Final Report of Special Problem

Projections of global climate change over the next century indicate that multiple stresses to coastal ecosystems are expected. Sea level rise is one effect of climate change that may significantly alter current estuarine habitats, resulting in the need to modify current management strategies. A one dimensional (1D) hydraulic analysis was completed for the Caloosahatchee Estuary to determine the potential effects of sea level rise on the salinity distribution in the estuary. Typically, water quality analysis of estuaries is completed with sophisticated three dimensional (3D) models that are proprietary. HEC-RAS version 4.1 is relatively simple, publicly available software that has water quality analysis capabilities applicable to estuaries. We applied the 1D hydraulic and water quality capabilities of HEC-RAS to evaluate salinity distributions in the Caloosahatchee Estuary. The model was successfully calibrated and thus sufficient for scenario analysis of changing sea level boundary conditions. Results showed that under current management strategies, a 0.9 m rise in mean sea level could result in a 4.5 ppt increase at the point of regulatory compliance. Under those conditions, the total managed inflow to the estuary would need to be increased from 14.2 m/s to 22.9 m/s to maintain current habitats. Additionally, a 0.9 m rise in sea level could reduce the rate of salinity reduction in the estuary under high flow conditions from 0.50 ppt/day to 0.28 ppt/day, with no observable effect on the rate of salinity increase under no flow conditions. IMPORTANCE OF COASTAL ESTUARIES Estuaries are semi-enclosed bodies of water subject to both tidal and freshwater inflows. As a result, estuary ecosystems are comprised of organisms that are tolerant to variations in salinity resulting from incoming and outgoing tidal flows. Estuaries inhibit eutrophication of marine water bodies by utilizing the nutrients transported by freshwater inflow. Estuaries protect marine water bodies by removing contaminants from freshwater inflows as well (Kennedy et al. 2002). The high nutrient loading common to estuaries results in highly productive plant communities. The plant communities found in estuaries provide a highly productive habitat for marine organisms. Coastal estuaries in the United States provide habitat for 75 percent of commercially harvested fish and shellfish, as well as provide a significant food source for migratory birds that travel the central flyway (Environmental Health Center, 1998). ESTUARINE STRESSES FROM CLIMATE CHANGE Global climate change is projected to alter sea surface temperature, hydrologic processes, marine water quality, and mean sea level. It is estimated that increases in sea surface temperature of up to 3°C can be expected by 2100 (IPCC 2007). Increases in sea surface temperature have the potential to: (1) decrease dissolved oxygen; (2) increase dissolved oxygen demand by increasing the rate of organic matter loading resulting from an increase in biomass production and subsequent decay; (3) reduce habitat for cool water species such as macroinvertabrates. As outlined by the Intergovernmental Panel on Climate Change (IPCC 2007) it is projected that global climate change will increase storm intensity resulting in higher annual precipitation, but also increase the frequency of drought, with significant variations in climate patterns across the globe. Estuaries could be impacted from increased storm intensity due to increased contaminant loading from urban runoff, increased water column stratification caused by variations in density of freshwater inflows versus brackish/saline water found in the estuary, and flushing of organic matter and organisms out of the estuary during flood conditions. If the frequency of drought were to increase in a region, estuaries could receive reduced flows from viable freshwater tributaries, desiccation of wetland soils, toxic levels of hypersalinity for certain seagrass species, and the reduction in the inflow of nutrients and organic matter (Mulholland et al. 1997). The IPCC estimates that sea level rise over the next century will range between 18 and 81 cm (IPCC 2007). The IPCC estimates that sea level rise will exceed currently observed trends because of the accelerating effect of climate change with increased global temperatures. Increases in temperature are expected to cause expansion of ocean water, melting of glaciers and ice caps, and portions of the ice sheets on Antarctica and Greenland to slide into the ocean (IPCC 2007). The IPCC estimates of sea level rise were completed for two scenarios. The first scenario, called the low scenario, assumes economic growth will occur at a slower than current rate, and the global economy will shift to be more servicebased. The second scenario, called the high scenario, assumes global economic growth will occur at the current rate, and the global economy will maintain the current focus with a fossil fuel incentive. The expected range of sea level rise for the low scenario is 18 cm to 38 cm, and 25 cm to 58 cm for the high scenario. Neither scenario of sea level rise incorporates the rise in sea level associated with polar ice sheets sliding into the ocean, which would cause an additional rise of 23 cm. Therefore, the upper limit of the high scenario, coupled with movement of the polar ice sheets, would result in a sea level rise of approximately 81 cm. The effects of sea level rise will be most devastating to coastal zones. The EPA estimates that nationwide 5,000 square miles (12,950 square kilometers) of land is within 2 feet (0.6 m) of high tide (EPA 2010). Therefore, the projected sea level rise will cause a significant loss of shoreline, including coastal wetlands. In addition to land loss, sea level rise will increase shoreline erosion, increase flooding in areas that are directly hydraulically connected to the ocean, and increase salinity in estuaries, rivers, and coastal aquifers (EPA 2010). EFFECTS OF SEA LEVEL RISE ON ESTUARIES IN THE SOUTHEAST UNITED STATES Estimation of the effects of sea level rise on estuaries in the Southeastern U.S. has typically been completed using statistical analysis. There have been few studies that determined the potential effects of alterations in the hydrology due to climate change, or the alteration in salinity resulting from sea level rise (Marshall et al. 2008). A series of studies were completed by the U.S. Geologic Survey (USGS) that incorporated the hydrologic alteration occurring from climate change, and associated changes in salinity due to sea level rise. The studies were completed to determine the potential effects on drinking water sources in the Southeastern U.S. An Artificial Neural Network (ANN) was developed by Conrad et al. (2010a, b) to determine the potential effects of sea level rise on the Grand Strand of the South Carolina coast and the Lower Savannah River. The ANN was developed with between 15 and 20 years of hourly streamflow, water quality data, and water-level data. The projected alterations to current hydrologic processes were incorporated into the ANNs, and the projected increase in mean sea level was evaluated incrementally. The results of the study for the Grand Strand of the South Carolina coast estimated that, near a municipal freshwater intake, a sea level rise of 0.3 m would increase the frequency of specific conductance above 2,000 μS/cm to 4 percent, which is a two fold increase. A 0.6 m increase in mean sea level was estimated to increase the frequency of specific conductance concentrations above 2,000 μS/cm to 9 percent. The results for the study of the Lower Savannah River provided similar results, indicating the relative magnitude of salination of drinking water sources. An evaluation of the effects of sea level rise on the salinity distribution of Florida Bay was completed by the Army Corps of Engineers (ACOE). The effect of sea level rise was analyzed by direct and indirect methods. The direct method analysis consisted of the development of a Sea Level Affecting Marshes Model (SLAMM), and altering the mean sea level at Key West to determine the resulting modification in salinity distribution in Florida Bay. An evaluation of the dependence of the effects of sea level rise to water table elevation in the Everglades was completed as well. The indirect method analysis was completed by altering the salinity of an open Gulf monitoring station, and determining the effects on the salinity regime in Florida Bay using univariate models. The study estimated that the effects of sea level rise of 50 cm would result in the increase of 1-2 practical salinity units (psu) of near shore embayments, and that the salinity of Florida Bay was much more sensitive to freshwater stage in the Everglades than mean sea level (Marshall et al. 2008). In my study, my overall goal is to provide a simple evaluation of changing sea levels on an important estuary in southwest Florida, applying a 1D hydraulic simulation model to make relative comparisons of salinity distributions under a range of sea level rise scenarios.