5 66 73.5 98
Grassland-Herbaceous 49 62 85 71 67 73.5 79.5 98
Mixed Forest 30 55 77 71 53.5 66 73.5 98
NoData 71 71 71 71 71 71 71 98
Open Water 98 98 98 98 98 98 98 98
Pasture-Hay 39 61 84 71 61.5 72.5 79 98
Shrub-Scrub 30 48 73 71 51.5 60.5 69 98
Woody Wetlands 72 80 93 71 82.5 86.5 90 98
255 71 71 71 71 71 71 71 98
Building 98 98 98 98 98 98 98 98
Pavement 98 98 98 98 98 98 98 98
Impervious 98 98 98 98 98 98 98 98
Pond 98 98 98 98 98 98 98 98
3.5 Regression Analysis
3.5.1 USGS Regression Equations
USGS regression-based flows were developed for 1D model stream studies. USGS developed regression
equations from a statistical analysis of empirical data from the USGS stream gage network. Regression
equations for rural areas between 0.1 and 7,485 square miles with impervious area less than 10% is
outlined in USGS report SIR 2023-5006, Magnitude and Frequency of Floods for Rural Streams in
Georgia, South Carolina, and North Carolina, 2017.
New Hanover County Northern Watersheds Hydrologic & Hydraulic Analysis Report
August 6, 2025 Page 10
In USGS SIR 2014-5030, Methods for Estimating the Magnitude and Frequency of Floods for Urban and
Small, Rual Streams in Georgia, South Carolina, and North Carolina, 2011, regression equations were
developed from a statistical analysis of empirical gage data for drainage areas between 0.1 and 53.5
square miles with percent impervious greater than 10%.
Regression based flows for each stream in this study were calculated in a spreadsheet,
“New_Hanover_Hydrology.xlsx” in the Digital Model Support Data.
3.6 USGS Gage Analysis
There are no USGS stream gages in the study area for use in gage analysis.
3.7 Calibration
No highwater marks were collected along the streams in this study and gage stage data is not available
for model validation.
3.8 Comparison to Effective FEMA Discharges
Final discharges were compared to calculated effective discharges from previous studies. Discharge
comparisons were made based on drainage area and are detailed in Table 4.
Table 4: Comparison to Effective FEMA Discharges
Stream
DA Study
(square-
miles)
DA Eff.
(square-
miles)
DA %
Difference
Study
Discharge
(cfs)
Effective
Discharge
(cfs)
Discharge
%
Difference
Kings Grant Tributary 1.5 1.32 12.8 1060 1091 2.9
Prince George Creek 2.5 1.6 43.9 760 1020 29.2
Pumpkin Creek 2.2 2.1 4.7 1190 1080 9.7
Wildcat Branch 0.6 0.5 18.2 500 688 31.6
The difference in the discharge estimates from this study compared to the effective FIS is primarily
attributable to the updated regression equations used in this study. Other factors include the use of
newer and higher quality terrain to develop drainage areas and more recent impervious area data.
Generally, discharge estimates from regression equations used in this study are lower than discharges
calculated in previous studies. An annotated Summary of Discharges Table presenting the study flows in
the format presented in the FIS is included in the digital supporting attachments.
4 Hydraulic Analysis
1D hydraulic analysis was performed in accordance with FEMA Guidelines and Specifications. Studied
streams were analyzed using detailed study hydraulic methods per the scoping datasets provided by New
Hanover County and/or ESP scope modifications approved by New Hanover County.
2D hydraulic analysis was performed per the North Carolina Floodplain Mapping Program, Rain-on-Grid
2D Base Level Engineering Analysis and Mapping requirements. These studies provide advisory level flood
information for flood awareness in the studied areas.
4.1 HEC-RAS Modeling
1D hydraulic analysis was performed using U.S. Army Corp. of Engineers, Hydrologic Engineering Center
HEC-RAS, version 6.4.1, applying steady-state hydraulic modeling techniques to develop flood profiles for
New Hanover County Northern Watersheds Hydrologic & Hydraulic Analysis Report
August 6, 2025 Page 11
various events. Additional flood profiles were developed for the future and ultimate buildout impervious
conditions. The modeled profiles are consistent with the effective FIS.
