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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. 3 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. 5 Summary of Shoreline Monitoring (Cont.) 6 Summary of Shoreline Monitoring (Cont.) (Profile 91.5 in front of array) 7 Summary of Shoreline Monitoring (Cont.) (Profile 93 +1,000 feet north of array) 8 Summary of Shoreline Monitoring (Cont.) (Profile 90 +1,200 feet south of array) 9 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) 11 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 13 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 16 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.) 17 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 18 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 24 •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.