Conservation Division

The goal simply stated is to develop a project that will improve tidal exchange with Nantucket Sound in order to address water quality issues (to improve marine fisheries) while protecting environmental values, preventing and relieving flooding, and reducing public health threats from mosquito-borne diseases. This report reviews background information, and new field results and analyses to form a conceptual plan for a new inlet at Rushy Marsh Pond. The conclusion is that such an inlet is technically feasible, could be constructed at reasonable cost, and is in principle permittable by relevant agencies.
 
Part 1 of the report discusses the prevailing coastal processes bearing on the Rushy Marsh Pond outer shore, that set limits on what can be done, and how, with regard to the new inlet. Rushy Marsh Pond lies on the south coast of Cape Cod in an enigmatic coastal setting (Fig. 1). Historical changes in the outer shore at Rushy Marsh Pond, leading to its present configuration, can only be understood and accommodated in the larger context of this part of Cape Cod. Over the past century it has been characterized simultaneously by low tidal and wave energy, but at the same time by active changes in the configuration of coastal landforms.
 
This seeming contradiction results from the unusually active tidal inlet hydraulic processes at the neighboring mouths of Popponesset and Cotuit Bays. The area also has an abundant offshore sediment supply resulting from converging littoral drift of sand from the west and from the east.
 
In essence, the observed history of coastal dynamics here results from an interplay between inlet hydraulics, wave attack, and weak littoral drift (Fig. 2). Human activities, such as dredging (which began as early as 1896) and the construction of groins and seawalls, are other significant factors. The natural inlet to Rushy Marsh Pond, depicted in several historical maps, closed a few years after an offshore island disappeared, probably related to dredging (Fig. 3).

Over the past three decades, the principal factor involved in evolution of the outer shoreline at Rushy Marsh Pond (loss of the hook) has been wave overwash of barrier spit remnants, pushing the sand into the abandoned, inactive inlet channel from Popponesset Bay, and onto the beach behind. Wave action around the diminishing hook caused short-term, localized erosion problems on the adjacent outer shore during the 1970s and 1980s, but these processes have completed their work and the present beach is now wider and more stable than before.
 
Part 2 addresses environmental conditions within the Pond itself, to identify the existing environmental issues, limits on what improvements can be expected with increased flushing, and conditions that will need to be addressed in engineering a suitably enhanced opening to the sea. Materials presented here are based largely on new field observations and calculations conducted expressly for this study.
 
Rushy Marsh Pond lies in a glacial moraine landscape (Fig. 4), in which kettle holes are variously filled with ponds, wetlands, or are drydepending upon their elevation, supply of water, and other factors. Rushy Marsh Pond has an area of 14.9-acre and is up to 7.5 feet deep (Fig. 5), with a surface-level elevation of about +2 to +3.3 feet (varying seasonally and in response to a drainage pipe through the barrier beach). The Pond contains up to 7.5 feet of mud, organic sand, and peat over the underlying gravelly glacial sand. During our study Pond water was highly stained a brown-red color, preventing light penetration beneath a few feet. This coloration is believed to result from dissolved tannic organic matter (probably from surrounding natural vegetation) rather than from aquatic microorganisms. Microscopic examination of a sample of discolored water revealed very few plankton algae. Nutrients in the pond water are not inordinately high.
 
Diurnal oxygen measurements support these observations, with a daily variation of less than 2 parts per million (ppm or mg/l) in the Pond watersuggesting moderate to low primary productivity and ecosystem respiration. The water is nearly fresh and usually well oxygenated to the bottom, with little stratification of temperature. During late summer and autumn when the pond surface elevation was low and tides in Nantucket Sound high, flow reversed tidally in the drainage pipe, introducing salt into the Pond. Measured salinities reached 6 parts per thousand (ca. 1/5th full-strength seawater.) During periods of salt stratification in the Pond, bottom levels of oxygen can become strongly reduced or entirely depleted (Fig. 6).
 
Nantucket Sound water adjacent to the Pond area has a salinity of 30.5 parts per thousand and transparency exceeding 6 feet. Diurnal oxygen variation is similar to that in the Pond (except in deep remnants of the former channel where seaweeds accumulate). Tidal range is typically from 1.7 to 3.4 ft. and correlates strongly with the tide measured at Nantucket Island, except tide range is 13% lower and a lag in high tide of 2 hours occurs.
 
The minimum stillwater elevation for ocean flooding across the dunes into Rushy Marsh Pond is +5 to +6 ft., with greatest flooding vulnerability across the sections of barrier beach at the north and south ends. All of Rushy Marsh Pond, the lowlands to the north and to the west, and the entire barrier beach lies within the 100-year flood zone mapped by FEMA. In addition, all of the barrier beach is also within the Velocity Zone, subject to storm wave activity.
 
Part 3 examines the technical feasibility of renewed exchange with the sea, and examines options for the design of a structure that would accomplish this goal. Since the closing of the Pond after 1911, there is no reason to believe an inlet would remain open at this site without structural enhancement.
 
