COARSE GRAIN SIZE AND MOBILE SEDIMENT RESULT IN MINIMAL CONTAMINANT STORAGE BEHIND A LOW-HEAD DAM IN RICHMOND, IN

A pre dam removal assessment of sediments stored behind a weir dam on the East Fork Whitewater River found lower contaminant concentrations than anticipated, likely due to coarse grain size and high channel bed mobility. The city of Richmond, Indiana is removing an approximately 2-meter high, run-of-river, dam to increase ecological connectivity, eliminate a safety hazard, and improve water quality of the East Fork Whitewater River. Over the past two summers, we conducted a pre dam removal study to document the impact of the dam on river morphology and water quality, and to characterize the composition and contaminant concentrations of the sediment wedge retained by the dam.

Our initial 2021 survey determined that the dam pool was approximately 370m long and contained approximately 7,400m3 of sand and gravel. Cores of dam pool sediment collected for chemical and grain size analysis found the sediment accumulated behind the weir dam to be bimodal, with a poorly-sorted pebble population mixed with moderately-sorted, coarse sand. Cores taken outside of the main channel were commonly capped with moderately well-sorted, fine sand.

Analysis of 12 sediment samples for metals, pesticides, PCBs, polycyclic aromatic hydrocarbons, total petroleum hydrocarbons, and cyanide found only trace amounts of metals and hydrocarbons, and did not detect any legacy pesticides or PCBs. This finding was surprising, given the extensive industrial history of the Whitewater Gorge.

Repeat surveys of 10 cross sections and the riverbed long profile found approximately 3,700m3 of sediment eroded out from behind the dam between June 2021 and July 2022. The peak flow for water year 2022 was equivalent to a 3-year flood. The lack of significant contaminant concentrations measured in the dam pool can likely be attributed to the absence of fine grained particles and the fact that sediment is mobile even during relatively small floods.