Presenter: Richard Styles (U. S. Army Engineer Research and Development Center)

Description:
Shoreline erosion due to commercial vessel wakes is well-documented along deep-draft navigation channels, but analogous erosive impacts remain unquantified for recreational vessels traversing interior waterways. Given the importance of coastal wetlands for storm protection and aquatic habitat, evaluating the contribution of small watercraft to marsh scarp retreat addresses a key knowledge gap for coastal management and restoration. This project relates wake characteristics to the normal force on vertical marsh scarps using data from several interior waterways in the United States, including a site near St. Augustine, FL, where rapid marsh scarp retreat has been observed. At each location, wake energy was characterized using a shore-normal profile of bed-mounted pressure sensors, and corresponding wake force measurements were collected using an array of submersible strain-gauge load cells mounted vertically on the scarp. These data indicate an order-of-magnitude relationship between wake energy flux and the impact pressure on the scarp. However, the results are strongly dependent on tidal stage, and the largest wake heights did not typically generate the largest impact forces. Peak forces on the scarp correspond to still-water elevations below the midpoint of the scarp, when breaking is most common. For still-water elevations above the scarp midpoint, wakes tend to reflect off the scarp without breaking; this results in smaller impact forces on the scarp face. These results suggest that scarp undercutting could be an important mode of wake-induced marsh erosion at fetch-limited sites. The undercutting process may be exacerbated in vegetated systems because the root mass helps to maintain the upper scarp’s stability even as the lower scarp is undercut by wake energy. Low-elevation breakwaters and nature-based features should therefore be evaluated as cost-saving yet equally effective alternatives to features which dissipate wake energy over the full tidal range.