Mississippi River Flood of 2011 and the Activation of the Birds Point-New Madrid Floodway: Observations and Modeling of a Levee Breach

During 2011, record flooding has occurred in many parts of the central United States. As the flooding reached record levels for the Mississippi-Ohio River confluence at Cairo, Illinois, the 61 kilometer long and 8 kilometer wide Birds Point-New Madrid Floodway (Floodway) was activated to provide a lowering of upstream water levels through a controlled demolition of approximately 3,300 meters of levee at 10:00 PM on May 2, 2011. Prior to activation of the Floodway, the U.S. Geological Survey (USGS) deployed 38 self-contained stage sensors throughout the Floodway to capture the change in water elevation through time at various locations. From April 29, 2011 to May 24, 2011, daily streamflow measurements were made upstream of the confluence of the Ohio and Mississippi Rivers, within the Floodway opening and outlets, and on the Mississippi River downstream of the Floodway opening. Additionally, velocity and bathymetric data were collected immediately downstream of the Floodway opening at Birds Point to characterize scour in the Floodway. The data provide a unique look at the impact of a controlled levee breach on river flows and hydraulics. The activation of the Floodway lowered the water level at Cairo, Illinois by 0.44 meters in the first 14 hours, while increasing the streamflow of the Mississippi and Ohio Rivers in vicinity of Cairo, Illinois by 9,200 cubic meters per second. On May 2, prior to the activation of the Floodway, the measured combined streamflow of the upper Mississippi and Ohio Rivers at Cairo, Illinois was 52,900 cubic meters per second with the Ohio River contributing 27,700 cubic meters per second. Following the controlled breach of the Birds Point levee (immediately downstream of Cairo, Illinois on the right descending bank) the night of May 2, 2011, the measured combined streamflow at Cairo, Illinois on May 3, 2011 increased to 62,100 cubic meters per second with the Ohio River increasing to 38,100 cubic meters per second, an increase of 10,400 cubic meters per second (37.5-percent increase). The Ohio River streamflow 57 kilometers upstream at Metropolis, Illinois was nearly steady during this time. Our preliminary interpretation is that a flow transient was introduced in the system by an immediate increase in downstream conveyance through the Floodway. The increased conveyance increased the local water slope, increasing the streamflow, particularly on the Ohio River. The inflow and outflow data for the Floodway, time series of water levels at 38 locations in the Floodway, and velocity mapping data collected during operation of the Floodway provide a unique data set to apply and calibrate two-dimensional unsteady- and steady-state hydraulic models, and evaluate how effectively they can be used in a complex flow environment. The USGS IRIC modeling platform is being used to construct both steady and unsteady two-dimensional models of the Floodway. Assuming the two-dimensional models can be successfully applied, these models will provide water-resources managers with simulation capabilities to evaluate the efficacy of future levee breach scenarios in mitigating flood risk.