In 1927 there was a large flood and people realised that the main flood risk wasn’t from the Mississippi river itself but from its main tributaries; The Missouri; Ohio; Tennessee. So levees were built along the whole course of the Mississippi and its main tributaries. Levee’s are used all over the world and were originally made from natural materials (more recently made from concrete) and are built along flood plains. They provide protection from the river when it is in flood as the flood water builds up inside the levees. Although this is quite and effective form of management in the Mississippi 1993 flood the levees broke and lead to the floodwater pouring out onto the cities. Levees must be made to be strong to be effective.
Between 1934 and 1943 it was decided to get rid of the floodwater faster it would be a good idea to straighten the river. By doing this they shortened a 530km piece of the river by nearly 300km. They rebuilt levees on the new artificial course but they had increased the gradient, distance and the speed of the river. In theory this would decrease the flood risk but this tactic has not always worked.
The Lyn River in the village of Lynmouth in North Devon is a small and quite docile river. The village of Lynmouth was small and quite but they had straightened their river so tourist accommodation could be built, as the meanders were inconvenient. In the first fortnight of August 1952 it was exceptionally wet throughout the Southwest of England leaving the ground saturated in Exmoor. On the 15th of August there was a huge amount of rainfall and the two main tributaries of the Lyn flooded. When this got to the village of Lynmouth the river was carrying so much discharge and was travelling so fast that it reverted to its old route causing catastrophic effects. This was because its old course ran straight through the village (the discharge was also carrying large boulders), this ruined homes and claimed about 20 lives. So perhaps straightening rivers is not always the right way to reduce flood risk. The Lyn since then has been allowed to flow in its natural course and there hasn’t been a major flood there since.
All these methods so far are examples of hard engineering but a popular form of soft engineering used on the Mississippi is Afforestation. This is where many trees are planted so that the roots soak up surface run off water lessening the volume of water in times of floods. This is a good way of reducing flood risk, as it is economically friendly and cheap and easy to maintain.
A form of Hard Engineering used all over the world is to build Dams that create reservoirs. Dams are expensive to build and maintain but are efficient in what they do. In times of high flow the dam will hold back water and in dry spells more stored water will be allowed to flow. The dams also pay themselves off my creating hydroelectricity, a cheap and economically friendly form of electricity that can supply the large surrounding towns. On both the Missouri and the Tennessee rivers extensive damming took place, 16 major dams between them were built and in the floods of 1993 they held back a significant amount of water and without them, the damage would have been a lot more.
A form of management on the River Tees, England was the Tees barrage. It took four years to build and contains 650 tonnes of steel. It has four massive flood gates, which are operated by hydraulic pistols. These make sure that the river is always at a constant level and during times of flood acts like a dam in the way that it holds make water. The Tees Barrage works mainly according to water pressure so it doesn’t really have to be controlled it works itself. It’s a good form of management as it reduces flood risk and has allowed investment of £500million to be put into housing, office, leisure, shopping and educational facilities. It also stops the river water mixing with the salt sea water and so the water is fresher.
A method that has been used in Exeter on the river Exe and on the Mississippi was building diversionary spillways. These are channels next to the river that can take flood water when it overflows. The Bonnet Carre spillway begins 50km north of New Orleans and ends in the Gulf of Mexico. This spillway has greatly reduced the flood risks at New Orleans and Baton Rouge. The spillway on the Exe River has also been effective in stopping the overflow of floodwater spilling into the town of Exeter.
Other methods of trying to reduce flood risk are mainly soft engineering. Another tactic used is dredging and is very effective this is where the sediment at the bottom of the river is constantly scooped out to maintain a deep water channel. This means more discharge can be held so in times of flood more water can be held in the river channel. Practising evacuations and having prepared emergency services is always a good precaution to have ready in case the river does actually flood. The methods mentioned in this paragraph have both been used of the management of the Tees.