Except for Spring Branch, new HEC-RAS models were developed with a single plan (“Multiple Profiles”)
and steady flow file, along with one universal geometry file (“LiDAR, Survey Structures & Channel”), to
model the six (6) profiles. The name of the flow file is dependent on the hydrology used to develop
discharges. An example is “2023 USGS Rural Regression SIR-5006” for discharges developed using the
USGS 2023 Rural Regression Equations.
For Spring Branch, the effective FIS flows were maintained, and the effective model was revised
upstream of Martin Luther King (MLK) Blvd. Model cross-sections upstream of MLK Blvd. were updated
with the NCEM QL2 lidar data. The revised model was run using HEC-RAS 6.4.1. No model changes were
made downstream of MLK Blvd. so the effective FIS information still applies.
For the 2D study areas, HEC-RAS, version 6.4.1 was used with rain-on-grid modeling techniques to
develop flood profiles for various annual chance events. Additional flood profiles were developed to
represent future conditions as increases to the 1% rainfall.
4.1.1 Existing Condition Profiles
For 1D models, six regulatory and non-regulatory flood events are modeled using the “Multiple Profiles”
plans while considering existing conditions hydrology. The six existing conditions profiles include the
10%, 4%, 2%, 1%, and 0.2% annual chance events (“10pct,” “4pct,” “2pct,” “1pct,” and “02pct,”
respectively).
For 2D models, ten flood events were modeled considering existing conditions hydrology. The ten
profiles include the 20%, 10% 4%, 2%, 1%, 1% minus, 1% plus, 0.5%, 0.2% and 0.1%.
4.1.2 Floodway Profile
For 1D models, the HEC-RAS “Multiple Profiles” plans also analyze the 1% regulatory floodway profile.
The 1% regulatory floodway profile is identified as “1pct_FW.” Using the 1% annual chance event as a
reference profile, the 1% floodway reflects encroachments on the 1% event used to develop the
regulatory floodway.
4.1.3 Future Condition Profiles
Consistent with the effective FIS, future conditions were modeled for the 1d models. Future conditions
are represented as anticipated increases in percent impervious which in turn are input into the
regression equations to determine future discharges for the 1% annual chance event. Impervious areas
of 10%, 20% and 35% were used.
For 2D models, flood profiles were developed for the 1% rainfall plus 10%, 20%, and 30% to represent
both increases in future development imperviousness and increases in future rainfall due to climate
factors.
4.2 Peak Discharges
The peak discharges used in the 1D HEC-RAS models are based on the output of the detailed hydrologic
analysis as presented in Section 3 of this report. Appendix A summarizes the discharges used for the 1D
hydraulic studies.
4.3 Detailed Study
Detailed 1D model study involves the use of lidar data supplemented with field survey data to conduct
detailed multi-profile, floodway/encroachment, and/or other hydraulic analyses. In detailed studies,
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draft floodplain boundaries for the 1% and 0.2% events, and a regulatory floodway, were developed
using GIS and/or other automated tools. Studied stream limits are shown in Table 5.
