Fluvial geomorphology Flashcards
who invented stream order system
Strahler
who invented stream magnitude system
Shreve
4 sources of streamflow in a small catchment
channel precipitation, overland flow, groundwater leading to baseflow, interflow/throughflow
what is the max rate for water entry into the soil called
infiltration capacity
3 factors influencing infiltration capacity
vegetation, soil, slope
which soil types allow the least and most infiltration
sand= most, silt- medium, clay= least
is there more infiltration in grasslands or forests
forests
which type of overland flow is most common in deserts
infiltration excess overland flow
what is the coefficient of runoff
ratio indicating the percentage of rainfall thats converted into overland flow (how much flows awat when all other factors considered)
what is the dominant method of water flow to rivers
throughflow
what type of pores can lead to bogbursts
matrix flow
name a process that can create macropores
bioturbation
what pores allow for rapid preferential water or chemical transport
macropores
in what climates is pipeflow common
arid and semi-arid areas
give Darcy’s law and what the letters stand for
q=KIA q- discharge per unit time K= saturated hydraulic conductivity I= hydraulic gradient A= area being tested
what does Darcy’s Law calculate
the amount of water flowing through a porous substance
what graph is used to calculate the amount of water in a river
hydrograph
what are the values on the axes of a hydrograph
time and precipitation
what is the stage of a river
its height
what do people mean when they say a 1 in 100 year flood>
1% chance of a flood in every year
why might rain gauges not measure accurately
other types of precipitation, local differences in rainfall
what device is used to measure river flow
river flow gauge
3 main methods of river discharge measurement
velocity-area method, acoustic gauging, dilution gauging
how do you determine the efficiency of the catchment
compare amount of water that entered the catchment as precipitation with the amount that came out
name 8 factors that affect evapotranspiration rates
solar radiation, air temp., evaporation surface temp, wind speed, humidity, turbulence, plant biology, water availability
how can potential evapotranspiration be measured
lysimeter-> block of soil isolated from surroundings and water balance measured
give 2 methods for measuring evaporation
atmometers- give direct readings of evaporation, evaporation pan- shows fall in water level
what technique is used to measure soil water content
gravimetric- particles weighed, heated and reweighed dry
when is the water table not flat
after heavy rain, lies in hollows at surface
what percentage of earths non-frozen freshwater supplies are groundwater?
97%
what is stemflow
flow of water down plants allowing water to reach the hillslope
what is interception storage
storage of precipitation on leaves and tree trunks
5 factors affecting surface entry of water
vegetation cover, soil texture, porosity, structure, compaction
does well-developed humus and leaf litter layers increase or decrease infiltration capacity
increase
what type of flow occurs in the different pore sizes
laminar in matrix pores, turbulent in macropores and pipes
which sized pore flow is determined by Darcys Law
matrix flow
when does lateral throughflow occur in the matrix
when hydraulic conductivity declines with depth
which pores are filled first when water enters soil
small pores, due to greater soil water tension
difference between aquitards and aquifers
aquifers- layers of rock porous enough to store water and permeable enough to allow water to flow through, aquitards - aquifers confined between impermeable rock layers
what is the baseflow on storm hydrographs derived from
groundwater sources- the “slowflow” element
what controls the amount of baseflow
seasonal variations of precipitation, evapotranspiration and vegetation
displacement flow
soil water rapidly pushed out by new water entering at the slope bottom
flow duration curves
hourly or daily flows grouped in discharge classes where the percentage of time that any particular flow is equalled or exceeded is plotted
what does a steep flow duration curve mean
highly variable flows with a large stormflow component
what does a gentle flow duration curve mean
large baseflow component
what is a river regime
a seasonal variation in river flow that tends to be repeated annually
how is a river regime often expressed
monthly discharge
3 things that can control the river regime
climate of region, catchment geology, soils
when is discharge greatest in glacial regions
early summer due to melt
fluvial flooding
river overflows its banks
pluvial flooding
heavy rainfall leads to concentrated overland flow inundating an area
groundwater flooding
concentrated zones of saturation-excess overland flow
coastal flooding
tidal surges, storm surges and tsunamis
4 major river regimes
- dominated by snow and ice melt
- temperate oceanic areas
- tropical
- equatorial
snow and ice melt river regime
major peak in late spring/early summer
temperate oceanic areas river regime
precip all year, autumn/winter maximum, evapotranspiration peak in summer
tropica river regime
high precip in summer, dry season in winter, all year high evapotranspiration
equatorial river regime
bimodal distribution of precipitation
flashy regimes
steep mountainous areas: intense precipitation episodes and large flood peaks
subdued regimes
lowland catchments with permeable rock: smaller flood peaks
how does sediment usually enter a river
in pulses, often seasonal
where does most river sediment come from
headwater in the mountaisn
