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HomeMy WebLinkAbout35 SE48_3115 ACRE 03_6_2019 Comments for NLC RESEARCH AND ENGINEERING, INC. Corporate Headquarters: 766 Falmouth Road, Suite A-1, Mashpee, MA 02649, (508) 539-3737 Gulf Coast Office: Executive Tower, 3500 North Causeway Boulevard, Suite 1480, Metairie, LA 70002 www.appliedcoastal.com March 5, 2019 Emily Molden Nantucket Land Council, Inc. 6 Ash Lane Nantucket, MA 02554 Re: Discussion of the Independent Review of Notice of Intent Application for Expanded Baxter Road and Sconset Bluff Storm Damage Prevention Project The following discussion and comments focus on the independent review of the Siasconset Beach Preservation Fund (SBPF) application to the Nantucket Conservation Commission for the Expanded Baxter Road and Sconset Bluff Storm Damage Prevention Project conducted by Greg Berman, Woods Hole Sea Grant and Cape Cod Cooperative Extension. In addition to the comments made on behalf of the Applicant in the February 7, 2019 letter from Epsilon Associates, Inc. Mr. Berman’s review builds upon his previous reviews of the 2016 and 2017 Annual Monitoring Reports which are associated with the existing 947-foot geotube revetment constructed by SBPF. The comments and questions he provides are similar in nature to those he previously expressed in his past reviews and highlight the deficiencies associated with the design and application to extend the geotube revetment to an overall length of 3,820 feet. Mr. Berman agrees that the current geotube system has resulted in increased erosional impacts on adjacent shorelines and that the current design and mitigation protocols do not function as originally portrayed by SBPF. Further expansion of the revetment system will not alleviate these design and implementation shortcomings, there have been no design changes made as a result of the 5-years of monitoring which has highlighted the adverse impacts to adjacent shorelines. The one substantial change that has been proposed is ‘adaptive’ management of the annual mitigation volume. That change to the mitigation protocol will result in reduced volumes of mitigation entering the littoral system, accelerating the erosive impacts observed over the past 5-years particularly if the length of the revetment is quadrupled. Mr. Berman agrees that to maintain the beach as the Applicant has suggested, is infeasible. The review begins with a discussion on the calculation method for determination of mitigation nourishment volumes. It is important to note that the method for calculating mitigation has not been a point of contention, rather the parameters or data used within the calculation are what has been debated at length through this and previous applications. SBPF utilized longer term erosion rates at the top of the coastal bluff, which minimize the rate of erosion which has accelerated significantly, predicating the need for the geotube revetment. The extended period incorporates episodes along the southern extent of the proposed project when shoreline was not eroding and dunes fronted the coastal bluff. The review highlights that there is a direct but temporally offset correlation between the retreat of the mean high water line (MHW) and the retreat of the top of the coastal bank. SBPF has over 20-years of data relative to the retreat of the MHW line but utilized the bank retreat rate which lags in response until the bluff slumps. This Page 2 of 2 method misses accelerations in erosion rates, which SBPF’s monitoring data from the quarterly reports clearly show an increase in erosion since the initial geotube system was installed. Mr. Berman’s review highlights the importance of calculating, the “right” compensatory nourishment volume and requiring that volume be put down every year as a minimum and then additional sand would be required to maintain the cover on the geotube revetment. Mr. Berman highlights the expanded project represents 7-percent of the approximately 10-miles of unarmored shoreline along the eastern coast of Nantucket. It is important to consider the significance that the 7-percent which will be covered by a revetment represents to the larger littoral system. The height of the bluff within the project area is the highest along the eastern shoreline, which means for every foot of erosion a larger percentage of sediment is contributed to the littoral system. The project area also has the highest rate of erosion, thus providing more sediment to the littoral system then other areas along the eastern coastline. The 7-percent of the coastal bluff within the project area provides a more significant contribution to the littoral system then the simple ratio of project length relative to the total length suggests. The overall contribution is likely on the order of 15 to 20-percent of the overall sediment contributed annually. The importance of delivering mitigation sediment during storms is highlighted in Mr. Berman’s review. He states, during storms is when additional sediment in the nearshore would have the biggest impact on preserving the upland and coastal resource areas. Which is confirmed by decades of scientific research and measurement of the impacts of sediment on the dissipation of wave energy during storm events. The Applicants contend the sediment situated offshore will have the same influence dissipating erosive effects of incoming storm waves and elevated water levels as sediment contributed directly to the swash zone from the natural bluff would, this is clearly not defensible or accurate. The mitigation template has been designed by the Applicant to provide 2.0 CY regardless of the magnitude and duration of a storm and is not replenishable at a rate to address back to back storms. This is a critical design flaw which has not been addressed, mitigated for, or minimized. As designed, the geotube system will have adverse impacts on adjoining shorelines. SBPF has made design changes to increase the depth of the toe of the geotube revetment. However, as mentioned above no design changes have been made to the compensatory mitigation design and protocols (except reducing the annual contribution through adaptive management). Mr. Berman’s review highlights the effects the geotube array can have upon the littoral system and the end scour that is already being experienced. The end scour increases the erosion of the bluff and jeopardizes the properties situated above that are not afforded the protection of the geotube system. The review ends with a review of the feasibility of maintaining the geotube revetment and meeting the failure criteria, including examples where similar approaches have been constructed and resulted in unintended adverse impacts to adjacent shorelines and properties, and it does not offer the protection that has been suggested by the Applicant. It is clear upon review and analysis of the SBPF’s data that the proposed geotube system will have negative impacts on neighboring beaches and shoreline properties due to the loss of sediment from the littoral system.