20181699 Ver 2_Monitoring Report_20220901

Pre -Construction Monitoring Report
For:
Shinn Creek Estates
Wilmington, New Hanover County, North Carolina
Prepared By:
Davey Resource Group
3805 Wrightsville Avenue
Wilmington, North Carolina
September 2022

Contents
1. Introduction 1
2. Methodology 1
3. Sampling Stations 1
4. Results 2
4.1 Water Temperature 2
4.2 Salinity 2
4.3 Specific Conductivity 4
4.4 Dissolved Oxygen 4
4.5 Turbidity 6
4.6 Rainfall 6
5. Marsh Vegetation 6
6. Summary 8
Tables
Table 1. Water Quality Sampling Results 3
Figures
Figure 1. Location of Sampling Stations 2
Figure 2. Temperature (°C) by Station 3
Figure 3. Salinity (ppt) by Station 4
Figure 4. Dissolved Oxygen (mg/L) by Station ID 5
Figure 5. Temperature and Dissolved Oxygen (mg/L) by Station ID (excluding reference) 5
Figure 6. Dissolved Oxygen (mg/L) and Salinity (ppt) by Station ID (excluding reference) 5
Figure 7. 30-Day Antecedent Rainfall 6
Figure 8. Marsh Extents, Upper Portion of Channel 7
Figure 9. Marsh Extents, Lower Portion of Channel 7
Appendices
Appendix A — Tidal Flow Analysis
Appendix A — Marsh Vegetation Images
Water Quality Monitoring Report — September 2022
Shinn Creek Estates
Wilmington, New Hanover County, North Carolina
i

1. Introduction
Davey Resource Group has contracted with Shinn Creek Estates Homeowners Association (HOA) to perform the pre -
construction monitoring within permitted navigation channel extending from the Shinn Creek Estates neighborhood
boat ramp to the Atlantic Intracoastal Waterway (AIWW). Data collected during this sampling period will be utilized
to develop a better understanding of the pre -construction conditions within the channel relative to subsequent post -
construction monitoring events and to meet Condition 1. of 401 Water Quality Certification No. WQC004925.
The Shinn Creek Estates water quality assessment incorporates eight (8) discrete sampling locations located on a
gradient from the open water of the AIWW to the head of the small boat canal and in the same location as the 2018
sampling event. This report summarizes the required chemical and physical parameters obtained during the pre -
construction monitoring event.
2. Methodology
Surface water quality samples were collected during the late morning of August 24, 2022 (coinciding with low tide).
Physiochemical water quality measurements (including water temperature, dissolved oxygen, salinity, and specific
conductivity) were collected in situ at each station using a YSI Model ProSeries Professional Plus (YSI) multiparameter
water quality probe. The YSI was field calibrated directly prior to sampling. Turbidity was measure using a Secchi
disk.
Table 1 provides Station ID nomenclature for each sampling point. A near surface ("a") and near bottom ("b") sample
were collected at all but two locations (SC1 and SC4), which at the time of sampling were less than 14 inches deep.
Meteorological conditions (including, air temperature, wind speed and wind direction) were recorded in situ. Daily
antecedent precipitation data was downloaded from the KILM weather station using the North Carolina State Climate
Office database (NC CRONOS).
Tidal flow analysis to the 15 acres of salt marsh connected to the Shinn Creek boat basin was performed by
Dieffenbauch & Hritz, PLLC and is included in Appendix A.
To evaluate the marsh vegetation adjacent to the project high -resolution drone imaging was collected on August 20,
2022. A mosaic of overhead images was georeferenced and overlayed on the permit drawings. The outer edge of
marsh was digitized for comparison to post construction monitoring flights. Duplicate flight paths will be flown in
subsequent post construction monitoring.
3. Sampling Stations
The Shinn Creek Estates project is located immediately west of the AIWW and within a small navigation channel
connecting the boat ramp of the Shinn Creek Estates neighborhood to the AIWW. The waterbody is an unnamed
tributary of the AIWW located within the Lower Cape Fear River Basin (Hydrologic Unit Code 03030005). Water flow
is principally directed by semidiurnal tides in the area, with the mean tidal range of approximately 4.48 ft at
Masonboro Inlet (NOAA). The water depth in the footprint of the proposed project ranges from 0 ft MLW to -5.0 ft
MLW. Physiochemical parameters (including salinity and dissolved oxygen) are affected by freshwater inflow,
temperature, Biological Oxygen Demand (BOD), tides, and wind forces. The channel is classified as SA Waters that
are directly contiguous with High Quality Water (HQW) of the AIWW.
Surface water samples were collected at a total of eight stations. Seven of the stations were located along a gradient
from upstream to downstream along the access channel to the Shinn Creek Estates neighborhood boat ramp
(beginning upstream of a 42" culvert (SC1) immediately north-northeast of the ramp) and extending south and east
Water Quality Monitoring Report — September 2022
Shinn Creek Estates
Wilmington, New Hanover County, North Carolina
1

