WRRC Seminar

November 21, 3:00pm - 4:15pm
Mānoa Campus, POST 126

Modeling specific groundwater problems in Hawaii: Borehole flow and groundwater inundation

Two specific problems involving localized groundwater flow processes are presented as examples of groundwater models at different scales and different approximations: Vertical flow of groundwater through monitor wells and increased coastal flooding from groundwater-table rise with sea level.

The Pearl Harbor aquifer in southern Oahu is one of the most important sources of freshwater in Hawaii. Salinity profiles collected from uncased deep monitor wells (DMWs) commonly are used to monitor freshwater-lens thickness. However, vertical flow in DMWs can cause the measured salinity to differ from salinity in the adjacent aquifer. Substantial borehole flow and displacement of salinity in DMWs over several hundred feet have been observed in the Pearl Harbor aquifer. A numerical modeling approach incorporated hydraulic characteristics and flux rates representative of the Pearl Harbor aquifer. Borehole flow caused by vertical hydraulic gradients associated with both the natural regional flow system and groundwater withdrawals was simulated. Model results indicate that, with all other factors being equal, greater withdrawal rates, closer withdrawal locations, or higher hydraulic conductivity of the well cause greater borehole flow and displacement of salinity in the well. Borehole flow caused by the natural groundwater-flow system is five orders of magnitude greater than vertical flow in a homogeneous aquifer.

In the wake of climate change, sea-level rise will impact many coastal areas in Hawaii. Global mean sea level may rise about 1 ft by 2050 and 3 ft by 2100, and cause substantial coastal erosion and flooding. However, it is widely unrecognized that low- lying coastal areas may also be vulnerable to groundwater inundation, which is flooding due to a rise of the groundwater table with sea level. Vulnerability to groundwater inundation in southern Oahu was assessed and results indicate that 0.6 m of potential sea level rise causes significant flooding. In fact, the flooded area considering groundwater inundation more than doubles previous predictions of flooding with future sea-level rise. Groundwater inundation has wide-reaching consequences for decision- makers, resource managers and urban planners, and requires a revised approach to adapt to climate change.

Event Sponsor
Water Resources Research Center, Mānoa Campus

More Information
Philip Moravcik, 808-956-3097, morav@hawaii.edu

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