Breaching and Environmental Impacts of Woodlands Lake Dam a High Hazard Dam

Woodlands Lake Dam is located in Irvington, New York, and is owned and operated by Westchester County Department of Parks, Recreation, & Conservation. Woodlands Lake dam is a run-of-river dam that impounds water from the Saw Mill River to form Woodlands Lake. The dam is constructed of stone and masonry and is believed to have been erected circa 1840. The dam has a single spillway that is partitioned into three overflow sections by two piers that support a vehicular bridge over the dam. The dam is considered to be a Class C high hazard dam. The existing structure has insufficient spillway capacity and major rehabilitation is required to bring the dam into compliance with New York State Department of Environmental Conservation regulations for a Class C dam.
Westchester County Commissioners Kathleen O’Connor and Hugh J. Greechan recommended and approved the decommissioning of the existing dam, to reclassify the dam to a Class D “No Hazard” classification. A project is underway to partially breach the existing dam and revert the Saw Mill River to its original condition as a flowing river through this reach. Under this project the impoundment will be eliminated, and the Saw Mill River will be diverted through a series of armor stone weirs, that naturally step down the water surface until ordinary high water is reached downstream of the existing structure. The armor stone weirs will facilitate fish migration through this reach and replace the obstruction created by the dam. The former impoundment will be converted to a floodplain planted with native vegetation to provide food and shelter to native wildlife.
A hydraulic model shows that the proposed breached structure will pass the 100-year storm with no more rise in water surface elevation than is typical for comparable sections of the stream and is consistent with New York State Department of Environmental Conservation (NYSDEC) regulations for Class D “No Hazard” dams. During the 100-year storm, the river will flood out of the new channel into the floodplain, upstream of the breached structure, until water reaches the overflow segments and will provide adequate capacity for floodwaters. The model shows that flooded areas due to the breach downstream of the structure will not be more than what is currently observed during the 100-year storm.