to the AIWW (SC7). An additional, reference station was sampled in the adjacent Shinn Point boat channel (next to
that boat ramp) located just to the south of the project area. This channel exhibits similar configuration and
geometry with the exception that it is regularly maintained via periodic dredging. All station locations are depicted in
Figure 1 below.
Figure 1. Location of Sampling Stations
4. Results
4.1 Water Temperature
Water temperatures on the date of the sampling ranged from 26.8°C to 28.6°C (X = 27.88°C). Temperature data were
narrowly distributed with no observed pattern or correlation between sites (Table 1) (Figure 2).
4.2 Salinity
Given its proximity to Masonboro Inlet, the sampling area consists of waters generally characterized as euhaline (i.e.
marine/oceanic waters) near the AIWW to polyhaline (i.e. estuarine waters) in the upper reaches of the channel.
Temporal and spatial variation in salinity levels can occur as a result of freshwater inflows from up -gradient sources
(particularly after larger precipitation events). At the time of sampling, salinity values ranged between 18.31 ppt and
27.28 ppt (X = 24.55 ppt) (Table 1) (Figure 3).
Water Quality Monitoring Report — September 2022
Shinn Creek Estates
Wilmington, New Hanover County, North Carolina
2

Table 1. Water Quality Sampling Results
Station
Date
Time
Temp
(°C)
DO
saturation
(%)
DO
(mg/L)
Specific
Conductivity
mS/cm
Conductivity
mS/cm
Salinity
(ppt)
Tubidity
(Secchi-
Inches)
SC1
8/22/2022
10:57
28.6
28.8
2.74
29.69
32.18
18.36
>13
SC2a
8/22/2022
11:08
27.3
25
1.75
36.07
38.28
22.58
30
SC2b
8/22/2022
11:10
26.8
35.4
2.35
43.96
46.47
18.31
30
SC3a
8/22/2022
11:21
27.4
38.1
2.61
39.4
41.9
25.86
>20
SC3b
8/22/2022
11:23
27.3
46.2
3.05
42.18
44.26
22.42
>20
SC4
8/22/2022
11:29
28.6
33.4
2.26
37.98
40.66
24.16
>14
SC5a
8/22/2022
11:38
28.6
44.6
3.04
39.84
42.47
25.18
24
SC5b
8/22/2022
11:40
28.3
49.5
3.39
41.48
44.25
26.54
24
SC6a
8/22/2022
11:47
28.3
49.1
3.3
40.86
43.4
26.09
20
SC6b
8/22/2022
11:49
27.9
41.6
2.8
41.87
44.2
26.78
20
SC7a
8/22/2022
11:52
27.7
66.7
4.54
41.03
43.16
26.19
>32
SC7b
8/22/2022
11:54
27.8
63.8
4.34
41.13
43.35
26.22
>32
SWRa
8/22/2022
12:26
28.4
62.7
3.5
39.72
42.31
26.28
30
SWRb
8/22/2022
12:28
27.3
42.9
2.91
42.66
44.48
27.28
30
* Values shaded gray indicate stations exhibiting DO levels below 5.0 mg/L.
Temperature (°C)
29
28.5
28
27.5
27
26.5
26
25.5
Temperature (°C) by Station
IiiiiIIiiiIi
C.ti c.5, 4 G�,a 4:9 h�� c)( 4., G�a 4 S��a L�� ��aA4Sa
Station ID
Figure 2. Temperature (°C) by Station
Water Quality Monitoring Report - September 2022
Shinn Creek Estates
Wilmington, New Hanover County, North Carolina
3

