SEDIMENT SLUICING TO MANAGE SEDIMENT ACCUMULATION 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 cement and masonry dam on the Mill River (140km² watershed) in Northampton, Massachusetts. The dam is located 4km upstream of the confluence with the Connecticut River. During the last 50 years sediment removal was by dry excavation at 8-10 year intervals. Approximately 17,000 m³ of sediment were removed each time and used as daily cover for a local landfill. Closing of the landfill has made this option impractical. A new strategy is being developed that combines mechanical sediment displacement with sluicing through the gate at the bottom of the dam during high flow events (>500cfs). In this model, an excavator will move sediment accumulated in the upstream part of the pond to the downstream section next to the dam. The displaced sediment will then be released from the pond through the sluice gate at the bottom of the dam during high flow events when transport through the downstream fluvial system is assured.

To test the viability of this technique, a sluicing experiment was conducted during a large storm event (100mm of rain in a 24 hour period) on September 30^th 2015. Peak stream discharge exceeded 3000 cfs and the sluice gate was opened for approximately 10 hours. Measurements of water and bedload sediment velocity were made, after peak flow, at five locations in the pond using a Teledyne RDI RiverRay Acoustic Doppler Current Profiler (ADCP). Water velocities in the pond approximately 100m upstream of the dam exceeded 45 cm/sec at a discharge of 1600 cfs. Stationary boat moving bed tests determined bedload sediment movement at velocities ranging from 0.45 to 6.0 cm/sec. However, bathymetric surveys conducted before and after the event did not show significant erosion of the pond bottom. It may be that high sediment inflow into the pond during this event balanced the sediment leaving the pond so there was no net sediment loss. Higher bed velocities may be needed to mobilize sediment at the bottom of the pond. This could be accomplished by mechanically moving more sediment to the area of the sluice gate prior to the event. This would reduce the cross section available for flow causing an increase in velocity.