Rivers

Rivers in ‘bank full’ stage

It is the flow stage of a river in which the stream completely fills its channel and the elevation of the water surface coincides with the bank margins.

Channelization

Channelization 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. In order for this to happen, various engineering methods are used to alleviate the problems of flooding.

Types of channelization

Channel managements are carried out to facilitating navigation, regulating the speed of river, protecting the river banks against erosion and controlling floods by managing the volume and speed of flow of water. There are 4 methods used, they are river re-alignment, river resectioning, vegetation planting and bank protection.

River re-alignment

The straightening of the river channels removes meanders and reduces the length of the river channel. It also increases the speed of flow, reducing friction. And because of this increase in the speed of flow, sediments get washed off easily. The river re-alignment involves the deepening, flattening and smoothening the river channels. An example of this is the Kallang and Singapore rivers. The advantages of this method it is that is able to hold more water. Thus reduces incidence of flooding in the upper part of the stream. Another advantage of this is that the speed of the river is able to carry flood water away quickly. However, the disadvantage of this method is that it is too costly and it cannot prevent large floods and it also increases the erosion of the river channels due to the increase in velocity.

Re-sectioning

Re-sectioning involves the widening and deepening of the channel it increases the channel’s ability to hold more water and decreases incidence of floods. Therefore, more surface runoffs enter the rivers. River banks and beds are replaced with cement and granite, this causes less friction between water and river bed and banks allowing the speed of flow to increase. The water flows away from the area faster. This method is usually used in urban areas to alleviate flooding.

Bank protection

Bank protection is built to prevent floods and reduces soil erosion along the banks. This process involves building artificial levees / dykes / embankments on the sides of river channel to increase the height of the river bank to contain flood waters. Walls of sand, stone, or concrete increase channel capacity to hold water but it is not very effective (built up of sediments on river bed, channel becomes shallower). Building gabions or revetments protects the river banks from being eroded while reducing the amount of sediments washed into rivers.

Planting Vegetation

Roots of trees and other plants hold the soil together firmly this protects the river banks, reduces amount of eroded sediments, Slows down the rate of surface runoff. It also prevents large amounts of water from entering river at one time as the roots of the trees absorbs the water in the river. The disadvantage of this however is that excessive shade prevents sunlight from reaching the river bed, killing aquatic plant and animal life. The destruction of habitats is due to the building of concrete banks. This is ineffective in urban areas and time is needed for these trees to mature.