30.00
25.00
Q 20.00
Q
Y 15.00
vf6i 10.00
5.00
0.00
Salinty (ppt) by Station
4, 4, 4, ���� c) �`'� �(h� ��a G�� ��a hc,•h��a�,\S`
Station ID
Figure 3. Salinity (ppt) by Station
4.3 Specific Conductivity
Conductivity is a measurement of the dissolved ionic compounds, and as such, increases with increasing salinity.
Uncharacteristic elevations in conductivity measurements can often reflect point source pollution, such as industrial
or municipal wastewater. Results from the August 2022 sampling event yielded conductivity measurements
characteristic of coastal systems and displaying similar spatial correlation between sites as that of salinity.
4.4 Dissolved Oxygen
The dissolved oxygen (DO) levels during the August 2022 sampling ranged from 1.75 mg/L to 4.54 mg/L (Table 1).
The North Carolina Water Quality Standard for surface waters is not less than a daily average of 5.0 mg/I with a
minimum instantaneous value of not less than 4.0 mg/I. DO levels less than 5.0 mg/L were observed at all of the
eight stations. Dissolved oxygen concentrations are influenced by both temperature and salinity (solubility of oxygen
decreases with increases in temperature and salinity). The purpose of this monitoring event was to identify any
spatial patterns in DO concentrations rather than temporal or seasonal differences. In 2018, a strong correlation was
observed between stations with the highest DO levels occurring within the AIWW and the lowest levels occurring at
the up -gradient sites (R2 = 0.9307). While this correlation remained in the 2022 results, it was reduced to R2 = 0.634.
Additional charts (Figure 4, Figure 5 and Figure 6) below graphically display DO concentrations, temperature, and
salinity by station (excluding the reference station).
Water Quality Monitoring Report — September 2022
Shinn Creek Estates
Wilmington, New Hanover County, North Carolina
4

2.5
0 2
0
1.5
0 1
p 0.5
0
29
28.5
0 28
a' 27.5
27
a
a£ 26.5
26
25.5
DO (mg/L) by Station
�Lti y(ya 5y(3a 4:9 y(,o y(�a 4 2 (9' 5`GQ �db' h0o5�''%449
Station ID
Figure 4. Dissolved Oxygen (mg/L) by Station ID
Temperature and Dissolved Oxygen by Station
Station ID
Temp (oC) D0 (mg/L) Linear (DO (mg/L))
5
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
Dissolve Oxygenn (mg/L)
Figure 5. Temperature and Dissolved Oxygen (mg/L) by Station ID (excluding reference)
5
c 4.5
4
c 3.5
o50 3
OX 2.5
v 2
> 1.5
a
1
0 0.5
0
(1
S
Dissolved Oxygen and Salinity by Station
pri
0,a ,,G�� 42' d'
¢3) (.a 60
(5a (roe
coco S S o 5
30.00
25.00
20.00 0_
oa
15.00
10.00 N
5.00
0.00
Station ID
SaIinity(ppt)DO (mg/L) Linea r(SaI nity (ppt).) Linear (DO (mg/L))
Figure 6. Dissolved Oxygen (mg/L) and Salinity (opt) by Station ID (excluding reference)
Water Quality Monitoring Report — September 2022
Shinn Creek Estates
Wilmington, New Hanover County, North Carolina
5

4.5Turbidity
High turbidity levels can affect dissolved oxygen levels, light penetration and ecological productivity and habitat
quality. To determine turbidity, Secchi depth readings were taken at each sampling station. Due to the shallow depth
at some stations, a Secchi depth reading could not be obtained. Results are displayed in Table 1.
4.6Rainfall
Rainfall and associated freshwater inflow can influence water quality data. As a result, daily precipitation data was
downloaded from the NC CRONOS website for the New Hanover County Airport Weather Station (KILM).
Precipitation for the 30-day antecedent period is graphically depicted below (Figure 7). A 0.75-inch rain event
occurred two days prior to sampling and a 3.08-inch rain event occurred five days prior to sampling. In the seven
days prior to sampling, approximately 3.93 inches of rain were reported at the KILM station.
3.5
3
2.5
z
0 2
1.5
c 1
a
0.5
0
30-Day Antecedent Rainfall
N N N N N N N N N N N N N N N N
N N N N N N N N N N N N CN N N CN
O O 0 0 0 0 0 0 0 0 0 0 0 0 0 0
N N N N N N N N N N N N N N N CN
\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \
00 O �--� m V1 T.,OCT)c-I M V1 m
N
N N N m
N. n r` N. W 00 00 00 00 00 00
DATE
Figure 7. 30-Day Antecedent Rainfall
5. Marsh Vegetation
To evaluate the extent of marsh vegetation adjacent to the proposed project high -resolution drone imaging was
obtained. Differences between the 2018 data and 2020 data are minimal. Some small areas of marsh vegetation
accretion has occurred in the vicinity of the existing oyster beds. Results are shown in Figure 8 and Figure 9 below.
Full resolution images are included in Appendix B.
Water Quality Monitoring Report — September 2022
Shinn Creek Estates
Wilmington, New Hanover County, North Carolina
6

