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HomeMy WebLinkAbout29 SE48_3115 SBPF Response to G Berman 2_7_19 PRINCIPALS Theodore A Barten, PE Margaret B Briggs Dale T Raczynski, PE Cindy Schlessinger Lester B Smith, Jr Robert D O’Neal, CCM, INCE Andrew D Magee Michael D Howard, PWS Douglas J Kelleher AJ Jablonowski, PE Stephen H Slocomb, PE David E Hewett, LEED AP Dwight R Dunk, LPD David C. Klinch, PWS, PMP Maria B. Hartnett ASSOCIATES Richard M. Lampeter, INCE Geoff Starsiak, LEED AP BD+C Marc Bergeron, PWS, CWS 3 Mill & Main Place, Suite 250 Maynard, MA 01754 www.epsilonassociates.com 978 897 7100 FAX 978 897 0099 21597/2018/Expanded Project/NOI/Conservation Commission Hearings February 7, 2109 Nantucket Conservation Commission Via Electronic Mail and U.S. Mail 2 Bathing Beach Road Nantucket, MA 02554 Subject: Response to Greg Berman Comments on the Expanded Baxter Road and Sconset Bluff Storm Damage Prevention Project (DEP File No. SE 48-3115) Dear Commission Members: On behalf of the Sconset Beach Preservation Fund (“SBPF”), Epsilon Associates, Inc. (“Epsilon”) submits the following response to technical review comments by Mr. Greg Berman, as well as a supplement to sand management reporting to clarify certain aspects of this issue raised by the Nantucket Conservation Commission (“Commission”) during the Public hearing process. Many of the questions raised have been answered on numerous previous occasions and therefore many of the responses here reference those previous responses. TECHNICAL REVIEW COMMENTS Mr. Berman provided technical review comments on the Notice of Intent (“NOI”), and subsequent materials, for the Expanded Baxter Road and Sconset Bluff Storm Damage Prevention Project (“Project”) to the Commission via correspondence dated February 1, 2019. He has reviewed this Project and submitted comments previously, and some of his presented below reflect points he has made in those reviews. SBPF was pleased to see that Mr. Berman’s review confirmed our assessment of the Baxter Road and Sconset Bluff Storm Damage Prevention Project in many important ways. At its essence, the Commission’s requirement is to determine whether the historic homes and infrastructure (road, utilities, bluff walk) qualify for using a Coastal Engineering Structure to protect them, and whether the proposed protection is using the “best available measures” to provide that protection. Nothing about erosion protection is easy, or simple, or perfect, and certainly not when it is being done in one of the most dynamic coastal environments on the eastern United States. SBPF Nantucket Conservation Commission 2 February 7, 2019 believes that the criteria for both State and local law and regulation are met here, and that Mr. Berman’s report supports that determination. Mr. Berman’s report confirms that SBPF’s sand nourishment numbers use the correct formula, and also that an adaptive formula is appropriate. Moreover, Mr. Berman confirms that principally the littoral drift defines the method through which the beach in front of the geotubes will be impacted. The sand released from the protected bank is a small percentage of sand resource when considering the full length of the Eastern shore (10 miles). Nevertheless, the sand being contributed through the mitigation program makes up for that impact. In addition, Mr. Berman points out that the timing of the availability of that sand (whether from the bank or from mitigation sand placed on the geotubes) into the system is not critical for protecting downdrift beaches and dunes. Mr. Berman confirms that there is a risk of adverse effects if the geotubes are uncovered during a storm (but not between storms). SBPF addresses this by maintaining a significant sand volume in the template, and by improving the protocol for re-covering exposed geotubes by purchasing equipment and making other changes to improve response times. That said, the sand in the template is designed to be contributed, so some of it will be washed away during storms. In addition Mr. Berman has raised a concern about the need to manage the ends of the Project since continued erosion in the area adjacent to these ends, something that we propose to address by adding soft coir rolls for return extensions as needed over time. Mr. Berman expresses a concern about the timeframe for which the geotube project may be effective and over which a walkable beach in front of the geotubes effectively can be maintained. He suggests monitoring the beach width and including a failure trigger from the outset of the project so all parties recognize the risks. SBPF believes that a walkable beach is maintainable and that the beach in front of the geotube system should remain for a long time based on the mitigation sand and the sand supplied through the littoral system from updrift beaches. However, the Commission need not decide what the actual timeframe will be, because the proposed Project includes failure criteria that would kick in based on the monitoring requirements. SBPF recognizes that if the success criteria included in the Project are not being met as measured by the monitoring reports, then the Project’s failure criteria are triggered requiring either an effective response approved by the Commission, or the potential removal of the Project. Nantucket Conservation Commission 3 February 7, 2019 Following are the topics and comments provided by Mr. Berman. Comments are presented in italics typeface followed by our response in normal typeface. 1. To determine nourishment volume the following parameters are needed: Length of coastal bank, Average height of coastal bank, and Rate of erosion. To calculate compensatory nourishment requirements the following equation is typically used: (Length of coastal bank) x (Average height of coastal bank) x (Rate of erosion) = (Volume of nourishment) We concur with Mr. Berman’s calculation and used that method for the proposed extension project. See Notice of Intent (“NOUI”) Section 2.3 Updated Bank Contribution Volume. 