HomeMy WebLinkAboutSE48_2824 2019 Annual Review Presentation Public Hearing 03_22_20212019 Annual Report
Sconset Bluff Geotextile Tube Project
March 22, 2021
Sept 2018
Key Findings
Monitoring Performed
Summary of Monitoring Results
Shoreline Monitoring
Sand Delivery
Bluff Monitoring
Underwater Video Monitoring
Annual Drainage System Report
Independent Review Comments
Outline
2
Key Findings
The Coastal Bank has been stabilized by the geotubes and plantings
on the slope above the geotube array.
Shoreline monitoring data shows the measured shoreline positions
and the rate of shoreline change are similar to historic patterns, with no
indication of accelerated erosion in front of or adjacent to the geotube
array.
2019 bluff survey indicates that sand off template is contributing
significantly more sand than the unprotected bluff over the last 6 years
(13.8 cy/lf/yr vs. 6.8 cy/lf/yr).
Template sand volume is conservative: about 33,020 cy remained on
the template after the 2018 / 2019 winter, and about 10,200 cy
remained after the 2017 –2018 winter.
No indication of adverse effects noted in underwater video monitoring
or wetland well monitoring.
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Monitoring Performed
The Geotube shoreline stabilization system has been in place since
Dec. 2013 / Jan. 2014 –over 6 years
Considerable survey work done over this period:
23 Shoreline Surveys
7 Underwater Video Surveys
Annual Drainage System Reports
Aerial Bluff Surveys
Quarterly submission of Work Reports
4
Summary of Shoreline Monitoring
The November 2019 shoreline position in the Project area was generally similar
(within about 20 feet) to the shoreline position in the ~2008-2010 timeframe.
The short-term variability shown by surveys since geotube installation in
January 2014 is similar to short-term trend variability (~2-to 3-year
periods) observed over many years of surveys before the geotubes were
installed. Surveyed post-geotube shoreline changes are not materially
different from previous observations as related to rates and duration of
shoreline change. No accelerated erosion post-geotube installation
in excess of historical observations is evident.
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Summary of Shoreline Monitoring (Cont.)
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Summary of Shoreline Monitoring (Cont.)
(Profile 91.5 in front of array)
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Summary of Shoreline Monitoring (Cont.)
(Profile 93 +1,000 feet north of array)
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Summary of Shoreline Monitoring (Cont.)
(Profile 90 +1,200 feet south of array)
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Summary of Sand Delivery &
Contribution from the Template
(May 2018 –Dec 2019)
10
Sand Volume at Start of Sand Year 10,200 CY 10.8 CY/LF
Sand Volume Delivered to Template 23,335 CY
Sand Contributed off the Template 515 CY 0.5 CY/LF
Sand Volume at End of Sand Year 33,020 CY 34.8 CY/LF
The following table summarizes the metrics from Sand Report
Tables 1 and 2.
Summary of Sand Contributed from the
Unprotected Bluff
May 2018 –Sept. 2019)
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Summary of Sand Contributed from the
Unprotected Bluff
(May 2018 –Sept. 2019 & Long-term)
12
Bluff Volume Loss in Unprotected Areas Adjacent to Geotextile Tubes for the Most
Recent Year, May 15,2018 -September 2019
Line Area
Volume
Eroded
(CY)
Length
(Feet)
Duration
(Years)
Annual Change
(CY/LF/YR)
1 North Unprotected Area -797 802 1.0 -1.0
2 South Unprotected Area -285 138 1.0 -2.1
3 Total Bluff Erosion for Adjacent Unprotected Areas -1,082 -1.2
Line Area
Volume
Eroded
(CY)
Length
(Feet)
Duration
(Years)
Annual Change
(CY/LF/YR)
1 North Unprotected Area 35,423 802 6.0 7.3
2 South Unprotected Area 5,577 192 6.0 4.8
3 Total Bluff Erosion for Adjacent Unprotected Areas 41,000 6.8
Bluff Volume Loss in Unprotected Areas Adjacent to Geotextile Tubes for the Six
Years since Geotextile Tube Installation,July 2013 -September 2019
Volume of Sand Contributed off the
Template & Unprotected Bluff
Unprotected Bluff Areas (2013-2019):
6.8 cy/lf/yr contributed
Geotube Area (2013-2019):
Average of 13.8 cy/lf/yr contributed
34.8 cy/lf in template (inc. ramps) as of Dec 2019
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Summary Underwater Video Survey
14
Underwater video surveys show that a productive cobble habitat
area is located just offshore from the geotextile tubes, with no
indication that this cobble habitat is being adversely affected by the
sand mitigation.
