Rivers Flashcards
what is river planform
the configuration of the channel in a plan view or aerial
what is an alluvial river
a river that flows through the sediments or alluvium
what is a reach
a length of river that has uniform appearance in both character and behaviour.
what is a segment
an alternating sequence of reaches
what is a geomorphic unit
a discrete channel or floodplain unit
what is lateral accretion
coarse sediment deposited inside a meander bend which gets incorporated into the floodplain.
what is vertical accretion
the vertical rise of a floodplain due to the overbank deposition of suspended load.
what are levees
a ridge along the channel with suspended and bedload couplets due to overbank vertical accretion
what is a backswamp
the lowest part of the distal floodplain which has fine grained sediments
what is a crevasse splay
when the levee breaches depositing bed load.
what is a cutoff
when a meander bend cuts through its neck leaving a paleochannel
what is the energy, sediment mix and valley setting of high energy floodplains
- > 300 W/m2
- non cohesive sands and gravels with some boulders
- confined and partly confined valleys on high slopes
what is the landscape position, formation and reworking processes and river type of high energy floodplains:
- uplands and headwaters
- vertical and abandoned channel accretion
- floodplain stripping, channel widening and floodchannels
- confined valley and partly confined
what sculpted features are found with high energy, high slope
waterfalls, step pool, cascades, rapids, riffles and runs
what mid channel depositional units are found
boulder mound, longitidunal bar, diagonal bar, transverse bar, islands and gravel/sand sheets
what bank attached features can be found
lateral bars, point bars, ledges, tributary confluence bars and benches
what sculpted fine grained erosional features can be found
sculpted point bars, sculpted lateral bars, runs and poolsha
what are the two mechanisms of accretion that build floodplains
lateral and vertical accretion
what is the energy setting, sediment mix, valley setting and landscape position of medium energy floodplains
10-300 W/m2, non cohesive sands and gravels, partly confined and middle to lower reaches
what is the formation and reworking processes and river types of medium energy floodplains
vertical and or lateral accretion
avulsion, cut offs, channel expansion and floodchannels
braided, meandering and wandering gravel bed
what is the energy setting, sediment mix, valley setting and landscape position of low energy floodplains
<10 W/m2
cohesive sands, silts and clays
wide, alluvial valleys on low slopes
lower valley reaches
what is the formation and reworking processes and river types of low energy floodplains
vertical accretion
abandoned channel and avulsion
anastomosing, discontinuous watercoarses
what are flood channels
a channel cut through the floodplain initiated upstream by flows shortcutting the floodplain pocket
what are cut offs
a shortening of the channel by cutting through the neck of a meander bend
what is avulsion
occurs when a river is ready to shift channel state. a decrease in channel stability over time may lead to a breach of avulsion threshold.
what are the 3 process zones in a catchment
source, transfer and accumulation
what does a squashed hydrograph mean
long distances between tributaries, not geomorpically significant and dont experience flash floods as often
what does a peaked hydrograph mean
short distance between tributaries, geomorphically significant and experiences flash floods.
how to calculate relief ratio
maximum relief / basin length
why are most rivers show a concave longitudinal profile
because they are trying to erode to base level
what makes longitudinal profiles irregular
geological and long term environmental changes
what is sediment limited transport
when a river doesnt have enough sediment to transport due to higher carrying capacity. these rivers will be erosional and incise (headwaters region)
what is transport limited transport
when a river cant move the sediment. sediment is too large (competence limited). the sediment supply exceeds the capacity of transport (capacity limited)
what happens along the drainage area of a catchment
bedload size decreases, stored alluvium increases and width, depth, flow velocity and discharge increases
what are the 5 scalar components of a river landscape
catchment
landscape unit - valley setting
reaches - gorge river, meandering stream
geomorphic units - benches, cutoffs
hydraulic unit - water characteristics such as turbulent flow
formula for total stream power
() = yQs
y =specific weight of water
Q = discharge
s = slope
what are imposed boundary conditions
they do not adjust over geomorphic time, produce the valley setting for a river (confined or unconfined) and contain the base level of the river
what are flux boundary conditions
dynamic with sediment and flow regimes. (EDIT)
a river with single channel, <1.5 sinuosity, reflect imposed boundary conditions and laterally stable
straight rivers
a laterally unstable, with multiple thalwegs, indicative of high and low flow conditions, sinuosity <1.5 and where flow diverges and rejoins multiple times
braided river (tasman river)
absent or discontinuous channels, laterally unstable and prone to channel infilling and incision
discontinuous coarse rivers (down south near bega)
usually single channel, high sinuosity and laterally variable. contains mixed or suspended load
meandering river (colo river)
up to 3 channels, bedload dominated, water flows around islands or vegetated bars
wandering gravel bed rivers (rivers in NZ)
a multi channeled, low sinuous river with laterally stable channels that are prone to avulsion. the channels are separated by floodplains
anastomosing rivers (outback aus)
what valley setting has discontinuous floodplains
partly confined
what valley setting has isolated or absent floodplains
confined
what valley setting has continuous floodplains
laterally unconfined
what are features of confined rivers
isolated or absent floodplains
found in headwater regions
sinuous <1.5
bedrock dominated
laterally stable
floodplains are prone to stripping
what are features of partly confined rivers
discontinuous floodplains
sinuousity <1.5 due to channel length = valley setting
found in moderate slopes
what are features of laterally unconfined rivers
abut valley <10%
continuous floodplains
found in low slopes
suspended load dominated
what is a planform controlled river
abuts the valley margin 10-50%, highly variable sinuosity with gravel, sand or fine grained material
what is a margin controlled river
abuts the valley marging 50-90%. sinuousity is low due to valley length = channel length and is bedrock and bedload dominated
gravel braided bars indicate?