Over the years several attempts have been made to re-open the connection to the sea, mainly for the purpose of draining the Pond to prevent flooding of surrounding lands and roads. These efforts have entailed a wooden flume (Fig. 7), and pipes of various diameters and lengths. These structures have crossed both the north and the south limbs of the barrier beach; and all of them have failed. Their failure resulted from too small a cross-sectional area; excessive length; inadequate engineered depth and scouring ability; siting in areas of active sand mobility; and a design impractical to maintain.
 
Nevertheless, a seasonal inlet structure is technically feasible. The preferred option suggested herein would incorporate aspects of successful natural and artificial inlets elsewhere: the appropriate balance between cross-sectional area and tidal prism; a cross-section form that promotes scouring; minimal inlet length; and siting where sand movement is minimal and existing features can provide shelter for the inlet channel. In addition, ease of maintenance is an essential design factor. Finally, it should be possible to easily close off the structure for storm surge flood control, inlet maintenance, and water quality management purposes. These factors would be incorporated into a new inlet sited at the former flume location, on the south limb at Oregon Way, where an easement for this purpose already exists.
 
Given the natural conditions at Rushy Marsh Pond, such an inlet would rapidly improve the Pond as a healthy coastal salt pond ecosystem. A restored alewife run could be expected and marine and estuarine fishes and invertebrates would return. Plant and animal biodiversity typical of a salt pond setting would become reestablished. Introducing seawater would flush out the tannic water currently filling the Pond. The new inlet would not significantly increase flooding danger; to the contrary it would alleviate seasonal flooding of neighboring lands and roads
 
Part 4 translates the above scientific and technical information into the realm of applicable regulations and funding that will, in the end, determine whether or not an inlet project can be built.

The overall purpose of this project is to provide environmental information for Rushy Marsh Pond and a conceptual plan for a new inlet to reconnect the Pond with the sea. More specifically, the goal is to develop a project which will improve tidal exchange with Nantucket Sound in order to address water quality issues for enhancing marine fisheries, while protecting environmental values. Other goals and benefits are to prevent and relieve flooding, reduce public health threats from mosquito-borne diseases and enhance biodiversity. Avid public support exists for such a project.
 
The preferred inlet option would be sited in the existing right of way across the south limb of the barrier beach near Oregon Way (Fig. 8). The inlet structure would consist of an open-top sluiceway built of pre-cast concrete sections. The sluiceway would be engineered to control or terminate flow, and to maximize ebb-flow scouring. The sluiceway would cross from the shore at the Pond side to the 4 MLW elevation contour location on the Sound side, with an engineered depth of about 2 feet MLW for the structure (Fig. 9). The unstructured channel connecting the sluiceway and the Sound, across the beach, would be scoured primarily by ebb flow from the Pond. A simple bridge of slab concrete across the sluiceway would provide owners access to adjacent private lands. This alternative out of four examined in depth provides the multiple benefits of marine fisheries, water quality and ecological enhancement, reduction of flooding, and public health threats. This alternative was also deemed feasible when analyzed under the eight feasibility factors used to assess each alternative, i.e., goals achievement, multiple benefits, environmental impacts/permitability, easements, capital costs, maintenance/operations costs, safety, and potential funding sources.
 
This project requires several permits from agencies responsible for the use and modification of coastal lands, wetlands, and waters. The following permits will be required: an Order of Conditions from the Conservation Commission, Certification from the Mass Environmental Policy Act (MEPA) Office, a Department of Environmental Protection (DEP) Section 401 Water Quality Certification, a DEP Ch.91 Waterways Permit and a new license or license amendment, a Mass Coastal Zone Management Consistency Concurrence, and a U.S. Army Corps of Engineers Section 404 PGP 11 or individual permit. In addition, due to agency discretionary power, a Development of Regional Impact may be required from the Cape Cod Commission, and an Environmental Impact Report may be required by MEPA. In conclusion, significant difficulties will be encountered in seeking needed permits; however, this project could be successfully implemented if arguments are accepted to restore marine fisheries and preserve functions of flood and storm damage control.
 
The cost estimate for the preferred alternative is $163,000 with yearly maintenance and operations costs of $8,472. Possible sources of support for the project include both technical and financial support at the local, regional, state, and federal levels. Funding sources include government agencies and nonprofits at all levels. The recommended funding strategy is to combine a variety of sources. Local non-profit funding now could be used to keep the project momentum going by developing a preliminary design, inventorying wetlands, completing pre-application regulatory review and developing and applying for funds from several sources. Several grant opportunities are now approaching and the Town should be targeted for requests in the fall. An estimated $10,000 now would provide the services to complete preliminary design and apply for construction funds.
 
 
This report was produced by The Coast & Harbor Institute and Robert L. Fultz Associates. The towns Project Monitor was Mr. Robert Gatewood, Conservation Administrator, Town of Barnstable.