Modeling Shoreline Rotation of Headland-bounded Littoral Cells due to Wave Climate Variability

A recent study by the USGS National Assessment of Shoreline Change project revealed an increase in erosion hazards along the U.S. Pacific Northwest coast since the 1960s and a pattern of counter-clockwise shoreline rotation within headland-bounded littoral cells (Ruggiero et al., 2013). Shoreline rotation at seasonal to annual scale has been hypothesized to be produced by the El Niño Southern Oscillation (ENSO) due to the generation of larger than average waves approaching the coast from more oblique angles during major El Niños. This change to the wave height and direction results in a greater alongshore component of wave energy flux and therefore a greater potential for strong gradients in longshore sediment transport in headland bounded littoral cells. Despite this relationship, the impact of an ENSO-driven increase in longshore sediment transport gradients on long-term shoreline change patterns has not been quantitatively explored. In this study, we first establish a relationship between ENSO and north eastern Pacific wave energy flux variability, and then use a one-line shoreline model to assess the effect of ENSO variability on seasonal to decadal coastal evolution. Analyses of long-term wave hindcasts reveal a wave climatology with southerly alongshore energy flux during summer months, and northerly flux during winter months. During major El Niño winters, the northerly flux is an order of magnitude greater than the climatological average. Additionally, decadal trends reveal net cumulative wave energy flux to the north, with values at 1 to 2 orders of magnitude greater than the annual variability. These conditions are used as inputs to a single-line contour model which computes gradients in longshore sediment flux, and the associated shoreline changes. Using this simplified approach we assess if gradients in longshore sediment transport is the most relevant process affecting morphological change on sandy beaches bounded by rocky headlands. We then vary the wave climatology to develop a probabilistic understanding of how these coastal systems may respond to future changes. Our approach allows us to investigate the relationship between offshore wave climate variability and littoral cell scale shoreline rotation to yield new insights into the dominant processes forcing shoreline evolution.