Fluvial and Alluvial systems

Question 1

Braided River Systems

Answer 1

• Multi-thread channels, high energy, steep valley gradients (<0.5o), large and variable discharges, non-cohesive banks, bedload transport
• Grade from gravel to sandy-bed rivers
• Large and variable discharge = seasonality in braided rivers – large flux of sediment movement and then not much
• Bedload transport – bed of river does the transporting
• Bedforms
• Braid bars, Dunes, scours, bedload armouring

Question 2

Braid Bars

Answer 2

• Creation
o 2 channels coming together creates eddy
o Causes scour
o Sediment is picked up and moved downstream quickly
o Acts as a point of nucleation
o Bifurcation will cause drag
o Will slow flow down and cause deposition as its not a flat smooth bed anymore
• Start of bar are lozenge shaped and flow bifurcates around it, causing deposition again
• Coarser in the middle – coarse material dropped first and then finer material placed on top – fine upward
• Bank bars are more restricted and do not move as much

Question 3

Pebble Clusters

Answer 3

• Pebble Clusters:
o Pebble clusters are an excellent example in how drag can influence what is entrained in to a flow and also be used as a palaeocurrent indicator (palaeoflow of river is from right to left according to the pebble cluster).
o Pebble clusters
o Bedflow armouring – boulder or oversized clast moved in rare weather event – regular flow packs smaller clasts behind it – larger ‘v’ on upstream, smaller fine grained sediments on downstream - stabilizes river system
o Larger pebbles on opposite flow direction side

Question 4

Trough cross bedding

Answer 4

o	3D bedform 
o	Mostly found in braided systems
o	High lower flow regime
o	Undulation ‘ice-cream scoop’ on surface
o	Ancient structure

Question 5

Overall Braided log?

Answer 5

•	Lots of bars
o	Transverse bars or Longitudinal bars
•	Coarser grained 
•	Straight multi-channeled
•	Grading up bars
•	Blocky grainsize profile
o	Waning into finer material
•	Little variation

Question 6

Meandering stream

Answer 6

• Single channel, high sinuosity (channel length/channel reach length > 1.3)
• Migrate by selective bank erosion, point bar deposition, meander cut off and avulsion
• With bankfull discharge cross channel shear - helicoidal flow
 Only time processes take place, during normal conditions sediment does not move
• Create terraces as they cut down through topography
• Important features
 Fining up point bars (deposition)
 channel avulsion
 How the banks are selectively eroded and migrated
 crevasse splays
 Scroll bars – surface impressions – channel margin held up by sandy levees which doesn’t compact at same rate as the mud of the flood plain so it protrudes as the channels migrate without it – shows paleochannels

Question 7

Helicoidal flow

Answer 7

• Helicoidal flow- faster erosion on the right and then the water ‘sloshes’ back and overturns to the right, causing deposition
• Outside of bend – faster, more erosion
o Leads to Dunes
o Ripples and bioturbation on inside bend

Question 8

Cravasse splay

Answer 8

• Crevasse splay – breach in levee leads to deposition of sands
o Usually on outside of meander bend
 Splays sand out into flood plain
 Well sorted, mature sands
 Can get sand in muddy flood plain environments therefore
 Flash flood like – horizontal expansion and deposition of load
 Thin and wedge away from channel
 Small scale lamination within

Question 9

Alluvial fan overview?

Answer 9

• Small discrete body of sediment – with apex and toe
• A semi-conical, downstream fining, sediment accumulation predominantly of alluvial origin, resulting from loss of transporting capacity due to horizontal flow expansion
• Stream emerges from a mountain belt and deposit broad cone shaped bodies of sediment
• Unconfined flow leads to rapid lateral expansion
o Friction causes fast deposition
• Radius typically 2-15 km
• Conglomerates, breccia, sandstones and mudstones
• Coarsest sediments found closest to mountain front (proximal) and finest further away (distal)
• alluvial from Latin “to wash against” vs. fluvial (Latin “river”)
• point-sourced from streams issuing from a drainage catchment.
• fan toe grades gently into a basin floor environment
• coalesced fans, particularly along faulted mountain fronts Bajada
• Source area: Mass wasting (e.g. mud flows, debris flows, rock falls) leads to gravity flows (mass flows). Stream water can later rework the deposits out across the fan.
• Slopes less than 10 degrees, <100km2
• Shape related to grain size