The Water Survey uses a GIS analysis tool called the Topographic Wetness Index to identify low-lying areas that could be subject to ponding during rain events. The index is particularly important because areas of low terrain with upslope—conditions that contributes to drainage areas—can be spotted outside of the Federal Emergency Management Agency’s mapped floodplain areas. The Illinois Urban Flooding report recommended the development of a statewide Topographic Wetness Index to help identify areas that might pond to aid in planning.

A user-friendly interactive map enables anyone to look up an address and zoom in to locate relatively low areas.

Communities can use the tool to locate low-lying areas where runoff might accumulate. Homeowners who are planning to build a shed on their property or plant a garden can learn where water accumulates and avoid those areas.