Channel Plan Forms (Braided and Meandering) PDF

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Channel Plan Forms Sinuosity Ratio: Ratio between distance along central line of river and length of the channel.  Shows extent to which river deviates from straight course  Actual channel length between two points : Straight line distance between two points

Coarser sediments = pebbles and cobbles; Finer sediments = gravel, sand, silt and clay

The larger the ratio, the more meandered the river is.  Meandering River: > 1:1.5 Straight Channels (rare)  Rivers in steep slopes  Rivers that are influenced by joints in underlying rocks 

Characteristics  Consistent flow of velocity and discharge; even erosion throughout

Meandering Channel: River with a sinuosity ratio exceeding 1:1.5  Hydraulic Action and Corrasion prevalent here  Number of meanders ↑es downstream 

Geometric Features o Symmetrical, consistent form  Same channel width and meander amplitude throughout o Alternating pool-riffle sequence o Wavelength of fully developed meander = 7 – 10x as great as channel o

Geometric Features: wavelength, amplitude, crest, trough, channel width of meander River Cliff = River Bluff Slip off slope = Point bar

width Hypothesised relationship between wavelength and

√ bankfulldischarge 

Reasons for Development o Response to excess of free energy in the stream o Velocity and discharge ↑ downstream. Meanwhile sediment calibre ↓ es, becoming finer, and requiring ↓ er energy to transport o Stream becomes underworked, causing free energy within the channel, which will be expanded uniformly throughout the stream o Balance between energy to expend and energy expended is maintained, thus developing a meander.

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Process of Development o In low flow conditions, straight channels have riffles (regularly spaced sediment bars built up from coarser sediments) on the river bed. o Pools, deeper parts of the channel occur between riffles. The floor has finer sediments there. o The alternating arrangement results in a pool-riffle sequence. o In this sequence the riffles slope alternately towards one bank and then

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the other so that the thalweg (area of maximum velocity) winds between them, thus deflecting the thalweg to one side of the channel or the other. Erosion is concentrated at the concave side of channel where river is deflected, such that hydraulic action is greatest at this area, hence eroding the bank to form river bluffs After this, water is dragged across the bed to convex banks. River competence falls due to losing energy to frictional drag, erosion conducted at the convex banks and increased surface contact with the channel banks Hence deposition occurs as critical settling velocity of coarse sediment is achieved, to form point bars at convex bank This overall helicoidal flow causes continuous erosion and deposition, at the concave and convex banks respectively.

Pool-riffle sequence vs. channel size: - Distance between top of a riffle and the next is on average 5 – 7x the channel width) - Riffles occur at shallow, symmetrical areas - Pools occur at deeper, asymmetrical areas

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It causes the former to gradually retreat and the latter to advance. Eventually meander amplitude and river sinuosity ↑es, forming a well-developed meander However, when the river is too sinuous, the meander necks are cut during high discharge, straightening the river channel and forming ox-bow lakes. These lakes may dry up over time to form meander scars.

* Ox-Bow Lake: water body that forms when a wide meander from the main stem of a river is cut off, creating a free-standing body of water 

Process of Development o Corrasion and hydraulic action erode outer bends of meander

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Deposition builds up point bars. The neck narrows as erosion continues. Eventually, in periods of high discharge, heavy erosion breaks the necks through Ox-bow Lake forms after the meander is sealed off by deposition of sediments. The river path then straightens.

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The lake may dry to form a meander scar.

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Braided Channel: channels characterised by sub-divisions of water flow, that have main channels separated into smaller, interlocking channels  Associated with lateral erosion  Unstable (and everchanging); individual channels may be abandoned, buried by sediment, or eroded. However, overall characteristic of sub-divisions of water persists Anabranches: series of smaller, interlocking channels separated by islands of coarse sediment called mid-channel bars Mid-Channel Bars: islands of coarse sediment in the middle of the channels, i.e. not in contact with banks 

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Geometric Features o Banks comprise incoherent, loose sand and gravel sediment that experience strong erosion during high discharge  Facilitates lateral erosion, widening the channel to form an inefficient channel with low hydraulic radius (high width-depth ratio; i.e. width is relatively large compared to channel depth) o

River regime is unstable or markedly seasonal, has fluctuating discharge  Allows for the erosion and deposition required to form braided channels  Braiding is common in semi-arid (prone to irregular downpours) and alpine (meltwater streams or glacial outwash respond to seasonal glacier melting) regions

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Transport large % of coarse bedload, deposited during low discharge to form mid-channel bars  Peri-glacial areas’ freeze-thaw weathering on steep slopes deposits a large amount of coarse, blocky debris into rivers

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Presence of low, elongated, unvegetated mid-channel bars of sand/gravel (incoherent sediments) and vegetated islands standing above water level  Due to deposition of load when critical settling velocity is achieved  Stability of mid-channel bars varies; some can withstand erosion

Process of Development o During high discharge, increased river velocity exceeds the critical erosional velocity of coarse sediment, transporting bedload downstream o During low discharge, energy ↓es, hence achieving critical settling velocity. Overloaded rivers deposit sediment to form mid-channel bars, starting from coarse bedload with higher settling velocity, to form the nuclei of bars. Finer sediments then become progressively trapped by coarse ones. o As discharge further lowers, the water level falls gradually, exposing the

Non-cohesive sediments are unstable at steep angles and collapse if the stream cuts deeply into them Width-Depth Ratio vs. Silt and Clay Content of Bank Material = Negative Relationship - Generally wide and shallow channels develop where the bank material is loose and non-cohesive - Deeper, narrower channels develop at banks of cohesive material

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bars, which also disrupt flow and lower velocity further  The mid-channel bars also divert flow towards banks. This narrows the river channel on either side, maintaining enough velocity to transport and erode sediment at the banks.  Thus lateral erosion continues, and maintains the width-depth ratio Some of the sand and gravel mid-channel bars will be eroded by anabranches on both side during subsequent floods when critical erosional velocity is exceeded, but others will grow and become vegetated, hence becoming stable as plants trap sediment...