November 2019 June 2016
Independent Reviewer Comments
15
Mr. Greg Berman provided an independent review of the 2019
Annual Review dated December 8, 2020. Epsilon Associates, Inc.,
provided a written response to those comments on December 21,
2020. The following slides cover the major topics raised by Mr.
Berman. Please see the response document for a detailed
response to all comments.
Sand Volume
Bluff Monitoring
Shoreline Monitoring
Sand Volume
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Mr. Greg Berman noted that the Sand Report Table 1 shows a 16,459
cy deficiency through 12/21/2019. This fact was noted to the
Commission in the transmittal of the Annual Review.
SBPF made a good faith effort to deliver the requisite volume of sand
to the template based on the 2018 Sand Report. As reported in the
2018 Sand Report there was a documented 12,682 cy deficiency.
For the 2019 sand year SBPF secured approximately 33,335 cy of
sand (various sources = 23,335 cy + ~10,000 cy of Polpis Harbor
sand). That would have resulted in only a 181 cy deficiency
33,335 cy –12,682 cy = 20,653 cy vs 20,834 cy required (= 181 cy)
Sand Volume (Cont.)
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Line Sand Amounts 12/13-3/31/14 4/1/14-3/31/15 4/1/15-3/31/16 4/1/16-1/31/18 2/1/18-4/30/18
1 Total Volume Delivered for Geotube Construction (See Line 5 in Table 4 and Line 5 in Table 6)12,653 0 2,931 0 0
2 Total Volume Delivered for Mitigation (see Tables 4-7)23,951 14,429 15,085 15,138 8,152
3 Total Volume Delivered to Bluff Face (Not Counted as Mitigation; See Ln 10 in Tbl 4 & Ln 12 in Tbl 6)2,600 0 4,469 0 0
4 Total Volume Delivered by Truck (Sum Lines 10-12)39,204 14,429 22,485 15,138 8,152
5 Required Mitigation Volume (22 cy/lf * Project Length of 852' for 3 tiers, 947' for 4 tiers w/ret.)18,744 18,744 20,834 20,834 20,834
Surplus Sand From Prior Year
6 Surplus Delivered in Prior Year (From Line 13 in Preceding Column)0 5,207 3,029 -583 0
7 Volume on Template at Start of Sand Year 0 5,900 8,500 15,000 17,000
8 Countable Surplus Present in Sand Template (Line 6; Not to Exceed Line 7)0 5,207 3,029 -583 0
9 Volume on Template at End of Sand Year 5,207 8,500 15,000 17,000 10,200
Bluff Erosion
10 Net Contribution from Erosion of Bluff Face (pre-veg & during 4th tier const.; see Table 6)0 2,138 2,138 0 0
11 Adjusted Required Mitigation Volume (Line 5 - Line 8 - Line 10)18,744 11,400 15,667 21,417 20,834
12 Total Volume Delivered for Mitigation (Line 2 above)23,951 14,429 15,085 15,138 8,152
13 Mitigation Surplus or Deficit (-) ( Line 12 - Line 11)5,207 3,029 -583 -6,278 -12,682
14 Mitigation Surplus or Deficit (-) to Refill Template to 22 CY/LF 755 -10,634
Table 1. Summary of Sand Delivery in Cubic Yards (CY), December 2013-April 30, 2018
Sand Delivery Summary
Base Required Mitigation Volume
Mitigation Volume Adjustments
Mitigation Volume Summary
Line Sand Amounts 12/13-3/31/14 4/1/14-3/31/15 4/1/15-3/31/16 4/1/16-1/31/18 2/1/18-4/30/18 5/15/18-12/31/19
1 Total Volume Delivered for Geotube Construction (See Line 5 in Table 4 and Line 5 in Table 6)12,653 0 2,931 0 0 0
2 Total Volume Delivered for Mitigation (see Tables 4-7)23,951 14,429 15,085 15,138 8,152 23,335
3 Total Volume Delivered to Bluff Face (Not Counted as Mitigation; See Ln 10 in Tbl 4 & Ln 12 in Tbl 6)2,600 0 4,469 0 0 0
4 Total Volume Delivered by Truck (Sum Lines 10-12)39,204 14,429 22,485 15,138 8,152 23,335
5 Required Mitigation Volume (22 cy/lf * Project Length of 852' for 3 tiers, 947' for 4 tiers w/ret.)18,744 18,744 20,834 20,834 20,834 20,834
Surplus Sand From Prior Year
6 Surplus Delivered in Prior Year (From Line 13 in Preceding Column)0 5,207 3,029 -583 -6,278 -18,960
