Boat wake impacts and their role in shore erosion process, Boston Harbor Islands National Recreation Area

FitzGerald, Duncan; Hughes, Zoe; Rosen, Peter

Boston Harbor Islands National Recreation Area is a developing national park. Park managers are mandated to protect natural values of the islands while increasing visitor access and recreation opportunities. The Boston Harbor Island National Recreation Area is an exceptional natural resource. The islands are recognized as being the unique result (in the U.S.) of coastal and glacial processes resulting from Ice Age (Pleistocene) deposition of drumlins. The islands provide an exceptional opportunity for the public to experience Boston Harbor (Figure 1) and enjoy its dynamic coastal landscape. The park has prepared a General Management Plan, which states the purposes of the park as: 1. To preserve and protect a drumlin island system within Boston Harbor, along with associated natural, geologic, cultural, and historic resources. 2. To tell the islands’ individual stories and to enhance public understanding and appreciation of the island system as a whole. 3. To provide public access to the islands and surrounding waters for the education and enjoyment of this and future generations. Certain areas of the park are intensively visited, while other areas are sheltered and remote, offering an innate reserve for natural resources. There are also many examples of cultural resources due to both its long occupancy by Native Americans and European colonization, which have been declared significant by Congress. Today these islands are diminishing in size due to rising sea level and coastal erosion. The park is known to provide habitat for nesting seabirds, harbor seals, more than 70 species of terrestrial birds, and state-listed plants. The configuration of the Boston Harbor Islands system, and the assemblage of natural, geologic, cultural, and historic features, (in addition to the proximity to a major metropolitan area), offers a resource that has no parallel in the United States. Recently, large ferries have played a growing role in the transportation infrastructure for both island access and regional mass transit. This study is motivated by the need for improved monitoring and understanding of vessel wakes and their impact on the shoreline. Recently formed and rapidly retreating bluff scarps in low energy areas of the Boston Harbor Islands have become a public concern for resource stewardship and may indicate that boat wake traffic is responsible for erosion in areas unaffected by the natural wave climate. Unfortunately, the inferences have been neither verified nor quantified for the relative contribution of the anthropogenic role. Elsewhere however, there is growing evidence that ferry wakes can be the cause of significant erosion (Parnell and Kofoed-Hansen, 2001). This study employs a variety of field studies and modeling techniques to identify and monitor sites of critical erosion along the Harbor Island shorelines; to quantify rates of shoreline retreat; and to assess the role of storms, boat wakes, and other short-term processes responsible for shoreline erosion. The work continues and expands upon a previous study of Boston Harbor shorelines (Permit BOHA-05514 ‘Baseline Study of Harbor Island Geomorphology and Retreat Rates of Selected Islands’). The project consists of three main sub-studies: 1) Shoreline Retreat and Bluff Erosion study - monitoring critically eroding sites on several of the islands through topographic surveys (Figure 1). 2) Mapping - undertaken on all islands in the Harbor. 3) Wave and Wake study- surface wave conditions and boat wakes are monitored using oceanographic instrumentation deployed on specially designed frames. Additional information has been obtained through numerical modeling of the wave climate. This report summarizes the research, the findings, and our dissemination activities.

Boat wake impacts and their role in shore erosion process, Boston Harbor Islands National Recreation Area

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