longitudinal bars
sand bed braided bars indicate?
transverse bars
what kind of bars can you find in a wandering gravel river
vegetated bars with fine grained sediments
what are wandering gravel bed rivers prone to
avulsion
what is the continuum of rivers through a system
plateau - chain of ponds, discontinuous water courses
steep headwaters - gorge, single channel, isolated floodplains
mid slope - partly confined margin controlled river
mid slope - partly confined planform controlled river
mid slope - high energy braided river
low slope - wandering gravel bed river
low slope - anastomosing
low slope - floodout
what is the main floodplain process in steep headwater regions
vertical accretion
what is the main floodplain process in mid slope regions
lateral accretion and braid channel accretion
what is the main floodplain process in low slope regions
vertical accretion
what are partly confined rivers prone to
channel expansion and contraction
what are braided channels prone to
thalweg shift
what are anastomosing rivers prone to
avulsion
what are meandering rivers prone to
meander lateral shift
what are forced units
geomorphic units that are created by resistance or obstacles such as large woody debris, vegetation or bedrock
what are compound units
a range of flow and reworking events that create multiple set of geomorphic units
what is the continuum of grain size in high energy high slope to low energy low slope in geomorphic units
coarse grained to fine grained
what is genetically linked in geomorphic units
processes that get reworked and affect another unit such as pools and riffles or scroll bars that build ridge and swales
what are the four categories of geomorphic units in order
sculpted erosional
mid channel depositional
bank attached depositional
sculpted erosional fine grained
what are the sculpted erosional geomorphic units that can be observed in high slope regions
waterfalls, step pools, cascades, rapids, runs, rifffles, pools and boulder mounds.
what is the form and process of waterfalls
stepped bedrock with high turbulent flow.
represents headwater knickpoint retreat and important for dissipating energy
what is the form and process of step pools
staircase like with bedrock, boulder clasts or woody debris creating areas of turbulent flow and laminar flow.
important in dissipating energy and development influenced by local sediment supply and transport conditions
what is the form and process of cascades
interlocking bed material, separated by tranquil flow
jet and wake action between larger clasts creates tranquil flow inbetween
what is the form and process of rapids
ridge like sequence of arranged boulders in irregular transverse ribs.
the boulders are structurally aligned during high energy flood events
what is the form and process of runs
featureless bed, smooth flow or imposed shallow channel like features that connect pools.
caused due to plane bed creating smooth conveyance of water
what is the form and process of forced riffle
gravel or boulder accumulation that act as steps due to increased roughness.
what is the form and process of forced pool
scoured features after a forced riffle caused by LWD
fine grain accumulation during low flow and then scoured during high energy events
what is the form and process of plunge pool
-pool developed after bedrock step
- corrosion, corrasion and cavitation creates scoured pool
what is the form and process of pot hole
- scoured feature in bedrock settings
- sculpted by corrasion (hydraulic and abrasive action of water)
what is the form and process of mid channel riffle
- an organised cluster of bedload that spans the channel and act as sediment storage zones that are tightly packed.
