WILTON – In October, the Select Board authorized the H.L. Turner Group of Concord to conduct an analysis of the New Reservoir Dam for breach and discharge conditions. On Monday, Senior Project Engineer Robert Carter presented his findings to the board.

In a letter to Town Administrator Paul Branscombe, Carter noted that a breach of the dam during a “100-year flood event,” would impact downstream residential and commercial structures. Those impacts change the dam classification from its current “significant” hazard to a “high hazard” dam.

By state regulations, a dam must have adequate discharge capacity to pass 250% of a 100-year event with a minimum of a one-foot of freeboard. “Under existing conditions the dam spillway does not have adequate discharge.”

Turner was asked to perform an analysis to determine the potential of lowering the spillway to achieve discharge capacity.

The analysis reveled the dam has a bottom discharge outlet at 604.73 feet elevation, with a gated inlet and a three-inch clear gap at the top of the gate at an elevation of 607.75 feet. Discharge from the drop inlet is through two culverts, one 24 inches in diameter, the other 16 inches.

“When inflow to the pond from the 100-year storm is multiplied by 2.5 and routed to the dam outlet under existing conditions, in the best case scenario the dam does not meet freeboard requirements and in the worst case scenario is overtopped by approximately 2.5 inches.”

If the invert of the gated inlet is lowered by eight inches to 605.50 feet, (lowering the normal pond elevation) and the gated section modified, the spillway passes the 250% of the 100-year flood.

When the preliminary dam breach model is revised, there are still impacts downstream. The only benefit gained by lowering the inlet to elevation 605 feet is the reduction at the extreme end of the model, the confluence with the Souhegan River.

An analysis of the dam, approved by the Department of Environmental Services in 2015, showed several low areas that need to be addressed, particularly the side of the dam adjacent to Sand Hill Road.

The proposed lowering of the drop-inlet invert appears to be achieved by removing a one stop log from the stoplog bay and constructing a more suitable screen/grating at the outlet to improve hydraulic performance and prevent clogging.

If the outlet is lowered by eight inches and the gated inlet improved, the dam passes the 250% test. The reduction of the pond level appears to be fairly easily implemented.

Carter suggested the new breach model be submitted to NHDES Dam Bureau for review.

Selectman Matt Fish said he made some measurements and observation and determined lowering the dam by eight inches would have only a few inch impact on the pond level.

The New Reservoir pond has been under discussion since last spring. It was determined that, since it is no longer a public water source and will probably never again be one, swimming and other uses cannot be prohibited.

A chain link fence along Sand Hill Road was removed during the summer and reinstalled behind the Recycling Center.

The question of parking is still under consideration, as well as the idea of making parking available there for people visiting Garwin Falls. A proposal for parking near the falls and the site of the “Old Reservoir” dam is also being discussed.