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Publication Date

2013

Document Type

Honors Project

Department

Engineering

Keywords

Salt marshes-Massachusetts-Bourne, Bogs-Massachusetts-Bourne, Wetlands-Massachusetts-Bourne, Wetland restoration-Massachusetts-Bourne, Sea level-Forecasting, Sea level-Environment aspects, Salt marsh ecology, Hydraulic models, Environmental impact analysis, Wetland management, Wetland mitigation, Adaptive natural resource management, Salt marsh, Coastal wetland, Sea level rise, Sea level rise prediction, Salt marsh development, Resiliency to sea level rise metrics, Adaption strategies, Hydraulic modeling, Impact assessment, Adaptive management

Abstract

One of the threats that projects in coastal wetland restoration are facing today is rapidly rising sea levels. Under the Wetlands Reserve Program (WRP), the Natural Resources Conservation Service (NRCS) will be restoring a 23-acre inactive cranberry bog, Barros Bog, located in the town of Bourne, MA to historic salt marsh wetland hydrology. The NRCS has been provided with two restoration designs for the removal of two dikes creating tidal restriction at the Bog. An understanding of the general trends in the response of restoration design to different sea level rise (SLR) scenarios will allow NRCS and Town of Bourne to better prepare for SLR and focus their efforts in preservation and maintenance of the marsh when and where it is most needed. This thesis explores the response of the restored marsh at Barros Bog to sea level rise based on literature review and hydraulic modeling of the two proposed restoration designs. The relative SLR scenarios for Barros Bog was predicted for 2014-2100. For year 2063, it was determined that the tide levels would rise by 0.85, 1.5 and 2.15 ft under low, intermediate and high SLR rates. Similarly for 2100, three scenarios were 1.77, 3.34 and 4.93 ft. The 30-day tidal data measured at the Bog site was then shifted up by the predicted SLR amount to get future tide levels. This predicted tidal flow data was used as an input to a hydraulic model to analyze SLR response. Hydraulic modeling demonstrated that under low, intermediate and high SLR ii scenarios for 2063, Barros Bog has sufficient tidal flow that will allow the bog to move upstream within the bog boundary. The salt marsh will spread out to the entire bog by this time and thrive in the current in upland areas. However, the downstream marsh that currently exists in the bay may get submerged, depending on what the vertical build up will be in the marsh. If the bog were to be left as it is today, by 2063, the lower bog would be submerged and the marsh would move upstream but much less than the marsh would under the restoration designs. Under the intermediate and high scenarios for 2100, most of the bog will be inundated with maximum water surface elevations of 6.5 ft and 8 ft, which is above the 6ft threshold water depth where Spartina Alterniflora, a major salt marsh plant, can survive [Redfield, 1972]. As a result, the salt-marsh will submerge if the vertical build up in the bog is not able to keep up with SLR. None of the SLR scenarios increase flood risks to surrounding residential areas. But under the high SLR scenario in 2100, there are possibilities of water flooding into Puritan Road and Queen Sewell pond upstream of the bog. Based on hydraulic analysis, the lifetime of the Bog is suggested between 50-90 years depending on the changes in SLR rates. Any SLR greater than the 3.34 ft (predicted for intermediate SLR in 2100), or after any time beyond 90 years, the bog will be partially or fully submerged if no significant sediment accumulation takes place. Additional knowledge about salinity levels, primary production and the sediment accretion rate in the marsh post restoration can be combined with results obtained here to make a better prediction for SLR response from the bog. As SLR is not seen as an immediate threat to marsh sustenance, a proactive adaptive management plan that includes regular site monitoring is recommended. A nested adaptive management plan with the goal to provide and maintain habitat for endangered turtle species in the marsh has also been developed. The adaptive management plan focuses on establishment of the marsh for the first five years, monitoring and understanding the marsh for the next twenty-five years and finally re-evaluating the SLR response of the marsh beyond the thirty years in comparison to the response predicted in the model presented here. Data obtained from monitoring and any new knowledge about SLR rates should be used to revise the adaptive management plan and its timeline.

Language

English

Comments

vi, 86 p. : col. ill., col. maps Honors project-Smith College, 2012. Includes bibliographical references (p. [82]-86)

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