29-20 Post Construction Monitoring Report, Year 1 Shinn Creek Estates HOA

Post Construction Monitoring Report
Year 1
Fo r:
Shinn Creek Estates
Wilmington, New Hanover County, North Carolina
Prepared By:
Davey Resource Group
3805 Wrightsville Avenue
Wilmington, North Carolina
October2023

Contents
Tables
Table 1. Water Quality Sampling Results......................................................................................................3
Figures
Figure 1. Location of sampling stations and channel footprint....................................................................2
Figure 2. Depths at which each sample was collected.................................................................................2
Figure 3. Temperature (°C) by station for 2022 and 2023 data....................................................................4
Figure 4. Salinity (ppt) by station for 2022 and 2023 data...........................................................................4
Figure 5. Specific conductivity (mS/cm) by station for 2022 and 2023 data................................................5
Figure 6. Dissolved Oxygen (mg/L) by station for 2022 and 2023 data........................................................5
Figure 7. Temperature and dissolved oxygen by station (excluding reference station) for 2023 data ........ 6
Figure 8. Salinity and dissolved oxygen by station (excluding reference station) for 2023 data.................6
Figure 9. Secchi depth by station on August 28, 2023.................................................................................. 7
Figure 10. 2023 and 2022 specific conductivity by station...........................................................................7
Figure 11. Marsh extent in the upper portion of the channel......................................................................8
Figure 12. Marsh extents in the upper portion of the channel....................................................................8
Figure 13. Oyster analysis stations, existing oyster beds and oyster placement areas................................9
Water Quality Monitoring Report — October 2023
Shinn Creek Estates
Wilmington, New Hanover County, North Carolina i

Appendices
AppendixA
— Tidal Flow Analysis
Appendix 8
— Water Sampling Station Images
Appendix 8
— Marsh Vegetation Images
Appendix C—
Oyster Shell Images
Water Quality Monitoring Report — October 2023
Shinn Creek Estates
Wilmington, New Hanover County, North Carolina

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 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
and 2022 sampling events. This report summarizes the required chemical and physical parameters obtained during
the post -construction year 1 monitoring event.
2. Methodology
Water quality samples were collected the same time of the year and with similar conditions as the previous year's
samples. The samples this year were collected during the late morning of August 28, 2023 which coincided 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, and Figure 1 shows sampling locations. Multiple
samples were collected at all but one station (SC1) where total water depth was -5 inches. Of the seven stations with
multiple sample depths, two samples were collected at four stations (SC3, SC4, SC5 and SC7) representing near
surface and near bottom. Three samples were collected at three stations (SC2, SC6 and SWR) representing near
surface, middle of the water column and near bottom. The depth at which each sample was collected is found in
Figure 2.
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 15,
2023. 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 -6 ft
MLW. Physiochemical parameters (including salinity and dissolved oxygen) are affected by freshwater inflow,
Water Quality Monitoring Report — October 2023
Shinn Creek Estates
Wilmington, New Hanover County, North Carolina 1

temperature, Biological Oxygen Demand (BOD), tides, and wind forces. The channel is classified as SA Waters that are
directly contiguous with High Quality Water (HOW) of the AIWW.
Surface water samples were collected at a total of eight stations. Seven of the stations are 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
to the AIWW (SC7). An additional reference station (SWR) was sampled in the adjacent Shinn Point boat channel next
to the existing boat ramp just south of the project area. This channel exhibits similar configuration and geometry
with the exception that it is regularly maintained via periodic dredging. Images of each station on the day of sampling
are included in Appendix B.
Figure 1. Location of sampling stations and channel footprint. Aerial imagery by Nearmap captured January 2023.
Sample Collection Depth (in)
o° 6 Gtia tin ati° a'ta '9 �Aa Gab 6� G6' Gr0% V60` A
G6a'0' 5 �o �`'� �0 �0o
e ci' 0 cio5 5' 5 5' 9 5 0 0 5
,o n' 91111151111 1
-8 -8 -8 -8
12 .15
-20 -19 •17 -19 -10
-23 -24
t -30 -30 -29
fi
o -4o
-50 -45
-60
•70 -67
Figure 2. Depths at which each sample was collected.
Water Quality Monitoring Report — October 2023
Shinn Creek Estates
Wilmington, New Hanover County, North Carolina
2

