Rivers Flashcards
What is the definition of river discharge?
The volume of water flowing through a river channel at any given point
Draw a labelled diagram of a storm hydrograph
Lag time Discharge (cumecs) Precipitation Time Peak discharge Bank fall discharge Rising limb Falling limb Stormflow Baseflow
What is lag time?
The difference between peak rainfall and peak discharge
What are the factors influencing discharge?
Urbanisation (impermeable rock)- Water unable to percolate through impermeable rock + drainage direct to river, surface run off, short lag time, discharge high
Permeable rock- water able to percoulate through, taking longer to reach the river, ground water flow, long lag time, low discharge
Vegetation- Water flows and infiltrates to the plant roots, through flow, long lag time, low discharge
Farmland- soil is compact, increased forgave run off, through flow, short lag time, high discharge
Intense rainfall- soil becomes saturated, large surface storage, less infiltration, larger surface run off, surface run off/ through flow, short lag time, high discharge
Thin soils- Water able to infiltrate, low capacity, large saturation, more surface run off, through flow, short lag time, high discharge
Draw a sketch of a subdued and flashy discharge
Subdued= large lag time Flashy= short lag time
What are the 4 types of erosion and explain them
Hydraulic action: The force of air/water forcing into cracks
Attrition: Rocks break into smaller fragments after colliding with each other
Abrasion: Rocks erode by rubbing against the bed
Solution: Acidic water reacting with the rock making it dissolve
+(vertical and latteral erosion)
High energy conditions
What are the 4 types of transportation and explain them
Traction: large boulders rolling along the river bed
Saltation: small rocks bouncing along the river bed
Suspension: Light minerals held amongst the water
Solution: dissolved material carried by the river
What is the drainage basin hydrological cycle?
The cycle of water on a drainage basin scale, this is an open system with both inputs and outputs
What are the features of a drainage basin?
Source Tributary Watershed river channel Confluence Area within dotted line= catchment area
Hydrological cycle key terms:
Input:
What is precipitation?
When water/sleet/snow/hail droplets get heavy enough, gravity encourages them to fall to the ground
Hydrological cycle key terms:
Surface processes:
What is surface storage?
What is soil moisture?
What is groundwater flow?
When water accumulates on the surface of the land e.g. puddles
Water held within the soil layer
What stored in the rock layer
Hydrological cycle key terms:
Transfer processes:
What is infiltration?
What is through flow?
What is percolation?
What is groundwater flow?
What sinking in to soil/rock from the surface
What flowing through the soil layer parallel to the surface
What seeping deeper below the surface
What flowing through the rock layer parallel to the surface
Hydrological cycle key terms:
Outputs:
What is evaporation?
What is Transpiration?
What is evapotranspiration?
What is condensation?
Water changes state from liquid into a gas and rises into the air
The loss of water from plants through their leaves back into the atmosphere
The sum of transpiration and evaporation
When water vapour cools and changes its state from a gas to a liquid
What is dynamic equilibrium?
When inputs=outputs
True or False: The global hydrological cycle is a closed system.
True
What is the global hydrological cycle?
The continuous cycle of water between the oceans, the land and the atmosphere on a global scale. this is a closed system with no inputs or outputs
What is the drainage basin hydrological cycle?
The cycle of water on a drainage basin scale, open system with inputs and outputs
What is pipe flow?
Water that travels through holes left by root systems and animal burrows
What is through fall?
Where precipitation is not intercepted but falls straight to the ground
What is stem flow?
Where the precipitation flows along the stems of a plant to the ground
What is water balance?
What is it used for?
The balance between inputs and outputs as the river level rises and falls throughout the course of a year
Used to predict flooding
What is the river regime?
The annual pattern of river discharge
What is the water balance formula?
What does each symbol mean?
P=Q+E+/-S
P- Precipitation
Q- Streamflow
E- Evapotranspiration
S- Changes in storage
What is hydrostatic pressure?
Exerted by a fluid at equilibrium at a given point within the fluids. Increases the proportion to depth measured from the surface due to the increased weight exerting a downward force from above
Draw the water budget graph
See notes
Mean precipitation (mm)
Mean evapotranspiration (mm)
Months
What are the differences between a subdued and flashy hydrograph?
Flashy: Subdued:
- Impermeable rock -Gentle sided basin
- Steep sided basin -Freezing temperatures
- Urban area -Long period of light rainfall
- Chalk bedrock -Sandy soil
- Intense rainfall
- Thin soils
- Deforested catchment
- Clay soil (Barton)
- Snow fall + warm spring
What is overland flow?
Rainwater flowing over the ground/surface
What is base flow?
Groundwater flow that feeds into rivers through river banks and river beds
What is antecedent discharge?