August 2018 Marsh Line
August 2022 Marsh Line
Figure 8. Marsh Extents, Upper Portion of Channel
Shinn Creek.Estates
Navigation Channel
August 2018 Marsh Line
August 2022 Marsh Line _---
Figure 9. Marsh Extents, Lower Portion of Channel
Water Quality Monitoring Report — September 2022
Shinn Creek Estates
Wilmington, New Hanover County, North Carolina
7

6. Summary
This report summarizes the quantitative results obtained during the August 24, 2022 sampling of waters within a
small boat canal leading to/from the Shinn Creek Estates neighborhood boat ramp and the August 20, 2022 drone
imaging. Based upon observed data, there remained correlation between station and dissolved oxygen as evidenced
by low DO levels in the upper reaches of the canal and relatively higher DO levels in stations located closer to the
waterway. As anticipated, salinity exhibited the same pattern as freshwater inflows up -gradient contribute to lower
salinity levels. Such a pattern, however, further highlights the correlation of DO levels relative to station location
given that solubility of oxygen decreases with increases in salinity. As a result, the correlation can be attributed to
physical mixing and aeration of surface waters resulting from tidal exchange. The upper reaches of the channel are
more restricted thus limiting the tidal prism and extent of tidal exchange at upstream sites.
Water Quality Monitoring Report — September 2022
Shinn Creek Estates
Wilmington, New Hanover County, North Carolina
8

Appendix A — Tidal Flow Analysis

DIEFFENBAUCH & HRITZ, PLLC
530 Causeway Drive
Suite D1
Wrightsville Beach, NC 28480
Office: 910-239-9277, Ext 2001
License; P-1171
September 21, 2022
Ben Stephenson
Shinn Creek HOA
6433 Shinn Creek Lane
Wilmington, NC 28409
RE: Shinn Creek Tidal Culvert
Existing Culvert Flow Analysis
Dear Mr. Stephenson,
Per your request, we performed a hydraulic analysis of the existing 48" concrete culvert which
conveys tidal waters to and from the upper creek and marsh areas north of the culvert. FHWA's
HY-8 culvert analysis program was used for the computations. No survey data was obtained for
the analysis. Below are the assumptions and results.
Analysis Assumptions:
• Four conditions used for flow calculations: 1 foot above invert (0 hrs), half full (2 hrs), full (4
hrs), and 2 foot of head (6 hrs) to simulate an average tidal cycle
• 41 feet — 48" diameter concrete culvert
• Flat (0%) slope
• 0.012 Manning's n
• "Grooved end projecting" for inlet and outlet treatments
• 6-hour tidal flow assumed for volume calculations
• Linear flow distribution between conditions
Analysis Results:
• 2 ft head (6-foot water depth):
• Full (top of culvert):
• Half full (2 feet above invert):
• 1 foot above invert:
85 cfs
71 cfs
21 cfs
6 cfs
• Total flow volume for first 2 hours following low tide: 101,000 cf ( 750,000 gallons)
• Total flow volume for entire tide cycle: 994,000 cf (7,400,000 gallons)

The hydraulic analysis was based on the listed assumptions and performed using FHWA's HY-8.
Many factors can affect the flow such as higher or lower tide cycles, condition of existing culvert,
wind direction and strength, receiving stream conditions, etc. However, the flow numbers
calculated should provide an acceptable reference point. If you have any questions, please feel
free to contact me.
Very truly yours,
DIEFFENBAUCH & HR1TZ, PLLC
JOHN C. DIEFFENBAUCH, P.E.
Vice President

Appendix B — Marsh Vegetation Images

' , •
1
it •
1? •
Shinn Creek
Navigation Channel
1 ik le
August 2018 Marsh Line
August 2022 Marsh Line

August 2018 Marsh Line
August 2022 Marsh Line
A4