WAVE RUNUP AND COARSE SEDIMENT TRANSPORT ON MIXED-SEDIMENT BEACHES FOR COASTAL EVOLUTION MODELING

Within the Salish Sea, relative sea level rise (SLR) of 10-143 cm is projected by 2100. This is expected to cause current extreme water levels like the 100-year storms to occur more frequently. The influence of SLR will increase the risk of coastal flooding and erosion impacting local infrastructure and ecosystem habitats. Existing models used to predict water levels and sediment transport at the shorelines are largely derived from open coast sandy low sloped beaches rather than steep mixed-sediment (sand and gravel) beaches with limited fetch characteristic of the Salish Sea.

Here, we present field measurements and model tests of extreme event wave runup and substrate response on mixed-sediment beaches aimed to refine predictions of coastal change in the Salish Sea estuary. The study was conducted between March 2018 and March 2019 on Whidbey Island, WA where some of the highest bluff recession rates in the Salish Sea occur and a region vulnerable to wave impacts associated with long-period ocean swell and locally generated wind waves. Wave runup elevations were surveyed with high resolution (2-4 cm) differential GPS and remotely monitored with an Autonomous Real-Time Ground Ubiquitous Surveillance (ARGUS) video system. The offshore wave field was measured with a high-resolution pressure sensor. Coarse sediment movements were tracked with cobbles imbedded with Radio Frequency Identification tags and morphologic and substrate changes were monitored with repeat structure-from-motion photogrammetry. Observations indicate that modest (1 m significant wave height) storm wave events can lead to runup of up to 1 m despite relatively short period (e.g.,<9 seconds) and cobble transport was sensitive to location and elevation on the mid to upper beach face. These results aim to help refine wave runup models (e.g., Stockdon et al. 2006, Xbeach) and test the scales of roughness that are important to predicting wave impacts and coastal change in steep, mixed-sediment beach environments.