Flow-Vegetation Interaction in a Living Shoreline Restoration and Potential Effect to Mangrove Recruitment

Hydrodynamic differences among shorelines with no vegetation, reference vegetation (mature mangrove), and vegetation planted on restored shoreline (marsh grass and young mangrove) were compared based on field observations 6.5 years after living shoreline restoration. Mean current velocities and waves were more strongly attenuated in vegetation (from channel to shoreline: 80-98% velocity decrease and 35-36% wave height reduction) than in bare shoreline (36-72% velocity decrease, 7% wave height reduction, ANOVA: p < 0.001). Normalized turbulent kinetic energy dissipation rates were significantly higher in reference vegetation (0.16 +/- 0.03 m(-1)) than in restored (0.08 +/- 0.02 m(-1)) or bare shoreline (0.02 +/- 0.01 m(-1), p < 0.001). Significant differences in the current attenuation and turbulence dissipation rates for the reference and planted vegetation are attributed to the observed differences in vegetation array and morphology. Although the hydrodynamic analyses did not suggest limitations to recruitment, mangrove seedlings were not observed in restored vegetation, while four recruited seedlings/m were counted in the reference vegetation. The lack of recruitment in the restored shoreline may suggest a lag in morphological habitat suitability (slope, sediment texture, organic matter content) after restoration. Although hydrodynamics suggest that the restored site should be functionally similar to a reference condition, thresholds in habitat suitability may emerge over longer timescales.