ADAPTING COASTAL CITIES TO RISING GROUNDWATER (Invited Presentation)

Rising seas are expected to cause rising groundwater levels within several kilometers of the coast in some regions. Rising groundwater, as well as increases in groundwater salinity, poses significant risks to cities and associated nearshore ecosystems. Shallower groundwater could increase the rate and extent of infiltration in urban sewer pipes, remobilize existing soil contaminants, cause foundations to heave, and increase liquefaction risks in seismic regions before emerging as surface flooding. Most cities do not have maps of depth to groundwater, and are not aware of these risks. In addition, historically-vulnerable communities of color often reside in low-lying areas with the highest risk. The dynamics of changing discharge rates and spatial patterns of groundwater flow are currently poorly understood at the metropolitan scale.

Public agencies in the San Francisco Bay Area have begun to consider these impacts and a range of possible adaptation strategies. Expanded pumping raises new risks of subsidence where urban fill has been placed on former wetland soils. Alternative strategies include practical designs that have already been used in Dutch cities, such as excavating artificial canals and ponds, and constructing floating urban blocks. Extensive systems of managed ponds can also create habitat and recreational benefits, as well as opportunities to control water quality impacts. Canals can be used to manage groundwater elevation in adjacent areas, avoiding displacement of low-income residents. Cases from the San Francisco Bay Area will be used to provide examples. Siting innovative urban design strategies will require higher-resolution mapping of groundwater elevations and future flow directions. Impacts on health and property values from mold as well as soil contamination may already be observable, and a seismic event such as the Christchurch, New Zealand earthquake could occur at any time. Further research is urgently needed to predict groundwater flow dynamics in heterogeneous urban contexts.