NCDOT 106-12 B-2500 Phase I Structure Monitoring 6/14/2017

Brittingham, Cathy
From: Herring, Kathy
Sent: Wednesday,June 14, 2017 10:24 AM
To: Huggett, Doug; Brittingham, Cathy
Subject: May scour report
Attachments: N CDOT_9May2017_B-2500_Structure_Scou r_Mon itori ng_Report.pdf
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Kathy Herring
Environmental Program Supervisor
Natural Environment Section, Project Development and Environmental Analysis
North Carolina Department of Transportation
919 707 6128 office
919 971 4367 mobile
mkherrincancdot.gov
1020 Birch Ridge Drive
1598 Mail Service Center
Raleigh, NC 27699-1598
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Email correspondence to and from this address is subject to the
North Carolina Public Records Law and may be disclosed to third parties.
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EPI May 31, 2017
ENGINEERING &
CONSTRUCTION
Kathy Herring
North Carolina Department of Transportation
Natural Environment Section
1020 Birch Ridge Drive
Raleigh, NC 27699
Subject: Bonner Bridge Mitigation Structure
Scour Monitoring and Inspection
Pamlico Sound, Oregon Inlet
(B-2500) Bridge Replacement Project Dare County, NC
Executive Summary
In 2012 CSA Ocean Sciences, Inc. was contracted by the North Carolina
Department of Transportation (NCDOT) to conduct seagrass mitigation to compensate
for losses anticipated to occur during the replacement of the Bonner Bridge over
Oregon Inlet (Fonseca, 2015). This project is part of a larger long-term effort to
determine the best strategy and methodology for seagrass or submerged aquatic
vegetation (SAV) restoration in North Carolina. A wave attenuation structure with hard
surface habitat suitable for colonization by algae, oysters and other sessile communities
was completed in February 2017. Due to the potential for scour in this dynamic system
the NCDOT has requested a structure scour monitoring regime for the B-2500 Bonner
Bridge wave attenuator (Figure 1). The structure monitoring plan will provide monthly
data and observations to help determine sand transport along bottom of the structure.
Field measurements, underwater photos and wind patterns will be used to determine
scour depths. This document summarizes data collected during the May 9, 2017 site
visit.
Project Purpose and Need
SEPI Engineering and Construction was contracted by NCDOT Natural Environment
Section to conduct monthly scouring monitoring, survey subsided units, check structure
integrity and collect water quality parameters. The main purpose of the site visit on
May 9, 2017 was to collect measurements at the scour monitoring stations, inspect and
document subsided reef units and a structural integrity check. Monthly field
measurements are compared to the established baseline to determine if the structures
bottom scouring will reach an equilibrium or continue to increase in depth. Subsided
units will continue to be monitored to determine bottom impact and structure integrity.
Task Completed by SEPI
• Provided two staff, multi-parameter water quality meter, secchi disk, leveling
rod with underwater photos, aqua scope and shallow draft vessel.
1025 Wade Avenue I Raleigh, NC 127605 I Phone: [phone] I Fax: [phone]
sepiengineering.com I @SEPlengineers

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SEPI
• Collected water quality parameters, measured scour depths and photo
documented scour monitoring stations.
• Conducted structural inspection, collected measurements and photo
documented subsided units.
• Video reviewed scour monitoring stations and subsided units.
Tools and materials used to collect scour measurements and inspect structure:
• (1) 25' Level Rod
• (1) 6' Level
• (1) GoPro Hero Camera
• (1) GoPro Studio
Methods
Scour Measurements and Structure Stability Inspection
Prior to collecting scour depth measurements water quality parameters (temperature,
dissolved oxygen concentration, conductivity, specific conductivity, salinity, pH and
Secchi depth) were collected on the north and south side of piling 51. Structure
monitoring and scour data collection was conducted during mid ebb tide switching to
low tide conditions. Data collection for scour monitoring occurred at seven (7) scour
monitoring stations (Figure 2, Table 1). In order to maintain consistency scour depth
measurements started at the north of the eco-disks then continued clockwise to the
west. Using a level rod, measurements were taken from the tops of eco-disks at all
stations to the depth of the scour below the bottom disks. These measurements were
compared to the established baseline to evaluate monthly scour depths (Table 3).
Additional measurements included the top of disk to the top of the piling. The top of
the disk to the bottom of the top clamp was measured as well. These measurements
are used to determine if the disks subside from a stationary point. Following manual
data collection, a level rod was submerged to the bottom floor and stations were
videoed for desktop review. During the video review process, the distance from the
bottom disk to the scour depth was noted. The reef units were inspected from an
underwater view to determine the extent of scouring and structural integrity.
Stations Scour monitoring
The measured water level depth was 5.0'on the south side of the structure at piling
51. Water levels remained high during field data collection due to North Northwest
(NNW) winds increasing throughout the day and gusting up to 20 mph. Due to
increasing winds the water clarity was at a minimum. The maximum secchi disk
depth reading was 35"at the beginning of the day. Acquiring clear underwater photos
was difficult due to decreased visibilities at the site. Collected field measurements and
photos were reviewed, compiled and compared to the established baseline.
2