Table 5: Detailed Study Stream Extents
Stream Name Downstream Limit Upstream Limit Length
(mi)
Acorn Branch Confluence with Smith
Creek
Approximately 3400 feet
upstream of N. Kerr Ave. 2.8
Acorn Branch Tributary
Approximately 3200 feet
downstream of Acorn
Branch Road
Just downstream of N. Kerr
Avenue 1.0
Island Creek
Approx. 1.1 miles
upstream of Sidbury
Road
Approx. 2.3 miles upstream of
Sidbury Road 1.2
Kings Grant Tributary
Approx. 250 feet
upstream of Gordon
Road
Approx. 500 feet upstream of
Sapling Circle 0.4
Kings Grant Tributary 2
Approx. 400 feet
downstream of Daisy Lee
Drive
Approx. 1200 feet upstream of
Farmington Farms Drive 0.8
Kings Grant Tributary 2A Approx. 500 feet
downstream of Voris Lane
Approx. 300 feet upstream of
Ogden Park Drive 0.4
Ness Creek Tributary 1 Approx. 300 feet
upstream of Rockhill Road
Approx. 2200 feet upstream of
Rockhill Road 0.4
Prince George Creek Just upstream of Sidbury
Road
Approximately 2900 feet upstream
of Sidbury Road 0.6
Prince George Creek
Tributary 4
Confluence with Prince
George Creek
Approximately 1100 feet upstream
of Enis Acres Drive 0.6
Pumpkin Creek Just upstream of Juvenile
Center Road
Approximately 2400 feet upstream
of Blue Clay Road 0.7
Pumpkin Creek Tributary 1
Just Upstream of
confluence with Pumpkin
Creek
Approximately 170 feet upstream of
I-140 0.7
Pumpkin Creek Tributary 2 Confluence with Pumpkin
Creek
Approximately 0.2 miles upstream
of confluence with Pumpkin Creek
Trib 2
0.2
Spring Branch At confluence with Smith
Creek
Approx. 2200 feet upstream of BLK
Jr. Parkway 2.2
Wildcat Branch
Approx. 500 feet
downstream of Castle
Hayne Road
Approx. 2800 feet upstream of
Castle Hayne Road 0.6
4.3.1 Terrain Model /Geometry Definition
For the hydraulic analysis, North Carolina Digital Elevation Models (DEM’s) in 5-foot cell size (developed
by NCEM from QL2 lidar data) were downloaded via the North Carolina Spatial Data Download site in
the NAD 1983 (2011) State Plane North Carolina FIPS 3200 (US Feet) coordinate system with a vertical
New Hanover County Northern Watersheds Hydrologic & Hydraulic Analysis Report
August 6, 2025 Page 13
datum of NAVD88 in feet. DEM data were combined, projected to NAD 1983 State Plance FIPS 3200 (US
Feet) for hydraulic modeling.
1D study stream geometry/alignment was rectified to the terrain. Hydraulic cross-sections were
supplemented with surveyed channel sections collected upstream and/or downstream of each structure
and at various locations between structures. Channel sections between the surveyed sections were
interpolated based on the surveyed channel banks and the top of banks determined from the bare-earth
lidar data. Additionally, structures were field surveyed to provide the roadway centerline and to define
the structure opening area.
2D models use the underlying terrain directly. Major roadway culvert crossings were modeled as SA/2D
connections with connection data taken from effective 1D models. The terrain was improved by adding
terrain modifications to mimic culvert crossings for smaller roadway and driveway culverts that
influence flow patterns in the 2D model.
4.3.2 Hydraulic Structures
1D Hydraulic structure model geometry was developed using field survey and engineering
reconnaissance data. Field survey was performed to provide the structure roadway/weir profile, define
the bridge opening area, culvert dimensions and inverts, and determine other needed modeling
parameters. Field survey data was supplemented with engineering field reconnaissance data focused on
structure hydraulic characteristics such as entrance loss coefficients, headwall, and abutment
information. Field survey sketches, photographs, and point data (spatial and tabular) are included in the
supporting digital attachments.
4.3.3 Manning’s n Values
Manning’s roughness coefficients (n-values) for 1D models were estimated based on field observation
and aerial photography inspection. N-values generally ranged from 0.04 to 0.055 within the channel and
from 0.04 to 0.15 for overbanks. Engineering field assessment sheets are included in the supporting
digital attachments.
In lieu of modifying the overbank n-value, buildings were incorporated into the 1D models as blocked
obstructions.
2D model manning’s roughness follow North Carolina Emergency Management recommended values
that were developed using various published references that are generally accepted by the engineering
community. Within HEC-RAS the 2021 NLCD was blended with New Hanover County impervious and
building datasets, and n-values were assigned relative to the landcover types shown in Table 6.
