Last night, the US Army Corps of Engineers blew up a stretch of levee to alleviate flooding in a nearby town. This is how I think it works.
The town that is threatened is Cairo, Illinois, which is located on the confluence of the Ohio and Mississippi Rivers (from North-East and North-West respectively, both flowing South). Water levels surrounding the town have been at record levels for a while. Engineers are worried that the town’s levees may fail, not necessarily due to overtopping but due to the duration of the flood levels. Long exposure to flood levels may induce failure mechanisms such as piping.
The levee that was blown up is on the Ohio River, opposite Cairo. My guess is that the photos in the WP article were taken from the town. The levee’s ‘hinterland’, apparently consisting of farmland will be inundated. This extra storage (150,000 acres, approx. 600 km2 which is quite a lot) will likely reduce water levels at Cairo, which suffers from backwater effects due to its proximity to the confluence. The question, of course, is by how much water levels will drop.
Below plot shows the flood hydrograph at Cairo, Illinois (from NWS website). The gauge is located on Mississippi river, just upstream of confluence i.e. on the NorthEastern side of Cairo. The forecast shown is unlikely to include effects of the new breach I think.
Observations show a sudden change in the shape of the hydrograph. I have no idea how steep the river’s slope is and therefore also no idea as to how far upstream the effects of the measure will reach. I do think that USACE had its timing right and that the extra storage will reduce the flood peak somehow. But there could be new peaks, either from upstream or from laterals.
(Thank you Kate and William for bringing this to my attention and discussing causes and effects. Kate and William being Katie Jagt and Wim Kanning of course. Who else?)