what conditions lead to the most sediment in rivers
250-350mm precip and sparse vegetation
what major factor can alter sediment yield in rivers
humans- ploughing, deforestation, construction
typical sediment in tropical
silt and clays
typical sediment in temperate
silts and clays, some sands and gravels in mountains
typical sediment in arctic and alpine
coarse and silt sized
5 degrees of freedom in channel morphology
slope, channel width, depth, channel planform, bed roughness
describe Horton’s stream order
source stream = first-order stream, two first-order channels meet to generate a second-order stream
what was Strahler’s adaptation to Horton’s stream order
main channel remains a first-order stream along its length
main stream lenght
distance of main river channel from source to length
total stream length
combined length of all components of the channel network
drainage density
drainage area divided by the total stream length
bank-full conditions
when discharge just fills the channel (flow resistance at a min)
what is the hydraulic radius of a river cross section
the area divided by the wetted perimeter
3 broad types of channel planform
straight, meandering, braided
what is channel sinuosity
ratio of the length of river between two points to the length of the valley between these two points
what is the sinuosity of straight channels
less than 1.5
what is channel multiplicity
total length of perimeters of sand and gravel bars divided by length of channel reach
how can channel multiplicity change
as water levels rise and fall
what is the imbrication of particles at a river bank
long axes of particles aligned along the flow direction
2 principal forces governing velocity of water flow and amount of flow in a channel
gravity and friction
what does the Manning equation measure
the mean water velocity in an open water channel
isovels
lines of continuous velocity
where are velocities usually greatest in a river
mid channel, close to stream bed
thalweg
area of maximum velocity in a river- moves from the inner to the outer bend
can can cause erosion in a river despite shear stress being quite low
turbulence and eddies
hydraulic sorting
finer particles moved downstream more easily
stream competence
largest size of particle a stream can carry as bed load
stream capacity
max volume of sediment a stream can carry
dominant discharge
the flood discharge that achieves the greatest total geormorphic work
dynamic equilibrium in river channels
channel form oscillates around an average
relaxation time
time taken for a channel to return to original form after a flood event
what factors usually change downstream
cross sectional area increases, boundary roughness decreases, velocity increases
3 processes forming potholes in bedrock channels
corrasion, corrosion and cavitation
autogenic river channel change
fluctuations in channel form about an equilibrium condition
allogenic river channel change
adjustments of channel form in response to a change through time in the sediment and water regime of a river
avulsion
process where channel shifts from old to new course, leaving intervening area of floodplain intact
effects of wetland removal
runoff conducted downstream faster in greater volumes, flashier regime
impact of river engineering (removing meanders)
flashier regime and more likely to flood
what is the wetted perimeter
the part of the perimeter in touch with the river
main type of flow in fluvial systems
turbulent
what does Reynolds number determine
if the flow is laminar or turbulent
what is a flow and riffle
pool: streaming flow
riffle: shooting flow
what is subcritical flow
tranquil, streaming flow, Froude number less than 1
what is supercritical flow
rapid or shooting flow, froude number over 1
hydraulic jump
sudden change from supercritical to subcritical flow
what does the Hjulstrom diagram show
the relationship between flow velocity and particle size (what velocity will cause entrainment)
3 transport methods for load
suspension, saltation and traction
effect of suspended load on the river flow
reduces the inner turbulence and makes it look muddy
why may water with high suspended load not mix easily with normal water
it is denser
3 main channel types
bedrock channels, alluvial channels, bedrock/alluvium mixed channels
7 storage locations for sediment in the drainage basin
slope storage, alluvial fan, colluvium, floodplain deposits, channel deposits, delta, lake bed
3 zones in a catchment
zone 1: production, zone 2: transfer, zone 3: deposition
4 factors controlling the reworking of sediment stores
type of store, volume and calibre of sediment, vegetation cover, distance from the channel
2 types of floodplains
lateral and vertical accretion floodplains
helicoidal flow
circular pattern flow that moves sediment around the bend
lateral accretion floodplain
meandering rivers, banks build outwards, scars of old meanders left behind,
vertical accretion floodplain
fine material that settles out of suspension during overbank flows, graded bedding, levees (coarser sediment dropped first)
features of a non-cohesive meandering floodplain
point bars, lateral accretion
features of a low-energy cohesive floodplain
natural levees, vertical accretion, back swamp, crevasse splay where natural levees are breached
paired terrace
matched with terrace across the valley
unpaired terrace
migrating river removes evidence of the opposite terrace
characteristics of braided rivers
abundant bedload, erodible bamks, high stream power, variable discharge
characteristics of meandering rivers
lower stream power, smaller sediment size, lower gradient
3 types of meandering rivers
fine-grained, low-sinuosity fine-grained, anastomising fine0grained
