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

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Coastal Resource Guide for Pleasant Bay and Chatham HarborSection 1Introduction11.1 Purpose StatementPleasant Bay is an exceptionally beautiful and environmentallysignificant estuarine system located on the eastern shore ofCape Cod in the Towns of Chatham, Orleans, Harwich, andBrewster. The Pleasant Bay system is also renowned for the diverserecreational activities it supports, including shellfishing, sailing,power boating, kayaking, wind surfing and bird watching. Thearea’s significance as an environmental and cultural resource isevidenced by its designation as an Area of Critical EnvironmentalConcern (ACEC) in 1987.The Pleasant Bay system is a collection of embayments, tidal riversand salt ponds that currently exchange water with the AtlanticOcean through two inlets in the Nauset Barrier Beach system. Thebarrier beach and inlet system follows a quasi-cyclical pattern ofinlet formation and migration, which influences the volume andrate of tidal exchange throughout the system. The configuration ofthe barrier beach and inlets greatly influences resource conditionswithin Pleasant Bay and adjacent Chatham Harbor.The dramatic changes in the Nauset barrier system observedover the past two decades, following the formation of inletsin 1987 and 2007, have included measurable changes in tiderange and volume, sediment movement and shoreline erosion inboth Pleasant Bay and Chatham Harbor. While these conditionsare continuing to evolve, they have brought into sharper focusa multitude of coastal resource management issues. Thesemanagement issues include protection of public and privateproperty and infrastructure from erosion, protection of publicaccess points, impacts to navigation, and impacts to fisheries,wetlands and other coastal resources.The Coastal Resource Guide for Pleasant Bay/Chatham Harbor isintended to provide a source of objective, scientific informationregarding the status of coastal resources in the Pleasant Bay andChatham Harbor system, and the dynamic forces and trendsaffecting resource conditions. The guide draws from a number ofrecent technical studies sponsored by the Pleasant Bay Alliance,as well as existing regional and state resources, to describe trendsin barrier beach migration, tidal dynamics, flushing, water quality,and related processes.In providing this information, the guide seeks to present acomprehensive and long-term perspective on past, current andlikely future resource conditions in Pleasant Bay and ChathamHarbor. The guide is intended as a resource to ConservationCommissions, planners, resource managers, property owners andinterested citizens. The guide provides the technical underpinningsfor development of management recommendations and bestpractices for coastal erosion protection, beach nourishment, anddredging, and for long-term planning to protect coastal resourcesand minimize coastal hazards.1.2 Study AreaThe study area for the guide includes the entire Pleasant BayACEC and all of Chatham Harbor southward to the inlet formed in1987. Marine waterways within the study area include ChathamHarbor, Pleasant Bay and Little Pleasant Bay, numerous saltponds and coves (Meetinghouse Pond, Arey’s Pond, Pah WahPond, Kescayogansett Pond, Quanset Pond, Crow’s Pond, RoundCove, Ryder’s Cove and Bassing Harbor) and three tidal rivers(The River, Namequoit River, and Muddy Creek.) The ACEC alsoincludes eleven freshwater ponds and lakes that have a perennialhydrological connection to Pleasant Bay. The study area, includingthe Pleasant Bay ACEC and watershed boundaries, is shown onMap 1.1.3 Organization of the GuideThe guide is intended to be a map-based resource to provide aspatial representation of data and information. Accordingly, eachsection contains a brief description of current resource conditionsand trends, citing relevant research and background sources. Eachsection also contains references to GIS maps, compiled at theend of this document, which provide a visualization of resources,conditions and trends.In addition to this Introduction, the guide contains the followingsections:Section 2. Barrier Beach and Inlet DynamicsSection 3. Wave and Tidal ConditionsSection 4: Sediment TransportSection 5. Coastal StructuresSection 6. Coastal VulnerabilitySection 7. Natural ResourcesSection 8. Bathymetry and NavigationA glossary of terms is provided at the beginning of the guide toassist readers who may be unfamiliar with some of the terms andacronyms used.The guide and a complete set of maps havebeen provided to each Alliance communityand to the Cape Cod Commission. However,due to the size and number of maps, it wasnot feasible to reproduce the entire guide forbroad distribution. The guide and all mapsmay be viewed and downloaded from theAlliance website:www.pleasantbay.org.Pleasant Bay Alliance 2011
