Hydro: Channel Planforms

Question 1

Provide one reason for meander formation

Answer 1

Need for stream to lose energy as its sediment size decreases while its discharge increases. River is underworked and needs to expend free energy.

Question 2

Formation of meander part 1 (pools and riffles four steps)

Answer 2

1) Process starts with presence of pools and riffles in straight channels caused by turbulence.
2) In low flow conditions, regularly spaced bars of sediment where coarser materials [cobbles and pebbles] has built up are known as riffles (shallow part)
3) pools in between riffles are floored by finer sediment [gravel and sand]
4) known as pool-and-riffle sequence

Question 3

Formation of meander part 2 (thalweg, erosion)

Answer 3

5) riffles slope down alternatively towards bank so thalweg winds between them
6) swing of flow induced by riffles causes zone of max velocity to be deflected to other side of channel
7) bank erosion concentrates on concave bank
8) impact of hydraulic action greatest at concave bank where thalweg is diverted against
9) forms river bluffs

Question 4

Formation of meander part 3 (loss of energy)

Answer 4

10) however, loss of energy due to frictional drag as water is dragged across the river bed to convex banks
11) expenditure of energy on erosion at concave banks and reduced depth at convex banks causes deposition of sediment load to form point bars
12) corkscrew helicoidal flow that causes erosion at concave bank
13) increase in meander amplitude and sinuosity will form a well developed meander over time

Question 5

What is sinuosity ratio?

Answer 5

Ratio of actual channel length to the straight line distance between 2 points.

Completely straight 1:1
Meandering 1:2 (or 3 or 4 whatever)

Question 6

What are braided rivers? Describe their discharge.

Answer 6

Braided channels are characterised by sub division of water flow, where there is a separation of the main channel into anabranches separates by mid channel bars. They experience great variations in discharge.

Question 7

What is the 1st main feature of a braided channel

Answer 7

1) banks are composed of loose and incohesive sands and gravels that experience strong bank erosion during high discharge. This widens the channels, resulting in high width depth ratio and an inefficient channel. (V little silt and clay and these materials are cohesive)

Question 8

What is the 2nd main feature of a braided channel

Answer 8

River regime is seasonal and unstable: fluctuating discharge enables erosion and deposition. Braiding thus common in semi arid regions prone to irregular, sudden downpours and where meltwater streams respond to seasonal melting of glaciers

Question 9

What’s the 3rd main feature of a braided channel.

Answer 9

Large amounts of course bedload deposited during low discharge to form mid channel bars eg. Peri glacial regions

Question 10

What’s the 4th main feature of a braided channel?

Answer 10

Presence of low elongated bars of sand and gravel called mid channel bars (both vegetated and non vegetated). Some are more stable & permanent than others and can withstand erosion better.

Question 11

What are the general processes involved in the formation of braided channels

Answer 11

Erosion and deposition

Question 12

Formation of braided channels part 1 (what happens during high discharge)

Answer 12

1) High discharge
2) increase in river’s energy
3) large amounts of sediment entrained as bedload
4) river banks undergo lateral erosion and collapse, giving a wider channel

Question 13

Formation of braided channels part 1 (what happens during low discharge)

Answer 13

1) low discharge
2) energy decreases
3) overloaded river starts depositing some load to form mid channel bars
4) deposition occurs starting from course bedload with high settling velocity to form the nuclei of mid channel bars

Question 14

Formation of braided rivers part 3 (mid channel bars)

Answer 14

1) as flow is disrupted and velocity decreases downstream, finer sediment accumulates and MCBs grow
2) the bar localised and narrows the river channel on either side to increase the velocity again to a point where load can be transported
3) with further decreases in discharge, water level falls and these bars are gradually exposed
4) some MCB composed of loose sediments, will be washed away during subsequent floods but others will grow and become colonised by vegetation.

Question 15

What are the differences between meandering and braided channels?

Answer 15

1) differences in sediment load amount and calibre
2) differences in width depth ratio
3) differences in channel gradient

Other differences:

• stability of channel
• stability of channel landforms
• proportion of bed as compared to suspended load

Question 16

Describe the differences in sediment load between braided and meandering channels.

Answer 16

Braided channels have a larger amount of load and coarser load (ie. Sand and gravel) and tends to roll or bounce as bedload. Proportion of bedload is higher than suspended load.
Meandering channels carry less load and load tends to be finer (ie. Clay and silt). Channels found in downstream regions and loads have undergone a lot more attrition to become reduced in size. Finer load have low settling velocity which means they get transported almost indefinitely and accumulate downstream. Therefore less bedload compared to suspended load.

Question 17

Describe the differences in width depth ratio between braided and meandering rivers.

Answer 17

Difference in channel morphology where braided rivers have a higher WD ratio than meandering ones. (Much wider and shallower)

• banks of braided channels are composed of incoherent sands and gravels that experience strong bank erosion during high discharge, widening the channel. WD ratio may exceed 300:1
• presence of helicoidal flow as undercutting and retreat of concave banks is balanced out by lateral accretion and advancement of convex banks. Thus only results in increase in sinuosity ratio.

Question 18

Describe the differences in channel gradient between meandering and braided rivers

Answer 18

Braided rivers are steeper than meandering ones. Braided rivers are more inefficient due to a high WD ratio. To compensate, braided rivers develop a steep channel by keeping a straight course. Acc to manning’s Eqn, this increases river velocity due to its positive rs with channel slope. Meandering rivers have a gentler channel slope since meandering course increases the distance of the channel while its height remains unchanged. The excess energy possessed also means less need for river to keep its course straight.