Riverine Flooding in Wissahickon Creek – Photo by Steven-L-Johnson on Flickr
A flood is the overflowing of a large amount of water beyond its normal confines. Flooding is a pressing problem in Philadelphia with several causes; a problem that is likely worsening as the climate changes. Geographic Information System mapping can visually exhibit the risks associated with flooding in the city.
One viewpoint through which to analyze the impacts of flooding is the Flood Insurance Rate Map (FIRM) database established by the Federal Emergency Management Agency (FEMA). The database depicts flood risk information and supporting data based on the 1-percent-annual-chance flood event (i.e. a “100 year flood”), and the 0.2-percent-annual-chance flood event (i.e. a “500 year flood”). The FIRM database is a geospatial translation of the flood hazard analyses published by FEMA. The dataset includes geospatial and tabular information such as hydrographic features (surface water), regions at risk of 18-inch coastal waves (modeled based on topography and vegetation), insurance risk zones (based on 100-year flood risk), flood control structures, levees, and pertinent geographic barriers such as political jurisdiction and measurement lines. FEMA does not provide the specific definition of a “flood control structure” but describes barriers such as flood walls as being one type of flood control structure.
Further risk data can be attained through localized analyses. The Pennsylvania Statewide Flood Study, made available by the Pennsylvania Emergency Management Agency (PEMA) estimates economic loss and housing damage due to flooding across the state. The corresponding flood study generated models of five scenarios (10, 50, 100, 200 and 500-year floods) based on streams and rivers with a drainage basin area of 10 square miles or more. Flood impact predictions are divided by census block.
Economic losses data from this dataset can be displayed based on total cost, building damage, content damage, and more. Damaged homes data can be displayed based on total number damaged, substantially damaged homes, and homes by percent damage for each of the affected census blocks. “Substantially damaged homes” are defined as structures that reside within the 1-percent-annual-chance floodplain for which the total cost of repairs is 50 percent or more of the structure’s market value before the disaster occurred. This designation does not include land value and does not depend on the specific cause of damage.
Other flood infrastructure can be considered in addition to the flood control structures and levees provided by the FEMA dataset. Specifically, the Philadelphia Water Department’s Green Stormwater Infrastructure (GSI) can be mapped across the city. Several datasets demonstrate these projects. The datasets I utilized were private GSI retrofits, public GSI projects, and public street projects. Detailed descriptions of each type of project are available on the Philadelphia Water Department website.