Table 6: 2D Manning's n-value Summary
NLCD Value Recommended
n-value Range of n-values Description
- - 0.15 NoData
11 0.03 0.025-0.05 Open Water
21 0.04 0.03-0.05 Developed, Open Space
22 0.1 0.08-0.12 Developed, Low Intensity
23 0.12 0.06-0.14 Developed, Medium Intensity
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NLCD Value Recommended
n-value Range of n-values Description
24 0.15 0.12-0.20 Developed, High Intensity
31 0.03 0.023-0.03 Barren Land (Rock/Sand/Clay)
41 0.13 0.10-0.16 Deciduous Forest
42 0.13 0.10-0.16 Evergreen Forest
43 0.13 0.10-0.16 Mixed Forest
52 0.1 0.07-0.16 Shrub/Scrub
71 0.045 0.025-0.05 Grassland/Herbaceous
81 0.06 0.025-0.06 Pasture/Hay
82 0.06 0.025-0.06 Cultivated Crops
90 0.12 0.045-0.15 Woody Wetlands
95 0.08 0.05-0.085 Emergent Herbaceous
Wetlands
- - 100 Building
- - 0.02 Pavement
- - 0.02 Impervious
- - 0.02 Ponds
4.3.4 Ineffective Flow Areas
For 1D models, ineffective flow areas were defined upstream and downstream of all bridges/culvert
crossings to simulate contraction and expansion at crossings. Stagnant and low conveyance areas
identified in floodplains were also modeled as ineffective.
4.3.5 Expansion and Contraction Coefficients
For 1D models, contraction and expansion loss coefficients were defined to be 0.3 and 0.5, respectively,
at cross sections “2,” “3,” and “4” of structures. Reach contraction and expansion coefficients were
generally defined as 0.1 and 0.3, respectively, and raised as judged appropriate where abrupt floodplain
expansion and contraction occurs. When reach cross-section coefficients were raised, an explanatory
note was added to the cross-section description in HEC-RAS.
4.3.6 Boundary Conditions
For 1D models, a downstream, normal depth boundary condition was set as the boundary condition
except when a hydraulic tie-in to another study was appropriate. Normal depth downstream boundary
conditions were defined by estimating a slope using the ground slope and energy grade line slope of the
downstream reach.
2D models also use normal depth boundary conditions, however, at model outlets along well defined
streams where 1D models exist, the normal depth outlet slope was adjusted so the modeled 1% water
surface elevation approximated the effective 1s model 1% water surface elevation.
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4.3.7 Floodway Development
For 1D models, floodways were developed using HEC-RAS method one (1) analysis. Modeled floodways
were adjusted manually with a target surcharge of 1.0 foot, while keeping all surcharges within
acceptable limits (0.00 to 1.00 foot). Notes were added to model cross sections where surcharges are
outside of the range of 0.00 to 1.00 foot. Floodway Data Tables consistent with the FIS format are
included in the supporting digital attachments.
4.4 Model Validation
The model validation process typically involves an optimization process for critical parameters of a HEC-
RAS model using observed historical event(s) and other published values such as high-water marks or
USGS Gage records. There are no known gages or high-water marks that could be used for validation of
the models. Therefore, the hydraulic modeling assumptions and parameters follow generally accepted
engineering parameters that are assumed to be valid for New Hanover County.
4.5 Revised Effective Models
To achieve tie-in to the effective floodway and meet the maximum floodway surcharge of 1.00 feet for
the new Kings Grant Tributary 1D model, the effective Kings Grant Tributary model floodway
encroachments were revised at cross-sections 10624 and 10414. No other changes to the effective
model were made.
The effective Spring Branch model was modified upstream of Martin Luther King Jr. Blvd. by realigning
the streamline and cross-sections based on the NCEM QL2 Lidar. The effective model was rerun in HEC-
RAS version 6.4.1.
4.6 Tie-ins
1D model tie-ins to downstream effective models are listed in the detailed tie-in table found in Appendix
B, “Tie-in Table.” Tie-ins to downstream effective models were made where applicable. Peak discharges
from the effective model were adopted at the tie-in cross-section to facilitate model tie-in per typical
NCFMP guidance. As a result, the discharges at those tie-in locations are higher than those calculated by
the regression equations used for this new study.
Backwater tie-ins are listed in Appendix C, “Backwater Tie-in Table.” Appendix C lists the tie-in for the
1% annual chance exceedance of the newly studied stream at the confluence with its receiving stream.
The tie-in is adequate if the newly studied stream 1% elevation is lower (or equal) to the receiving
stream Base Flood Elevation (BFE).
4.7 Comparison to Effective FEMA Study
Base Flood Elevation (BFE) comparisons were completed for Spring Branch (upstream of Martin Luther
King Jr. Blvd.) and Wildcat Branch (upstream of cross-section 11,000). All other modeled streams do not
have corresponding effective FEMA studies for comparison. BFEs were compared to effective water
surface elevations at each modeled cross-section.