- deposition imposed by sediment storage during high energy events and low flow events maintain it.
what is the form and process of mid channel pool
- tranquil flow at low flow occuring at concave banks in alluvial rivers
- during high energy events, scouring occurs and sediment stored is stripped.
what is the form and process of longitudinal bar
- tear drop shaped bar with coarse grain to fine grain anatomy
- there is too much sediment for river to transport around the bedload deposit and fine material is trapped in the wake.
what is the form and process of transverse bar (linguoid bar)
- abrupt channel expasion, with avalanche face which is ramp shaped. arcuate shaped
- flow moves through centre of bar, and sediment deposited on lee side. this moves the bar downstream as a rib
what is the form and process of diamond bar
- diagonal shaped spanning channel
- indicates reworking of riffles and oblique flow creates bars
what is the form and process of boulder mound
- boulders without matrix
- forms during high flow events and wake allows smaller clasts to form behind larger boulders
what is the form and process of sand sheets
- homogenous sheet of sand draped over entire bed
- formed when transport capacity is exceeded or competence is decreased. moves downstream as a pulse
what is the form and process of forced mid channel bar
- induced by diverging flow around vegetation or LWD.
- coarse grained to fine grain, and may scour the bed in the secondary flow depending on flow stage
what is the form and process of lateral bar
- attached to the bank or bench, found at low sinuous reaches
- formed by lateral and oblique accretion
what is the form and process of scroll bar
- an elongate ridge along the convex bend and develop ridge like shape
- associated with laterally shifting point bars and formed due to helicoidal flows
how does a ridge and swale sequence form
lateral shift of point bar that contains scroll bars creates ridge and swale topography as is moves laterally.
what is the form and process of point bar
- found in convex bends of meandering systems. fine grained as it progressives downstream.
- laterally shifting from deposition on convex bank and erosion on concave bank.
what is the form and process of tributary confluence bar
- formed immediately at mouth of confluence with poorly sorted gravels
- generation of secondary flows at high flow events promotes sedimentation
what is the form and process of ridges
- found on top of bars in elongate shape. may be formed downstream of vegetation
- reflects channel adjustment during high flow events
what is the form and process of chute channel
- cuts through a bank attached bar
- scours during over bar flow and reworks if constant
what is the form and process of bench
- sediment storage, stepped unit with obliquely accreted mud deposits in suspended systems and sand deposits in bedload systems.
- formed by oblique and vertical accretion during small to medium flooding events in widened channels. during rising floods, bedload is deposited and during waning stages, suspended load deposits, creating flood couplets
what are benches important for
channel contraction in over widened channels
what is a flood couplet
layer of bedload sand or gravel overdraped by suspended load (mud or silt)
what is the form and process of ledge
- flat topped, elongate unit and has same materials as floodplain
- erosional due to channel widening and unpaired ledges reflect lateral shift.
what is the form and process of sculpted lateral bar
- forms along low sinuous fine grained reaches
- formation due to straight reaches beginning to develop sinuous path.
what is the form and process of sculpted point bar
- inclined toward channel at low angle
- sculpted from sediment of adjacent bank and may form from oblique accretion of suspended sediments
how are levees formed
overbank accretion with vertical accreted coarse grained sediments
what way is the helicoidal flow downstream on a right hand bend
anti clockwise
what happens in the shear zone
sediment transport from thalweg to convex bank through helicoidal flow
what is distal fining
along the floodplain the grain size fines as it moves from proximal to distal floodplain
what is upward fining flood couplets
scouring during beginning stages of flood, with deposition of coarse grained suspended load during waning stages followed by fine grained suspended load
what does counterpoint accretion create
concave bank benches making 20% of floodplain in actively moving meandering rivers
what is lateral migration
related to lateral shift of rivers, as bends get tighter, potential for thalweg to erode is greater.
what is engineering time
event driven changes due to extreme events such as 1 in 100 year flood events
what is geomorphic time
process-form over timeframes of individual events to lagged responses over tens, hundreds or thousands of years
what is geological time
over time frames of millions of years, with tectonic setting exerting primary control on topography and imposed boundary conditions
how do confined rivers evolve
over time, incision and valley expansion allows room for floodplains to develop and to become partly confined. this alters the imposed boundary conditions.
how are partly confined rivers evolved
from low sinuousity to meandering streams
how do laterally unconfined rivers evolve
changes to flux controls, such as reduced energy resulting in braided river into wandering gravel bed or increased sediment in passive meandering into an active one
what are direct factors on river evolution
riparian vegetation removal, dams, channelisation and gravel extraction
what are indirect factors on river evolution
catchment deforestation and urbanisation causing increased sediment yields, increased incision, stream power and discharge
what is a sediment slug
a pulse of sediment through a river causing channel widening as it fills pools and aggrades the bed. homogenises floodplains
