Influence of living shoreline elements on wave run up elevations

Abstract

Nature-based coastal protection, also known as engineering with nature or living shorelines, is becoming increasingly popular due to its dual benefits of reducing coastal flooding and providing ecological and recreational opportunities. In many coastal areas experiencing chronic erosion, changes in sediment supply, composition, and grain size are significant contributing factors to shoreline recession. One living shoreline strategy to consider includes the application of cobbles over more traditional sand nourishments. On sandy beaches that experience high-energy wave conditions, the introduction (or reintroduction) of cobbles can mitigate backshore erosion. Cobble-backed beaches have been found to mitigate the effect of coastal erosion and flooding in laboratory settings and field observations, and they have recently been piloted in locations such as Cape Lookout State Park in Tillamook County, Oregon, and Surfers Point in the City of Ventura, California. However, there are no formal engineering guidelines stipulating the calculation of wave run-up on cobble-backed beaches. This study applies three different wave run-up equations on a living shoreline design (i.e. mixed sand and cobble berm-backed beach) in Malibu, California, and compares the predicted run-up levels with existing condition flood levels for typical and eroded conditions. The different wave run-up equations were designed for cobbles only, revetments, and composite beaches, respectively, where the composite beach equation was most applicable to project design. For typical beach conditions (higher levels of sediment accretion resulting in shallower beach face and berm slopes), all three equations showed a reduction in wave run-up values. When applied to worst-case conditions (i.e. scoured by a creek channel and steeper fronting beach slopes), the equation most applicable to the design showed the highest reduction of total water levels. A sensitivity analysis found that the cobble-backed beach equation predicted the most consistent values of run-up (run-up values changed the least), even when input parameters (slope and water depth) changed. This study shows that cobblebacked beaches hold promise to mitigate coastal flooding in appropriate areas, in addition to being a natural solution for areas experiencing erosion. This study also points to the need for more studies and field observations to validate the run-up levels determined here.