The level of discharge before the storm
What is GPE?
Potential energy (stored energy)- determined by the altitude of the source in relation to base level
What is the amount of kinetic determined by?
Gradient + Velocity
When does deposition occur?
Low energy environments
Lower course
When there is a sudden reduction in gradient
River enters a lake or sea
Discharge has been reduced following a period of low rainfall
Shallower water
Sudden increase in the calibre/volume of sediment
What is the wetted perimeter?
The part of the cross sectional area in contact with water
What is the cross sectional area?
The total area within the cross section of a river channel
What is the hydraulic radius?
Measures the efficiency of a river
Cross sectional area divided by the wetted perimeter
high value = efficient
What is channel efficiency?
The ability of a river to move water
What is channel roughness?
The S.A of a river bed e.g. angular rock
What are the 3 channel processes?
Transportation
Erosion
Deposition
What are the factors influencing channel efficiency?
Channel shape
Channel roughness
Channel slope
Sketch the Hjulstrom curve
see notes
X axis- river velocity (cm per sec)
Y axis- Size of particles (mm)
Mean or critical erosion velocity curve- Velocity needed to entrain sediment (pick up sediment) of various sizes
Mean fall or setting velocity curve- velocity at which particles start to become deposited
Particles transported
Particles eroded
Particles deposited
this velocity is required to erode and entrain the smallest material clay particles is highest as they tend to stick together
What are the characteristics of the upper course within the long profile?
Vertical erosion- high amounts of friction
Large bed load size- river doesn’t hold enough energy to move large fragments
Large gradient
High amounts of friction as the channel depth is very shallow
Large GPE- as the altitude is high in relation to the base level
Stones are very angular as there is less attrition/abrasion within the upper course
What are the characteristics of the middle course within the long profile?
Velocity increases due to the introduction of tributaries
High kinetic energy due to velocity + gradient
Stones become less angular due to abrasion
Discharge increases a both the velocity and volume increase, due to the widening and deepening of the channel
What are the characteristics of the lower course within the long profile?
Cross sectional area increases as depth/width become larger increasing the carrying capacity
Low friction as water ti deeper
Gentle gradient as the depth increases due to erosional processes
Smaller bed load size as sediment gets eroded by attrition
Increased width, chunks of the bank and are eroded through hydraulic action + solution
What is the river capacity?
The total sediment load of a river at a particular point or location
What is the river calibre?
The size of a particular pebble/particle
What is the river competence?
The size of the largest sediment particle that can be carried by that river at a particular time or location
What is the hjulstrom curve?
The graph shows the relationship between the velocity of a river and the particle sizes transported. It shows the velocity needed to erode, transport and deposit material of different sizes
What is sediment entrainment?
The action of picking up sediment so that it can be transported
What is meant by spatial variations in load?
The level of load a river can carry depending on the drainage basin size
What are the landforms of erosion?
Waterfalls Gorges Interlocking spurs V-shaped valleys Potholes
What are the landforms of erosion and deposition?
Meander
Oxbow lakes
Floodplains
Braided channels
What is name for the deepest and shallowest part of a river channel?
Deepest= pool Shallowest= riffle
What is the thalweg?
Fastest flow of water in a channel, centripetal force swings the thalweg into the opposite bank
Draw the 3 stages of meander initiation
See notes, Pool Riffle Thalweg Centripetal force
Draw a leader cross section
See notes Low efficiency Thalweg Deposition Erosion Slip off slope Undercut by lateral erosion Small river cliff
What is helicoidal flow?
Helicoidal flow is the cork-screw-like flow of water in a meander
Explain the formation of meanders
1) alternating pools and riffles
2) pools are deeper and able to hold more energy than riffles due to less friction
3) as the river meets a riffle (low efficiency/low hydraulic radius) the thawed flows around it
Erosional landforms:
Give a description of a V shaped valley and interlocking Spurs
Draw a labelled diagram
Explain the formation of both
V shaped valley- a steep sided valley within the upper course
Interlocking Spurs- river bends and winds around hard rock on the upper course
See notes
V shaped valleys form when erosion takes place which is predominantly vertical
Interlocking Spurs are formed when the river is forced to wind around a hill or ridge of land taking the path of least resistance, protrusions of land are called Spurs
Erosional landforms:
Give a description of potholes
Draw a labelled diagram
Explain the formation of them
They are cylindrical holes drilled into the rocky bed of a river, by turbulent high velocity water loaded with pebbles
Upper/early middle course
See notes
Pebbles become trapped in slight hollows and vertical eddies in the water which are strong enough to cause sediment to grind a hole into the Rock by abrasion
Attrition smooths and rounds the pebbles caught in the hole to reduce the size of the bed load
Erosional landforms:
Give a description of waterfalls and Rapids
Draw a labelled diagram
Explain the formation of both
Waterfalls -A waterfall is a place where water flows over a vertical drop or a series of drops in the course of a stream or river.
