WETLAND IMPACTS AND COASTAL BLUFF STABILITY STILL PLAGUE SBPF/DPW DRAINAGE PROJECT PUBLIC HEARING CONTINUED TO JAN. 7 20015 By Peter B. Brace Although stability of the coastal bank drew a fair amount of attention, the impacts of a bluff drainage system proposed by the town for a northern section of Baxter Road by the town, kept this public hearing open beyond the Dec. 17 Conservation Commission meeting. Engineer Arthur Gasbarro of Nantucket’s Blackwell & Associates returned to the ConCom on Wednesday with revisions and what he thought would be all the answers to commission and citizen questions, along with requested drawings showing the locations of 207 bluff drainage wells sunk into this coastal bank between 2000 and 2007. But Gasbarro and longtime Siasconset Beach Preservation Fund engineer Mark Haley of Haley & Aldrich, Inc., of Boston, Mass., found themselves answering or attempting to answer even more questions on the Department of Public Works proposed system of catch basins, infiltration trenches and discharge pipes designed to curb the top-down erosion of the coastal bank between 90 and 106 Baxter Road. With a perched vegetated wetland on the west side of Baxter Road within the wetlands buffer zone that’s within the proposed project area, perched meaning a layer of clay beneath it holds its water in place, several commissioners and members of the public are concerned about how drainage pipes installed near this 1.5 acre wetland might affect its water levels over time. The DPW hopes to grade the ground on both sides of the road and install soil and asphalt berms to move water toward catch basins on either side of the road, collecting runoff from storm and rain events. Water would flow from the catch basins to Stormceptors, special catch basins, which would filter suspended solid contaminants from the water and remove oil from it before sending the water into three-foot diameter “drainage-storage” pipes running horizontally along the west side of Baxter Road. From these pipes, the water would be carried down to 12 six-inch diameter discharge pipes 60 feet apart from each other into the coastal bank, to be released slightly below beach and sea level to percolate through the sand and finally into the ocean without seeping out of the bank and thereby weakening the overall stability of the bank. SBPF/DPW is proposing three or four of these discharge pipes for each of the three installations. One of these discharge pipes would serve as a test well before full installation of this system commenced, to gauge water composition, groundwater mounding, groundwater dispersion, well response and aquifer properties, to determine the feasibility of installing such discharge pipes into the coastal bank. However, not all of the precipitation falling in the project area would be collected, said Gasbarro. “It’s a limited road layout and a limited depth, [so] we don’t have the capacity to collect all of the water,” he told the commission. On bank stability, both Gasbarro and Haley had answers. “The wells are about 110 feet back from the edge of the bank and each well is 60 feet apart, and it is my opinion it’s not going to impact the bank at all,” said Gasbarro. “I think it’s important to note, regardless of where you stand with the erosion, the town has a responsibility here. “I believe this proposal here is the best. It’s going to serve to protect your interests and to protect private property.” In backing up Gasbarro, who noted the town would keep Baxter Road free of catch basin-clogging debris, regularly vacuuming out catch basins and cleaning the Stormceptors’ filters, DPW director Kara Buzanoski told the commission this proposal is a common method her department uses all over Nantucket to keep island roads dry. “This installation is like what we’re starting on Raceway Drive,” she said. “This is how we get the water off the roadway. The one difference is the depth of the wells. If the commission feels the wells are problematic, we would still be able to do the drainage from the roadway.” The commission, as expected had plenty of questions and comments. ConCom member Ashley Erisman said she’s concerned about the lack of vegetation proposed in the plan and asked the applicants for information on watershed contribution from the lawns in the project area. Raising his voice a bit and definitely showing signs of frustration, commissioner Bam LaFarge questioned the distance between the edge of the bank and the location of the discharge wells, saying he measured it himself and found the distance to be less than 110 feet and that 80 feet is closer to the truth. “I think you’re working off of maps that are several years old,” said LaFarge. “You better go back and measure with a tape. I’m very sorry we as a group haven’t gone out there to do a viewing, but that ain’t 110 feet. I just don’t understand why you’re giving us inaccurate information. I’m not going to vote on anything that’s not measured on the ground.” ConCom members Sarah Oktay and Ernie Steinauer stressed that the commission didn’t want the DPW to drain out or overfill the nearby wetlands and endanger the integrity of this resource. Gasbarro assured the commission that this wouldn’t happen. “What we have here is very large wetland system, really an acre and a half, and then it ties down to a larger wetland; we’re not going to drain an acre and a half of wetland,” said Gasbarro. “This isn’t a small little kettle hole or a vernal pool that we’re going to punch a hole in the middle of and drain it.” Haley then told the commission that, if they found any of the wells nearby the wetlands to be causing problems with this wetland, that they could be closed off as needed. Comments from the public mirrored the concerns that the commission had, such as Nantucket Land Council Resource Ecologist Emily Molden’s request for more precise information on how the wells close to the wetland would impact it. “My biggest concern at this point is that the infiltration itself with the gravel does not go down beyond that confining layer (the layer of sediment that holds the wetland water in place),” said Molden, who added that she’s concerned about how the standing water on the west side of the road would fare. “I would recommend that they monitor the wells monthly, not the twice-a-year program that is currently proposed, or quarterly at the least.” Erisman, whether following up for Molden or not, asked for more data on wetland impacts. “I would like to see some sort of input for modeling of the wetlands with a rain event to see how the wetlands are impacted,” she said. Charley Walters asked what the criteria would be if this drainage system failed and who would pay for its repair or removal, to which Haley replied the town would be the entity to cover this, as it is their project. Realizing that the town needed more time to respond to all the new questions and requests for more data, Gasbarro asked for and got the public hearing continued to the commission’s Jan. 7, 2015, meeting.
Posted on: Fri, 19 Dec 2014 11:49:01 +0000
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