River management case studies
- The US needed to prevent the yearly floods and tame the river to make it navigable in order to develop
- Before management schemes were implemented the river constantly shifted its channel and eroded its banks
- They used stone dykes to trap sediment and provoke the river to erode vertically so that the channel was deep enough for paddle steam boats to use
- More wing dykes were constructed along with reserviors, levees and channel straightening, channelisation (concrete matressing) and dregding were also used -----> this all made the river faster as they increased the gradient along the rivers long profile
- All of this management, like all river management in the America, was completed by the US Army Corps of Engineers and costs $180 million a year in maintainence as the force of the water sweeps away thousands of dollars worth of management each year. It is hard to manage rivers as they constantly change (in a state of dynamic equilbrium) and so management techiniques are based on guess work and trialed in labs. Some people, though, think that management of the Mississippi has made the floods worse......
- 1993 = 3 months of torrential rain -----> defences were not designed for the such large size of flood that occured and the local people chose not to pay for the levees to be heightened. The levees failed. However, many think that if the levees didn't fail then the flooding would have been worse as they are believed to constrict water movement, block up the channel and increase pressure.
- Floodplain development has raised the flood risk as concrete increases surface runoff by reducing infiltration. Also the removal of vegetation reduces the interception store and, because there is nothing to trap the sediment, can raise the level of the river bank. Drains etc, which are designed to imitate the natural processes like throughflow,are a lot more efficient and so the water enters the channel quicker. Therefore scientists conclude that floodplains should not be built on as they are a natural flood defence that is supposed to flood.
- The high flooding of the River Rhine in 1993 and 1995 , in combination with the growing awareness of global climate change, made the public and respective authorities realise that constantly raising the height of levees and dykes, for example, is neither economically or environementally sustainable and that, instead, it is more appropriate to allow the river more room so that it can deal with a higher discharge at a lower water level. This reflects a new philosophy that we should adapt to the shape and behaviour of the river basins nto alter them to suit us. This has been approached by:
- Landuse change and relocation of habitats - not allowing building developments to be constructed on flood plains as they are supposed to flood
- Floodplain land use zoning - land is being zoned for uses that will not be damaged by winter floods like forests and parks etc.
- Afforestation - the planting of trees has increased the interception store, prevented the net movement of sediment and so reduced the amount of water and sediment reaching the river
- Room for the River scheme which includes:-
- An increase in water meadows which can be allowed to flood when necessary. The sealing of the soil surface with tarmac or concrete in vulnerable areas is being limited to slow the water run off into the rivers
- Ground coverage of vegetation with woodlands and grasslands is being increased
- The use of fertilisers on soil is being carefully monitored because these affect the soil structure and its ability to retain water
- To allow more space for trees on the floodplain, metres of silt accumulated over many years has been stripped and deep trenches constructed
- All of these soft engineering methods have increased the time taken for water to enter the channel, reduced the amount of water that does enter the channel, created a channel that has a larger cross section and so can accomodate a larger volume of water and moved people away from the most vulnerable areas - remeber that disasters, like a flood, only occur when people come in close contact with a risk!
- Some hard management options are still being used though like the building of flood relief channels to siphon off the Rhine flood water when the delta becomes overloaded, making the course of the river straighter and shorter and increasing the height of some of the levees.
- The River Quaggy runs through southeast London and since the 1960's it has been heavily managed by building artifical channels and culverts to divert the flow beneath the surface as it passed through Greenwich.
- The areas of Lewisham and Greenwich have become more densely populated and the flood risk has increased, due to the continued development, and so more is needed to be done to protect the surroundign area. Further widening and deepening of the channel were considered but instead teh Environment Agency decided a softer option was most appropriate. A solution was proposed by the local residents, who formed the Quaggy Waterways Action Group, that would improve the local environment whilst also provided protection against floods.
- The plan was to bring the river back above ground once again , cutting a new channel through Sutcliffe Park, and creating a new multi-functional open space. This method improves both the flood management and quality of the park. A culvert did remain to take soem of the excess water, during times of flood, underground but a new lake was built to allow the are to deal with the majority of the excess water when the river floods.
- The park itself was lowered and shaped to create a new floodplain where watercould naturally collect, instead of rushing downstream through the previous artifical channels to flood Lewishantown centre. The parks flood storage capacity is equivalent to 35 Olympic swimming pools, has reduced the risk of flooding for 600 homes and businesses in the local area and created a diverse environment for wildlife
- By reducing the river to a more natural course and including a flood storage area, the scheme has created a wetland environment with reedbeds, wildflower meadows and trees. This scheme won the Natural Environment category in the 2007 Waterways Renaissance Awards and the Living Wetland Award.
Floods are part of the natural water cycle and flooding is, for the most part, steered by natural processes.
Uncontrollable climatic factors are not the only reasons for the increasingly frequent occurrence of flooding and a number of human-made factors have made the problem worse.
Apart from the river bed, there are four important factors which improve the water storage effect of a river's catchment area and help to control flooding levels:
When water storage in vegetation, soil, ground and drainage networks is overloaded the drainage situation changes dramatically.
Deforestation in the Alps has reduced interception and soil storage of water and increased rates of surface runoff.
Urbanisation of the floodplain, with water flowing off roofs and roads into drains leading directly to the river has greatly increased river levels after heavy rain.
The Rhine, for several decades, has been put into a kind of straitjacket. In the past, excess water would flow out over marshes and floodplains. These acted like sponges, soaking up the water, but since then some of the land has been drained, cemented and asphalted for buildings and roads.
The embankments have been strengthened and raised to protect residential and industrial areas, but raising them has closed off former flood meadows. Steep concrete flood walls along the upstream river banks channel flood water quickly from the upper reaches of the river but this has shifted the flooding problems down stream. (Politicians came under pressure to make riverside land available for local businesses or housing).
The river Rhine is a major shipping highway. To enable larger barges to use it and to speed up the journey time it has been strengthened, deepened and canalised. When a storm takes place the flow of water (or discharge) does not increase straight away. There is a gap, called a 'Time Lag' between the high rainfall and the peak discharge. A river with a short time lag and high discharge increases the danger of flooding. Stretches of the Rhine have been straightened and banks heightened, cutting some 50 kilometres off the river's 1,320 kilometre meander to the sea. This has doubled the speed of the water's flow from Basle, at the Swiss border, to Rotterdam. Now, when there is heavy snow or rain upstream, water cascades down to flood at the mouth or half-way along, instead of soaking into marshes near its source.
Building Hydro-electric power stations along much of the upper Rhine has increased the problem. Since the 1950's, the upper Rhine, along the French-German border, has been changed with the construction of 10 hydro-electric power stations. The 'Power Project' involved building a 'new' river parallel to the old Rhine and the construction of these H.E.P. stations created a deeper, faster Rhine.
Changes in farming practices have made fields less absorbent, as hedges and forests have been chopped down to create prairies farms. The drainage of swampy areas, and pumping out the ground water for irrigation purposes have dried out the land even further. The extensive network of cemented farm roads act as extra drainage channels.