Sources of Map Data

Changes to coastal tidal inundation and episodic coastal flooding were estimated by increasing current-day conditions by the projected changes to sea level for each scenario and year. Inundation was defined as land with elevations less than the local mean higher high water tidal datum. Episodic coastal flooding was defined as the FEMA 1% annual chance floodplain, also referred to as the “100-yr floodplain.” The extent of flooding was determined using standard flood modeling practices and high resolution/high accuracy topographic data. Flood depths were determined by subtracting water surface elevations from the topography.

Impacts were evaluated by intersecting the flood extents with building footprint data. First floor elevations were estimated using lowest and highest adjacent grade relationships for each structure, with grade elevations derived from the building footprint and digital elevation model. Flood depth was attributed to each structure, and then potential damages were estimated by application depth damage functions sourced from FEMA Benefit-Cost Analysis Flood Module. All structures were assumed to be slab-on-grade construction. Structures having basements were differentiated in the depth-damage function analysis. Damages were generalized into three categories: minor (>25% damages), moderate (>25%, <50% damages), and severe (>50%) damages.

Floodplain:

Floodplain elevations were provided by the Federal Emergency Management Agency (FEMA). Storm surge elevations were sourced from the regional storm surge modeling effort completed in 2011.

Elevation:  

Elevation data for floodplain and inundation modeling were sourced from Anne Arundel County. These data were collected by the Maryland Department of Natural Resources. The dataset was derived from countywide high-accuracy/high-resolution LiDAR ground elevations measured in 2004. The vertical accuracy of this data set was tested to have a root mean square error of 14.3 centimeters (5.6 inches). These data were processed from a tile format into a continuous elevation model.

Tidal Inundation:  

Ground elevations less than the elevation of the mean higher high water (MHHW) tidal datum were labeled “inundated.” MHHW is defined by NOAA as “the average of the higher high water heights of each tidal day observed over the National Tidal Datum Epoch.” The elevation was established using the NOAA Vdatum tool. This software application provides conversions between tidal and geodetic datums in overwater areas. A continuous MHHW surface for Anne Arundel County was developed through a standard application of this tool. 

Building Footprints:   

Building footprints were sourced from Anne Arundel County. These data were originally developed from 2002 orthophotography and later updated against 2007 orthophotography. Changes in the built environment subsequent to 2007 are not reflected in this data set.