SEDIMENT SLUICING TO MANAGE SEDIMENT ACCUMULATION IN PARADISE POND, NORTHAMPTON MA

Sediment sluicing is a viable alternative to dredging for management of accumulated sediment in small impoundments on dammed rivers. Although a high ratio of watershed to pond area can expose the pond to high sediment loads, the dynamics of the river provides good opportunities for sluicing. Paradise Pond (4ha) is created by a 5m high dam on the Mill River (watershed 140km²). Mean annual discharge is roughly 100cfs and varies from less than 10cfs to peak flows exceeding 2000cfs. The flood of record (7000cfs) occurred during Tropical Storm Irene in 2011. A prime location for water power a dam has been located here since the mid 1600’s.

Monitoring stations to measure discharge, turbidity, and cross sectional profile changes have been installed both upstream and downstream of the site. Bathymetric changes within the pond are monitored on an annual basis using a depth sounder and the volume of sediment moved during sediment redistribution operations is determined by comparison of sequential Digital Elevation Models created from drone surveys using structure from motion analysis of air photos. In a “normal” year 1500 – 2500m³ of sediment accumulates in the pond, but unusually high flow events bring in much more. It is estimated that Tropical Storm Irene deposited 15,000m³ of sediment.

The sluicing technique involves lowering pond level during the late Fall (November) and redistributing sediment with a bulldozer. The sediment is moved into the stream flowing through the upper part of the pond where it is transported to the dam. A 105cm pipe located at the base of the dam is used to control water level While the sluice gate is opened to sluice sediment during high flow events (>1000cfs) when the pond is full, it is most effective to release sediment during sediment redistribution operations when the pond is nearly empty. The released sediment accumulates in the splash pool and the immediate downstream channel, to be flushed downstream during the next high flow event.

Permitting for the project was difficult as the downstream channel was diverted as a flood control measure in 1941. Sediment accumulation in the diversion channel would increase flood risk. In the end regulators recognized that given the grain size of the released sediment and the gradient of the channel this was unlikely and careful monitoring would be a sufficient safeguard.