TOWN, SBPF PROPOSING DRAINAGE SYSTEM TO DRY UP TOP-DOWN RUNOFF EROSION IMPACTS ON NEARBY WETLANDS, THE ISLAND’S AQUIFER AND SPECTER OF FAILED SIMILAR PROJECT WEIGH ON CONCOM By Peter B. Brace Waiting for the Department of Environmental Protection to issue its ruling on SBPF’s June 17 superseding order of conditions request must have been too much for this pro-hard-armoring lobby. Such a weight of ennui bore down on the Siasconset Beach Preservation Fund and Nantucket’s Department of Public Works that they just had to file some sort of notice of intent with the Conservation Commission to continue combating erosion or go crazy. Well, actually, the current filing by the town, represented by SBPF at the Dec. 3 ConCom meeting, was deemed a necessary step toward slowing erosion of the coastal bank from the top down, regardless of DEP’s pending decision. Given that rainwater during heavy rain events runs right over the edge of the bank, harrowing deep gullies down its face, SBPF and the town needed an active solution to work in concert with their three-layer geotube wall 900-feet long, as it does nothing to prevent erosion from above. SBPF/DPW is proposing a second attempt in 14 years at drying up runoff flowing eastward from both sides of the road and the road itself, over the edge of — and out of, at various points — the coastal bank by installing infiltration trenches and pipes, catch basins and Stormceptor drains with filters in them in three locations on the west side of Baxter Road between 90 and 106 Baxter Road. As explained by one of SBPF’s long-time engineers, Mark Haley of Haley & Aldrich, Inc., of Boston, Mass., each installation would employ grading of the ground on both sides of the road, and soil and asphalt berms to move water toward catch basins that would collect runoff from storm and rain events. The 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 six-inch diameter discharge pipes 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. The applicants also want to install an overflow pipe that would discharge water into the wetlands on the west side of Baxter Road between #92 and #96, should water flowing into these three installations ever overwhelm them. One of the issues that might be spooking the commission a wee bit is SBPF’s failed similar attempt at diverting runoff from the surface down through the clay layers of the coastal bank along Baxter Road with the installation of 207 wells in this area. Fifteen wells were installed in 2000 with another 192 in 2004 in a project employing PVC piping perforated on one end and non-perforated on the other. SBPF placed the pipes in the coastal bank to prevent runoff from percolating into the ground, hitting impermeable clay layers and emerging out of the face of the bank to cause erosion. With the perforated sections of these pipes above the clay layers in the ground, theoretically the water was to have entered the pipes and flowed down through them and into the ground below the clay, causing no erosion. However, Haley admitted at the meeting that these pipes silted in, failing to perform as proposed and could not be repaired or replaced. At this Dec. 3 meeting, the commission asked SBPF for a plan showing the locations of all these wells for the next meeting. Individually, the commissioners had their own concerns. Commissioner Ashley Erisman asked if any native plantings would be included. ConCom member Sarah Oktay asked SBPF to include the town engineer’s report in whatever they filed for the Dec. 17 meeting, and if there’s any change in the groundwater elevation on the west side of Baxter Road. And toward that end, Chairman Ernie Steinauer wanted to know the depth of the island’s aquifer, whether it was at or near sea level, stating that all of the water must be well filtered, and then asking how the project would affect how water flows into wetlands on the east side of the road. “We don’t want the wetlands to not have enough water,” said Steinauer. SBPF’s Nantucket engineer, Arthur D. Gasbarro III of Blackwell & Associates, assured Steinauer there wouldn’t be any negative impacts on the wetlands. However, there are those who do not share Gasbarro’s confidence in this proposal to protect nearby wetlands. These include the Nantucket Land Council, Steinauer, Oktay and Nantucketer Dirck Van Lieu, who are concerned that this subterranean removal of surface water along Baxter Road would unnecessarily deprive wetlands on the west side of the road of water that they’re currently receiving. Oktay noted it would be easy to use a “spring gauge” to measure the height of water in wetlands within the project area. Land Council Resource Ecologist Emily MacKinnon clarified that the Land Council is looking for more information on what the existing zone of contribution is now for water flowing into the wetlands so the amount of water SBPF’s project is going to be removing can be documented. She added that the Land Council is also anxious that the wetlands on the southern half of the project area are delineated on existing maps in 2013 as being 25 feet west of Baxter Road, but that she’s witnessed standing water as close as 10 feet from the road. The ConCom continued this public hearing to its Dec. 17 meeting at 4 p.m. in the Training Room on the second floor of the Public Safety Building at 4 Fairgrounds Road. — PHOTO, taken two weeks ago, shows one of the crevices at the edge of the bluff deepening and widening. Schematics are part of the submission to the Conservation Commission.
Posted on: Thu, 11 Dec 2014 11:15:46 +0000
Trending Topics
Recently Viewed Topics
© 2015