KARST TERRAINS AS FORGOTTEN FLOODPLAINS

Flood risk models in karst landscapes often use methods that do not capture the complex linkages between surface water and groundwater flow. Therefore, published maps for access to the National Flood Insurance Program may exclude those who experience regular inundation. In karst, point-source recharge in sinkholes and sinking streams rapidly conveys runoff to the subsurface. When this recharge rate exceeds the capacity of underground conduits, or when the receiving body of water for karst springs is flooded, the potentiometric surface can rise and flood low-lying areas across a karst basin. The nature of how these floods occur differentiates karst from floodplains in riverine settings.

Using simple GIS-based inundation models, we demonstrate how Orleans Indiana, in the classic karst landscape of the Lost River watershed, has been systematically excluded from flood insurance risk maps because FEMA-produced models do not consider the town flood prone, despite well-documented evidence to the contrary. Our models connect missing elements to the National Hydrography Database and, when compared to satellite ortho photography, demonstrate that published flood insurance rate maps significantly underpredict the possible scope of floods in this watershed.

Recently proposed highway infrastructure in southern Indiana included alternatives that would have bisected the Lost River watershed and could have potentially increased flooding concerns. Unless comprehensive studies detail the actual risk of floods as 1) part of the natural landscape response in karst, 2) a natural consequence of more extreme events as climate changes, and 3) a consequence of impaired flow routes possible from road construction, towns such as Orleans may experience amplified economic exclusion.