Manitoba is once again contending with floods this spring, and surging water levels in the Assiniboine River near Portage La Prairie have forced the province to consider doing deliberate damage — namely, breaking a dike.
Dikes, also known as levees, are natural or manmade structures that slope down to a river, typically running parallel to the water’s flow. Dikes mitigate the effects of rising rivers on nearby roads and properties, but by containing the flow of a river, they also increase the water’s speed in a flood, which can cause the erosion of other dikes downstream.
Dikes are not invincible. High water levels can overwhelm them and inundate big areas of land. Authorities might consider a planned dike breach if they feel that not doing so will overwhelm other dikes along the river and jeopardize a larger territory. The release of water in a planned dike breach is often less damaging than an unplanned breach.
Initiating a dike breach is never an easy choice — the philosophy behind it is to allow flooding across a small land mass in order to protect a bigger one. Homes are inevitably damaged. In the case of Manitoba, the province has said the dike breach will swamp 150 properties in order to save 850.
Dikes are typically broken with an explosion of dynamite. Manitoba authorities are planning to cut a 65-metre hole in the Assiniboine dike to prevent the Assiniboine River from spilling its banks further east. The move will divert water to the Elm River and eventually the La Salle River.
One of the most famous levee breaches was undertaken during the Great Mississippi Flood of 1927, the worst river flood in American history. Officials blew up levees near Caernarvon, La. to divert the torrent of water to the Gulf of Mexico and prevent massive flooding in New Orleans. The blast proved pointless, however, as unplanned dike breaches upstream were ultimately responsible for keeping the water from inundating New Orleans.