Living shoreline stabilization has become a popular practice in shoreline restoration and bank protection; however, there are still many uncertainties regarding effective site design using living materials. For example, natural wave-breaks may be formed of created reefs, but the optimum water depth for hydrodynamic influence may differ from the preferred depth to ensure organism recruitment. The objective of this research is to understand how water depth relative to the crest of submerged artificial oyster reef structures influences nearshore hydrodynamic processes and sediment transport or retention in nearshore areas. A field study, sited in a microtidal estuary on the Atlantic coast of Florida, applied a controlled, two-way block experimental design testing varied configurations of oyster reef structures constructed by bagged oyster shell: continuous vs. gapped structures located close (3 m) and far (12 m) from the edge of existing black mangrove vegetation. Bed elevations, sediment texture and organic matter content, and hydrodynamic data were measured in each treatment and control plot and compared before and after structure implementation over three years. Distance from shore influenced reef submergence such that far from shore structures were sited in deeper water and subtidal while closer to shore structures were in shallower water and were intertidal. The difference in water level over reefs influenced wave attenuation rates from the channel to shoreline, both at the individual plot level and across the treatments. More variable and lower attenuation rates were observed over structures far from shore in deeper water (range of mean attenuation, η = -6 – 36%). Shallower nearshore structures were associated with greater mean attenuation rates (η = 30 – 70 %), which also outpaced attenuation observed before structure placement (η = 0 – 10%). Repeat Real Time Kinematic (RTK) surveys before and after reef creation indicated that sizable sediment deposits formed in the areas between the created reefs and the shoreline, totaling a net gain of 418.5 m3 of sediment, an approximate mean deposition depth of 7.9 cm, within three years of reef creation. Accretion at gapped and continuous reef plots were similar whereas accretion behind far from shore reefs was about 20 % greater than behind near to shore reefs. Texture of nearshore sediments coarsened across all plots after structure placement, changing from majority fine sand (150-250 μm) to medium sand (250-500 μm). For example, fine sand composed of 69 % of nearshore sediments prior to structure placement, but only 11 % after three years. Additionally, mean organic matter content of shoreline sediments increased three folds, from 10.6 g/kg to 32.0 g/kg, while no change was documented seaward of reef structures. Study results can be applied to improve designs of created oyster reef as self-sustaining natural infrastructure. The results suggest that design water levels and thus placement of created reefs relative to shoreline vegetation can be flexible, as the far from shore reefs enhanced sediment retention and deposition nearshore despite the lower wave attenuation. Greater flexibility in living shoreline designs allowed under common permits would allow artificial reefs to be sited to better optimize conditions for oyster recruitment, which will ensure the longevity and ecological function of the infrastructure.
| Identifer | oai:union.ndltd.org:ucf.edu/oai:stars.library.ucf.edu:etd2020-2450 |
| Date | 01 January 2022 |
| Creators | Vien, Peter |
| Publisher | STARS |
| Source Sets | University of Central Florida |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Electronic Theses and Dissertations, 2020- |
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