Groundwater Accelerates Sea Level Rise
Regardless of the causes, southeast Florida, with a population of 5.6 million (one-third of the State’s population), is among the most vulnerable areas in the world for climate change due its coastal proximity and low elevation (OECD, 2008; Murley et al. 2008), so assessing sea level rise (SLR) scenarios is needed to accurately project vulnerable infrastructure (Heimlich and Bloetscher, 2011). We know that sea level has been rising for over 100 years in Florida (Bloetscher, 2010, 2011; IPCC, 2007). Various studies (Bindoff et al., 2007; Domingues et al., 2008; Edwards, 2007; Gregory, 2008; Vermeer and Rahmstorf, 2009; Jevrejeva, Moore and Grinsted, 2010; Heimlich, et al. 2009) indicate large uncertainty in projections of sea level rise by 2100. Gregory et al. (2012) note the last two decades, the global rate of SLR has been larger than the 20th-century time-mean, and Church et al. (2011) suggested further that the cause was increased rates of thermal expansion, glacier mass loss, and ice discharge from both ice-sheets. Gregory et al. (2012) suggested that there may also be increasing contributions to global SLR from the effects of groundwater depletion, reservoir impoundment and loss of storage capacity in surface waters due to siltation. The loss of groundwater, mainly from confined aquifers, is troubling, and currently completely unknown. The contribution of carbon dioxide, commonly occurring in deep groundwater is also unknown.
To gauge the risk to property in southeast Florida, Southeast Florida Regional Climate Compact and Florida Atlantic University reviewed twelve different projections of SLR and its timing. The consensus was 3” to 7” by 2030 and 9” to 24” by 2060. From the literature review and analysis, it was concluded that approximately 3 ft. of sea level rise by 2100 would a suitable scenario and time frame to illustrate the methodology presented in this article. To allow flexibility in the analysis due to the range of increases within the different time periods, an approach that uses incremental increases of 1, 2, and 3 feet of SLR was considered for risk scenarios.
An issue normally ignored in sea level rise projections is groundwater. The importance of the groundwater table in the model is that it is responsible for determining the soil storage capacity. Soil is composed of solids, water, and air (voids). Soil storage capacity depends on physical and chemical properties, water content of the soil, and depth to the water table or confining unit (Gregory et al 1999). As the rain infiltrates the soil, unsaturated pores quickly fill up, effectively raising the water table (Gregory et al 1999). For example efforts, a groundwater surface elevation map was derived based well site information available from the USGS (http://groundwaterwatch.usgs.gov) that had a minimum of 35 years of continuous data. Using GIS, an inundation model was created in GIS by subtracting the groundwater surface model from the digital elevation model with the difference in elevation being the soil storage capacity. The photo shows the evolution of these features as applied to a section of northwestern Miami-Dade County. What this indicates it that the impact of sea level rise on low-lying inland areas may be far different that the projections using the bathtub models. It also means that wellfields, sewer mains, roadways and storm water systems will be affected far more quickly than projected from bathtub models. The method used here suggested that the estimated may be off by a factor of two of three.