Lake Barcroft Watershed Improvement District

By Mary Ann Francis

Barcroft News Staff

 

What happened when the trash diverter at Potterton Bridge was overwhelmed by heavy rain on top of heavy snow on Feb. 22, and how will we fix it?

These were the main topics at last month's Lake Barcroft Watershed Improvement District board meeting. The board even had a videotape showing the diverter's performance as water, at a level seen only two other times in recent memory, flowed over and through it

 

What Happened

Ken Young, an engineer and owner of GKY and Associates of Springfield, discussed the hydraulic action of the storm and its impact on the trash diverter. GKY and Associates is the WID's longtime engineering consultant and the firm that designed the diverter.

              Young exmplained that the diverter is designed to work for "high frequency," common events throughout an average year of precipitation. It is not designed to trap debris from the biggest storm of any given year. In major storms, debris is expected to flow over the top of the diverter.

The water that flowed through the runs and the dam on Feb. 22 was the equivalent of 3 to 3 ½ inches of rain because the ground was so frozen it could not absorb water. In spite of this heavy water flow, the diverter stayed in place (It had recently been reinforced with brackets and other improvements to prevent tipping.) but the rock under it washed out.

The water scoured out a large hole under the diverter that filled with large rocks as the access road caved into it. Hydraulic forces on the rocks then pushed them farther downstream.

How To Fix It

First, those rocks have been replaced with 150 tons of riprap.

              Also discussed was installation of a floatable boom to reduce the catch debris that would otherwise float through the diverter be washed downstream. The boom would be made of a material called Tuffboom, specifically designed for this use. It's half the weight of water so only half would be visible above the Lake. Use of a boom is under study.

The cost of the improvement program for the trash diverter, including the boom and the riprap fill, is estimated at $15,000. Upstream improvement to control trash will also be reviewed, but the primary focus is to improve the diverter.

Other WID News

The WID has been out during the past few weeks with equipment collecting the heavy salt and sand from the roadsides to prevent it from washing into the Lake during spring and summer rain. In addition, security at the WID compound is being improved. In February two LBA outboard motors and some landscaping equipment were stolen by someone who cut through the fence at the WID compound. Three sea containers have been purchased, two by LBA and one by the WID, to store larger industrial items that cannot be left outside. These containers will replace the makeshift sheds currently in use. The containers will be put in place shortly now that the snow has melted.