2. Erosion rates are typically calculated at MHW as this is the datum by which the Massachusetts Shoreline Change Project mapped high water shoreline across the entire state. … This project area has had extensive data collection for multiple decades, which should provide much more accurate erosion rates for the site. The large volume of data available has lead various parties to use different shoreline change rates (ex. bank retreat vs MHW change). We appreciate the comment and agree there can be certain conventional advantages to using a MHW reference. For instance, CZM recommends using MHW as a reference datum for LIDAR and photogrammetry data because those are remote sensing methods that have difficultly capturing data below MHW even at lower stages of the tide. Even when these data are taken at low tide, wave action can obscure the beach surface below the MLW line making it difficult to detect by either photogrammetry or LIDAR. Therefore, MHW is the better reference datum for those remote sensing technologies. At Siasconset, however, we have actual bluff survey data that captures data along the Coastal Bank the resource area for which sand mitigation is required. The bank contribution volume was calculated using long-term data sources from 1994-2017, and so incorporates data sources prior to the LiDAR data from 2000. The bank contribution rates (as detailed in Notice of Intent “NOI” Section 2.3) uses data available for Sconset Beach includes shoreline surveys back to 1994 and represents a much more robust data set than is typically available for coastal projects. The bank recession rate was determined for the area from 107-119 Baxter Road and for the area from 71-85 Baxter Road. (71 Baxter Road is the southern extent of top of bank erosion that is discernible on aerial photographs.) The bank recession rate was determined using the same methodology described above, where the bank retreat Nantucket Conservation Commission 4 February 7, 2019 distance was measured in GIS using top of bank lines digitized from 1994, 2003, and 2017. The 2017 top of bank line was derived from the August 2017 aerial survey of the bank. As shown in Table 2-1 below [reprinted from the NOI], the distance- weighted average annual rate of retreat for the area from 71-85 and 107-119 Baxter Road is 2.8 ft/yr. This number is significantly less than the average annual bank retreat of 4.6 ft/yr previously calculated for those areas covered by the Existing Project, which reflects the higher bank erosion in the Existing Project area. Table 2-1 Average Bank Retreat Rates, 107-119 Baxter Rd and 71-85 Baxter Rd Average Annual Bank Retreat (1994-2017) for 107-119 Baxter Road 2.0 Average Annual Bank Retreat (2003-2017) for 71-85 Baxter Road 3.5 Average Distance-Weighted Bank Retreat 107-119 & 71-85 Baxter Road 2.8 3. While the erosion rates at these two locations are certainly linked there is a somewhat convoluted correlation between them (e.g., 2’ of erosion at MHW does not immediately equal 2’ of loss at the top of the bank). Erosion rates and the related potential compensatory nourishment volumes were not calculated as part of this review. The regulations require that the mitigation volume compensate for the loss of the sediment source from the Coastal Bank that would otherwise be contributed to the littoral system of the protection system were not in place. Therefore, the use of Coastal Bank recession rate is the most applicable as compared to the recession rate at MHW. The process of Coastal Bank erosion involves toe scour which leads to over steepening of the banks, then at a certain slope the bank is too steep and it erodes en mass. Over a long period of time the toe recession rate and the top of bluff recession rate are essentially congruent. Therefore, the method used for the proposed Project accurately presents Coastal Bank recession rate. 4. This project, at 947’, is about 2% (or about 7% at the proposed 3,820’) of the approximately 10 miles of the mostly unarmored eastern shoreline of Nantucket. While the erosion rates along this shoreline can be highly variable, it is highly likely that much of the beach sediment at the site has come from updrift areas, as opposed to the site. There is a very large natural volume of sand moving along this stretch of shoreline which helps preserve the width of downdrift beaches and dunes. However, even with this natural volume and the artificially placed sediment nourishment, sand cover on all portions of the geotube array appears to have been difficult to maintain. Nantucket Conservation Commission 5 February 7, 2019 We concur that long-shore sediment transport means that sand on the beach in front of the geotube system has been deposited from updrift areas. The purpose of the proposed mitigate sand volume is to compensate for the annual average volume of sand that would be contributed to the littoral drift system if the geotube system were not in place. Monitoring over the past several years documents that the average annual volume of sand delivered to the template and washed off the template and contributed to the littoral drift system was 13.5 cy/lf/yr for the period 2016 – 2018 (these three years exclude construction years and were 11.3 cy/lf in 2016; 8.9 cy/lf in 2017, and 20.3 cy/lf in 2018) which is consistent with the calculated rate of 12 – 14.3 cy/lf/yr. Regarding the issue of maintaining sand cover on all portion of the geotube system, the sand cover is designed to be washed away during larger storms. This is the method through which sand that would otherwise be contributed from the natural erosion of the bank is supplied to the littoral system. This means by definition that the geotubes will be exposed after a storm event until they are re-covered. If the sand on the face of the geotubes were not being washed away during larger storms the sand template would not be functioning as intended. See discussion in the attached memoranda regarding template sand management and measures to maintain sand cover. 