GEOMORPHOLOGY OF RUNNING WATAER Flashcards
Most important agent of landscape formation
water
other factors may be locally dominant
distribution of earth’s water
fresh water = 3%
saline = 97%
distribution of fresh water
ice-caps and glaciers= 68.7%
ground water = 30.1%
other = 0.9%
surface water = 0.3%
distribution of surface water
rivers = 2% swamps = 11% lakes = 87%
why does the total amount if water on earth’s surface stay constant?
due to processes associated with the hydrological cycle
what are the interconnected elements of the hydrological cycle?
- precipitation
- interception
- evaporation
- evapotranspiration
- filtration/percolation
- surface runoff
- ground water
precipitation
clouds form due to ascent of saturated air parcel and associated cooling
Rain/snow results from coalescence of cloud water droplets.
Fog/mist is the same, except that the cooled air is found close to ground surface
spatial variation
Distribution and intensity of rain is highly variable, e.g. rain shadow; high spatial variability
areas in the rain shadow (it rains on one side of the mountain and the water cant pass to the other side of the mountain) are often very dry
spatial vaiability
means that some places get more rainfall than others
temporal variability
differences over time
short-term variab
seasonality
long term variab
intensity of precipitation
can also be variable
how much precip falls in a set amount of time??
intensity of precipitation
can also be variable
how much precip falls in a set amount of time??
precip variability in arid regions
lower precip
more variable precip
less reliable precip
interception
- rain doesnt always fall directly on the grounf
- vegetation cover - precip is intercepted by plant foliage
geomorph importance of interception by foliage?
vegetation protects the land surface from direct impact of rainfall - reduces raindrop impact
vegetation also protects the soil through the binding action of plant roots
evaporation
-causes the huge difference between precip inputs and outputs along with transpiration (evap - storage and trans - comsumption)
evapotranspiration
accounts for 84% of precip in Africa
runoff ratio/ co-eff
ratio of total rainfall to total runoff in a catchment
low in semi arid seasonal rainfall climates such as across large areas of southern africa
Infiltration
depending on rainfall intensity and the nature of soil some precip enters soil by infiltration
what is infiltration capacity
maximum rate of absoption – if this is exceeded then surface water runoff happens
what factors influence infiltration capacity
soil conditions and land-use
-coarse vs fine grained soil
-heavy used land
less permeable surfaces - outcome may be erosion and transport of material
what happens to water that doesnt evap or runoff
infiltrates/percolates downwards to become groundwater
groundwater
precipitation that passes from soil later into underlying bedrock and meets zone of saturation (surface of which is referred to as the water table)
ground water variability
water table follows the general shape of surface topography
water table follows the general shape of surface topography
original ideas about runoff
American hydrologist: RE Horton
overland flow occurs on a slope in response to precipitation intensities that exceed infiltration capacity
Hortonian overland flow
(infiltration excess overland) flow envisaged to occur across the landscapeand be main cause of the rapid rise of river levels during storms
problems with hortonian flow model
1) surface runoff is rarely simultaneously widespread in the landscape
2) rainfall intensities do not always exceed infiltration capacity even during severe storms
additional mechanism to explain how runoff occurs
saturation overland flow
difference between hort and sat flow
hort = rainfall intensity is the initiating factor
sat = rainfall duration is the initiating factor
interflow/through flow
lateral movement of water within soil that may produce pipes or even tunnels which transmit water at considerable speed
movement of water on slopes is a combination of flow types
hortonian overland flow
saturation overland flow
through flow
ground water flow
ultimate destination - channel/river
what is water measured as once in rivers?
discharge
the volume of water flowing through the stream per unit time
extreme discharge -
flood
changes in discharge over time are recorded on a …
flood hydrograph
discharge formula
cross-sectional area x velocity
Q
what is the flood hydrograph?
record of changes of discharge over time
what causes variation is flood hydrograph shape
conditions in the catchment
why do different surfaces have different infiltration capacities?
differences in interception (evaporation) and infiltration
what to consider when looking at contrasting flood hydrogr
natural vegtation vs agricultural vs urbanisation
different graphs flood hydros
size of drainage basin – small vs large
vegetation –
bare vs forest
valley side steepness –
steep vs gentle
soil type –
impermeable vs permeable
what processes of water do geomorph work in channels
erosion
transport
deposition
erosion
after weathering
running water erodes rock
–abrasion
–undercutting
stream ability to erose is dependent on type of flow
types of stream flow
laminar
turbulent
entrainment
once eroded material is transported in the direction of flow
types of transport in a stream
in solution
suspended load
bedload
saltation
jumping of particles
bedload
carried along stream bottom
suspended load
carried either in suspension or by saltation
solution transport
sediment is dissolved in stream water and carried along steam in solution
competence of a stream
ability of a stream to transport sediment
what is stream competence dependent on
velocity of flow
volume of flow
periodicity of flow
periodicity of flow
what results in deposition
loss of stream competence
what causes loss in stream competence?
changes in discharge
change in gradient of river bed
what does a Hjulstrom diagram depict?
relationship between erosion, transport and deposition
zones of a drainage basin and dominant processes
erosion - headwaters
transport - middle reaches
deposition - lower reaches - floodplains and deltas
what does the nature of relationship between particle size and velocity of flow determine
whether a particle will be eroded, transported or deposited
Hjulstrom diagram parameters
x axis = grain size
y ax = stream velocity
headings top x axis - fine particles - coarser particles
why is a greater velocity needed for fine particles?
clump together and form a relatively large mass
what are three human processes that impact the environment (fluvial systems)
land use changes
urbanization
dam construction
what are the impacts on fluvial systems by three human processes
hydrolog cycle is modified
- -discharge
- -channel form
- -sediment load
changes in streams following landuse changes…
in annual and seasonal discharge (may be positive or negative) e.g. in fynbos catchments the effect of a change from fynbos to forest plantation (increased biomass) is to reduce runoff
–flood characteristics (increases or decreases)
–water quality e.g. fertilizers
– erosion (increases)
effects of dam construction (check written notes for better explanation)
Decreased sediment load downstream of the impoundment results in ‘clearwater erosion’ (i.e. incision of stream below dam wall due to increased competence)
Decreased sediment supply to the coast impacts beach and dune systems (sediment ‘starvation’)
Shrinking deltas and accelerated relative sea level rise
Mass of water as well as increased pore water pressures in underlying geology may increase risk of slope failure (e.g. Vaiont, Chinese examples)
