FORECASTING COASTAL GROUNDWATER RESPONSES TO SEA-LEVEL RISE IN SEATTLE, WASHINGTON USA

Rising relative sea levels due to climate change and other processes can raise groundwater levels and alter groundwater-related processes. Shoaling groundwater levels may increase the risk of low elevation coastal areas to groundwater-related flooding, liquefaction hazards, and damage to infrastructure. Understanding coastal groundwater systems can help coastal communities manage, prevent, and prepare for these hazards. We developed groundwater flow models for the Seattle, Washington area to understand how the hydrogeology influences hazard feedbacks with sea-level rise, integrating the effects of the geologic, topographic, and hydroclimatic setting. The hydrogeology of Seattle, Washington, is a complex and heterogeneous framework of glacial-sourced sediment with perched and unconfined aquifers further fractured and complicated by local faults. Steep topographic relief protects much of the Seattle area from the direct effects of sea-level rise. However, low-lying areas, such as the Duwamish River basin, are at a higher risk of tidal and storm-based flooding, although the role of groundwater in such events is currently unknown. Hydroclimatic seasonality also creates periods of intense rainfall that could increase groundwater levels in wet season months, contributing to the severity of compound coastal flooding risks. Climate change projections indicate significantly increased recharge that could further exacerbate groundwater feedbacks in coastal flooding. We used the groundwater flow models to forecast the magnitude and frequency of these compounding roles of groundwater on hydrologic hazards in the Seattle area. We first needed to construct a three-dimensional hydrogeologic framework based on well-log lithologies. We then computed how the seasonal groundwater dynamics respond to changing climatic forcings for multiple sea-level rise scenarios. Preliminary results identify present-day and future regions with an increased threat of groundwater flooding exposure and changes in groundwater discharge patterns.