River Management Strategies

CHIJ St. Joseph’s Convent

Chapter 6- Rivers

[Effectiveness of River Management Strategies]  Channelisation is the process of changing the natural course of a river in order to make it flow in a specific

path so as to reduce the possibility of flooding. Meth od

Description & Effectiveness

Limitations/ Ineffectiveness/ Drawbacks

 It refers to the straightening and shortening of

the river channel.  It shortens the river channel by removing

While the strategy is effective/ successful in the area where it is implemented, there are still some drawbacks to its successful implantation:

meanders along a winding river course and hence reduces length of the river channel.

Short term measure as rivers (such as the Mississippi River) will develop meanders over time. These will become pronounced, making the flow slower and the areas around the river vulnerable to flooding. 

 e.g. the Mississippi River (USA) has been

shortened by 240km after re-alignment by cutting off the meander through its neck  Strategy is effective as the shortened and



1.

deepened river carries away the sediments faster and allows faster flow of the river, therefore, minimising flooding risk. (link to reduced flooding MUST be explicit)

Effective in one (localised) area flooding problem may persist downstream especially since smoother channel and faster flow may increase sedimentation downstream -

Other examples:  Waikanae River re-alignment at Jim Cooke Park (New

Zealand) 

this makes the downstream section of the river shallower, reducing its capacity to hold water, causing the river to overflow its banks and hence flooding

Expensive method as resources like funds and manpower are required to shorter the course of the river Results in the loss wetlands/ habitats for marine life 

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Meth od

Description & Effectiveness

by shortening the course of rivers, the area of wetland shrinks because the river flows over a smaller land area now

Limitations/ Ineffectiveness/ Drawbacks

 It refers to the widening and deepening of the



river channel to increase its ability to hold water.

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 1st method: dredging channel to remove excess

sediment from the river bed, thus deepening -

e.g. Singapore River & Kallang River in Singapore (re-sectioned through dredging)

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smoother channel and faster river flow may increase sedimentation further downstream

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this makes the downstream section of the river shallower, reducing its capacity to hold water, causing overflowing and flooding

 2nd method: river bed and banks are smoothened by

replacing the soil of the river banks and bed with cement and granite

2.

 Chances of a river overflowing its banks and causing

flooding are reduced since the river channel is able to hold more water (with the deepening).  Smoothening of river channel leads to lesser friction

between water and river bed and banks and the speed of river will increase and water will flow away from the area more quickly.



Only effective in one area flooding problem may persist further down stream especially since smoother channel and faster flow may increase sedimentation downstream

Expensive method as resources like funds and manpower are required for regular maintenance such as dredging results in the loss wetlands/ habitats for marine life - e.g. by shortening the course of rivers, the area of wetland shrinks because the river flows over a smaller land area now 

 Effective short-term measure as the increased river

flow prevents river from reaching flood height, so that the areas along the river is protected from flooding.

Meth od

Description & Effectiveness

Limitations/ Ineffectiveness/ Drawbacks

 involves manipulating the banks to protect



Dykes have not been very successful because of the continual build up of sediments on the river bed, making the channel shallower. this has caused water levels to rise higher than the dyke walls over the years



Higher dykes have to be built regularly or the sediments have to be dug up and removed from the river bed



Expensive method as it incurs high maintenance costs

them from being eroded  refers to the building of artificial walls of sandbags,

rocks, concrete metal (known as levees/ embankments/ dykes) to raise banks and increase river depth, thus increasing the waterholding capacity of river channel  One method of bank protection is the building of

artificial levees or dykes - refers to walls of sandbags, stone or concrete are built along rivers that often overflow their banks

3.

dykes have built in China for centuries in the lower course of the Yellow River -

Mississippi River that runs through New Orleans in America

 May not be effective in cases of heavy rainfall when

the volume of water in the channel may overwhelm effectiveness of the dykes in retaining water in the channel. (e.g. dykes along the Mississippi River in New Orleans burst during Hurricane Katrina in 2005)

 Concrete structures, like gabions and revetments,

are built along a river channel - Purpose: to divert the flow of water from the river banks to the centre of the channel - this protects the river banks from being eroded by the force of running water - thereby reducing the amount of sediments that flow into the river ♣

Meth od

Effective strategy as a deeper river channel (with less sediments), increases the capacity of the river to hold water. This prevents water from overflowing its banks during heavy rain, hence reducing the chances of flooding. Description & Effectiveness

e.g. The West London Dyke is 2374 metres long, and runs along the west bank of the North Thames River Limitations/ Ineffectiveness/ Drawbacks

 It is the planting of vegetation such as



Weight of vegetation could also contribute to bank stresses, causing bank collapse and eventual sediment build-up in river channel.

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Planting of vegetation may also result in the loss of marine life. - ecosystems may be destroyed in the process of channelisation - excessive shade prevents sunlight from reaching the river bed and this kills aquatic plants and affects aquatic food chains

mangroves to protect river banks against erosion (i.e. minimise bank erosion).  Vegetation also slows down the rate of surface

runoff. -

4.

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prevents large amounts of water from entering the river at any one time reduces the amount of eroded sediments being transported into the river thus, the channel does not become shallower, reducing the chances of floods

 Effective as a longer term solution as vegetation

slows down the rate of surface runoff - roots of trees and other plants anchor/hold soil together firmly, stabilising the banks - prevents large amounts of water from entering the river at any one time - reduces the amount of eroded sediments from being transported into the river - thus, the channel does not become shallower, reducing the chances of floods  Thus, planting of vegetation prevents the silting

of rivers (i.e. piling up of sediments) and reduces the chances of floods  E.g. Pasir Ris Park (Singapore), Sungei Api Api

Meth od

Description & Effectiveness

Limitations/ Ineffectiveness/ Drawbacks

 Dams can be constructed to temporarily store

rainwater that falls upstream during heavy rainfall  Dam construction is multi-purpose and not a

solely channel management strategy -

can provide flood protection, maintain normal running water functions, store irrigation water, and create hydroelectric power

5.

 E.g. Three Gorges Dam (China)

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constructed across the Yangtze River due to the 1998 Yangtze River floods about 325km upstream from Shanghai and is controlled by sluice gates to control river flow effective as frequency of flooding downstream has reduced in 2007, during the heavy summer rains, the Yangtze did not flood as the dam released limited amounts of water into the Yangtze and trapped excess rains in the huge reservoir behind it

 Success of dam is often enjoyed at the expense of

good farming land - upstream: good farming land behind the dam has to be permanently flooded - downstream: prevents deposition of silt (i.e. very fertile soil) in the lower course of the river, reduces the fertility of the soil and affects agricultural productivity in the floodplains in the lower course  In China, the huge weight and water fluctuations in

the reservoir have caused crumbling cliffs, severe soil erosion, fatal landslides and ecological damage in the areas around the dam.