Rasters for the effective BFE were collected from the NC FLOOD database. Effective elevations were
extracted, and differences were calculated at each streamline/cross-section intersection. To compute
the approximate BFE change, effective water surface elevations were subtracted from the new study
water surface elevations, where a positive result is an increase in BFE and a negative result is a decrease
in BFE.
Figure 4 depicts an example of BFE change near the cross-sections, determined by comparing effective
and new water surface elevations. Table 7 present BFE comparison results.
New Hanover County Northern Watersheds Hydrologic & Hydraulic Analysis Report
August 6, 2025 Page 16
Figure 4: BFE Comparisons Using New and Effective Water Surface Elevations
Table 7: BFE Comparison Summary
Stream Name
Length of
Study
(mi.)
Cross-
section
Comparison
Locations
Average
Water
Surface
Change
(ft)
(FY2016 Map Maintenance -
Effective WSEL)
Maximum
Water Surface
Increase (ft)
Maximum Water
Surface Decrease
(ft)
Spring Branch
(upstream of MLK Jr.
Blvd.) 0.23 3 -0.2 0.21 -0.84
Wildcat Branch 0.63 12 -0.31 0.25 -0.64
4.7.1 General BFE Variances
Variances from effective water surface elevations are due primarily to two reasons:
• Updated Regression Equations: As noted previously in the report (Table 4), discharges for this
study were calculated using the most recent set of USGS regression equations. These recent
regression equations tend to estimate flows lower than the previous study except where
impervious increases have occurred since the previous study resulting in the use of the urban
regression equations.
• Updated Terrain: The base terrain information used in the development of the new hydraulic
models is a dataset not previously used for NCFMP study in this area. Significant changes in the
terrain were noted across the study area, which may affect how water surface elevations are
calculated.
References
1. USGS Scientific Investigation Report (SIR) 2023-5006, Magnitude and Frequency of Floods for
Rural Streams in Georgia, South Carolina, and North Carolina, 2017-Results, 2023.
2. USGS Scientific Investigation Report (SIR) 2014-5030, (2014), Methods for Estimating the
Magnitude and Frequency of Floods for Urban and Small, Rural Streams in Georgia, South
Carolina, and North Carolina, 2011.
3. North Carolina Cooperating Technical State Mapping Program, Riverine Hydrologic and
Hydraulic Engineering Guidelines and Standards Revised August 7, 2017.
4. FEMA Flood Insurance Study, A Report of Flood Hazards in New Hanover County, North Carolina
and Incorporated Areas, Revised January 17, 2025.
ENGINEER CERTIFICATION
This is to certify that all work performed during completion of this flood study was performed by ESP
Associates, Inc. and was done in accordance with contract 23-0496 for New Hanover County. All work
has been accomplished in accordance with sound and accepted engineering practice within the contract
provisions for respective phases of the work.
Signed this 6th day of August, 2025
___________________________
Matthew Dudley, PE, CFM
Director – Surface Water/Hazards/GIS
ESP Associates, Inc.