5. Most important would be finding the “right” compensatory nourishment volume and requiring that volume be put down every year as a minimum. Then more sand may be needed if filling the sand template requires more than that minimum volume. Ex.8.8cy/lf/yr as a minimum to be placed each year and 22 cy/lf/yr as a minimum template volume to maintain. Whatever volumes are decided, they may need to be adjusted based on how often the geotubes get exposed. We concur that the “right” compensatory volume be placed to maintain the template. That is why the SBPF proposed the adaptive sand management protocol that in summary involves:  Upon completing construction, the template would be filled to stockpile 22 cy/lf of sand,  During the winter storm months use that stockpiled sand to maintain geotube coverage after erosion events,  Then after spring monitoring is complete refill the template to 22 cy/lf to replace the volume of sand contributed by the template to the littoral drift system. Nantucket Conservation Commission 6 February 7, 2019 This protocol results in the “right” volume of compensatory sand and is adjusted on a year-to-year basis to match what was lost off the template. Based on the historic data, it is expected that the average amount of sand to re-fill the template will be 8.8 cy/lf, however with the adaptive approach the amount that actually is washed away will be added based on the annual 3D survey. 6. Previous comments have indicated that the project site has not experienced a significant storm event since the installation of the geotube array. However, now data is available with the geotube array experiencing several larger storms. While the array has not experienced a tropical storm (i.e. hurricane) of significance, some recent winter events would qualify as “testing” the array. The initial geotube array was installed 12/2013-1/2014. Since that time 5 of the top 10 water levels have been observed at the Nantucket Tide Gauge, since measurements began in 1965 … We agree that the existing geotube system has been tested by significant coastal storms, and that it has held up well. It serves as good demonstration project, and based on its performance only minor adjustments to the design of the extension Project are proposed, most notably lowering the lowest tier to elevation -3 feet MLW datum from 0 feet MLW datum. As described in the attached memoranda, the March 2018 storms presented timing challenges to re-cover the exposed geotubes in between storm events. However, the total volume of sand delivered to the template and eroded from the sand delivery points (i.e. sand volume contributed to the littoral drift system) was about equal to the volume of sand that needed to be re-placed pursuant to the Order of Conditions (“OOC”). For this reason, the littoral system was not negatively affected by the inability, on occasion during the succession of March 2018 storms, to re-cover portions of the geotubes in time for the next storm. The SBPF has also proposed changes to the protocol and secured dedicated equipment and equipment operators to overcome the timing challenges of re-covering geotubes experienced during the March 2018 storms. 7. The existing (and proposed) project has been designed so that the sand placed on the face of the geotubes is eroded during storms and contributes to the littoral drift system. This is a good feature of the design [underlined added] as it provides sand to the littoral system during storms, as during storms is when additional sediment in the nearshore would have the biggest impact on preserving the upland and coastal resource areas. Another aspect of this design is that, after the sand on the face of the geotubes has eroded, the array is exposed and interacting with the waves as a CES. Also, by stabilizing the toe of the bank with geotubes less sediment is eroding than would occur naturally (see figure to the right). Despite some sediment being lost on Nantucket Conservation Commission 7 February 7, 2019 top of the geotubes, a more natural bank would not preserve the sand template above the geotubes. This would cause the bank to erode much more sediment, compared the geotube array, and migrate landward. As there has been sediment “left behind” in the array’s template during some years, this region may benefit by the applicant placing a minimum yearly amount of sediment as mandatory, compensatory nourishment. This requirement would not be affected by how much sand remains in the template. More sand than the minimum may be required to keep the template full. Keeping the geotubes covered (i.e. maintaining the sand template) mitigates for how the geotubes affect local wave processes, while the minimum volume would serve downdrift beaches and dunes. We agree with the author’s comment that this is a good design and that the geotube system maintains the position of the bank by preventing erosion, as depicted in the graphic on page 5 of his comments. The SBPF also agrees with, and is committed to providing compensatory sand to maintain the template and contribute sand to the littoral drift system. The difference is approach, 1) provide the “right” amount of sand as the author referenced previously, or 2) as the author suggests in this comment, provide a yearly mandatory volume of sand. We believe the proposed adaptive sand management protocol is the best approach to place the “right” volume of sand in any given year. The proposed protocol requires the placement of 22 cy/lf of sand on the template when it is built, then replaces the volume of sand lost over a winter storm season to “refill” the template to 22 cy/lf. This sand management approach meets the criteria mentioned by the author to: 1) keep the template full, 2) provides an on-site stockpile of sand keep the geotubes covered to mitigate for how the geotubes affect local wave processes, and 3) supply the minimum volume needed to maintain downdrift beaches and dunes. 8. … photographs from during and shortly after the March 2018 storm was made available. Some of these images show, what appears to be, classic images of end scour (top and middle images) and flanking caused by a hard structure interacting with the waves. What complicates this location is the clay outcrop (lower right corner of middle image). The area between the clay and geotube is “between a rock and a hard place” with the unconsolidated sand of this portion of bank eroding much more quickly due to increased wave turbulence on each side. There is little doubt that the exposed geotube is partly to blame for the scour, but the natural clay headland likely shares the blame. The clay head made it impossible to build the correct returns at the northerly end of the existing geotube system, and instead the returns were placed to abut the clay head. Therefore, when the clay head eroded, the northern end of the system was left unprotected as if there were no returns. It is expected that the returns for the existing geotube system will need to be lengthened periodically as the bank adjacent to the Nantucket Conservation Commission 8 February 7, 2019 system naturally erodes over time. This Commission recently permitted SBPF to do exactly this type of return extension to protect the existing pilot project system, and something similar will be needed going forward for the extension Project as well. The northerly extension proposed for this Project will cover the area where the clay head previously existed before it eroded away to avoid this phenomena from happening in the future. The procedures in the attached memoranda and the previously submitted Sand Template Protocol address measures to keep the geotube system covered and quickly re-covering exposed tubes after erosion events. 