4. Results
4.1 Water Temperature
Water temperatures on the date of the sampling ranged from 29.0°C to 31.8°C (X = 30.34°C). Temperature data were
narrowly distributed with no observed pattern or correlation between sites (Table 1) (Figure 3). The water
temperature is 2.46°C higher on average than the data collected last year.
4.2Salinity
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 29.40 ppt and
33.20 ppt ( X = 31.63 ppt) (Table 1) (Figure 4). Salinity values were overall higher than last year, averaging 7.08 ppt
higher.
Table 1. Water Quality Sampling Results
Station
Station
Date
Date
FTimeDO
(in)
Temp
(C)
Saturation
(070)
DO
(mg/L)
SpecificDepth
Conductivity
mS/cm
Conuctivity
mS/cm
Salinity
(ppt)
Secchi
(in)
SC1
8/28/2023
1049
-5
31.0
43.1
2.71
46.8
52.0
30.24
-5
SC2a
8/28/2023
1054
-12
29.8
52.8
3.3
47.4
51.7
30.76
-23
SC21b
8/28/2023
1055
-30
29.3
30.0
1.95
48.1
52.0
31.23
-23
SC2c
8/28/2023
1058
-67
29.0
23.2
1.49
48.3
52.0
31.40
-23
SC3a
8/28/2023
11:24
-5
30.3
50.3
3.19
48.5
53.4
31.44
-15
SC31b
8/28/2023
11:25
-15
30.3
39.3
3.12
48.4
53.3
31.4
-15
SC4a
8/28/2023
11:38
-8
30.5
49.69
3.15
48.7
54.0
31.67
-24
SC41b
8/28/2023
11:39
-23
30.3
45.8
2.97
48.8
53.9
31.75
-24
SCSa
8/28/2023
11:57
-8
30.7
59.4
3.92
50.2
55.8
32.71
-15
SCSb
8/28/2023
11:59
-19
30.7
53.0
3.47
50.2
55.6
32.68
-15
SC6a
8/28/2023
1208
-8
31.6
86.8
5.41
50.7
57.1
33.06
-18
SC61b
8/28/2023
1209
-17
31.8
91.6
5.62
50.8
57.6
33.14
-18
SC6c
8/28/2023
12:10
-29
30.3
77.9
4.82
50.9
56.2
1 33.20
-18
SC7a
8/28/2023
1217
-8
29.7
108.8
5.89
50.3
54.8
32.81
-23
SC71b
8/28/2023
1219
-19
29.7
76
4.8
50.1
54.5
32.5
-23
SWRa
8/28/2023
1259
-10
30.6
62.5
3.9
45.5
50.4
29.4
-28
SWRb
8/28/2023
1301
-24
30.2
50.6
3.26
46.3
50.9
29.87
-28
SWRc
8/28/2023
13:02
-45
30.3
54.4
3.48
46.4
51.1
30.00
-28
*Values shaded gray indicate stations exhibiting DO levels below 5.0 mg/L.
Water Quality Monitoring Report - October 2023
Shinn Creek Estates
Wilmington, New Hanover County, North Carolina
3

Temperature (-C) by Station
• 2022 •2023
32
31
30
U 29
v
�s 28
m
0 27
E
26
25
24
sc1 SC2 SC3 SC4 SC5 SC6 SC7 SWR
Station
Figure 3. Temperature(T) by station for 2022 and 2023 data.
Salinity (ppt) by Station
■ 2022 ■ 2023
35
30
25
o9 20
T
E 15
10
5
0 - —
sc1 scz sc3 sca scs scs sc7 svura
Station
Figu re 4. Salinity (ppt) by station for 2022 and 2023 data.
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 2023 sampling event yielded conductivity measurements
characteristic of coastal systems and displaying similar spatial correlation between sites as that of salinity. Specific
conductivity was overall higher this year than last year (Figure 5).
Water Quality Monitoring Report — October 2023
Shinn Creek Estates
Wilmington, New Hanover County, North Carolina
4

Specific Conductivity (mS/cm) by Station
.2022 .2023
e 60
U
50
'W —
U
a 30
c
U 20
U
K
u 10
N 0
SC1 SC2 SC3 SC4 SC5 SOB SC7 SWR
Station
Figure 5. Specific conductivity (mS/cm) by station for 2022 and 2023 data.
4.4 Dissolved Oxygen
The dissolved oxygen (DO) levels during the August 2023 sampling ranged from 1.49 mg/L to 5.89 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 six out of eight stations.
In 2022, all stations fell below the 5.0 mg/L threshold. DO values for 2023 were higher at every station except for
SC1, and overall, 2023 DO values were higher on average than in 2022. DO concentrations are influenced by both
temperature and salinity (solubility of oxygen decreases with increases in temperature and salinity). The purpose of
this monitoring event is to identify any spatial patterns in DO concentrations rather than temporal or seasonal
differences. In 2022, a weak correlation was observed between stations with the highest DO levels occurring within
the AIWW and the lowest levels occurring at the up -gradient sites (Rz = 0.634). In 2023, this correlation increased to
an Rz value of 0.8375. Additional charts (Figures 6, 7 and 8) below graphically display DO concentrations,
temperature, and salinity by station (excluding the reference station).
6
5
J 4
3
O
02
1
0
Dissolved Oxygen (mg/L) by Station
• 2022 •2023
SC1 SC2 SC3 SC4 SC5 SC6 SC7 SWR
Station
Figure 6. Dissolved Oxygen (mg/L) by station for 2022 and 2023 data.
Water Quality Monitoring Report — October 2023
Shinn Creek Estates
Wilmington, New Hanover County, North Carolina
61

