Created on Thursday, 17 April 2008 05:43
Last Updated on Thursday, 17 April 2008 05:43
By Lisa Loomis
Waitsfield has hired an engineering firm to assess the safety of the dam at the north end of the town pond after a sinkhole developed and a culvert collapsed last week.
The town hired the firm after the Vermont Agency of Natural Resources issued the town an enforcement order for failure to obtain the appropriate state permits to work in a state wetland. The state found that the town had reacted to a potential dam failure at the pond site by dewatering the lower pond in a Class II wetland.
The town pond between the movie theater and Route 100 is separated from a lower pond and wetlands by a dam at the north end. Last week town select board member Charlie Hosford and town administrator Valerie Capels came to the pond at the request of adjoining property owner Russ Bennett.
"The embankment had caved in and the culvert was crushed. The water was flowing over the trash guard in the town pond, and in the lower pond the water was six inches above the exit impoundment. We decided we need to hear from Fred Viens, who built the pond, so that we could understand how it was built and what he thought about the cave-in," Hosford explained to the board on April 14.
"We were concerned that we didn't know where the water was running. The water levels in both ponds were high and rain was predicted all weekend. We felt we had a public safety issue on our hands," Hosford said.
"We determined we needed to see up the culvert and remove the top of the standpipe. To do that we had to release water from the beaver dam at the lower pond," Hosford said.
By Thursday morning, April 10, water had been released from the lower pond; by Thursday afternoon the town had received a visit from state officials; and, on Friday, the town received a notice of enforcement action from the state for beginning the work without state permission.
The Section 1271 enforcement order requires the town to hire an engineer qualified in dam safety to assess the condition of the dam and requires that any drawdown procedures be coordinated with the ARN stream alteration department. The town was also required to immediately stabilize 200 feet of disturbed channel downstream of the dam outlet structure and put in "blast mats" to hold disturbed earth in place.
"The major problem is really the dam. We might want to consider dewatering the pond and returning it to more of a catch basin. I share responsibility for not properly contacting ANR before we released the water from the lower pond. At this point, the pond needs to be drained incrementally until we can get to the culvert," Hosford said.
PIPES THAT FAIL
"The dam is a 30-year-old Soil Conservation Service standard design, built with corrugated metal pipes that fail after a certain point, and our dam is no exception. I don't know what kind of maintenance could have been done to prevent it, but now the problem has to be solved. The pond will have to be drawn down and the lower pond as well so that the pressure they exert on the dam will remain relatively equal," Capels explained, after meeting with an engineer from Weston and Sampson on April 16.
The engineer Shawn Patneaude, in his explanation of the scope of the project, notes that: "The dam is an earthen embankment with a corrugated metal riser mated to a corrugated metal outlet conduit, and an earthen emergency spillway. There is no accessible low-level drain valve or gate. The state engineers have speculated that the sinkhole is the result of a condition called piping. This condition occurs when the embankment is eroded from the inside where the eroded sediment flows through a break in the outlet conduit. Weston and Sampson have encountered these specific conditions on dams very similar to this one."