Channelisation

Question 1

Channelisation

Answer 1

Attempts by humans to modify the shape and size of the river channel in order to increase the capacity and hydraulic efficiency of the channel.

Question 2

Methods of channelisation

Answer 2

1. re-sectioning
2. re-alignment
3. dredging

Question 3

Re-sectioning

Answer 3

This involves changes to the rivers cross-sectional profile, by widening / deepening the river. As a result, the river has:
- a greater capacity so it can hold more water before overflowing its banks onto the floodplain
- a greater efficiency so it allows water to flow at high velocities and so water can be removed from this section of the river more quickly, reducing flood risk at the channelised section.

Question 4

Re-alignment

Answer 4

This involves the straightening of a river channel, either by replacing an entire stretch of meandering river with a straighter channel, or creating cut-offs across meander bends.
- this improves the river’s efficiency by increasing river velocity. Velocity is increased as the removal of meanders results in an increase of channel gradient.

Question 5

Dredging

Answer 5

Dredging is the removal of accumulated silt and sediment from the bed of the river by diggers and pumps. Rivers naturally deposit material that has been eroded from upstream. Overtime, this can cause the channels cross-sectional area to decrease, reducing the capacity of the river.
- this can also dislodge sediment from the beds and put it back into the main current.

Question 6

Impacts of channelisation

Answer 6

1. Instability
2. Increased flood hazard
3. Aesthetic consideration

Question 7

Instability

Answer 7

Natural river channels have dynamic nature due to processes of erosion.
Engineering schemes immediately change the characteristics of the river and as a response to the unnatural changes, the river may flow faster from one reach of the river creating a new regime downstream.

Question 8

Increased flood hazard

Answer 8

Moving a greater volume of water quickly to an unchanged part of the river channel will increase the risk of the river exceeding its bankfull discharge and inundating its floodplain.
Urban areas have concrete-lined, narrow channels. When flow is concentrated in an engineered channel there is no floodplain storage, which means that flood peak is greater and lag time is reduced.

Question 9

Aesthetic consideration

Answer 9

Heavily engineered channels are rarely attractive in appearance. Natural features like meander bends, sparkling riffles, deep pools, and wetland environments give an area scenic and amenity value for the local community eg river used for sport such as angling or canoeing.

Question 10

Impacts to river ecology

Answer 10

1. pools and riffles when providing a ranging of habitats for plants and animal life
2. channelisation usually modifies river banks
3. plant cover on river banks reduces erosion

Question 11

Pools and riffles provide a range of habitats for both plant and animal life

Answer 11

• Riffles or pools are natural elements of river meanders, and they provide a range of habitats for plants and animals.
• Coarse gravel beds are used by fish laying their eggs, while pools provide shelter for resting fish, especially in times of low flow where these are shaded by overhanging vegetation.
• The riparian environment will be directly impacted under many engineering schemes eg improved floodplain drainage makes it drier so the flora found in marshes and wetland will suffer.

Question 12

Channelisation usually modifies the river banks…

Answer 12

either through regrading of their profile or the clearance of vegetation. Bank vegetation provides food and shelter for insects, mammals and birds that make sure of the river.

Question 13

Plant cover on river banks…

Answer 13

reduces erosion as roots bind the soil particles and plants shade the river, helping to moderate changes in stream temperature.
- this latter point is critical to many plants and animals that are sensitive to wide fluctuations in light and temperature conditions.

Question 14

The River Main Drainage Scheme Public Enquiry

Answer 14

Designed to improve the land drainage for 4,000 hectares along a 20km section of the River Main, County Antrim. It ran from ballymena and into the Lough Neagh. The proposal involved the enlargement, degrading, and realignment of river channel. The enquiry considered the views of locals, such as local and national representatives of angling clubs, local farmers and farming organisations. This potential economic impact included changes such as restoring pools and replacing boulders for fish shelter, retaining some river bends, and replacing trees on the banks.