Rapids- a fast-flowing and turbulent part of the course of a river.
See notes
Waterfalls- Soft rock is undercut (1).
This leaves a layer of hard rock which overhangs the layer of soft rock (2).
The water flows over the overhang and creates a plunge pool in the soft rock below. As the water hits the plunge pool it will erode it by both hydraulic action (the force of the water against the bedrock) and by abrasion (where the material carried by the river acts like sand paper against the riverbed) (3).
Eventually the overhang will collapse due to the erosion of the soft rock beneath it. The waterfall then retreats up stream (4).
This creates a steep, gorge-like valley.
Rapids- They are caused by different resistance among various rocks, that leads to sudden drops and rises in the river bed, creating fast/turbulent flow of water
Give a description of braided channels
Explain their formation
A river becomes braided as it sub divides into many smaller streams
In between, small islands form although streams are constantly shifting
made of loose sands and gravel
Braiding occurs as rivers made up of easily eroded material e.g. sands and gravel break down and variable discharge causes braiding to be common in the snout of glaciers and semi-arid regions. The river has a low carrying capacity due to it being broad and shallow (low hydraulic radius), this means that it constantly gets overloaded by water. As a result the river sub divides into smaller and faster streams. There is increased sediment deposition at times of high discharge.
Draw a diagram of a braided channel
see notes
point bar
Give a description of Floodplains and Levees
Explain their formation
Draw a diagram
A floodplain is a large area of flat land on either side of the river which constantly gets flooded
Levees- Larger, heavier sediment deposited at the top of a bank creating a larger natural embankment
Floodplains are caused as erosion on the outside of the meander (lateral) causes channel migration. The river alluvium then becomes deposited building up the level of the floodplain- through sediment accretion of fertile land
Levees- As a river overflows, friction increases, the coarsest sediment is deposited first
See notes
What is a delta?
Draw a diagram of a delta
A delta is a land form created by deposition of sediment as the channel progresses from the mouth to slower moving/standing water
Occurs when rivers meet an ocean, sea estuary, lake or reservoir
see notes
low tidal range
delta growing into sea/lake
little or no currents
Deposition of silt fills in sea or lake forming a delta
Dis tributary
River water carrying large amounts of silt from upper parts of the valley
Give an explanation for delta formation
When a river reaches a sea/lake mouth, the energy is absorbed by the sea/lake with a lower velocity. As a result the rate of deposition is greater than the rate of removal, a large quantity of sediment in still conditions causes it to build up rising above sea level.
Flocculation occurs as sea water meets fresh water, where particles on suspension E.g. Clay/silt react with the salt, becoming heavy enough to sink and become deposited. The river then braids in order to reach the sea, this increases the rate of sediment removal and the river gains more energy.
What are top set beds?
What are Foreset beds?
What are Bottom set beds?
Larger and heavier particles are first to loose energy
Medium calibre particles travel a little further before deposited , they are deposited at more steep angled (wedges) of sediment
Finest particles which travel the furthest distance before low velocity and flocculation leads to their deposition
What are the features of a Arculate delta?
Arcuate Deltas:
Most common deltas
Curving the shoreline, distinct pattern of drainage more gravelly deposits
E.g. Nile Delta
Longshore drift keeps the seaward edge relatively smooth in shape
What are the features of a birds foot delta?
Draw a diagram
Deposited material divides the river into smaller distributaries
E.g. Mississippi River
See notes
What are the features of a Estuarine Delta?
Draw a diagram
Deposited material divides the river into smaller distributaries
See notes
What are the features of a lobate delta?
Draw a diagram
Formed when a river flows into a lake
See diagram
What are the 5 landforms in the upper course?
- V shaped valleys
- Interlocking Spurs
- Potholes
- Waterfalls+Rapids
- Gorges
What are the 2 landforms in the middle course?
- Meanders
- Oxbow lakes
What are the 4 landforms in the lower course?
- Floodplains
- Deltas
- Levees
- Braided channels
What is the base level of a river?
The lowest point a river can erode down to
Which course is there the most erosion?
Upper course
What happens to GPE as it moves into the lower course?
There is very little GPE as the distance to base level is very small, meaning vertical erosion is minimal. High kinetic energy used to erode banks not the beds, bed is smoother therefore less erosion.
Draw the graph of a graded profile
See notes
The long profile is smoothened out by the river
What conditions does river rejuvenation occur?
- a fall in sea level relative to the level of land
- a rise of the land relative to the sea
Enables a river to renew its capacity to erode as its potential energy is increased
What are the landforms created by river rejuvenation
- Knickpoints
- River Terraces
- Incised meanders
What is river rejuvenation?
