East Harbor Water Level Manipulation

East Harbor is a back barrier coastal lagoon and salt marsh within Cape Cod National Seashore (Massachusetts, USA) that has been undergoing partial tidal restoration since 2002. The current tidal exchange has been sufficient to elevate salinities in the open lagoon but is still too constrained by the present infrastructure to create high tides sufficient to flood the peripheral marsh areas. Consequently, in cooperation with the Town of Truro, an adaptive management strategy using a reverse one-way tide gate was implemented in 2011 to increase lagoon water levels so that portions of the peripheral marsh could be flooded in a way that let high tides into the system while blocking their escape. The increased flooding of the marsh, above and beyond what the current engineering of the system could provide by opening the restrictive culvert, in June through August raised porewater salinities in many areas and resulted in decreases in the cover of fresh- and brackish-water plant taxa - a necessary precursor for the establishment and expansion of native halophytes.

 

Diagram of reverse tide gate installed in East Harbor

photo by Stephen Smith

 

Salt killed cattail (July 2011) after approximately 1.5 months of reverse tide gate operation in East Harbor.

photo by Stephen Smith

 

The use of a reverse tide gate in East Harbor has accelerated the process of restorative vegetation change that could not be achieved with the present structural limitations on tidal flow. The ideal solution is to facilitate even more tidal exchange so that water levels may reach elevations similar to those obtained using the one-way gate. In the case of East Harbor, the construction of a larger conduit (e.g., multiple culverts, open channel, etc.) is limited by the amount of undeveloped land that could be used for such purposes. Thus, an adaptive management plan centered on manipulating water levels could be quite useful from the standpoint of advancing restoration while any future changes in tidal exchange capacity are deliberated. From a broader standpoint, this technique might be applicable to other locations with similar conditions and/or management history.

 

Increased flooding height and duration did not adversely impact the native halophyte Spartina alterniflora (cordgrass) during the summer of 2011.

photo by Stephen Smith

 

For more information about this project please contact Stephen Smith (Plant Ecologist) at e-mail us.

 

Areas of salt water inundation through the use of the reverse tide-gate in 2011.

photo by Stephen Smith

 

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