9. According to the “Baxter Road and Siasconset Bluff Stabilization Project January 2018 – March 2018 Work Log” the first work performed in March of 2018 was a sand delivery of 434 CY on 3/12. An additional 4,312 CY was delivered between 3/14-3/16. This means that after the geotubes were exposed (likely on 3/3/18) they continued to be exposed and interact with the series of storms for another 5 days, and then the shorter event on 3/13 likely interacted with the geotubes as well. While the exposed geotubes might not be visually appealing, they cause relatively little harm during quiet periods. It is when the geotubes interact with storm waves and currents that they have the most negative impacts on coastal resource areas. See the attached March 2018 Sand Management Memorandum that describes the work SBPF did to re-cover exposed geotubes during the March 2018 storms. A key take away from the efforts described in this memorandum is that the volume of sand contributed from the sand delivery point (at the southern end of the system) to the littoral drift system was nearly equal to the sand volume required in the OOC; the difference is that the sand was released from a single location (the southern end of the Project) during the storms over that three week period rather than from the face of the geotube system. That period of time, March 2018, is not a failure by the SBPF to provide an adequate volume sand to littoral drift system during this sequence of storms, rather it shows the commitment by SBPF and C+C to persevere during an intense storm season to comply with the intent to the OOC. March 2018 did however identify a logistical challenge to re-cover expose geotubes when there is a succession of winter storms. To address that issue SBPF and C+C has revised their template management procedures to more quickly access and re-cover exposed geotubes. See the attached Future Maintenance Procedures Memorandum for details about dedicated equipment and personal. Please note the procedures described in the Future Maintenance Procedures Memorandum supplement the Template Sand Protocol previously submitted to the Commission. In addition, it should be noted that the amount of sand contributed off the template, even in this above average erosion year, exceeded the amount of sand contributed Nantucket Conservation Commission 9 February 7, 2019 from the adjacent unprotected bank to the north and south. In other words, even though there were a number of days during March 2018 when a number of sequential large storms occurred before the geotubes could be re-covered with sand, thereby potentially reducing the amount of sand that would have washed away on those days, and even after factoring in the sand washed away from the southern delivery point, the total volume of sand that did wash away over the 12 month period (as measured by 3-D survey) was still greater than the volume of sand that would have been contributed by this portion of the bank if it were not protected by the geotubes. 10. While maintaining a beach in front of a Coastal Engineering Structure is theoretically possible, at some time in the future (likely tens of years, not hundreds) it will not be feasible. As was previously mentioned, the proposed project would be about 7% of the mostly unarmored eastern shoreline of Nantucket and much of the beach sediment in this area has likely come from updrift areas, as opposed to being placed at the site. The compensatory nourishment volume described in a previous section is intended to put the amount of sediment into the system that would have eroded naturally. This is the volume of material that would be coming out of an eroding bank. This is not the volume of material that would be needed to prevent the bank from eroding any further. As the author describes this reach of the shoreline represents about 7% of the eastern Nantucket shoreline (3,820 feet / 50,280 feet). That means 93% of the shoreline remains as is to provide sediment to support the long-shore sediment transport system. Additionally, the compensatory sand volume eroded off the template will be contributed to the littoral drift system by cross-shore, diffusion, and long-shore mechanisms to contribute the same volume of sand on an average annual basis that would have been contributed to the littoral drift system from the 3,820 feet of unprotected Coastal Bank. The recent surveys by the Woods Hole Group, Inc. show that as the sand migrates off the template it builds up on the Coastal Beach until finally being washed out to the near-shore environment. This movement of sand off the template by cross-shore and diffusion mechanisms plus the sand contributed from the updrift shoreline by long-shore transport will help to maintain the width of the Coastal Beach going forward in time. As described herein as well as in the NOI and at the Public Hearings, the mitigation volume of sand is contributing sand to the littoral system at a greater volume than the unprotected bank, but the Project has no effect on the updrift shoreline’s capacity to supply sediment to the Project area. Given that, 1) the sand eroded off the template mitigates for the protected bank, and 2) the updrift landforms continue to supply sand equal to the present rate, the beach in front of the geotube system should remain for years to come. Nantucket Conservation Commission 10 February 7, 2019 The Coastal Beach profile in the Project