Environmental Law Blog
Floods of '08
As Missouri prepares to take on flood water from its northern neighbors, many are left wondering how a 500-year flood could happen twice in 15 years.
The terms "100-year" and "500-year" flood are used to describe the estimated probability of a flood event happening in any given year. Using historic weather and hydrograph data, experts can determine the estimated rate of flow or discharge of a river or creek. A 100-year flood has a 1 percent probability of occurring in any given year, and a 500-year flood has a .2% probability. Though unlikely, it is possible to have two 100- or even 500-year floods within years or months of each other. This is why we are hearing about the 100-year flood of 1993 and the 500-year flood of 2008, even though the floods are merely 15 years apart.
What does this have to do with environmental law, you ask? In 1944, Congress enacted the Flood Control Act, which charged the U.S. Army Corps of Engineers with the duty to provide quality, responsive engineering services to the nation including, among other things, planning, designing, building and operating massive civil works projects. Congress mandated the Corps to manage these projects to serve the dominant purposes of flood control and navigation, and to incidentally benefit a host of secondary purposes including fish & wildlife, water supply and recreation. Although the Corps must sereve many masters, the law is crystal clear that flood control is paramount when purposes collide. Everything else must yield.
Currently, the Corps is working with state and local levee districts to coordinate flood-fighting activities in Indiana, Minnesota, Wisconsin and Iowa. Immediately following the Floods of '93, the Corps planned and executed flood control strategies in all of the areas affected by the Floods of '93. Now we will find out if the Corps' work was sufficient.
The St. Louis Post Dispatch published an article this morning, highlighting Missouri's expected flood levels due to the extreme saturation and catastrophic flooding going on up North. As that water reaches Mississippi River towns, all Missourians can do is wait and hope the Corps's post-1993 flood improvements can withstand the pressure of all that water. Meanwhile, Missourians along the Missouri River and many other swollen tributaries are bracing for flooding, and hoping that this disaster doesn't spread any further.
Technorati Tags: 100 year flood, 1993, 2008, 500 year flood, army corps of engineers, flood control, mississippi river, water, floods, moagoenvironmentallaw, environmental law, environmental protection
Posted by on June 16, 2008 10:31 am :: Comments (1) :: Permalink
1 Response to "Floods of '08"
says:
June 24, 2008 10:21 pm
Thanks. Very informative. One question: Does this mean that we may be recalculating the probability that these waters will occur? So maybe the 500-year flood is really a 100-year flood and maybe the 100-year flood is really pretty common.
Martin, thanks for asking. Experts are constantly reassessing flood forecasting and risk based on rainfall, predicted weather, and a host of other factors including an often-evolving "stage-discharge relationship." Over time, the relationship between a certain amount of water in a river (discharge) and the level to which the water rises (stage) has changed. The same amount of water in a river does not necessarily equate to the same flood height today as it did years ago. Today, the stage is usually higher even though the discharge is the same. There are many reasons for this--some hotly debated--and they can be better explained by scientists like those at the USGS. Check out this interesting "Citizen's Guide" from Washington to gain a better understanding of the stage-discharge relationship. Recently, FEMA has begun recalculating the likelihood of flooding along our major waterways. The results have been controversial, but it seems likely that the notion of a 500-year flood is evolving along with the stage-discharge relationship for some waterways.
