Amphibious City: Sustainable Adaptations to Sea Level Rise in Seattle���s Interbay Area

Thesis (Master's)--University of Washington, 2016-06

Development of a Conceptual Site Model for Evaluating Seawater Intrusion under Sea Level Rise Scenarios Using Analytical Methods

Island County is located in the Puget Sound of Washington State and includes several islands, the largest of which is Whidbey Island. Central Whidbey Island was chosen as the project site, as residents use groundwater for their water supply and seawater intrusion near the coast is known to contaminate this resource. In 1989, Island County adopted a Saltwater Intrusion Policy and used chloride concentrations in existing wells in order to define and map ���risk zones.��� In 2005, this method of defining vulnerability was updated with the use of water level elevations in conjunction with chloride concentrations. The result of this work was a revised map of seawater intrusion vulnerability that is currently in use by Island County. This groundwater management strategy is defined as trigger-level management and is largely a reactive tool. In order to evaluate trends in the hydrogeologic processes at the site, including seawater intrusion under sea level rise scenarios, this report presents a workflow where groundwater flow and discharge to the sea are quantified using a revised conceptual site model. The revised conceptual site model used several simplifying assumptions that allow for first-order quantitative predictions of seawater intrusion using analytical methods. Data from water well reports included lithologic and well construction information, static water levels, and aquifer tests for specific capacity. Results from specific capacity tests define the relationship between discharge and drawdown and were input for a modified Theis equation to solve for transmissivity (Arihood, 2009). Components of the conceptual site model were created in ArcGIS and included interpolation of water level elevation, creation of groundwater basins, and the calculation of net recharge and groundwater discharge for each basin. The revised conceptual site model was then used to hypothesize regarding hydrogeologic processes based on observed trends in groundwater flow. Hypotheses used to explain a reduction in aquifer thickness and hydraulic gradient were: (1) A large increase in transmissivity occurring near the coast. (2) The reduced aquifer thickness and hydraulic gradient were the result of seawater intrusion. (3) Data used to create the conceptual site model were insufficient to resolve trends in groundwater flow. For Hypothesis 2, analytical solutions for groundwater flow under Dupuit assumptions were applied in order to evaluate seawater intrusion under projected sea level rise scenarios. Results indicated that a rise in sea level has little impact on the position of a saltwater wedge; however, a reduction in recharge has significant consequences. Future work should evaluate groundwater flow using an expanded monitoring well network and aquifer recharge should be promoted by reducing surface water runoff.

Surface Elevation Change Processes in the Snohomish River Estuary, Washington

Estuaries provide crucial ecosystem functions and contain significant socio-economic value. Within Washington State, estuaries supply rearing habitat for juvenile salmon during their transition period from freshwater to open sea. In order to properly manage wetland resources and restore salmon habitat, the mechanisms through which estuaries evolve and adapt to pressures from climate change, most notably eustatic sea level rise, must be understood. Estuaries maintain elevation relative to sea level rise through vertical accretion of sediment. This report investigates the processes that contribute to local surface elevation change in the Snohomish Estuary, conveys preliminary surface elevation change results from RTK GPS monitoring, and describes how surface elevation change will be monitored with a network of RSET-MH���s. Part of the tidal wetlands within the Snohomish River Estuary were converted for agricultural and industrial purposes in the 1800���s, which resulted in subsidence of organic soils and loss of habitat. The Tulalip Tribes, the National Oceanic and Atmospheric Administration (NOAA), Northwest Indian Fisheries Commission (NWIFC), and the Environmental Protection Agency (EPA) are conducting a large-scale restoration project to improve ecosystem health and restore juvenile salmon habitat. A study by Crooks et al. (2014) used 210Pb and carbon densities within sediment cores to estimate wetland re-building capacities, sediment accretion rates, and carbon sequestration potential within the Snohomish Estuary. This report uses the aforementioned study in combination with research on crustal movement, tidal patterns, sediment supply, and sea level rise predictions in the Puget Sound to project how surface elevation will change in the Snohomish Estuary with respect to sea level rise. Anthropogenic modification of the floodplain has reduced the quantity of vegetation and functional connectivity within the Snohomish Estuary. There have been losses up to 99% in vegetation coverage from historic extents within the estuary in both freshwater and mesohaline environments. Hydrographic monitoring conducted by NOAA and the Tulalip Tribe shows that 85% of the historic wetland area is not connected to the main stem of the Snohomish (Jason Hall 2014, unpublished data, NOAA). As vegetation colonization and functional connectivity of the floodplains of the Snohomish estuary is re-established through passive and active restoration, sediment transport and accretion is expected to increase. Under the Intergovernmental Panel on Climate Change (IPCC) ���medium- probability��� scenario sea level is projected to rise at a rate of 4.28 mm/year in the Puget Sound. Sea level rise in the Snohomish Estuary will be exacerbated from crustal deformation from subsidence and post-glacial rebound, which are measured to be -1.4 mm/year and -0.02 mm/year, respectively. Sediment accretion rates calculated by Crooks et al. (2014) and RTK GPS monitoring of surface elevation change of the Marysville Mitigation site from 2011-2014 measured vertical accretion rates that range from -48-19 mm/year and have high spatial variability. Sediment supply is estimated at 490 thousand tons/year, which may be an under-estimate because of the exclusion of tidal transport in this value. The higher rates of sediment accretion measured in the Snohomish Estuary suggest that the Snohomish will likely match or exceed the pace of sea level rise under ���medium-probability��� projections. The network of RSET-MH instruments will track surface elevation change within the estuary, and provide a more robust dataset on rates of surface elevation change to quantify how vertical accretion and subsidence are contributing to surface elevation change on a landscape scale.