area is unique formed by the dynamic and erosive nature of the wave climate at Siasconset, the beach has a very steep foreshore slope offshore typically to about -7 to -10 feet MLW before becoming more gradual. There is not a typical equilibrium beach profile with a berm/bar configuration. This profile and the area’s wave action will have more of an effect on beach width going forward in time than the presence or absence of the geotube system. Continued placement of compensatory sand is expected to slow the beach loss and maintain a walkable beach longer than if no mitigation sand is contributed to the system, and as described above the primary source of sand in front of the geotube system comes from updrift landforms and is expected to continue to be deposited in front of the geotubes maintaining a walkable beach over the long term. 11. If a geotube expansion is approved erosion will continue in adjacent areas, as is occurring now with the current extent of the geotube array. With erosion continuing to occur (or made worse) at the end of the structure, properties adjacent to the structure will often request an extension of the CES to cover their property (aka “chasing erosion”). The geotube array has been designed with returns so that it is not compromised by scour. One of the dangers of “holding the line” with a CES is that the array will artificially protrude further seaward than the rest of the shoreline. Flanking may occur if adjacent properties continue to erode naturally, while the project site maintains a shoreline position further seaward to protect the homes. Flanking could require returns to be extended landward over time in order to protect the house, allowing the property to protrude further seaward than the rest of the shoreline (see image below). By definition the Coastal Bank stabilization provided by the geotube system only protects that reach of the Coastal Bank from erosion, adjacent unprotected reaches of Coastal Bank will continue to erode. SBPF expects there will be the need to extend returns in the future as the immediately adjacent bank erodes. In fact, the existing OOC was recently amended to extend returns for the pilot project for exactly this reason, and we suggest that similar return extensions be a part of the proposed 4,000 linear foot system as well. The length of the proposed geotube system, approaching 4,000 linear feet also helps to ameliorate the potential flanking as compared with sorter shoreline stabilization installations intended to “hold the line” for a short shoreline reach. The author included a photo of a single house protected by a revetment and protruding seaward from the adjacent shoreline. This is an extreme example and depicts the “groin effect” that a short system can cause. Although included as an extreme example, it really does not illustrate the conditions along the Siasconset Bluff Project area. First, the proposed extended geotube system “holding the line” for about 8/10ths of mile is not Nantucket Conservation Commission 11 February 7, 2019 expected to result in a “groin effect” due to its length whereby long-shore sediment transport will continue to move sediment along an elongated bulge as the adjacent shoreline erodes landward. Continued placement of compensatory sand is expected to yield shallow sloped junctions between the shoreline in front of the geotube system and the shoreline in front of eroding shoreline, instead of sharp nearly right angle transitions. Second, the example does not appear to include compensatory sand mitigation. The continued placement of sand mitigates the “starvation” effect caused by groins and mitigates the effects of flanking by diffusion. 12. If erosion continues at these rates, the returns will have to be extended significantly over this time frame … Any project undertaken at this location should have both short and long term plans, which would include the eventual criteria for retreat (of the homes, road, and/or array). Having a planned trigger for eventual removal of the geotubes is reasonable foresight, and beach width is a valid indicator of the likelihood of negative impacts. If it is the intention for the beach width to be the trigger for removal of the geotubes then this information should be included in any long term plans. The SBPF understands this may not necessarily be a permanent fix as compared to longer-term erosional trends. It is expected to be a positive shoreline protection program that should last for decades, and as such has a meaningful and positive impact on an otherwise threatened historic residential community. The Project need not be permanent to be worthwhile, in the same way that an effective medical procedure is considered worthwhile if it adds to a person’s lifespan and quality of life. The bank had been stabile for a long period of time and as documented by the Massachusetts Office of Coastal Zone Management in a 2013 comment letter, increased erosion of the Siasconset shoreline has been observed since he mid-1970’s. The reason for this change is unknown, hypotheses include ideas such as a shift of offshore shoals, or a shift in currents, or even the dredging of kelp beds – all potentially effecting location, directions, velocity of currents. The construction and maintenance of the geotube system is proposed for as long as success criteria, as defined by the Nantucket Conservation Commission, can be maintained. Existing OOC (SE48-2824) Special Condition Number 36 requires addressing any failures and if necessary removing the geotube system. Thus, the SBPF in accepting the OOC has accepted the potentiality that at some unknown time in the future that retreat of homes and the road may be required. In closing, the SBPF has provided herein responses to Mr. Berman’s technical assistance review, and has addressed lingering issues regarding the template maintenance that we thought Commission members may still have. This Nantucket Conservation Commission 12 February 7, 2019 correspondence plus the NOI and several additional information / response to comments submissions throughout the Public Hearing process, we believe, provides the Commission with all the information it needs to make a permitting decision. The record for this filing (SE48-3115) describes the Project purpose, proposed