SEPI
Summary
Cumulative monthly winds from January 1, 2017 to May 9, 2017 exhibited
multidirectional wind patterns, from the north-northeast (NNE), north-northwest
(NNW) and south-southwest (SSW). Extended periods of winds ranged from 15 -20
mph with occasional winds of 20 -25 mph, gusting to greater than 30 mph from the
SSW (Chart 1). Prevailing winds between the April and May 2017 site visit were from
the SSW direction. Sustained winds were between 20 and 30 mph, gusting to over 35
mph (Chart 2). Scour depths at the wavebreak structure increased in all locations
except the south side of piling 76 (Figure 2, Table 1). The largest scour depth observed
on both sides of the structure was at piling 25. This site had accumulated sediment to
the bottom the structure in March and has since scoured to a depth -3.0'on both sides.
Pilings 1 and 50 increased -1.7' and -1.79' on the north since the baseline was
established. Less of a scour increase was observed on the south of these two locations
with depths of-0.75' and -1.19'. Piling 51 scoured less overall with scour depths of -
0.6' north and -0.25' south. Piling 52 has increased to a scour depth of -1.37' north
and 1.67 south since monitoring began. Piling 101 has increased -0.82' north and -
0.72' south. Overall seasonal trends, show an increased scour depth on the north side
of the structure at pilings 25, 50, 52 and 76 (Figure 3). North scour depths at pilings
1, 51 and 101 were similar during the March and April visits and increased during May
data collection. The south side of piling 76 scour depths have been similar since data
collection began. All locations seemed to be experiencing flow beneath the bottom eco-
disk from all directions. Even though some have slipped down a foot or so, all reef units
are still fully functioning as wave attenuators.
Subsided Units and Structure Stability Inspection
Structural inspection began with collecting notes and walking across the top eco-disk
of all 101 ReefMaker units. Subsided units were evaluated to determine extent of
subsidence and bottom scour depths (Figure 2). Collected measurements included, top
of the eco-disk, to the top of piling, and distance from top of disk, to the bottom of the
top clamp (Figure 1, Table 2). These measurements were collected to compare the
extent of subsidence since the field visit on March 21, 2017. Following data collection,
all subsided units were videoed for desktop review. During the video review process,
the distance from the bottom disk to the scour depth was noted. The overall structural
integrity was checked to determine the degree of subsidence and impact per individual
ReefMaker unit.
Summary
During subsided reef units' inspection, a total of nine (9) units were evaluated and
inspected. The largest cluster of subsided units were located at the beginning of the
western wing. These reef units were located at pilings P57, P60, P65 and P66. The
location with the most subsidence measured was 2.1' at piling 25. The south side of
piling 65 had a scour depth increase of -1.6' since the March site visit. Pilings 8, 25,
33, 57, 60, 66 and 94 had increased scour depths ranging from -0.3, to -1.25'. Other
than the newest subsided unit at piling 96, no other location increased in subsidence.
All locations seemed to be experiencing flow beneath the bottom eco-disk from all
directions. During desk top review all stations were inspected and appeared to be
intact.
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SEPI
We appreciate the opportunity to work with you. Please contact us if you have any
questions regarding the information we have provided.
Sincerely,
Jason Hall
Environmental Project Manager
5030 New Centre Drive, Suite B
Wilmington, NC 28403
Direct: 910-523-5715
Cell: 919-868-6044
Attachments:
1) Figure 1- B-2500 Mitigation Structure Project Vicinity Map
2) Figure 2 - B-2500 Mitigation Structure Monitoring
3) Figure 3 Monthly Scour Depths
4) Table 1 - Scour Monitoring Stations
5) Table 2 - Subsided Units
6) Table 3 - Baseline
7) Chart 1 - 1Jan2017 - 9May2017
8) Chart 2 - 19April2017 - 9May2017 - Oregon Inlet Wind Directions
9) Site Photos
References
Oregon Inlet Marina Wind Rose Plot [Charts]. (2017). Retrieved from
http://www.mesonet.agron.iastate.edu/sites/dyn_windrose.phtml?station=ORIN7&ne
twork=NC
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Figure 1 :
Project Vicinity
Project Vicinity
Alligator Rivet
National
Vlildlae Refuge STIP B-2500 SAV WAVEBREAK
Dare County INSPECTION AND SCOUR MEASURMENTS
Bombing
Range
DARE COUNTY - NORTH CAROLINA
t,; May 2017
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W �E
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SAV Wavebreak Site
Source:Bing Maps
Figure Date:04/05/2017
0 1,500 3,000 6,000
Feet