MODELING SEDIMENT TRANSPORT DURING SLUICING EVENTS IN PARADISE POND, NORTHAMPTON, MASSACHUSETTS

The viability of sluicing sediment from a small impoundment during high flow events is being investigated as an alternative to dry dredging. Paradise Pond (4 ha) is impounded by a 60m long, 5m high dam on the Mill River in Northampton, Massachusetts. The pond is located 4km upstream of the confluence with the Connecticut River and lies within a 140km² watershed. A dam has been located at this site since the late 1600’s and for at least the past 50 years, accumulated sediment has been removed every 8-10 years by dry excavation. Each dredging needs to remove at least 17,000m³ of sediment in order to maintain the pond for recreational boating. Rising costs associated with the closing of local landfills that accepted the dredge spoil, together with concerns regarding downstream sediment starvation, has made this option unsustainable. A new two-part plan combines mechanical movement of sediment to the upstream side of the dam during a partial pond drawdown, with release of that sediment through the sluice gate at the bottom of the dam during high streamflow events. Releasing sediment to the downstream reach in this manner closely mimics the natural pattern of sediment transport through a fluvial system.

A sediment transport model (MIKE 21 Curvilinear Modeling System by DHI) is being used to evaluate the flow conditions under which sediment will be released through the sluice gate and move through the downstream reaches. The model will be used to establish the minimum flow conditions required to move sediment through the sluice gate, and more importantly the flow required to move the released sediment through a downstream diversion channel that is part of a flood control system that protects the City of Northampton from flooding. The model is being calibrated using data from a 100mm, 24hr rain event that occurred on September 30^th 2015 that caused peak flow to exceed 3000cfs. Instruments on the pond and just downstream of the pond recorded flow and turbidity data throughout the event that included a 10hr period when the sluice gate was open. A Teledyne RiverRay Acoustic Doppler Current Profiler (ADCP) was used to collect velocity and moving bed data in the pond just upstream of the open sluice gate. Measured moving bed velocities in excess of 6cm/sec suggest significant volumes of accumulated sediment can be removed using this method.