Temperature and Dissolved Oxygen by Station
aaaaonTemperature -DO ..... Linear(Temperature) • • • • • Linear (DO)
5
31.
31.0
630.5
0230.0
m
`ai. 28.5
o 29.0
t-
28.5
28.0
R' = 0.8375
SC7 SC2 SC3 SC4 SC5 S06
Station
•R' = 0.0007
SC7
6.00
5.00
4.00
Ja
3.00 E
O
200
1.00
000
Figure 7. Temperature and dissolved oxygen by station (excluding reference station) for 2023 data.
33.50
33.00
32.50
32.00
131.50
Z 31.00
c
30.50
ur 30.00
29.50
29.00
28.50
Salinity and Dissolved Oxygen by Station
- Salinity -DO ..... Linear (Salinity) • • • • • Linear (DO)
SC1 SC2 SC3 SC4 SC5 SOB SC7
Station
6.00
5.00
4.00
a
3.00 E
O
200 0
1.00
0.00
Figure 8. Salinity and dissolved oxygen by station (excluding reference station) for 2023 data.
4.5 Turbidity
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. Comparing 2022 secchi
data with 2023 suggests that the water was more turbid during the 2023 sample collection than in 2022. This could
be due to recent rain events and mixing. In 2022, a rain event occurred two days before sampling. While in 2023, a
rain event occurred one day before sampling. Light, periodic precipitation was also present during sampling in 2023.
Results are displayed in Table 1 and Figure 9 below.
Water Quality Monitoring Report - October 2023
Shinn Creek Estates
Wilmington, New Hanover County, North Carolina
1.4,

Secchi Depth (in) by Station
.2022 .2023
Sc1 SC2 sm SC4 SC5 SCE SC7 sm
0 r
-5
-10
-15
r:
w -20
2s
-30
-95
Figure 9. Secchi depth by station on August 28, 2023.
4.6 Rainfall
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 10). Over 0.8 inches of rain fell
the day before samples were taken. On the day of sampling, light rain fell periodically. This was not recorded in KILM
but was experienced on -site at the time of sampling. In the seven days prior to sampling, a total of 1.08 inches of rain
fell with most of that occurring the day before sampling. These rainfall conditions are similar to last year when a 0.75-
inch rain event occurred two days prior to sampling.
0.9
0.8
C_
07
C
A 0.6
a 05
d 0.4
N
'0 03
H
0.2
0.1
L
Daily Total Precipitation (in) from KILM
,n r ,n r m r ,n r m r ,n r m M ,n ,n ro M ,n m m M r m n, M m r ,n r m
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 N N
Figure 10. 30-Day precipitation.
5. Marsh Vegetation
To evaluate the extent of marsh vegetation adjacent to the proposed project, high -resolution drone imaging was
obtained. The imagery was captured on August 15, 2023, which was a clear day. Based on the on -site visit and drone
imagery, the marsh line has not significantly changed since August 2022. Results comparing 2018, 2022 and 2023 are
shown in Figure 19 and Figure 20 below, and images of the marsh captured on August 28, 2023 can be seen in
Appendix C.
Water Quality Monitoring Report — October 2023
Shinn Creek Estates
Wilmington, New Hanover County, North Carolina 7

Figure 11. Marsh extent in the upper portion of the channel. Drone imagery captured on August 15, 2023.
Figure 12. Marsh extents in the upper portion of the channel. Drone imagery captured on August 15, 2023.
Water Quality Monitoring Report — October 2023
Shinn Creek Estates
Wilmington, New Hanover County, North Carolina

