Coastal Resource Guide Pleasant Bay and ... - Town to Chatham

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Coastal Resource Guide for Pleasant Bay and Chatham HarborA full description of Nauset Barrier Beach and inlet formationand migration is found in a study commissioned by the PleasantBay Alliance (Giese, 2009). The study builds upon earlier work(Giese,1978) and analyzes current aerial photography andmore than 150 years of historical data to assess the likely futuremovement of the outer beach and inlet. The data show that theformation of the inlet in 2007 is a continuation of the historicalcycle, and that the cycle occurs in two distinct phases.Phase 1 is an inlet formation stage in which a new breachingevent launches a period of multiple inlets and changes in tides andtidal channels. The system is presently in Phase 1. It was initiatedin 1987 with the formation of a new inlet east of ChathamLight. Phase 2 is an inlet migration phase, in which a single stableinlet migrates southward. Under this scenario a single stable inletcould be in place in twenty years and begin a southward migrationin thirty years, potentially ending up somewhere betweenMinister’s Point and Chatham Light in fifty years (Figure 1).2.3.2 Tide Gage Monitoring and AnalysisFrom 2005 through 2010, tide gage data was collected fromMeetinghouse Pond located at the headwaters of the PleasantBay system in Orleans by Graham Giese (Provincetown Center forCoastal Studies) and Kelly Medeiros (Cape Cod National Seashore).The tide gage was deployed in the spring of 2005, 2006 and 2007,to allow the collection of data during at least one full lunar cycleeach year. However, because of the breaching of Nauset Beachin April 2007, followed by formation of a new tidal inlet, the tidegage was left in place after the 2007 deployment until commercialmarina reconstruction required its removal in December 2010. Thetide gage was redeployed in April 2011 at a nearby location.The purpose of collecting the tide gage data was to monitordifferences between the tides of Pleasant Bay and those of theopen Atlantic Ocean outside of the Bay. Initially the monitoring wasundertaken in an attempt to anticipate inlet formation. Changesin tide range and phase (see Section 3), as well as changes intide distortion, can give valuable information about Pleasant Bayhydrodynamics and, in so doing, give indications of conditions3Figure 1. Three images show successive morphological changes consistent with the initiation of a future wave-dominated inlet migration phaseof the system’s development. In each, the barrier beach north of the inlet has become increasingly well developed, producing (a half-centuryfollowing the 2007 breach) a well developed re-curved spit and tidal inlet in the general location of today’s Tern Island (Source: Giese, 2009).Pleasant Bay Alliance 2011
Coastal Resource Guide for Pleasant Bay and Chatham HarborFigure 2 shows projected changes in the Nauset Barrier Beach and Inlet System. (Source: Giese, 2009; Mark Adams)that could lead to changes in the barrier beach inlet configuration(Figure 2).An abrupt increase in tidal range followed the formation of anew tidal inlet in 2007 (Figure 3). After this inlet formed, thegage was maintained in order to record the expected ensuingtidal changes. They show that not only did tide range increaseapproximately half a foot directly following formation of the 2007inlet, but it continued to increase at an average rate of about atenth of a foot per year during the following three years. Thesechanges indicate increased flushing of the Bay resulting fromimproving tidal connections with the ocean.Pleasant Bay Alliance 20112.3.3 Light Detection and Ranging (LIDAR)Additional measurements of the Nauset barrier shoreline have beencollected through aerial imagery and a technology known as LightDetection and Ranging (LIDAR).LIDAR is an optical remote sensing technology that uses lasers todetect the distance to an object or surface. LIDAR measurementstaken in the vicinity of Pleasant Bay appear like aerial photographs,but also contain detailed elevation data on the depth or height oflandforms.4
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