Exploring the Interplay Between Successive Storms and Basin Drainage Topology During the 2008 Extreme Flooding Conditions in Eastern Iowa

Historical and real-time data collected during the 2008 floods in Eastern Iowa show that small basins in the region experienced mild flooding conditions with small return periods (< 50 years), while large basins experienced extraordinary flooding conditions (> 100 years). The individual storms that preceded the flood were not extraordinarily large; rather, the week prior to the flood was characterized by a series of storm systems that successively hit the region. This observation suggests the hypothesis that extreme flooding conditions in locations draining large basins can be created by consecutive "ordinary" storms rather than by extraordinarily large storms that have been the focus of previous studies. Current analyses linking storm return periods to flood return periods do not account for the possibility that large floods can be created by consecutive storms. We perform a series of numerical simulations that show that the rainfall totals observed in June do not account for the extreme flooding that was observed, instead the timing of the individual events was compounded by the topology of the river network draining the landscape. This result highlights the need for a more precise understanding of the changes in rainfall patterns in the presence of climate change. Our simulations indicate that large basins are more sensitive to how rainfall falls over the basin rather than to the total amount of rainfall.