Hydrodynamic Impact of Boat Wakes to Shoreline Ecotones: A Field Experiment and Proposed Boat Wake Hazard Model

Abstract:

Vegetation fringing open water bodies provides shoreline defense during extreme events but can be subject to erosive removal when magnitude and frequency of hydrodynamic forces exceed shoreline resistivity. Boat wakes may represent a significant hydrodynamic forcing to some estuarine shorelines but are often poorly characterized. In this study, we investigate the hydrodynamic impact of boat wakes on nearshore velocities and waves by analyzing boat wakes generated in-situ from 50-200 m of a shallow estuary shoreline in Mosquito Lagoon, the northernmost water body of the Indian River Lagoon system on Florida’s east coast. Nearshore orbital velocities and wave heights induced by boat-generated waves are compared to ambient conditions (tidal current and wind-driven waves) along a gradient of distance from shoreline to parameterize the effect of offshore generation distance on incoming wave energy at the shoreline. Findings indicate that even boat wakes generated from the furthest offshore distances measured in this study (~200m), produce an appreciable hydrodynamic signature at the shoreline. Wake event duration and number of waves per wake event increase with distance from shore, while wake magnitudes (peak wake-induced velocities and wave heights) are significantly greater when wakes are generated closer to shore. Although magnitudes of offshore-generated wakes are smaller, the prolonged propagation time suggests that they may have an equally detrimental effect on shoreline stability. This finding could suggest that offshore boat traffic may have potential to induce shoreline sediment erosion. The potential findings will allow for understanding relative risk of boat wakes within estuarine shorelines. A model is proposed to characterize the likelihood of wave hazard to shorelines with comprehensive boat wake data and calculate an index of potential boat wake hazards for an area depending on frequency and magnitude of boat passes. The potential for boat wake hazards is calculated at each shoreline grid from the summation of composite risk using a distance decay function. We find that locations of high boat wake risk, as indicated by the model, are well correlated with locations of severe erosion.