Project design, alternatives considered in lieu of the proposed Project, anticipates potential impacts and provides mitigation measures based on results of monitoring the existing geotube system (SE48-2824), and documents compliance with the performance standards for Coastal Bank and Coastal Beach. Therefore we respectfully request that the Commission issue an Order of Conditions authorizing construction and maintenance of the proposed geotube extension Project with practical special conditions to protect relevant interests of the Wetlands Protection Act and the Nantucket Wetlands Protection By-Law. Please contact me at (978) 897-7100 or via email at ddunk@epsilonassociates.com with questions of comments regarding this matter. Sincerely, EPSILON ASSOCIATES, INC. Dwight R. Dunk, LPD, PWS, BCES Principal cc: MassDEP-SERO J. Posner, SBPF A. Gasbarro, Nantucket Eng. & Survey S. Cohen, Cohen & Cohen Law, PC G. Wood, Ruben and Rudman, LLP G. Thomson, W.F. Baird & Assoc. L. Smith, Epsilon R. Hamilton, Woods Hole Group encl. 1. Expanded Baxter Road and Sconset Bluff Storm Damage Prevention Project (DEP File No. SE 48-3115) – March 2018 Sand Management 06 February 2019 2. Expanded Baxter Road and Sconset Bluff Storm Damage Prevention Project (DEP File No. SE 48-3115) – Future Maintenance Procedures Memorandum, 06 February 2019 M E M O R A N D U M Date: 06 February 2019 To: Nantucket Conservation Commission From: J. Feeley, Cottage + Castle, Inc. D. Dunk, Epsilon Associates, Inc. Subject: Expanded Baxter Road and Sconset Bluff Storm Damage Prevention Project (DEP File No. SE 48-3115) – March 2018 Sand Management During the Public Hearing process for the Expanded Baxter Road and Sconset Bluff Storm Damage Prevention Project (“Project”) the Nantucket Conservation Commission (“Commission”) and members of the public asked about template sand management both with regards to the March 2018 sequence of storms and going forward, relative to managing a longer sand template. We believe this topic has been addressed thoroughly during the lengthy Public Hearing process. In closing out the Public Hearing however, we take this opportunity to again address these topics in a manner that: 1) summarizes the information previously submitted to the Commission; and 2) answers any lingering questions about template sand management relative to the existing template (SE48-2824) and the proposed extension (SE48-3115). This memorandum is submitted to describe efforts to replenish the sand template during March 2018, and the Future Maintenance Procedures Memorandum dated 06 February 2019 addresses procedures to manage the existing sand template and the longer template for the proposed Project. March 2018 Sand Management The work records describing the March 2018 period were provide to the Commission in April 2018 in accordance with Order of Conditions (“OOC”) reporting requirements. These records were also summarized in response to comments on the extension Project (SE48-3115) as part of the response to comments letter dated 02 November 2018. Below we re-present this same information in a slightly different format to better describe the SBPF’s response to the sequence of coastal storms in March 2018. There were four storm events in March 2018 that caused major wave action along the Siasconset Bluff and existing geotube Coastal Bank stabilization system. The following is a brief description of the site conditions prior to, and after, the storms and the actions to re-cover the geotube system. 2 Photos are also included below which document the area of exposed geotubes in March, all of which have been previously submitted. March Storm Events and Maintenance: March 3, 2018 Major Storm – Prior to the March 3rd storm, the geotube system was covered, and an additional 2,500+ cubic yards (“cy”) of sand was available for contribution to the littoral drift system at the southern end of the sand template below the sand delivery point. This storm exposed the entire length of the second and third tier of geotubes, the southern and northern returns and approximately 100 feet (“ft”) of the fourth tier of the geotube structure. The storm also removed the 2,500+ cy of sand from the southern end of the template (that was contributed to the littoral system). Please note, this 2,500 cy of sand is more than what would be contributed from the front of the entire template when the second (and maybe third) tiers(s) are exposed during a new storm. March 8, 2018 Minor Storm – The geotubes were exposed following the March 3rd storm, in the areas described above, and they remained uncovered up to and during the March 8th storm because the beach was inaccessible, and work conditions were unsafe, for maintenance due to wave run-up. Therefore, this storm occurred at a time when the geotubes were uncovered so that any sand that would have been contributed off the face of the system during this storm was not available at that time. March 12 to March 16, 2018 Sand Deliveries – During the period of March 12th to March 16th, 4,746 cy of sand was delivered to the sand template, via the Baxter Road delivery area, for template replenishment (see Table 1. SBPF Sand Deliveries 12 March 2018 to 16 March 2018 below). Re- grading to cover the exposed geotubes did not occur during this period because C+C’s heavy equipment subcontractors were not able to perform the work due to scheduling conflicts caused by the high volume of building that was occurring on the island. A large percentage of the 4,746 cy of sand delivered to the sand template at the southern end of the system was, however, made available for contribution to the littoral drift system during the next storm event (see March 16th delivery photo below). Table 1. SBPF Sand Deliveries 12 March 2018 to 16 March 2018 Vendor Invoice # Date Quantity (CY) PM Reis Trucking 133673 3/12/2018 434 PM Reis Trucking 133683 3/14/2018 1,020 PM Reis Trucking 133696 3/15/2018 1,754 PM Reis Trucking 133710 3/16/2018 1,538 Total 4,746 3 March 13, 2018 Major Storm – For the reasons stated above, the geotube system was still exposed as it had been prior to the March 8th storm in the areas described above. However, the 434 cy of sand delivered to the southern end on March 12th were available and consumed in the storm. March 18 – 20, 2018 Template