U. of I. civil and environmental engineering professor Marcelo Garcia, an expert in river hydrology, described flooding along the Mississippi River and the measures taken to control it.
Photo courtesy of the department of civil and environmental engineering
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In May, the U.S. Army Corps of Engineers opened spillways and intentionally breached levees to reduce the impact of floods along the Mississippi River. This protected large cities but inundated small towns and farmland. In an interview with News Bureau physical sciences editor Liz Ahlberg, U. of I. civil and environmental engineering professor Marcelo Garcia, an expert in river hydrology, described flooding along the river and the measures taken to control it.
What kind of flood-prevention measures have been engineered along the Mississippi River?
Most measures have been structural in nature and they involve the use of levees as well as reservoirs to mitigate the impact of floods. The levee system of the Mississippi River has evolved along with its surrounding civilization. In 1717, Georges de la Tour, the French engineer who laid out the city of New Orleans, constructed the first levee along the riverfront. Levee construction has continued to this day.
In the late 1800s, the states north of the confluence of the Mississippi River with the Ohio River drained swamps and flood plains into the tributaries of the Mississippi River, resulting in increased flood heights in the lower reaches of the river. Levees south of Cairo, Ill., had to be constructed higher and stronger.
How have these measures altered the natural progression of the river?
The main channel of the river has been separated from its floodplain so when flooding occurs the water cannot spill over into the flood plain. The interaction of rivers with their flood plains is a natural cycle that does not occur once levees are built. Floodplains include low-lying areas that work as wetland buffer zones during floods. Since the riparian wetlands were converted into agricultural land for cultivation, the floodplain no longer plays its natural role.
A levee system provides local protection but can increase water levels upstream of its location. Sometimes, when levees exist on both sides of the river, a bottleneck is created and the water levels increase resulting in a "backwater effect" that can raise water levels for many miles upstream, causing flooding of areas that might not have flooded before the downstream levees were built or raised higher.
How does opening spillways or tearing down levees upstream help prevent flooding downstream? What are the pros and cons of such an action?
When a levee is intentionally breached or a side spillway is opened, the water that flows out of the river provides relief downstream because the flow discharge down the river is reduced and the flood stage diminishes accordingly. This practice was recently implemented when the U.S. Army Corps of Engineers opened the Morganza Spillway for the first time to reduce the risk of flooding New Orleans and Baton Rouge. On May 2, 2011, the Birds Point-New Madrid floodway in Missouri was also activated by the intentional breaching of the levee, to prevent the flooding of Cairo, Ill., on the other side of the Mississippi River. While the people of Cairo were grateful, the levee breaching flooded about 200,000 acres of agricultural land in Missouri.
The human and economic costs are extremely large. The critical question is whether agricultural damages in floodways will be limited to one year's crop or will instead persist over multiple years or even generations.
How do the costs and manpower of attempting to control the river compare to those if the Army Corps of Engineers abandoned all such efforts?
The costs of continuing to develop and implement flood control technologies are relatively modest in comparison with the millions of dollars, sometimes billions of dollars in the case of Hurricane Katrina, caused by flood-related damage. At the University of Illinois, we are currently developing a method to assess the carrying capacity of rivers during floods. Allowing occasional flooding by means of "fuse" levees in case of extremely high water levels may be more cost-effective than safeguarding against it by building fortress-like levees that are rarely, if ever, needed.
We use terms "25-year" or "100-year" floods yet they seem to be happening at a greater rate than their names would imply. Why?
A 100-year flood has a statistical probability of 1 percent of being equaled or exceeded in any given year, estimated with a statistical analysis of river and stage-flow measurements taken in the past. This does not imply that flooding events of the same magnitude might not occur more frequently, since this depends on rainfall amounts, snowfall and ground conditions. This is obviously very frustrating to the public when it suffers the effects of floods that, according to a statistical hydrologic analysis, should not be occurring so frequently. There is an element of risk when dealing with natural phenomena. In any river system, flooding will certainly occur; we just don't know when. We need to prepare communities for more extreme flooding events.