Proceedings of the 2019 Mississippi Water Resources Conference

Year: 2019 Authors: Martin S., Temple N., Palino G., Cebrian J., Sparks E.

In the wake of the Deepwater Horizon oil spill, large-scale breakwater projects have been constructed to restore and conserve marshes across the northern Gulf of Mexico. These breakwater projects are often termed living shorelines, due to the perceived increase in productivity around the breakwaters and within the fringing marsh shoreward of these structures. However, evaluations of the effectiveness of breakwaters at preserving natural shorelines are limited. To evaluate the effectiveness of large-scale breakwaters at protecting or restoring marshes in high wave energy environments, we conducted experimental plantings and a shoreline monitoring program landward of six year old breakwaters (OBW), recently constructed breakwaters (RBW), and reference no breakwater sites (NBW) along Bon Secour Bay, AL. The OBW, RBW, and NBW complexes cover 0.6km, 3km, and 1.2km of consecutive shoreline, respectively. Within the OBW and NBW sites, eight replicates of planted (4m2 of nursery grown S. alterniflora sods planted in checkerboard pattern), natural stand, and no vegetation treatments were randomly distributed throughout each site. Within the RBW sites, an additional planted design was also established (clumped plantings), yielding four shoreline vegetation treatments. Each plot was visited quarterly with a suite of vegetative measurements taken, including: percent coverage, species diversity, biomass, porewater DIN, and soil organic matter. Additionally, the perimeter of all of the natural S.alterniflora patches within each site was field mapped using an RTK GPS and validated with drone imagery to compare S.alterniflora area across breakwater treatments. Preliminary results indicate a positive effect of breakwaters beginning 5 years after construction on naturally colonized S.alterniflora, but no effect on planted vegetation using the fixed monitoring plot data. However, to date, the RTK GPS and drone imagery have shown no discernable effects of the breakwaters on enhancing shoreline vegetation. If these trends continue throughout the duration of the monitoring, they will show that large-scale breakwaters could have marginal effects of preserving and enhancing fringing marsh vegetation in high wave energy environments.