7 Volume on Template at Start of Sand Year 0 5,900 8,500 15,000 17,000 10,200
8 Countable Surplus Present in Sand Template (Line 6; Not to Exceed Line 7)0 5,207 3,029 -583 -6,278 -18,960
9 Volume on Template at End of Sand Year 5,207 8,500 15,000 17,000 10,200 33,020
Bluff Erosion
10 Net Contribution from Erosion of Bluff Face (pre-veg & during 4th tier const.; see Table 6)0 2,138 2,138 0 0 0
11 Adjusted Required Mitigation Volume (Line 5 - Line 8 - Line 10)18,744 11,400 15,667 21,417 27,112 39,794
12 Total Volume Delivered for Mitigation (Line 2 above)23,951 14,429 15,085 15,138 8,152 23,335
13 Mitigation Surplus or Deficit (-) ( Line 12 - Line 11)5,207 3,029 -583 -6,278 -18,960 -16,459
14 Mitigation Surplus or Deficit (-) to Refill Template to 22 CY/LF 755 -10,634 12,186
Mitigation Volume Summary
Table 1. Summary of Sand Delivery in Cubic Yards (CY), December 2013-March 31, 2019
Sand Delivery Summary
Base Required Mitigation Volume
Mitigation Volume Adjustments
2018
2019
Bluff Monitoring
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Bluff monitoring shows accretion
on the unprotected bluff adjacent
to and above the geotube array.
There is no sediment source to
rebuild (accrete) sediment on the
bluff face.
Two potential reasons:
(a)vertical accuracy, or
(b)a slight increase at the toe of
slope from the littoral system.
The conclusion drawn from this
data is that the bluff face
adjacent to and above the
geotube system was fairly stabile
during this monitoring period.
Shoreline Monitoring
19
Comment: The natural changes in this dynamic area continue to overshadow
any signal that might be from the geotube project. No additional shoreline
change can be attributed to the project at this time with the available data
provided.
SBPF concurs this is a dynamic reach of shoreline and that no adverse shoreline
change can be attributed to the project at this time. The slope above the
geotube system was fairly stabile during this monitoring period.
Comment: The main uses of compensatory nourishment are to: ensure the
beach in the immediate vicinity of the project does not drop and change the
coastal processes of the immediate area, keep the geotube covered so it does
not interact with waves/currents, and to make up for any reduction in sediment
available for downdrift beaches.
We concur with this assessment of the purpose of the compensatory sand
nourishment. The average annual contribution rate off the template was 13.8
cy/lf/yr over the past 6 years vs. the unprotected bluff which was 12.0 cy/lf/yr.
The comparison of MLW positions from WHG data shows there is no
accelerated loss of beach in front of or adjacent to the geotube system.
Shoreline Monitoring (Cont.)
20
Shoreline Monitoring (Cont.)
21
Profile 93 –1,000 ft. North
Profile 92 –100 ft. North
Shoreline Monitoring (Cont.)
22
Profile 91.5 –Geotube
Profile 91.35 –Geotube
Shoreline Monitoring (Cont.)
23
Profile 90.9 –100 ft. South
Profile 90 –1,200 ft. South
Closing
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•Geotextile tubes have stabilized the base of the bluff; face of
bluff has been stabilized through vegetation and stormwater
drainage system.
•From 2013-2019, 13.8 cy/lf/yr of sand contributed off the
template (with more available).
•From 2013-2019, unprotected bluff has contributed 6.8 cy/lf/yr
of sand.
•As of Dec. 31, 2019, ~33,000 cy (~34.8 cy/lf) remained in the
sand template.
•Data shows and the independent review concurs that shoreline
monitoring data show no indication of accelerated erosion
within or directly adjacent to geotextile tubes.
•Independent review concurs no evidence of harm observed in
underwater video monitoring.