The Carlisle Mosquito Online

Friday, October 3, 2008

Wetlands damaged when storm pummels Hanover Hill

The timing could not have been worse. When the storm of September 6 and 7 dropped over four inches of rain on the Hanover Hill subdivision project off Westford Street, a large quantity of silt was washed from the unfinished dirt roadway and drainage structures into the

The detention basin between Gormley Way and Common Drive B (C.D. B.) failed during heavy rainfall in early September, causing silt to flow toward Curve Street and into a wetland. (Map drawn by Maya Liteplo)

wetlands. “The whole floor of the wetland was covered with silt,” said site contractor John Durkin, Sr., adding that the layer was as thick as six to eight inches in places. Conservation Administrator Sylvia Willard estimates that about half an acre was covered.

“Was there a mistake made, or was it an ‘act of God?’” was one of many questions posed by the Conservation Commission (ConsCom) on September 25 when they met with the developers to review the damage, assess repair efforts and plan the next steps. “We made every provision we could,” added John Durkin, Jr., also a contractor on the project. The developers had advance warning of the storm, but even with “trucks lined up bringing fill” they could not complete the detention basin in time.

What happened

Stamski and McNary engineer George Dimakarakos distributed photos of the site and described the events causing the damage, which were reconstructed as follows: during the storm, a large amount of water rapidly drained out of a wetland on the east side of Hanover Road. The water funneled toward an intermittent stream that crosses the roadway. Haybales placed in the streambed to protect the wetlands during road construction may have caused the water to back up and flood over the dirt roadway. The water then ran downhill into an unfinished detention area on the west side of Hanover Road between Gormley Way and common drive B (see map at right).

The berm of the detention basin was only about half built and was still bare earth. Water overflowed the berm, washing it away. Extensive gullying also washed out the 36-inch cement outlet pipes. The water laden with silt surged over a line of haybales placed beyond the detention basin, depositing silt in the wetlands located over 100 feet away.

Restoration

The ConsCom was told that a number of other areas of minor erosion had occurred and been fixed, but the area of most concern to the commission was the damaged wetland. The developer’s wetlands biologist, David Crossman, had inspected the site on September 18, and in a memo to the ConsCom he documented the cleanup operations he saw underway, “The crew was removing the siltation with shovels and buckets. They carried the buckets to a Bobcat, which then took the silt out of the [wetlands] buffer zone.” He said that on the following day, “a vacuum machine was brought on-site to aid in the removal of the silt. Most of the crew was still removing the silt with shovels and buckets, while a few worked with the hose from the vacuum machine. This activity continued throughout the weekend (September 20 and 21.)”

Durkin, Jr. noted that the workers were told not to remove any of the dark wetland soil beneath the layer of silt. He said, “It’s pretty much a skim-coat of silt that remains in the wetland.” The developers said that it took 15 to 18 workers a week to remove the silt.

Willard estimated the silt had been in the wetlands, “at least a week and a half,” and plants may have been smothered by the dense silt, which had a consistency similar to clay. Many of the ferns were vacuumed up along with the silt. However, since it was late in the growing season, it is hoped that most of the wetland plant cover will re-grow next spring from roots or seeds.

ConsCom comments

There was general agreement that the storm had been a freak event. Commissioner Tricia Smith, however, offered that she has worked on commercial construction projects where the sedimentation basins were completed before the majority of the site stripping and earth moving was begun, thereby reducing the risk of erosion from storms.

Commissioner Peter Burns wondered, “Should they have come to notify ConsCom before responding [to the problem]?” The developers did not inform the commission of the situation or the repair efforts until Monday, September 22, by which time the work was mostly complete. However, Commissioner Tom Brownrigg noted, “I was impressed with the amount of effort they put into this.” Commissioner Kelly Guarino agreed: “I’m impressed by how conscientious they were in cleaning it up.”

Next steps

In preparation for more rainfall, the developers had added or reinstalled temporary haybale check-dams along the roadway to forestall erosion. Some of the bare land near the roads had been seeded, additional rows of haybale siltation barriers had been placed between the detention basin and the wetlands, and the incomplete berm had been temporarily covered with tarp. Work continued to complete the drainage structures and detention basins. Plans call for part of the roadway to be paved before winter.

The ConsCom decided to hire an independent wetland expert, John Rockwell, to assess the site, both now and in the spring. The developers were asked to return in early November to review project status before winter. ∆


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