PO Box 7030
Charlotte, NC 28241 – 7030
(803) 802-2440
License: F-1407
Status: CURRENT
Service: Engineering and Land Surveying
Appendix A
Summary of Discharges Table
Summary of Discharges for Detailed Study Streams
Basin
10%4%2%1%0.20%
Just below North Kerr Ave. (SR-1322)W_AB_12 0.85 340 450 540 630 840
Just below Acorn Branch Road (SR-1370)W_AB_10 1.05 413 538 630 730 970
Approximately 1,900 feet downstream of of Acorn Branch Road (SR-1370)W_AB_08 1.61 536 698 820 950 1270
Approximately 6,050 feet above confluence with Smith Creek W_AB_06 2.12 617 810 960 1120 1500
Approximately 4,450 feet above confluence with Smith Creek W_AB_04 2.99 715 956 1150 1350 1850
Approximately 3,450 feet above confluence with Smith Creek W_AB_02 3.41 757 1019 1230 1450 2010
Approximately 1,950 feet above confluence with Smith Creek W_AB_00 3.64 788 1060 1280 1510 2090
Acorn Branch Tributary
Just bleow Acorn Branch Road (SR-1370)W_ABT_04 0.49 122 177 230 280 420
Just below Cornelius Moore Ave.W_ABT_02 0.53 129 187 240 300 440
Just upstream of confluence with Acorn Branch W_ABT_00 0.66 147 212 270 340 500
Island Creek
Approximately 2.1 miles upstream of Sidbury Road W_IC_28 2.03 297 426 550 670 990
Approximately 1.6 miles upstream of Sidbury Road W_IC_26 2.44 333 475 610 750 1100
Approximately 1.3 miles upstream of Sidbury Road W_IC_24 2.50 341 487 630 770 1130
Approximately 0.9 miles upstream of Sidbury Road Tie-in (Effective
flows used)3.14 450 672 870 1103 1799
Just downstream of Sapling Circle (SR-2666)W_KGT_21 0.50 469 526 560 600 660
Approximately 350 feet upstream of GoRoadon Road W_KGT_19 0.60 474 545 590 640 730
Just upstream of Gordon Road Tie-in (Effective
flows used)1.32 508 777 934 1091 1562
Just downstream of Farrignton Farms Drive (SR-2772)W_KGT2_08 0.31 227 284 330 370 460
Approximately 1,470 feet upstream of confluence with Kings Grant Tributary W_KGT2_06 0.37 257 319 370 410 510
Approximately 700 feet upstream of confluence with Kings Grant Tributary W_KGT2_04 0.70 443 531 590 650 780
Approximately 400 feet upstream of confluence with Kings Grant Tributary W_KGT2_02 0.72 444 533 600 660 790
At confluence with Kings Grant Tributary W_KGT2_00 0.84 491 589 660 730 880
Just downstream of Ogden Park Drive W_KGT2a_04 0.24 272 311 340 360 410
Just downstream of Farrignton Farms Drive (SR-2772)W_KGT2a_02 0.25 281 321 350 370 420
At confluence with Kings Grant Tributary Tributary 2 W_KGT2a_00 0.31 337 381 410 440 490
Just upstream of Rockhill Road Tie-in (Effective
flows used)0.59 125 177 224 277 402
Approximately 2,500 feet upstream of Sidbury Road W_PGC_06 0.11 131 159 180 200 240
Approximately 700 feet upstream of Sidbury Road W_PGC_02 2.19 311 445 570 700 1030
At Sidbury Road Tie-in (Effective
flows used)2.49 400 630 780 1020 1650
Approximately 1,950 upstream of confluence with Prince George Creek W_PGCT4_04 0.16 62 91 120 150 220
At confluence with Prince George Creek W_PGCT4_02 0.22 75 109 140 180 260
Approximately 1,270 feet upstream of Blue Clay Road (SR-1318)W_PC_08 0.14 57 83 110 130 200
Just downstream of Blue Clay Road (SR-1318)W_PC_06 0.52 126 183 240 290 430
Just downstream of Just downstream of Juvenile Center Road W_PC_04 0.90 176 254 330 400 600
Downstream of Juvenile Center Road Tie-in (Effective
flows used)205 354 455 605 1095
Just downstream of I-140 W_PCT1_04 0.60 371 454 510 570 700
At confluence with Pumpkin Creek W_PCT1_00 1.10 603 719 800 880 1050
At confluence with Pumpkin Creek W_PCT2_00 0.20 87 127 170 200 310
Approximately 1,400 feet upstream of Castle Hayne Road W_WC_02 0.30 154 206 250 290 400
Approximately 400 feet downstream of Castle Hayne Road W_WC_00 0.60 278 363 430 500 660