6. Oyster Beds
Oyster beds are well known to promote marsh stability and improve water quality. In early 2023, loose oyster shells
were deposited along the shorelines of the Shinn Creek Estates Navigation Channel in areas where oysters are absent
to encourage the growth of spat (oyster larvae). Figure 13 shows locations of existing oyster beds, oyster shell
placement areas and oyster analysis stations. During the on -site data collection on August 28, 2023, three oyster
evaluation points were established: 01, 02 and 03. At each of these stations, the strategically placed oyster shell was
evaluated for spat and shoreline coverage. Images were taken on the day of data collection of the oyster shell
placement areas and are shown in Appendix D. Inspection of the loose oyster shell material revealed no evidence of
spat growth. Since the oyster shells were placed in early 2023, it is possible that some larvae could still attach in the
fall season. However, the oyster shell coverage appeared to be sporadic, and much of it was covered in sediment
which could potentially have implications on spat attachment. It is suggested that oyster shell bags be placed in the
shell placement areas to increase the amount of spat attachable substrate for the next spawn season.
Figure 13. Oyster analysis stations, existing oyster beds and oyster placement areas. Drone imagery captured on August 15, 2023.
7. Summary
This report summarizes the quantitative results obtained during the August 28, 2023 sampling of waters within a
small boat canal leading to/from the Shinn Creek Estates neighborhood boat ramp and the August 15, 2023 drone
imaging. Based upon observed data, there remained correlation between station and dissolved oxygen as evidenced
by lower DO levels in the upper reaches of the chanel and relatively higher DO levels in stations located closer to the
waterway. Salinity exhibited the same pattern as freshwater inflows up -gradient contribute to lower salinity levels.
DO values and salinity levels were higher on average in 2023 than in 2022. It should be noted that the channel was
not excavated to the complete permitted width and depth and additional maintenance dredging should contribute to
increasing the tidal prism and exchange in the upper reaches of the channel.
Water Quality Monitoring Report — October 2023
Shinn Creek Estates
Wilmington, New Hanover County, North Carolina 9

Appendix A — Tidal Flow Analysis

2 ,
September 18, 2023
Ben Stephenson
Shinn Creek HOA
6433 Shinn Creek Lane
Wilmington, NC 28409
RE: Shinn Creek Tidal Culvert
Existing Culvert Flow Analysis - Update
Dear Mr. Stephenson,
arEPPERaAUM & HRI1Z PLLC
6321 Shinn Creek Lane
Wilmington, UC 28409
Oaicr. 910-239.9277, at 2001
License: P-1171
Per your request, I am providing the Shinn Creek HOA an update on the culvert analysis we
performed in 2022 and subsequent letter dated September 21, 2022, tided "Shinn Creek Tidal
Culvert — Existing Culvert Flow Analysis". As stated in the letter, we made numerous
assumptions for the flow analysis and none of those assumptions have changed. Therefore, no
further calculations were performed. Furthermore, unless there is a modification to, or
replacement of, the existing 48" concrete culvert in the future, the analysis results will be valid
indefinitely.
If you have any questions, please feel free to contact me.
Very truly yours,
DIEFFENBAUCH & HRTTZ, PLLC
3P/-
!FFENBAUCH, P.E.
a President
Attachments: Shinn Creek Culvert Letter

DIEFFENDAUC14 & HRM PLLC
530 Causeway Drive
Suite DI
Wrightsville Beach, NC 28480
Office: 910-239-9277, W 2001
Llcensc: 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: I 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 (0n/o) 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): 85 cfs
• Full (top of culvert): 71 cfs
• Half full (2 feet above invert): 21 cfs
• 1 foot above invert: 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 & HRITZ, PLLC
3FFENBAUCH, P.E.
e President

Appendix 8 — Water Sampling Station Images

SC1 on August 28, 2023.
SO on August 28, 2023.
SC5 on August 28, 2023.
SC7 on August 28, 2023.
SC2 on August 28, 2023.
SC4 on August 28, 2023.
SC6 on August 28, 2023.
SWR (Reference Point) on August 28, 2023.

Appendix C —Marsh Vegetation Images

Marsh line near SC2.

�,syr ry,lu
y

�9
I
1
I il'.. 1 ifl 11
1
�f
)rl
Al'G{��a{���I) Marsh line near SC2.

Marsh line near 01.

Marsh line near SC3.

Marsh line near between SO and SC4.

4 ; r 761 --qmllf
-o.
r

Appendix D — Oyster Shell Images

Oyster shells near 01 and 02.

a
. Z. �
r� � y
�
rr= ♦
1'/ �
A-
Y
x
Piles of oyster shell material found near 01.

r IV
!Y
t � v
w
F4
AIZA3
Cf
f a
4
Oyster shell material close-up near 01.

Owl
TT R A
Y sue, '"y � � � ► 0
i+fP,
Oyster shell material close-up near 02.

Existing beds and shell placement area near 03.

Close-up of placement area near 03.