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Flood diversion tactics put engineers in the front line

Engineers were heroes and villains at the same time in the United States this week, following an intense operation to demolish part of a levee at Birds Point, Missouri, after the region was inundated with extreme rainfall.

The US Army Corps of Engineers decided to breach the levee after days of discussion over whether it was right to activate a 90 year old flood storage area. That decision saw the city of Cairo, Illinois, saved from flooding as muddy waters from the bulging Mississippi River were diverted onto 53,300ha of Missouri agricultural land.

The event illustrated the delicacy and difficulty of deliberately sacrificing land to flooding in an extreme event. Strong feelings on both sides of the argument came to a head when Missouri Republican congressmman Steve Tilley who represents a nearby constituency told journalists that economically deprived Cairo was more deserving of flooding than Missouri’s farmland. He said: “I’ve been there. Trust me. Have you been to Cairo? OK, then you know what I’m saying.”

The 56km long Birds Point New Madrid Floodway was designed in 1928 to channel flood waters in the event of a levee breach from Birds Point to New Madrid further downstream.

This allowed the waters to bypass a particularly narrow section of the Mississippi River, as well as Cairo, which was then a major steamboat port.

However, the floodway has only been operated once since then, in 1937, and today it contains homes as well as farm land. Missouri and Illinois residents argued emotively over whose settlement should be saved, and Corps commander Major-General Michael Walsh voiced the difficulty of the decision.

“Making this decision is not easy or hard - it’s simply grave,” he said. “The decision leads to loss of property and livelihood either in a floodway, or in an area that was not designed to flood.

“I have to use this tool. I have to activate this floodway to help capture a significant percentage of the flow. I don’t have to like it but we must use everything we have in our possession, to prevent a more catastrophic event.”

The incident will interest to those involved with the Environment Agency’s TE2100 flood risk management plan for the Thames Estuary, for which the Environment Agency is preparing an implementation plan.

The Agency proposed sites along the estuary at Erith Marshes, Aveley and Wennington Marshes, Dartford and Crayford Marshes, and Shorne and Higham Marshes as potential locations for flood storage.

However, consultation respondents argued that “inundation of predominantly freshwater areas with saline/brackish water would be harmful to their ecology”, and that infrastructure including Crossness sewage treatment works and a planned energy-from-waste plant would be at risk at the Erith Marshes site, as well as Darent Industrial Estate on Crayford Marshes.

The Agency subsequently concluded that flood storage is not currently feasible due to cost and safety issues, damage to habitats, and concerns over the reliability of forecasting the timing and shape of tidal surges. It admitted that the recovery period for ecosystems in flood storage areas would be “several years”.

The TE2100 Plan states that flood storage remains one of its “end of the century options for future review”. But the Agency warned following its consultation that continuing development on its selected undeveloped marsh areas is gradually reducing the potential for both tidal and fluvial flood storage in the region in future.

MWH executive technical director David Balmforth says that any requirements for flood storage in extreme events must be considered at the first stage of development, and structures located and designed accordingly.

Designating areas for flood storage after they have been developed is more problematic, he says.

Unless that forethought happens on the Thames estuary marshes, the Environment Agency’s future review may find that designating flood storage areas will be as difficult and unpopular an option on the Thames in 2100 as it has been this week on the Mississippi.

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