This story originally appeared on the Prairie Research Institute blog and is used with permission.
After a downpour in early June, Illinois State Water Survey (ISWS) hydrographer Ryan Meekma compared images from the Topographic Wetness Index (TWI), which outlines low-lying areas in Illinois that could flood, with actual flooding at a gun range in Champaign, Illinois, to study the tool’s effectiveness.
On June 3, a storm dropped 2.9 inches of rain in three hours, flooding areas indoors and out at the Police Training Institute Tactical Training Center. Observations showed that as much as 5 inches of rain fell in some locations. The water reached 22.5 inches in one spot, with nearly 12 inches of water inside one of the buildings. Rainwater fell with such force and moved so quickly that it moved tons of gravel and two 150-pound railroad ties.
The TWI map and the on-the-ground flooding were similar in extent, demonstrating that the TWI can be used to show property owners where flooding is likely after a storm. This information is especially important for areas that are outside of the Federal Emergency Management Agency’s mapped floodplain areas, according to Kingsley Allan of the ISWS.
The Institute property is not mapped despite its proximity to a nearby stream. It’s likely that some of the river tributaries have larger drainage areas than expected, with possible flooding in areas that are not shown on floodplain maps, Allan said.
Although the TWI does not account for various storm durations and intensities, the index is useful to indicate locations that might flood after a storm. The user-friendly, interactive map enables anyone to look up an address and zoom in to locate relatively low areas on their property or in their community.