Approximately 1.25 miles upstream of the confluence with Northeast Cape Fear River Tie-in (Effective
flows used)1.41 460 717 870 1026 1501
Wildcat Branch
Prince George Creek Trib 4
Pumpkin Creek
Pumpkin Creek Tributary 1
Pumpkin Creek Tributary 2
Ness Creek Tributary 1
Prince George Creek
Kings Grant Tributary
Kings Grant Tributary 2
Flooding Source/Location
Drainage
Area
(Miles2)
Peak Discharges (cfs)
Acorn Branch
Kings Grant Tributary 2A
Page 1 of 1
Appendix B
Tie-in Table
Study Upstream Model Downstream Model Station Q - 1%FW Width FW 10%4%2%1%0.2%1%-
10%IMP
1%-
20%IMP
1%-
35%IMP
New Study Island Creek -39792 1103 66 24.25 21.19 22.02 22.63 23.25 24.67 24.17 24.95 25.63
Effective DS Model -Island Creek 39792 1103 66 24.25 21.19 22.02 22.63 23.25 24.67 24.17 24.95 25.63
New Study Kings Grant Trib -10624 1091 80 37.71 33.91 36.34 36.57 36.73 37.18 36.73 36.85 37.13
Revised Effective Model -Kings Grant Trib 10624 1091 80 37.71 33.91 36.34 36.57 36.73 37.18 36.73 36.85 37.13
New Study Ness Creek Trib 1 -3203 277 30 21.01 17.77 18.73 19.65 20.88 23.88 24.96 25.46 25.89
Effective DS Model -Ness Creek Trib 1 3203 277 30 21.01 17.77 18.73 19.65 20.88 23.88 24.96 25.46 25.89
New Study Prince George Creek -43962 1020 320 32.16 31.42 27.91 31.58 31.66 31.85 29.03 29.26 29.59
Effective DS Model -Prince George Creek 43962 1020 320 32.16 31.42 NA 31.58 31.66 31.85 NA NA NA
New Study Pumpkin Creek 9910 605 160 31.95 29.16 31.02 31.33 31.50 32.16 31.92 32.11 32.32
Effective DS Model Pumpkin Creek 9910 605 160 31.95 29.16 NA 31.33 31.50 NA NA NA NA
New ESP Study WildcatBranchDS_101 11000 1026 55 23.94 21.76 22.87 23.40 23.78 24.73 23.78 24.29 24.83
Effective Model WildcatBranch 11000 1026 55 23.94 21.76 22.87 23.40 23.78 24.73 23.78 24.29 24.83
Tie-In Table
Page 1 of 1
Appendix C
Backwater Tie-in Table
New Study Downstream Elevation
(NAVD88) (ft)Receiving Stream Main Stem BFE
(NAVD88) (ft)
Acorn Branch 6.1 Smith Creek 9.2
Acorn Branch Tributary 12.9 Acorn Branch 20.2
Kings Grant Tributary 2 29.2 Kings Grant Tributary 36.7
Kings Grant Tributary 2a 38.2 Kings Grant Tributary 2 38.2
Prince George Creek Tributary 4 26.3 Prince George Creek 31.7
Pumpkin Creek Tributary 1 24.2 Pumpkin Creek 31.1
Pumpkin Creek Tributary 2 33.1 Pumpkin Creek 33.2
Backwater Tie-in Table
Attachments
Digital Support Data – Hydrology
• Watershed shapefiles
o Sub-basins developed using USGS StreamStats and ArcGIS.
o Watershed delineation points developed using StreamStats and ArcGIS grid.
• Impervious shapefiles
o Impervious areas from 2016 NLCD.
• Microsoft Excel spreadsheets
o New Hanover County discharge calculations
o 1%+/- calculations
o Effective flow interpolation calculations
Digital Support Data - Hydraulics
• 17 HEC-RAS Models (14 1-D and 3 2-D)
o RAS Mapper includes mapping files for the following:
▪ Draft 100-year event floodplain “1%Floodplain” shapefile – 1D models
▪ Draft 500-year event floodplain “Point2%Floodplain” shapefile – 1D models
▪ Model Streamline “River” shapefile – 1D models
▪ Model cross-section line “XS” shapefile – 1D models
▪ Floodway boundaries file “Floodway” shapefile – 1D models
Digital Support Data – Field Data
• Engineering Field Assessment Sheets
• Note the FLOOD database included in the GIS Mapping “1D” folder contains all field survey
photos, sketches, and collected point data
Digital Support Data – FIS Documents
• Annotated FIRM Panels depicting updated mapping
• Floodway Data Tables for new 1-D study reaches
• Summary of Discharge Data for new 1-D study reaches
Digital Support Data – GIS Mapping
• FLOOD Database gdb and supporting files including raster datasets for 1-D study reaches
• Advisory Flood Database gdb and raster mapping datasets for 2-D study areas
• Work Map Exhibits