Maintenance – Template maintenance commenced on March 18th and continued through March 20th. During this period:  the northern ramp was rebuilt,  the northern and southern returns were covered, and  approximately 550 ft of the southern portion of the geotube structure (second and third tiers) was covered (approximately 400 ft of the northern portion of the structure remained un- covered). Maintenance ceased on March 20th due to another storm that was to make landfall on the island on March 21st. The second and third tiers of the northern portion of the structure were exposed over approximately 400’ due to the work stoppage. A large percentage of the 4,746 cy of the sand delivered to the template at the southern end of the structure, during the period March 12th to March 16th was available for contribution to the littoral drift system. It should be noted that the amount of extra sand still at the southern end and claimed by the storm approximates the amount of sand that would have been claimed from the northern section of the geotubes if they had been re-covered. March 21 to March 22, 2018 Minor Storm – The storm re-exposed approximately 550’ of the face of the second tier of the geotubes on the southern portion of the template that had been covered during the period March 18th through March 20th. The storm also claimed a portion of the 4,746 cy of sand delivered to the southern end of the system. March 26, 2018 to April 11, 2018 Template Maintenance – Maintenance recommenced on March 26th and was fully completed on April 11th. It took longer than normal to re-cover the full-length of the geotube structure because the remaining large volume of sand delivered between March 12th and March 16th had to be evenly distributed over the full length of the Project. The succession of March storms and the unavailability of C+C’s heavy equipment subcontractor in March had delayed the maintenance and sand distribution. See the attached March 2018 calendar presents a succinct summary of events, and identifies the storm dates as well as sand delivery and template management dates. 4 Summary and Conclusion: Following the March 3rd storm, the second and third tier, the southern and northern returns and approximately 100’ of geotube structure’s fourth tier were exposed. This condition persisted, for the reasons stated above, until March 18th (14 days total) when the northern and southern returns and 550’ of the southern portion of the geotube structure were re-covered entirely. However, it is important to remember that there was a large additional volume (2,500 cy) stored at the southern delivery point and available to erode off the template in to the littoral system before the initial March storm plus additional sand was delivered to the template during this period on four different days totaling an additional 4,746 cy. Approximately 400 ft of the second and third tiers on the northern portion of the structure that were originally exposed during the March 3rd storm remained exposed until maintenance was completed on April 11th. In addition to the contribution from the sand that was on the face of the structure before the March 3rd storm, and following partial maintenance completed before the March 21st storm, the 2,500+ cy of sand available at the southern end of the structure below the delivery point was contributed to the littoral system. This volume plus a significant portion of the 4,746 cy that was delivered to the template between March 12th and March 16th, estimated to be 60% or approximately 2,850 cy of sand, eroded off the template at the sand delivery point. Combined is equates to 5,550 cy of sand contributed off the template from the delivery point instead of the face of the template. These contributions helped mitigate the fact that a portion of the face of the geotubes were exposed in March 2018. Special Condition 32.(b) of the Order of Conditions (SE48-2824) reads in part; “ … Throughout the winter, place additional sand on an as-needed basis, in accordance with the replenishment trigger in the Milone & MacBroom November 12, 2013 letter (i.e., if half the vertical height of the lowest Geotube is exposed, place a minimum of2 cy/If). …” SBPF worked during March 2018 to recover exposed geotubes as quickly as possible after each erosion events. The existing template is +950 feet long, thus pursuant to the Special Condition 32.(b) after each erosion event approximately 1,900 cy of sand should have been placed on the template following each erosion event to re-cover exposed geotubes. Review of the attached calendar shows there were three storm periods – March 3 major storm, March 8 minor storm, and March 13 major storm – between March 1st and March 20th. As described above during this time approximately +7,250 cy of sand was prepositioned (2,500 cy) and delivered to the template (4,746 cy) during that same time period. During those three storm events approximately +5,550 cy of sand was eroded out of the delivery point and contributed to the littoral drift system. 5 This equates to placing the required volume of sand on the template 2.98 times (approximately 3 times during March 2018) [5,550 cy / 1,900 cy = 2.98]. Therefore, the overall contribution of sand volume required by the OOC Special Conditions 32.(b) [2 cy/lf following each of the three erosion events] was contributed to the littoral system from a single point rather than along the face of the template following those three storm periods. Thus, although the SBPF was not able to re-cover the geotubes prior to each storm due to un-safe work conditions and equipment availability, the criterion of the replenishment volume was met during March 2018 because of the availability and contribution of sand from the plume of sand at the southern delivery point. Last, following after the storms of March 21 and 22, template maintenance was performed over the period March 26th to April 11th to fully re-cover the exposed geotubes. There were no storms during this period and maintenance to re-cover the tubes was completed on April 11th. Grading maintenance continued during the period April 12th to April 30th to level the template on top of the geotube structure and evenly distribute the sand. 6 SCONSET BLUFF PHOTOS MARCH 2018 Conditions Prior to 3/3/18 Storm Northern