ONE-DIMENSIONAL BED LOAD SEDIMENT TRANSPORT MODEL AND SEDIMENT BUDGET FOR THE DAM REMOVAL-EFFECTED CONOCOCHEAGUE CREEK FLUVIAL SYSTEM

Following the 2005 breaching of the Birch Run Dam on the Conococheague Creek in south-central Pennsylvania sediment-trapped contaminates being evacuated from the site heeded operations of the downstream Chambersburg Borough Water Treatment Plant after high flow events. Still today, signs of excessive physical erosion are observable in the form of channel avulsions and sand deposition downstream. This study investigates bed load sediment transport rates within the former reservoir with the goal of determining the sources of sand-sized sediments currently causing downstream aggradation. To do so, a one-dimensional bed load transport model was assembled to predict the transport of sands in reaches throughout the site. The model applies one-dimensional transport equations formed by DuBoys (1879), Shields (1936), and Rottner (1959) in Visual Basic Code for use with Microsoft Excel. Following this, differencing of transport rates between adjacent reaches was used to create a sediment budget for the site. Bed load sediment sampling of the stream in its current state was conducted to check and correct the assembled predictive model. As dam removal is a relatively new method of restoring the nation’s water resources, few studies have been done that explore the long-term effects of removals/restorations conducted in the past.

After dam removals sediment and contaminants are released from former impoundments as the channel adjusts to a lower energy state. This and past studies have found that one-dimensional models are sufficiently capable of estimating sediment transport of stream reaches after removals. For this site, results of predictive modeling are compared to collected field data for an accuracy assessment and improvements are recommended for further studies. The areas found most capable of contributing sediment are immediately upstream of the dam breach, within the wetland formed in the former reservoir and at the upstream extent of the former reservoir.

The completion of this site study suggests that methods of better controlling long-term erosion after dam removals are needed as signs of unnaturally high sediment transport persist almost ten years after removal. In close, results of this study can be helpful in better protecting habitats downstream and improving restoration methods for future removals.