Decline in base level, increasing the rivers potential energy
Explain why land can rise: (positive isostacy)
Positive isostacy occurs as an ice sheet weighing down the land melts, therefore the weight gets removed causing the land to rise after it has sunk
Collision/Destructive plate boundary
Explain why sea levels fall: (negative Eustacy)
Occurs due to an ice age freezing the water
What are the 7 natural causes of flooding? And give key words
Soil depth- (infiltration, capacity, saturation, overland flow, through flow)
Vegetation- (Interception, soil moisture extraction, transpiration, evapotranspiration)
Geology- (groundwater flow, impermeable, permeable, percolation
Slope angle- (Gradient, Overland flow, surface run off, steep/shallow)
Rainfall: excessive or intensive- (Precipitation, saturated/unsaturated, Baked ground)
Snow melt- (Saturated/unsaturated, rapid, frozen soil, surface run off, infiltration, capacity
Drainage density [total length of streams/total drainage basin area]- (impermeable, excessive, carrying capacity, runoff, increased discharge)
What are the 6 Human causes of flooding?
Give key words
Urbanisation: Developing on flood plains- (Vulnerable, flat land, low lying)
Artificial conduits e.g. drains and sewers- (Lag time, infiltration, capacity)
Channel straightening- (Artificial, increased velocity, downstream flooding)
Deforestation- (Soil moisture extraction, interception, through flow, evapotranspiration)
Farmland: Overgrazing and ploughing- (Compact, Infiltration, capacity, run off)
Dam failure- (Artificial, rapid increase, discharge, hard engineering)
What is the definition of flood?
An overflow of water breaching the warring capacity of a river
What is meant by the primary causes of a flood?
Usually the result of climatic factors
What is meant by the secondary causes of a flood?
Tend to be drainage basin specific e.g. topography/geology
What are the 2 types of Incised meanders?
Entrenched Meanders - River wear- Durham
Ingrown meanders- River Wye, Tintern Abbey
Comment on the cross section of an entrenched meander
What are the causes of entrenched meanders?
Entrenched meanders have a symmetrical cross section
Occur due to:
-Very rapid incision due to rapid uplift
-Sides being resistant to erosion
Comment on the cross section of an ingrown meander
What are the causes of ingrown meanders?
Produce an asymmetrical cross profile
Ingrown meanders occur due to:
Uplift of land, or incision by the river is less rapid (erodes laterally)
What are river terraces?
They are level/steps on the sides of rivers marking former floodplains which are caused by rejuvenation due to vertical erosion
How are river terraces formed?
River creates a floodplain
River is rejuvenated
Incised GPE and vertical erosion so river incises downwards
Creates a new floodplain at a lower level, leaving marks of levels/steps of the former flood plain
What are paired terraces?
Levels on either side of the floodplain which are the same height
What are unpaired terraces?
Form when lateral migration is dominant compared to incision.
What are the 3 causes of river rejuvenation?
Dynamic change
Eustatic change
Static change
What is dynamic change?
Upwards movement of land, raising the height above sea level, increasing GPE
What are knickpoints?
A knick point is a sudden break or irregualrity in the gradient along the long profile of a river.
shown by waterfalls/rapids etc.
How are knickpoints formed?
Base level falls (positive isostasy/negative ecstasy)\
river begins to cut down to new base level with renewed energy due to more GDP, carrying out more vertical erosion
The prices begins at sea level and begins to cut back up the long profile (headward erosion), moving upstream in the rivers course until an equilibrium in terms of GPE is reached
The knickpoint represents where the old long profile meets the new/regraded one
What is orogenesis?
plate movement, plate compression resulting in mountain building
What is a case study for an ingrown meander?
River Gwash Rutland
Runs through the village of Braunstone in Rutland
Fall in discharge means river channel becomes meandered, downcutting was marked by the ingrown meander
What is the flood magnitude?
The size/extent of the flood
What is the flood frequency?
The occurrence of a flood
What is the recurrence interval?
The interval at which particular levels of flooding will occur
What 6 factors influence flood risk?
Urbanisation Preparation/defences Economic development, infrastructure- better prep Population density, more people at risk Frequency of flooding Magnitude of flooding
Comment on the frequency of large and small magnitude floodings
Large magnitude floodings don’t happen very often e.g. every 100 years
Small magnitude floodings are more frequent e.g. every 2 years
What is the recurrence interval formula?
R= n+1
—–
m
m= rank
A longer recurrence interval = soft management instead of hard
What is sustainable development?
Development now that does not compromise the needs of future generations. It must ensure that economies, society and environment are preserved and maintained now and in the future
