Resource Assessment to Evaluate Ecological & Hydrodynamic Responses to Reinstalling a Water Control Structure in the Muddy Creek Dike
University of Massachusetts Dartmouth
School for Marine Science and Technology(SMAST)
Applied Coastal
Research & Engineering (ARCE)
FINAL REPORT - DECEMBER 2008
Study Purpose
The purpose of this project was to assess the wetland and habitat resource impacts associated with reinstallation of a dike in Muddy Creek. The dike would restore the upper Creek to a freshwater system for the purposes of natural nitrogen attenuation in a heavily overloaded subembayment of Pleasant Bay. The resource impacts were balanced against nitrogen attenuation and associated habitat restoration impacts to determine the overall feasibility of re-installation, in concert with other nitrogen management strategies under consideration by the towns.
The study provided critical baseline information on the current extent of wetlands and related resources in the vicinity of the shoreline of Muddy Creek, and documented anticipated changes to those resources and to private property and other upland areas bordering Muddy Creek should a water control structure (WCS) be installed within the existing earthen dike to enhance nitrogen attenuation in the upper portion of Muddy Creek. Finally, the report detailed a monitoring program that could be used if the dike were to be re-installed.
The concept of installing a water control structure was among a series of options identified by the Massachusetts Esutaries Project (MEP) for dealing with excessive nitrogen loading from surrounding land uses, which has resulted in severe eutrophication in Muddy Creek. Background on how community efforts to study and address nitrogen loading and other water quality concerns led to consideration of the WCS and other remediation alternatives is included in the report.
The significant nutrient impairments of the upper and lower basins of Muddy Creek are well documented. Muddy Creek nutrient related water quality has been studied by the Town of Chatham and the Alliance for almost a decade. Habitat assessments and water quality modeling by SMAST and ACRE scientists and engineers for the Town of Chatham (2000) and later by the MEP confirmed a significant level of nitrogen impairment and loss of habitat that requires a reduction in watershed nitrogen load for restoration of this system. Total Maximum Daily Loads (TMDL) established for upper and lower Muddy Creek require a 75% and 100% reduction in watershed septic load, respectively.
The report identified many possible ways to address nutrient-related habitat impairment: enhancing tidal flushing, enhancing freshwater attenuation, sewering, and other measures to control the flow of nutrients from watershed sources. As part of the analysis of restoration alternatives, the partial conversion to freshwater, was identified as having the greatest improvement in flushing along with ability to preserve salt marsh in the Lower Creek. The point where this separation was modeled was approximately at the location of the pre-existing earthen dike. This effort was a screening analysis to determine if further investigation was worthwhile and it was envisioned that further study would be needed to fully evaluate the concept. This study, along with parallel studies to evaluate other restoration alternatives, such as enlarging the Route 28 culverts, will help inform the towns’ selection of the appropriate remediation strategy.
It is important to monitor the response of the Muddy Creek System should the water control structure be reinstalled in the dike at mid-estuary. Monitoring will support both "adaptive management" of the system, where it may be necessary to refine the water level in the upper basin over the first few years of emplacement. In addition, confirming the attenuation of nitrogen within the upper basin will aid the Towns in meeting the MassDEP TMDL for this system and water quality measurements are needed to confirm compliance. The five year monitoring plan that is recommended after the installation of the Dike is summarized below.
