On our trip to the Singapore River, we found a river channelisation method used in Singapore,
which is the re-sectioning [Widening and deepening] of the river channel.
Why re-sectioning?
- It increases the channel's ability to hold water, thus decreasing the chances of flood.
- It increases the amount of surface runoff from the surrounding areas into the river because more surface runoff can now enter the river without causing it to flood.
- It prevents soil erosion as the soil of the river banks and bed are replaced with cement and granite. By doing so, it also increases the speed of the river and allow water to flow away from the area more quickly as there is less friction between the water and river bed and banks.
Limitations:
Flooding downstream
The increased speed of the river after channelisation may result in flooding downstream. As a result of channelisatiom upstream, the volume of water flowing downstream is increased. However, the channel in the area downstream may not be able to contain the increased flow of water. The channel overflows and flooding results.
Accumulation of Sediments:
Rivers have higher speed and energy to carry sediments downstream due to channelisation. THese sediments are washed into the sea and onto the sea bed, therefore blocking or intercepting the sunlight reaching the seabed. This affects the growth of marine life like corals and seagrasses. Construction of dykes also cause sediments to accumulate behind/along the structures, causing the riverbed to become shallower, increasing chances of overflowing.
Loss of Marine Life and Wetlands. :(
In the process of channelisation, ecosystems are destroyed. In the case of Resectioning, changing the course of the river and lining the natural banks of the river with man-made concrete affects the flora and fauna as habitats are detroyed. Channelisation also decrease the number of welands. IN the case of re-alignment, shortening the course of rivers decreases the area of wetland as the river flows over a smaller land area now.
Other channelisation methods include:
Re-alignment:
- River realignment is the straightening of the river channel. It removes meanders and the length of the river to allow the river to flow faster and wash away the sediments that have been accumulated on the river bed, which minimises the chances of flooding.
- Like resectioning, it also deepens the channel to allow it to hold more water to decrease chances of flooding.
Limitations of Re-alignment:
Rivers that carry large volumes of water (such as the Mississippi, Amazon, Mekong, Ganges) can not be re-aligned and it is best to keep people from building in their flood plains as this would reduce the loss of life.
Bank protection [Dykes, Gabions, Revetments]:
- Reduces soil erosion along river banks by building levees. These stone or concrete walls are built along rivers that constantly overflow their banks.
- Like resectioning and re-alignment, it increases the rivers capacity to contain more water to reduce flooding.
Limitations of Bank Protection: - Continual buildup of sediments make the rivers shallower, therefore causing the water levels to rise and waste the effort and energy and cost of building the dykes.
Other examples of River channellisation methods
Lower Latrobe River :
-channelisation occured in the 1800s
- artificial meander cutoffs were used
- caused 25 per cent reduction in channel length
-increased in mean channel width of river
-increased channel instability and incision of up to 1.05 m. - Hydraulic changes
- results in a threefold reduction of over-bank flow duration
-extensive channelisation works have reduced the frequency of minor flooding and the period of floodplain inundation
-Limitations : these channelisation methods and other human impacts have severely degraded the stream environment
Africa, Ogunpa river:
-channelisation occured in 1999
- Project" of "Ogunpa" these had been bedeviled by series of problems since 1977
- To this end 10 billion Naira contract was awarded to finish the Channelisation of "Ogunpa"
- watershed management was used, with detailed plans on conserving the natural vegetation
-reduced surface runoff and soil errosion
- channelisation works have reduced the frequency of flooding in the country
-Limitations : Buildings might be needed to be torn down to replace the area with natural vegetation. After which, a period of time would be needed for plants to grow, allowing floods to continue for a longer period of time. Large amount of money is also needed to manage watershed.
THANK YOU!
