QUANTIFYING THE “PERFECT STORM” FOR SEDIMENT ENTRAINMENT AT THE CONFLUENCE OF NONTIDAL STREAMS AND TIDAL ESTUARIES

The capacity of moving water to mobilize sediment has relevance to watersheds, lakes, rivers, and coastal estuaries because of the implications to water quality, aquatic habitat, public safety, and human health. Sediment entrainment in non-tidal stream delta areas can influence estuary water clarity conditions and contribute to loadings of pollutants adorbed to particles or released from interstitial water in bed sediments. Previous research and observations have indicated that problematic concentrations of bacteria in non-tidal and tidal waters can be associated with sediment entrainment and transport dynamics. The sediment associated bacteria is of interest to coastal managers because the source may be one of the factors driving shellfish harvest closures due to detection of contamination in some estuaries along the Maine coast. Quantification of sediment entrainment and transport in confluence areas where non-tidal streams outflow into estuaries is particularly challenging because of shifting boundary conditions related to stream inflow responses to precipitation runoff events and semidiurnal tidal fluctuations. The related research summarized here responds to the interest in quantification of conditions conducive to sediment entrainment in estuary tributary deltas that appear as submerged backwater conditions at high tide and flowing stream channels during mid- and low tide time periods. The study location is the confluence of Crippens Brook with the Jordan River estuary in Trenton, Maine. Conditions when relatively high vulnerability to entrainment exist are estimated based on time series of stream inflow and tide conditions combined with hydraulic evaluations using delta sediment grain size and topography measurements. The frequency that entrainment event conditions occur are estimated based on watershed runoff simulations using extended precipitation and tide time series data at hourly time steps. The presented work helps fill a knowledge gap in the understanding of sediment dynamics in estuary tributary deltas potentially linked to bacteria contamination problems affecting shellfishing areas in Maine estuaries.