SIMULATIONS OF GROUNDWATER INUNDATION INDUCED BY SEA-LEVEL RISE AND HIGH TIDES REVEAL WIDESPREAD FLOODING IN THE WAIKIKI AREA ON OAHU, HAWAII

According to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), global mean sea level is expected rise by as much as 0.98 m by the year 2100. However, global ice loss is currently exceeding researchers’ expectations, indicating that rates of sea level rise (SLR) will surpass IPCC projections. Hydraulic connectivity between the ocean and coastal groundwater, in conjunction with SLR, will produce flooding in low-lying areas regardless of surficial connection to the coastline as groundwater levels progressively breach the land surface. Such flooding will be exacerbated during spring-tide stages and diminished during neap-tide stages. Here, we describe a modeling approach that simulates narrowing of unsaturated space and flooding generated by SLR-induced lifting of coastal groundwater. The methodology combines terrain modeling, groundwater monitoring, estimation of tidal influence, and numerical groundwater-flow modeling to simulate future flood scenarios considering user-specified tide stages and magnitudes of SLR. We apply the methodology to the Waikiki area of Honolulu, Hawaii and find that SLR of nearly 1 m generates GWI across 23% of the 13 km² study area, threatening $5 billion of taxable real estate and 48 km of roadway. We further compare the method to alternate approaches used to simulate GWI including hydrostatic modeling and production of 1D analytical solutions employed by Rotzoll and Fletcher (2013).