PROJECTIONS OF 2069 WATER TABLE AND SEAWATER INTRUSION IN MIAMI FLORIDA USING THE UNITED STATES GEOLOGICAL SURVEY’S URBAN MIAMI-DADE MODEL

Miami-Dade County’s coastal location, low elevation, and high water tables make it vulnerable to sea level rise. With sea level rising, water tables are increasing and leaving less room for water storage during rainfall, storm surge, and king tides events. When infiltrated water exceeds the storage capacity, it overflows to the surface and streets and causes flooding. Flooding is becoming more frequent and causing not just economic damage, but also health risk as groundwater intercepts infrastructure like septic tanks. Sea level rise also poses challenges for water managers due to saltwater intrusion.

To understand changes in water tables and saltwater intrusion, the United States Geological Survey (USGS) developed the Urban Miami-Dade Model (UMD) in 2016. This model is complex due to its many inputs; it uses Modflow-NWT and incorporates the Surface-Water Routing (SWR1) and Seawater Intrusion (SWI2) packages. SWR1 is designed to simulate surface-water flow and groundwater interaction in areas where there is significant management of surface water by canals and control structures as in Miami-Dade. The SWI2 package simulates variable density flows and is useful in plotting and understanding saltwater intrusion. In its original form, UMD was helpful to understand water table and seawater intrusion up to the year 2040, but there were no future scenario runs attempted beyond the year 2040.

In this research, we conducted runs from 1996 up to 2069 with two sea level rise scenarios. We updated coastal boundary conditions, rainfall, and land use data. Because the boundary conditions change dramatically with sea level rise over 75 years (the length of the desired simulation period), we decided to split the 75-year long period into four shorter runs, and we updated the boundary and initial conditions for each run. The future scenario runs 2055-2069 were performed in three different ways -- with historical rain, updated rain, and no pumpage.

We processed the results using Python scripts to create head and depth to water maps and used Excel spreadsheets to create cross-section maps. The maps are helpful to identify areas most vulnerable to flooding. Wet seasons head maps can be used to assess flood risk and to update the current Florida Building Code (FBC). Furthermore, this model can be adapted to other coastal areas.