Tidal circulation and flushing characteristics of the Nauset Marsh System : report to the Town of Orleans

Abstract

Various interested bodies (i.e., National Park Servce, Cape Cod Commssion, and the Town of Orleans) charged with management of the Nauset Marsh system on Cape Cod, MA, commissioned a study of the estuarine circulation within the Nauset system. Recent signficant morphological changes in the system have changed mixing processes and residence times for the embayment. This study specifically addressed the differing water circulation and residence times arising from a migrating single inlet (dominant condition) and dual inlet (1992-1996) situations. These residence times are to be used by the Cape Cod Commission to identify nitrogen-sensitive sub-embayments based on various assumptions of build-out and nutrient loading. The Nauset Marsh system has experienced considerable development in recent years; proper management of this resource area requires knowledge of the consequences of such development. Application of field observations of bathymetry, sea surface elevation, temperature, salinity and currents, leads to better understanding the physics of the system. These data, analyzed in various forms, served as input data for a numerical, two-dimensional circulation model of the embayment. The circulation model provided flow and discharge data with which the residence times were calculated. Bathymetric measurements defined the volumes of the various sub-embayments to be used in the calculation of residence times. Residence times were calculated for six sub-embayments of the system, defined on the basis of their common hydrodynamic and morphologic characteristics. Two scenarios were evaluated: one for the present single-inlet system, which is near typical for most system states, and one for a dual inlet system such as existed for a period of time from 1992 through 1996. Residence times were evaluated for twelve cases, to demonstrate the range of residence times that can be defined based on varing assumptions. For instance, residence times can be defined on the basis of mean low water volumes or mean water levels, the latter being the more conservative (yielding a longer residence time). In addition, residence times depend on whether spring tides, neap tides, or average tidal conditions are used. We provide data on all three conditions: the neap tidal case is the most conservative in the sense of providing a longer residence time. This case can serve as the basis for flushing if conservatism is desired. Finally, residence time can be defined based on the amount of time it takes for water to renew itself with water from adjacent sub-embayments, or more conservatively assuming renewal from the offshore waters (which are presumed to be cleaner). Based on these various inputs, assumptions and calculations, residence times for Salt and Mill ponds under conditions of a single inlet are the longest of the various sub-embayments. Town Cove is still relatively quickly renewed, though not as fast as the main channels serving the system. Flow pattern under dual-inlet condition does seem to be partioned well, with the northern inlet serving the northern part of the system and the southern inlet serving the southern part of the system, with litte hydrodynamc communication between the two divisions. This new hydrodynamc behavior results in shorter residence times under dual inlets than under a single inlet. Calculations indicate that the slowest flushing occurs in Mill and Salt ponds. The main body of the embayment, consisting of narrow channels between well-flushed salt marsh and tidal flats, flushes rapidly. Two-dimensional calculations show that Town Cove also flushes relatively rapidly, on average. However, its greater depth and occasional temperature stratification create conditions which might accumulate nutrients in bottom sediments, which, when released, can cause decrease in water quality (such as plankton blooms). A more sophisticated low-trophic level ecosystem model combined with vertical hydrodynamic structure could clarfy the dynamics of this process. This study provides a defensible basis for evaluating nutrient loading and potential eutrophication arising from development in the watershed around Nauset embayment. However, since morphological changes occur on a rapid basis in this area, the issue of residence time should be re-examined periodically. For instance, rapid onshore migration of the southern barrier beach is threatening closure of the south chanel, a condition which could adversely affect water quality in Nauset Harbor in the near futue. A process should be established to examine the sensitivity of residence times for rapidly changing morphology.