How to reduce the risk of Floods in Rivers?
In many cases, the runoff collected in the streets flows through drains into artificial channels, large tubes or tunnels underground, which conduct rainwater to the nearest river. These channels suffer degradation with time, accumulating rubbish and soil which diminish their capacity to fulfill their purpose. They need constant and permanent maintenance and cleaning in order to have room for the maximum flow of water.
The amount of water being used and discarded as sewage by the population increases as more people occupy the region. In many parts of the world the same drainage system conducts both rainwater and sewage. These systems become overloaded, increasing the risk of floods. And with more of the city's surface becoming impermeable from asphalt and construction, combined with the removal of more and more vegetation, more rainwater stays above ground and needs to be drained off, instead of being absorbed into the earth.
The rain sewers that were sufficient for carrying the runoff several years ago are no longer enough to deal with the waters generated today, and when they fill, water overflows into the streets. And when these waters reach the river, they encounter the results of erosion and man's careless dumping of solid wastes into the watercourse. The accumulation of sediments, garbage and rubble in the riverbed limits the volume of liquid it can carry without overflowing, again contributing to the risk of floods.
Options do exist for recuperating the natural components of the drainage system, thus increasing a rivers carrying capacity and reducing the risk of floods. In many cases it is possible to clean up the riverbed, and with less complications for the local population than would come from reconstructing the artificial drainage components.
One technique for accomplishing this purpose is mechanized dredging. In larger bodies of water, a barge with a crane mounted on its deck can dig up the material from the riverbed. The mud is transferred either to the riverbank or to other barges which serve as transport, then hauled to some final destination such as a landfill. In smaller, shallower rivers, the crane can be located on the edge of the waterway, or actually move along the floor of the riverbed itself, lifting the silt up to the banks, where trucks can take it away. But digging in the river and piling the extracted soil in a new location can cause impacts on the local ecology.
Another technique for removing sediments from rivers and reducing the risk of floods is suction or hydraulic dredging. The machinery for this type of operation moves a large hose along the river bottom, through which a pump pulls the sediment from the bed to the banks, mixed with water. The solid sludge settles out in a pre-constructed pond to be removed in trucks to a final disposal area. The water is returned to the source river after a settling process to exclude sediment. Again, the environmental impacts can be great.
In some situations, where the rivers are shallow and the access to the watercourse is not too difficult, an alternative technique may be possible. In some countries, sand is traditionally extracted by manual methods from bends in the river, where it accumulates in great quantities, and is then sold as construction material. Unskilled laborers construct a one-man dredge to haul the sand to the riverbank. This sometimes consists of a 20 liter rectangular can for paint or cooking oil on the end of a pole, dragged along the bottom by hand.
Proposals are being studied to institutionalize and organize these operations as a preventative program to decrease the accumulation of soil and reduce the risk of floods. Of course this calls for serious regulation of this potentially complicated activity, just as in the other more industrialized forms of mineral extraction. With the orientation of a geologist or river engineer as to how much sediment can be removed, to what depth and at what location, cooperatives of sand miners can be taught how to extract the accumulations from the bottom of the river, and how to commercialize this material collectively to generate a higher income. At the same time, they become qualified participants in protecting their city's population and maintaining or increasing the volume of water that their river can conduct.
Of course, the best solution for reducing the risk of floods in rivers is to avoid the accumulation of river bottom sediments from the beginning. By maintaining large areas of vegetation and trees in urban regions, especially along riverbanks, much of the soil will be held in place and not washed into the channels. In this way, the rivers maintain more of their capacity to carry away excess water and avoid flooding. These green areas also help absorb more rainwater into the soil, recharging the aquifers and diminishing surface runoff, which means less water in the drainage systems during storms. Experience shows that preventing the cause of a problem is generally more cost-effective than trying to resolve it after it has become serious.
Written by: David Michael (Terespolis, Brazil)
Edited by: Rajesh Bihani ( Find me on Google+ )