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Pleasant Bay New Inlet Hydrodynamic and Water Quality Analysis

Updated hydrodynamic and water quality analyses of the Pleasant Bay estuary was performed to determine the present state of the system since the formation of the north beach breach during the April 2007 Patriot?s Day northeast storm. For the purposes of this memo, the time period after the north beach breach will be referred to as ?post-breach?. The hydrodynamics of the post-breach system were also compared to 2004 pre-breach conditions, using data and model results from the Massachusetts Estuaries Project report of Pleasant Bay (Howes et al., 2006). The formation of the north inlet in 2007 provided an increase in tidal exchange for the Chatham Harbor/Pleasant Bay system. The post-breach conditions represent an increase in tidal currents within much of Pleasant Bay for both maximum ebb and flood conditions. Within the vicinity of the new breach, the increase in tidal currents through the inlet throat and the reduction of these high currents led to the observed formation of well-defined flood and ebb shoals. A significant increase in ebb tide currents along the western shoreline of the barrier system north of the new breach likely has been responsible for much of the observed shoreline erosion in this region. In general, the model indicates that the north inlet primarily services the Pleasant Bay portion of the total tidal flushing volume of the complete estuarine system. The orientation of the developing tidal channels through this inlet also appears to support this hypothesis. As of November 2007, accretion associated with the north inlet shoals had no influence on the federal navigation channel to Aunt Lydia?s Cove. In addition to the update of the water quality model to include the north breach, two model scenarios were run to simulate water quality conditions under two different possible future hydrodynamic conditions. The first model scenario is designed to simulate the likely worst case hydrodynamic conditions for the Pleasant Bay system. This scenario is based on the location of the southern inlet prior to the 1987 breach. Tidal flushing in Pleasant Bay is reduced in this scenario due to two main factors: first, a long inlet channel which has a greater drag on hydrodynamic flow; and second, a reduced tide range resulting from the greater influence of Nantucket Sound. For the single north inlet scenario, a hydrodynamic model was developed based largely on the model for 2007 conditions, but also an estimate of how the north-south inlet complex will change through the coming decades. The estimate of the future condition of the inlet was in turn based present and historical data including the record of past breaches, present shoreline erosion/recession of Nauset Beach, and present inlet cross-sectional area.

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