Geol 33 Environmental Geomorphology
J Bret Bennington
Flood control and its difficulties
Because rivers are important sources of water, power, and transportation, and because floodplains are so fertile, humans have a tendency to occupy the land immediately adjacent to rivers in floodplains. Unfortunately, as with beaches and barrier islands, these lands are inherently unstable - in this case prone to flooding. Because we do not want the destruction and loss of life the results from flooding, efforts to control flooding are as old as civilization itself.
Effects of flooding
In 1993, the northern Mississippi River and Missouri River basin experienced over a month of fairly continuous rain due to a southerly bend in the jet stream. This saturating rainfall produced the most extensive flooding in North America in recorded history. Particularly hard-hit were Illinois, Nebraska, Missouri, and Iowa. This flood was an economic and personal disaster for millions of people and provides abundant examples of the problems associated with flooding.
Flooding caused bridges to close and prevented the flow of goods and traffic across large portions of the Mississippi. Barge traffic on the Mississippi was stalled for months as dams and locks on the river became choked by debris. Bridge failures resulted in disruption of rail traffic across the river. Damage to water treatment plants caused loss of fresh water supply to many cities and towns, including those far from rising flood waters. The 200,000 people living in Des Moines were without water for a week and many businesses were forced to close. Hundreds of other businesses were destroyed by floodwaters in river communities. Crop losses totaled in the billions.
Flooded sewage treatment plants released untreated sewage into the floodwaters, spreading potentially dangerous fecael bacteria. Fuel oil and pesticides were also dumped and carried by the flood waters. Standing water provided abundant breeding areas for mosquitos.
Flooding washed fish and nutrients into formerly isolated lakes and ponds. Temporary wetlands attracted waterfowl and the exploding fish populations attracted eagles and hawks. Flood sediments rich in nutrients blanketed farmlands, increasing future harvests (although current year crops were lost).
Flood control and mitigation
The most sensible way to mitigate flooding is to work with the river and allow it to flood along undeveloped regions of the channel called floodways. Creating floodways requires relocation of existing homes and businesses or the conversion of farmland back into natural habitat. Communities that take this approach to flood control often design the floodways as riverside parks with shallow depressions that can serve as temporary reservoirs for flood waters.
Urban areas built adjacent to rivers (ex. St. Louis, New Orleans) cannot easily be relocated. These areas are often protected by a large, reinforced concrete wall parallel to the banks of the river.
The floodwall along the Mississippi in downtown St. Louis is 52 feet high. On August 1, 1993 the river level climbed to a peak of 50 feet above normal - just 2 feet below the top of the wall. This illustrates the main problem with floodwalls - they can be overtopped. Had the St. Louis floodwall been compromised, downtown St. Louis would have been severely damaged. At one point during the 1993 flood in suburban St. Louis a leak in the wall developed that flowed at a rate of 40,000 gallons per minute. Fortunately, engineers were able to plug the leak with thousands of sandbags and 6 tons of stone.
Another problem with floodwalls is that, if flooding occurs, they prevent the floodwaters from returning to the channel when the river level subsides. Floodwaters become trapped and persist for much longer than they normaly would.
This is an extension of the natural levee built along a river to prevent high water from leaving the channel. This works well for small flooding events, but like floodwalls, levees are always in danger of being overtopped by unexpectedly large floods. Also, levees can fail if floods persist for too long and the levee becomes highly saturated and weakened. During the flood of 1993 many large, expensive levees failed and were unceremoniously washed away.
Another drawback of both floodwalls and levees is that they do not let the floodplain function as a natural resevoir to contain flood waters. This causes the excess discharge of the flooding river to move downstream until it finds a place not protected by artificial levees. In a way, these structures only move the flooding problem farther downstream.
During the floods of 1993 the lower Mississippi did not experience flooding. This is due in part to the failure of many of the upper Mississippi flood control structures, but also to the fact the the other main tributary of the Mississippi - the Ohio River - was flowing at below normal level in 1993 (the Ohio valley did not get the rains that the Missouri valley did). Because of this the lower Mississippi was able to accomodate the excess water from the upper Mississippi without flooding.
The natural reservoir function of floodplains can be taken up by artificial reservoirs created behind dams. In fact, the majority of dams and reservoirs built in the world are built primarily for flood control. The reservoir allows you to store water during times of high discharge and release it during times of low discharge. In effect, you store the floodwaters and let them out slowly in a controlled manner later on. An added feature of dams is that you can use the controlled discharge to generate electricity, and you can use the stored water for irrigation during dry seasons.
Dams have some big problems. First, like levees, they can be overwhelmed by large floods. This can be very dangerous if the dam fails. For example, 2200 people died in 1889 in Johnstown Pennsylvania when a dam on the river failed and a 36 foot high wall of water swept down a populated valley.
Dams also have the problem of their resevoirs being non-permanent. As the stream empties into the resevoir it dumps its load of sediment. The water leaving the reservoir through the sluice gates is mostly sediment free. So, all reservoirs in the world are slowly filling up with sand and mud. Most have projected useful lives of only a few to five hundred years or so. What do we do when they fill up? We donāt know.
Dams have other major problems besides these. The mixed blessing of dams is perhaps best illustrated by the High Aswan Dam built along the Nile River in Egypt. The High Aswan Dam was completed in 1970. It greatly increased the supply of irrigation water and electrical power to Egypt, and it stopped the seasonal flooding of the Nile River. But it created unforseen new problems as well that possibly outweigh its benefits...
Problems created by the Aswan Dam
The abundance of everwet irrigation canals has caused a sharp increase in the disease called schistosomiasis, caused by a parasite that is spread by snails living in the ditches.
Because the Nile does not flood anymore, its floodplain no longer receives the annual supply of fertilizing silt that has made it highly productive farmland for the last 5,000 years. Farmers must now use chemical fertilizers which are both expensive and hazardous to personal and environmental health.
The Nile is no longer supplying sediment to its delta, so it cannot built out into the sea. The sea is eroding the delta back, and normal subsidence of the shoreline is causing large areas of former land to sink below sea level. Farmland is thus being lost, even though the dam was designed to increase available farmland.
Finally, irrigation is causing the buildup of dissolved mineral salts in the soil. The annual flooding of the Nile used to flush the fields and cleanse the soils and waterways of salts. Now without this flushing, the irrigation water evaporates, leaving dissolved mineral salts behind. To rid the soils of these salts, the fields must be flushed with excess irrigation water every year. This works, but it causes an additional problem. The salts do not go away, they are simply carried down to the water table below the roots of the crops. However, the water table does not drain, and the excess irrigation is causing it to rise. When it reaches the crop roots, expensive, large scale draining systems will be needed to allow farming to continue.
Most of these problems are not peculiar to the Aswan Dam. Similar problems occur wherever flooding is controlled, or where agricultural water is supplied principally by irrigation and groundwater drainage is poor.
So you see, flood control is a mixed blessing. However, the alternative is not clear. Do we move people and cities away from floodplains? Do we accept floods and rely on insurance to repair the damage? Can we sacrifice short term economic gains in agriculture to avoid long term problems, problems that might not develop for hundreds of years?