BIRDS POINT NEW MADRID FLOODWAY ACTIVATION: FLOOD IMPACT ON THE NATURAL AND BUILT LANDSCAPE

In the spring of 2011 massive, coalescing storm fronts in the upper Mississippi, and Ohio River basins caused extreme weather conditions that flooded the bottleneck section of the Mississippi River near Cairo, IL. In order to alleviate flooding the USACE activated the Birds Point New Madrid Floodway by intentionally breaching the levee, releasing floodwaters onto active agricultural fields. The breach successfully decreased the flood stage south of Cairo IL and eased the hydraulic pressure on downstream systems. However, regionally extensive landscape modifications occurred, and these impacts on the landscape remain visible and costly. Flood waters rushing through constrained channels surpassed geomorphic thresholds and created erosional scours in the immediate vicinity of the breached sections and natural topographic highs. Rushing floodwaters overcame the shear strength of the cohesive soil causing concentrated erosion that generated deep scour holes, rills, and gullies. While a ramp like structure was created near an inflow/outflow section, the conduit was not formed from the main channel to the breach greatly reducing the potential alteration of the area. In addition of erosion, inflow and outflow of waters formed large deposits of sands and gravels blanketing the vicinity of the impacted areas. While the costs of these immediate damages were assessed, erosion and sedimentation effects are estimated at an order of magnitude more costly than a single year’s crop, and more persistent. Therefore, the success or failure of the New Madrid Floodway activation is directly related to the magnitude of geomorphic effects, alteration persistence, and diminished future agricultural production. Subsequent and more frequent activations of the floodway may cause remediated areas to form preferential pathways for surface waters to erode. Headward erosion and the botroidal features captured in imagery may produce a landscape texture that is persistent to remediation and detrimental to the primary agricultural purpose of the area. The geomorphic costs of future activation are indeed longer lasting than those routinely calculated in flood damage assessment.