ramp and return Northern end viewing south Midpoint viewing north Midpoint viewing south 7 Southern end viewing north South ramp and return Post Storm Photos 3/4/18 – The beach in front of the dune and Project was not accessible. Exposed third tier viewing south Northern end 8 Northern returns Midpoint viewing south Southern returns Southern end viewing north 9 3/6/18 Post Storm Photos Northern end viewing south Northern return and ramp Northern end viewing south Midpoint viewing north 10 Midpoint viewing south Southern end viewing north South return and ramp area 11 3/16/18 Sand Delivery to Southern End of Structure 3/20/18 Maintenance Photos Taken Prior to 3/21 to 3/22/18 Storm Northern return and ramp Northern end viewing south 12 Midpoint viewing north Midpoint viewing south Southern end viewing north Southern return and ramp area 13 Post Storm Photos 3/23/18 Northern return and ramp Northern end viewing south Midpoint viewing north Midpoint viewing south 14 Southern end viewing north Southern return and ramp area Post Partial Maintenance Photos 3/26/18 Northern return and ramp Northern end viewing south (1) 15 Northern end viewing South (2) Southern return and ramp area Post Maintenance Photos 4/11/18 Northern return and ramp Northern end viewing south 16 Midpoint viewing north Midpoint viewing south Southern end viewing north Southern return and ramp area MARCH 2018 SBPF MAINTENANCE ACTIVITIES SUN MON TUE WED THU FRI SAT 1 2 3 Geotubes covered Geotubes covered; ~2,500 cy sand stockpiled below the southern delivery point MAJOR STORM +/- 950 ft geotunes exposed 4 5 6 7 8 9 10 Beach Inaccessible Beach Inaccessible Beach Inaccessible Beach Inaccessible Minor Storm Beach Inaccessible +/- 950 ft geotubes exposed Beach Inaccessible 11 12 13 14 15 16 17 434 cy of sand delivered to geotubes MAJOR STORM +/- 950 ft geotubes exposed 1,020 cy of sand delivered to geotubes 1,754 cy of sand delivered to geotubes 1,538 cy of sand delivered to geotubes 18 19 20 21 22 23 24 Template maintenance Template maintenance Template maintenance Minor Storm +/- 400 ft geotubes exposed Minor Storm 25 26 27 28 29 30 31 Template maintenance 760 cy of sand delivered Geotubes covered M E M O R A N D U M Date: 06 February 2019 To: Nantucket Conservation Commission From: J. Feeley, Cottage+Castle, Inc. D. Dunk, Epsilon Associates, Inc. Subject: Expanded Baxter Road and Sconset Bluff Storm Damage Prevention Project (DEP File No. SE 48-3115) – Future Maintenance Procedures Because of the unforeseen subcontractor scheduling conflicts, and to avoid template maintenance delays in the future Cottage+Castle, Inc. (“C+C”) retained an independent heavy equipment operator as a dedicated operator for the Project, and it purchased a Caterpillar D6N XL Dozer, with a Trimble Computerized Grade Control System in May 2018. The following sections present the protocol details that Sconset Beach Preservation Fund (“SBPF”) will implement for future maintenance of the sand template in accordance with the Order of Conditions (“OOC”). This supplements the Template Sand Protocol previously submitted to the Commission (see Attachment 1 to the response to comments package dated November 2, 2018). To ensure sand template maintenance requirements can be accomplished within ten business days following erosion events and sand deliveries, C+C purchased a Caterpillar D6N XL Dozer, with a Trimble Computerized Grade Control System, and retained an independent heavy equipment operator, and backup operators, who are assigned as dedicated operators for the SBPF sand template. The primary dedicated operator is also an experienced diesel mechanic. Additionally, a maintenance protocol was developed to ensure sand template maintenance can be accomplished rapidly, efficiently and without interruption for the existing sand template and future expansion. As stated above, the following is information regarding the equipment and maintenance protocol, and supplements the Template Sand Protocol submitted to the Commission on November 2, 2018: Equipment In May 2018 after the March 28 to April 11 maintenance work, , C+C purchased a Caterpillar D6N XL Dozer (“D6N”), with a Trimble Computerized Grade Control System for dedicated use at the Sconset Bank sand template. The D6N is a large dozer that is highly versatile and well suited to perform the maintenance and grading work that is required at the existing and expanded template. The Trimble Dual Mast Grading System uses laser points and a computer system to automatically lift 2 and tilt the dozer’s blade to accurately establish a desired grade and pitch to achieve design grades. These features allow for rapid and precise grading over long distances which will significantly further assist in the efficiency of covering exposed geotubes following erosion events as soon as practical. See Photos 1 and 2 below. Dedicated Personnel C+C has retained Dan Baird Construction, Inc. (“Baird Construction”) on Nantucket to serve as the Project’s dedicated heavy equipment operator. This arrangement ensures that storm maintenance will not be delayed by scheduling conflicts that occurred in March 2018 when non-dedicated heavy equipment subcontractors were used for site maintenance. In the event Baird Construction cannot not provide maintenance services, AH Construction also on a Nantucket company, is the dedicated backup heavy equipment subcontractor. Alternative Equipment In the event C+C’s D6N is temporarily out of commission, Baird Construction has its own grader and excavator that may be used to perform post storm maintenance, both of which have been used in the past. The equipment can be quickly mobilized if required. Additionally, AH Construction has loaders and excavators which could be called in to assist in an emergency. Photo 1. Caterpillar D6N XL Dozer Photo 2. Trimble Dual Mast Laser Grading System 3 Post Storm Inspections Following storm events, or abnormally high tide events that cause run-up to the base of the geotube structure, inspections are performed within 24 hours of the event to determine what maintenance is required. Immediately thereafter, Baird Construction who is already on alert, is notified to schedule and perform the maintenance as soon as practical within the proposed ten business days after the geotubes are exposed.