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Hydrology and fluvial geomorphology (River Channel (Land forms (Waterfalls…
Hydrology and fluvial geomorphology
Drainage Basin
System
Output
Evaporation
the change of state from liquid to gas
Transpiration
loss of water from vegetation from the stomata
Evapotranspiration
loss from both evaporation and transpiration
Store
Interception
Vegetation storage
Groundwater storage
Water in pores and spaces in rocks in the underground aquifier
Water table
level of saturated soil above the aquifier
Soil moisture storage
The amount of water held in the soil at any particular time
Surface storage
Water lies above the earth surface
Channel storage
Water contained in a river channel
Flow/transfer
Below ground
Percolation
The gravity flow of water within the soil(vertical flow)
Infiltration
water on the ground surface enters the soil
Throughflow
The lateral movement of water through the soil, down the slope towards a river channel
Above ground
Stemflow
Water runs down the tree trunk into the ground
Surface runoff/overland flow
the flow of water over the earth's surface(horizontal flow)
Channel flow
water flowing through the river channel
Throughfall
precipitation that makes it directly to the land surface
Dripflow
water flow off the leaves and drip into the ground
Input
Precipitation
Hail
Rain
Sleet
Snow
Drainage patterns
Rectangular
form on flat land
water follow through retangular network of fratures or faults
Trellised
streams follow downhill slope along the areas of eroded bedrock
Dendritic
most common pattern
form in areas with uniform bedrock
can be eroded equally in all direction
example of rocks
sedimentary rock
volcanic rock
Granite
Radial
stream radiate outwards from a central high point
for example volcano, dome
Deranged
no clear pattern
for in areas with with a lot of geological disruption such as earth quake, volcanic activities, tsunami and plate movements
The flood hydrograph
Annual hydrograph
shows pattern of seasonal variation in a river of the whole year
peaks in summer - snow melts or summer monsoon
Storm hydrograph
shows how river discharge responds to a rainfall event
Components
Peak rainfall
when the amount of rainfall is the highest during the event
Peak flow/peak discharge
highest discharge
Lag time
time between peak rainfall and peak flow
indicates how quickly the precipitation is reaching the river channel after a rainfall event
rising limb
rising discharge
steep=precipitation reach river channel quickly
falling limb
decreasing discharge
steep=water reach the mouth quickly
Influences on hydrograph
Drainage basin characteristics
Drainage basin size
greater catchment area
more water collected
high peak rainfall
rainfall event may affect only one part of the basin
long distance for water to travel
long lag time
Drainage density
high density
short distance for water to travel
water reach channel quickly
flashy hydrograph
high, sharp peak flow
short lag time
low density
delayed hydrograph
long lag time
low peak flow
water reach channel slower
long distance for water to travel
Drainage basin shape
elongated basin(left)
flashy response
short lag time
low peak flow
round basin(right)
long lag time
high peak flow
Soil type(porosity, permeability)
clay
low infiltration and percolation capacity
tiny pore spaces(low porosity)
low permeability
causes rapid overland flow
sandy soil
fast infiltration capacity
large pore spaces(high porosty)
high premeability
less surface runoff
more throughflow and baseflow
thin soil
low storage capacity
Rock type
permeable and porous rocks
store more water
more baseflow
delayed hydrograph
impermeable and non-porous rocks
low water storage
more surface runoff
flashy hydrograph
Slopes
steep
greater surface runoff
water move very fast so little time for infiltration
gentle
water move down slower so more time to infiltrate
more surface storage
Vegetation type
dense vegetation
greater interception
sparse, thin vegetation
less interception
Land use
deforestation
less interception
replace normal ground with impermeable surface(concrete, roof)
high surface runoff
less infiltration
storm drains direct to river
replacement of natural woodland by permanent pasture
less interception
Climate
Precipitation type
rain reaches the river channel quickly
snow slows down the system; may be released quickly as it melts
Precipitation intensity
intense rainfall could exceed the infiltration capacity of the soil causing rapid surface runoff which reaches the river channel quicker
steep rising limb
short lag time
Temperature
winter
lacks precipitation input
spring, early summer
sudden input from melting snow and summer monsoon
Antecedent moisture
moisture retained in the soil before rainfall event
effects infiltration capacity(saturated at different rate)
River Channel
Processes
Transport
Solution
dissolved sidiments
Suspension
suspended by the river flow
Saltation
bouncing of small particles
Traction
rolling of large sediments
Deposition and sedimentation