Coastal Resource Guide for Pleasant Bay and Chatham HarborSection 2Barrier Beach and Inlet Dynamics2.1 PurposeThe purpose of this section is to illustrate the nature of change inthe configuration of the Nauset Barrier Beach and inlet system, andthe inner shoreline areas of Pleasant Bay and Chatham Harbor.2.2 BackgroundTogether, Pleasant Bay and Chatham Harbor encompass seventymiles of shoreline, including the backside of the Nauset barrierbeach and twelve and a half miles around six islands in PleasantBay. This expanse of shoreline includes distinct barrier beach andinland/estuarine shoreline types. The intensity of wave energyvaries for different shoreline types, from the quieter enclosedshores of salt ponds to more exposed shores subject to wavesgenerated over a longer distance.As described more fully in Section 7, the Nauset Barrier Beachseparating Pleasant Bay and Chatham Harbor from the openAtlantic Ocean is itself part of a much larger system of beachesformed of sediments eroded by wave action from the glacialdeposits of outer Cape Cod and carried southward by littoraldrift (Giese, 1978). Relative sea level was about 15-20 feet lowersome 5,000 years ago than it is today. As sea level gradually roseand the cliffs of Truro, Wellfleet, Eastham and Orleans retreated,some of the eroded sediment moved southward to Nauset Beach.Through time, Nauset Beach elongated and migrated westward asstorm waves eroded beaches and dunes and storm overwash andtides transported sediments into Pleasant Bay, causing the barrierto roll over itself (Howes et al, 2006).Section HighlightsPleasant Bay and Chatham Harbor encompass seventymiles of varied shoreline.The Nauset Barrier Beach and inlet system is amajor influence on tide range and natural resourceconditions in Pleasant Bay. Tide ranges increased halfa foot following the formation of the 2007 inlet, andcontinued to increase a tenth of a foot per year in thethree years following.The barrier beach and inlet follows a 150-year cycle ofinlet formation and migration. In accordance with thiscycle, a single stable inlet could be in place in twentyyears and begin a southward migration in thirty years,potentially ending up somewhere between Minister’sPoint and Chatham Light in fifty years.The inner shoreline of Pleasant Bay is more stablethan the outer beach shoreline. A comparison ofrates of change from 1868-2005 along twenty-fivemiles of shoreline where both the High Water Line(HWL) and the marshline were delineated showedthat approximately six miles of the HWL and fourteenmiles of the marshline exhibited statistically significantshoreline change over this timeframe.Nauset Beach is shown cluttered with clumps of peat deposited onthe ocean-facing beach following a coastal storm. This photo providesfurther evidence of the coastal process known as “beach rollover.”The peat was initially formed in the marshes along the bayside of thebarrier beach. As the ocean facing beach erodes and overwashes, thebeach migrates landward (rolling over on itself) ultimately exposing thepeat to the open ocean. Photo: Ted KeonPleasant Bay Alliance 20112.3 Key Studies: Methodologies and FindingsA number of studies have compiled and analyzed current andhistorical data regarding the Nauset Barrier Beach and inlet system.These studies have helped inform our understanding of the trendsinfluencing barrier beach and inlet migration and their effects inPleasant Bay and Chatham Harbor.2.3.1 Barrier Beach and Inlet MigrationPleasant Bay tides are controlled by an inlet system that generallyfollows an approximately 150-year cycle of inlet formation andmigration. Figure 1 depicts this cycle. As the barrier beachelongates it moves the inlet further south of the main basin ofthe Bay. The tidal exchange between ocean and Bay decreases asthe barrier beach lengthens, creating hydraulic inefficiency. Thisinefficiency is overcome eventually by the formation of newnortherly inlets, closer to Pleasant Bay. Over time the northernmostinlet becomes the dominant, and eventually the only inlet,as the barrier island forming the southern inlet erodes and movesinland. The first phase of the cycle ends—and sets the stage forthe following one —as the single inlet migrates southward farenough to recreate conditions of hydraulic inefficiency conduciveto the formation of new inlets.2
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