Posted on | May 20, 2011 | Comments Off
The drama on the Mississippi has spawned a lot of good writing on the disconnect between how nature thinks about flood plains versus how we think about them. As Anne Jefferson wrote, the bits next to our rivers tend to be the most desirable places to live:
People are drawn to settle on floodplains – because rivers provide transportation corridors, but mostly because floodplains are the most fertile soils on Earth. Those fertile soils are there because flooding creates floodplains, bringing in the sediment and nutrients that make rich, agriculturally-productive soils.
But precisely the same reasons that make them so attractive also make them the vulnerable:
The river gave their land value (whether they realized the soil connection or not), but it also gave them misery. They wanted the benefits of the river, but not its floods.
Robert Simmons at NASA’s Earth Observatory blog had an interesting followup comment in part to Anne’s piece and in part to other discussions on the EO blog:
In response to Map of the Ancient Mississippi a few of you left comments to the effect of “no one should live in floodplains.” It’s an appealing notion, but I think it’s unrealistic.
Simmons’ accompanying map shows why it is unrealistic – much of the development of the midwest is “in floodplains”.
It’s a notion that applies here in central New Mexico, where much of the core of Albuquerque has been built in the natural Rio Grande flood plain. In an arid climate the flood story is different in fundamental ways from wet climate rivers. Far more of our water management infrastructure is built around the “not enough” part of the water problem, rather than the “too much” side of the natural range of variability.
But we flood too, and building in the flood plain extracts a very similar price. Much of the history of early 20th century water development in the middle Rio Grande Valley is a history of managing the wet side of the natural range of variability – figuring out how to farm in the flood plain while managing the inevitable flood problem. The picture above is from the great flood of 1941. Peak flows through Albuquerque of 25,000 cubic feet per second* are tiny compared to what we’re seeing on the Mississippi, but it’s all relative. That was more water than the system at the time could handle, and led to the flooding you see in the picture and, eventually, construction of Cochiti Dam upstream from Albuquerque.
Before Cochiti was built, according to an analysis done for the US Army Corps of Engineers by Mussetter Engineering (pdf), flow here regularly topped 10,000 cfs, but hasn’t since.
Today, the Army Corps water management guidelines put the maximum permitted flows at just 7,000 cfs through Albuquerque, though they try to keep it below 5,000 cfs because of aging spoil bank levees and irrigation structures along this stretch of the river that have difficulties with water that gets any higher.
Our river is pinned between levees through town, and most folks here, to the extent they think about this at all, think the river is “over there” and don’t realize they’re living in a natural flood plain.