Flow
Slow
pick up less load, more deposition
Fast
pick up more load
Load particle size
Small
requires less velocity of river flow to be picked up
Clay
requires high velocity of river flow because it sticks together
Large
requires high velocity of river flow to be transported
Erosion
Attrition
bedload collide with one another
Hydraulic action
weight and force of river flow penetrates cracks in bed rock
Solution/corrosion
erosion from acid
Abrasion/corrasion
bedload are dragged and scrape the river bed
Cavitation
air bubbles in the turbulent flow implode, creating a small cavities in the rock
Land forms
River cliff
water flow fastest on the outer bend of the river, causing lateral erosion and undercutting
Point bar
water flow slowest on the inner bend, causing deposition
Waterfalls and gorges
differential erosion causes by the layers of high resistant rock over a less resistant rock. a step is formed
falling water is turbulent and explodes in all directions. Undercut and plunge pool are formed
large, high resistant rock are unsupported due to the undercut and falls down into the plunge pool
waterfall retreats upstream(headward erosion) forming a gorge
Riffle and pool
riffle
shallow section
accumulation of sediment
pool
deep seciton
found where erosion is greatest
Floodplain
the area which is often flooded when the water bursts the river bank(this area catches all the water)
located in the lower parts of the river valley: almost flat land
edged by bluffs where they meet the valley sides
deposition happens on the floodplain as water lose energy very quickly, this creates layers of alluvium and form a flat, fertile land
Levees
an elevated bank
formed during flooding by deposition of sediments
heavier sediments are deposited first(closer to the bank) and finer sediments are deposited further away
Deltas
formed when a lot of sediments are deposited at the mouth of the river
river flow slows down and deposit its load
Flocculation: salt in salt water join with the fine clay forming a larger particle (when river meets salt water)
Types of channel
Straight channel
rare in nature/mostly man-made
may follow the route of a fault line
found in low energy rivers
Meandering channels
have an asymmetrical cross profile
thalweg follows a sinuous path through the alternating pool and riffles
forms oxbow lake overtime
Braided channel
a channel is divided by bars and island made up of sediments
found in rivers with extreme discharge(unstable/keep changing)
River flow
Patterns of flow
Helicoidal flow
corkscrew shape
found around meander bend
Turbulent flow
rough, shallow channel with complex and rapid changes in chape
high/increasing velocity
spinning eddies are found here
a lot of air bubbles causing cavitation
Luminar flow
Smooth, shallow, straight channel
slow veloity
found in lower course
Thalweg
line of fastest flow
River velocity
speed of river flow in m/s
Factors affecting it
volume of water in the channel
more water(less wetted perimeter) - faster flow
channel shape
shallow,wide - more friction - slow flow
gradient of the channel
high gradient speeds up the flow
The channel roughness
rough channel slows down the flow due to friction
River discharge
volume of water passing a point in a given unit of time
Q=AV
Q=discharge(Cumecs)
A=channel cross sectional area(square meters)
V=velocity(m/s)
Human Impacts
Effects of land-use changes
Deforestation
reduce interception
increase overland flow
increase rates of soil erosion
fill channel = less channel capacity
less transpiration
Afforestation
less soil erosion
increases interception
reduces overland flow
more transpiration
Urbanization
construction of impermeable surfaces - concrete floor, metal roof, storm drain
Increases overland flow
rain falls on the roof is directed to the river by storm drain
Water abstraction
reduce channel flow
water is removed from aquifer
drop in water table
Impacts of floods
Population loss
damage ecosystems
damage land
water degradation
waterborne diseases
economic looses
Prevention of floods and damage reduction
Soft engineering
Afforestation
increase interception
protect, stabilize soil
Changing farming practices
Examples
contour ploughing
cover crops
terracing
Floodplain retreat
Government agencies buy floodplain land from landowners and let it flood. example - Mississippi
Hard engineering
Dams
control the flow of water in the upper catchment
Straightening/dredging river channel
speed up the flow of water so it drains faster
it's actually not effective
reduces channel capacity
increases erosion of the channel
makes conditions worse further down the river
needs to be done often in order to be effective because the river will force its way back to a meander form
Artificial levees, flood walls and embankments
increase channel capacity
sandbags can be used in emergency situation
Diversion spillways
take excess floodwater away from the main river channel
Forecasts and warnings
satellite imagery
rainfall radar
rain gauges
discharge recorder
