The Oyster Pond Environmental Trust Newsletter Fall 1996
OPET, P.O. Box 496, Woods Hole, MA 02543-0496
The following are quotes taken from Hydrodynamic Study and Weir Design, Oyster Pond, MA September, 1996 by Aubrey Consulting, Incorporated. The Pond Management Committee is presenting this by way of an update of the progress for the design and installation of the planned weir. If you would like to see the entire report, Barry Norris and Carl Breivogel each have a copy. [To improve readability, water levels have been converted here to mean ocean level (MOL)].
-B. Norris
The rationale behind the proposed pond management strategy is to return Oyster Pond to the ecologically stable system that existed prior to man-induced changes in the early 1980s. As the first step of the pond management, a weir has been designed to inhibit tidal exchange between Vineyard Sound and Oyster Pond for all but the higher Spring tide conditions. The intent of this structure is to stabilize the salinity conditions within the Pond and maintain a long-term average value between 2 and 4 ppt. In addition, dredging activities required in the Trunk River channel will allow tidal waters to enter the lagoon during most high tides. Allowing an increase in tidal action through this region will prevent fine sediment deposition and should improve water quality in this region.
...the maximum elevation between the upstream and downstream water levels (at the weir) is 0.30 feet, with a mean difference of 0.18 feet. [due to a slight tidal effect in the lagooned]
"...The formation of the sill (at Trunk River) inhibits the exchange of tidal waters between Vineyard Sound and the Oyster Pond system. To change the hydraulic control from this sill to the proposed weir, dredging of the Trunk River channel to an elevation of 0.6 ft above MOL or below will be required. Having the hydraulic control moved upstream to the Oyster Pond outlet will allow the lagoon to maintain the existing salt marsh plant species...
The weir also will prevent storm surge flood waters from entering Oyster Pond until the water elevation in Vineyard Sound exceeds 0.9 above MOL. During typical storm conditions, the weir will prevent pond flooding more effectively than the existing Trunk River system. However, severe storm surge conditions can overwash the barrier beach separating Oyster Pond from Vineyard Sound. During these severe conditions, the weir could inhibit the super-elevated pond from draining rapidly. Therefore, the weir has been designed with a removable gate to enhance relaxation of flood waters following severe storm conditions.
Concerns about anadromous fish also were considered. The weir design will cause a maximum height differential between the pond and lagoon of 0.3 ft. This differential is within the range of typical fish ladder steps utilized for herring runs along the south shore of Cape Cod...
...This system will emulate the Oyster Pond conditions prior to the man-induced changes in the early 1980's. Once the weir is installed and the Trunk River channel is dredged, the following changes can be expected:
Trunk River and the lagoon area will be subjected to more frequent tidal exchange with Vineyard Sound, where nearly half the high tides will cause some flow reversal in Trunk River.
...Once a stable salinity regime has been established, seasonal and/or post-storm review of salinity data should be utilized to adjust the weir elevation..."
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