IMPACT OF ANTECEDENT GROUNDWATER HEADS AND TRANSIENT AQUIFER STORAGE ON FLOOD PEAK ATTENUATION IN AN UNCONFINED KARST AQUIFER: STUDY OF THE UPPER SUWANNEE RIVER, FLORIDA, USA.

Flood peak attenuation is an important aspect of understanding flooding and its effects. Few studies exist that look at the effects of ground-surface water interactions in regards to peak attenuation, and fewer still focus on karst environments. In the karstic, variably confined Suwannee River Basin, discharge, river stage, and water table data that were collected over a ten-year period were analyzed to determine the relationship between antecedent groundwater head and flood peak attenuation. Flooding causes high hydraulic heads in the river, which rise faster than corresponding groundwater heads. Springs which normally feed groundwater into the river reverse flow, and conduits allow for large amounts of river water to be absorbed into the aquifer matrix. Peak discharge in floods that occurred when antecedent groundwater heads were low were attenuated downstream. In contrast, peak discharge in floods that occurred when antecedent groundwater heads were high lacked attenuation Because most flood discharge models do not consider how transient storage of floodwaters in aquifers can attenuate flood peaks, predictions and flood warnings may be inaccurate in basins that promote peak attenuation, such as unconfined karstic basins. In addition, understanding these interactions is paramount in determining pollution risks to karst aquifer systems.