The Marine Modeling and Analysis Programs (MMAP) branch of the Coast Survey Development Laboratory is using hydrodynamic models to simulate coastal flooding for a variety of processes. The purpose of these models is to provide accurate water levels both over land and water due to extreme events. Coastal inundation has a significant impact on both the natural and built environment. Understanding how and where this flooding may occur can enable coastal managers to mitigate coastal hazard risk.
Coastal inundation modeling is the application of computational models to determine the extent of flooding due to a wide range of processes. High water levels are a significant coastal hazard threatening coastal communities that requires mitigation planning. However, processes that contribute to coastal inundation vary widely. They include long term sea level rise, annual climate variability, monthly tidal cycles, and short term meteorological events. Furthermore, their effects can vary significantly across an ecosystem due to the local topography. Therefore, high resolution computational models are being used to simulate coastal inundation by representing the land and water areas affected by these events.
MMAP scientists are using unstructured grids to create highly refined models of the coast in order to simulate flood propagation. The complex geometry of the shoreline includes many natural and anthropogenic features such as barrier islands, tidal inlets, jetties, and levees. The flexible nature of an unstructured grid allows for these features to be accurately represented in the computational model by using varying nodal spacing and element size. This results in a model that can simulate inundation in the complex coastal environment by accurately representing the features that exist.
Modeling of coastal water levels depends not only on the resolution of the topography but also on the elevation data used to populate it. The grids developed by MMAP use the latest available hydrographic surveys to provide bathymetric depths. This is combined with the best available topographic data, which could include high resolution LIght Detection And Ranging (LIDAR) data, the United States Geological Survey (USGS) National Elevation Dataset (NED), and other sources.
Bathymetric data are generally relative to local tidal datums such as Mean Lower Low Water (MLLW), while topographic data are relative to ellipsoidal or orthometric datums such as the North American Vertical Datum of 1988 (NAVD 88). In order to construct a continuous bathymetric/topographic dataset, the VDatum vertical datum transformation tool is used to adjust all elevation data relative to a common datum such as NAVD 88. This common vertical datum allows for inundation to occur across a continuous elevation dataset at the land/water interface. This is key to accurately predicting flooding depths at the coast.
Inundation Modeling Projects
MMAP is cooperating with several NOAA partners to study the capability of unstructured grid finite element hydrodynamic models to model hurricane storm surge. Developmental projects are underway for the North Carolina barrier island/sound system and the Florida Panhandle/Mobile Bay region. These projects use a highly refined computational grid populated by a continuous elevation dataset. Boundary conditions and meteorological forcing from operational NOAA models are being tested to examine model skill for historical storms such as Isabel (2003) and Ivan (2004).
MMAP is also working with the National Centers for Coastal Ocean Science (NCCOS) and external academic partners to study the ecological effects of sea level rise. A pilot project is in development for the North Carolina sound system to combine a hydrodynamic model of water level and current with ecological models of landscape scale habitat change. The hydrodynamic model is based upon an unstructured grid and continuous elevation dataset and provides water level time series and potential flooding as a result of sea level rise. Long-term scenarios of sea level rise can be simulated to identify areas at potential risk and provide coastal managers with tools to assess possible changes.