Rivers Flashcards
What is a drainage basin?
Area of land drained by river and its tributaries.
What is a tributary?
Smaller river or stream which joins with main one.
What is a confluence?
Point at which two rivers meet.
What is a source?
Place where main river begins.
What is the mouth?
Place where river flows into sea or ocean.
What are the inputs to the simple open system of the drainage system?
Precipitation.
What are the flows from store to store in a simple open system of the drainage basin?
Surface runoff
Soil (unsaturated)
Ground (saturated)
River channel
What are the outputs in a simple open system of the drainage basin?
Evapotranspiration
River discharge
What is river discharge?
Amount of water passing any point in river at certain time.
What is river discharge measured in.
Cubic metres of water per second (cumecs).
How is river discharge calculated?
Multiplying cross-section of area of river channel at certain point by speed (velocity) of river at same point.
Cross-sectional area obtained by multiplying width of river by average depth. Speed (velocity) of river recorded using flow metre that when dipped into river gives digital reading of speed of flow in metres per second.
What are the components of a drainage basin?
Inputs - precipitation
Storage - interception, surface storage, soil moisture, groundwater
Flow (transfer) - infiltration, surface runoff (overland flow), percolation, throughflow, groundwater flow
Outputs - evaporation, transpiration, river carrying water to lake or sea
What is precipitation?
Rain, hail, sleet or snow.
What is interception?
When trees or vegetation delay or slow down rate which rainfall reaches ground.
What is surface storage?
Water may not be able to infiltrate immediately and stored temporarily on surface.
What is infiltration?
Downward movement of water from surface into soil.
What is soil moisture storage?
Water stored in soil.
What is percolation?
Movement of water from soil into underlying rocks.
What is groundwater?
Water stored within underlying rocks.
What is groundwater flow?
Water in underlying rocks moving towards river channel.
What is throughflow?
Water moving through soil towards river.
What is surface runoff (overland flow)?
Water moving on surface as infiltration is restricted.
What is evapotranspiration?
Includes total water lost by vapour due to evaporation and transpiration.
What are the expected characteristics of the upper course of a river?
Narrow channel
Shallow channel
Vertical erosion
Hydraulic action, abrasion and attrition
Transport of load by saltation, some traction at high flow
Large sized, angular load
What are the expected characteristics of middle course of a river?
Channel widens due to lateral erosion
Less hydraulic action
All methods of transport can be seen, but suspension most common
Erosion and deposition seen on meanders
Load becomes smaller and more rounded
What are the expected characteristics of the lower course of a river?
Channel is deepest and widest
Some lateral erosion
Deposition more common than erosion
Load now small in size, mostly sand, silt and clay
Load moved mostly by suspension
What is the Bradshaw Model?
Another way of showing how river’s characteristics change upstream and downstream, given natural conditions. Wider the triangle, more there is of that particular item.
What are the 4 main types of erosion?
Attrition
Abrasion or corrosion
Hydraulic action
Solution or corrosion
What is attrition?
Collision of rock fragments (suspended in water) against one another. Rock particles broken into smaller pieces and become smoother as process continues.
What is abrasion/corrasion?
Grinding of rock fragments, carried by river, against bed and banks of river. Action causes river channel to widen and deepen. Grinding most powerful during flood, when large fragments of rock carried along river bed.
What is hydraulic action?
From of mechanical weathering caused by force of moving water. Can undermine riverbanks on outside of meander or force air into cracks within exposed rock in waterfalls such as Niagra.
What is solution/corrosion?
Process by which river water chemically reacts with soluble minerals in rocks and dissolves them.
What are the 4 types of transportation of a river?
Solution
Suspension
Saltation
Traction
How does solution transport particles in a river?
Soluble minerals dissolve in water and carried in solution. Is a chemical change affecting rocks such as limestone and chalk, my result in discoloured water. E.g. rivers of Mournes often appear yellow/brown as stained from iron coming off surrounding peat bog.
How does suspension transport particles in a river?
Smallest load (sand and clay), held up continually within river water. Makes water appear opaque. Some rivers carry huge quantities of suspended materials.
How does saltation transport particles in a river.
Bouncing of medium-sized load, small pebbles and stones along river bed.
How does traction transport particles in a river?
Rolling large rocks along river bed. Requires a lot of energy. Load carried called bed load. Largest bed load only moved in severe flood.
How does deposition occur?
Velocity of river reduced, energy of water decreases, water can no longer erode or transport material, load dropped, starting with largest (and heaviest) particles.
What makes deposition likely to occur?
River enters lake or sea, slowing flow.
Area of shallow water, slowing flow.
River floods onto floodplain, flows very slowly.
Load increased, e.g. after landslide.
How do waterfalls form?
Are alternating layers of hard and soft rock.
As river passes over exposed, soft rock in river bed, it erodes at faster rate than harder rock, so step in river bed develops.
Force of hydraulic action and abrasion deepens step until waterfall formed.
Eventually erosion makes deep pool under waterfall, plunge pool, hard rock hangs over it.
When it becomes too unstable, overhang collapses and waterfall retreats, leaving gorge.
What are meanders?
Bends that develop in river channel as gradient (slope) of river evens out.
Continually-changing features due to differences in velocity of water across river channel.
How do meanders change shape?
Where water flows fastest, spirals downwards, causing vertical erosion, deepening river channel, creating river cliff on bank.
Opposite, water flows very slowly, doesn’t have energy to erode bank.
Water cannot hold up load carrying, deposits from heaviest to lightest material until only smallest clay particles left in suspension.
Leads to top-sided cross-section through meander.
Over time builds up to create clear slip-off slope.
Meanders constantly being formed and reformed.
Bends get bigger and even cut off, creating ox-bow lake.
What is a floodplain?
Area around river covered in times of flood. Large, flat area of land formed by erosion and deposition.
Erosion of outer bends if meander gradually wear down and flatten land on either side of channel.
When river overflows, travels across neighbouring land.
Very fertile, rich alluvium deposited by floodwaters make good for agriculture.
Floodplain become wider (erosion) and deeper (deposition).
What is a levee?
Raised bank formed in banks of river, in lower course.
How are levees formed?
Repeated flooding of river.
When river floods, during high energy, water and load leaves channel.
Drop in energy caused by friction, biggest, most coarse material dumped close to river banks.
Reparation g flooding causes levee to build up over time.
What are the physical causes of flooding?
Heavy rainfall > excessive surface runoff > flash flood
Prolonged rainfall (monsoon climate) > saturates ground > slow, long flood
Melting glacier or snow > create high river discharge in spring > river overflows
Underlying rock impermeable or clay soil, ground frozen > reduces infiltration into ground, precipitation likely taken to river as surface runoff > high risk regular flooding
What are the human causes of flooding?
Deforestation > less root support for soil, less water used by plants, big raindrops no longer broken up by leaves > mor surface runoff > more flooding
Urbanisation > more tarmac and concrete create impermeable surfaces > more surface runoff > more flooding
Narrowing river channel (building bridge) > river reduced capacity hold water > more surface runoff > more flooding
Building levees > provides short term protection, but… > more surface runoff > more flooding
What is the context to the Somerset Levels?
Low-lying region in South West England.
Has naturally high water table and poor drainage, prone to flooding.
In winter 2013-14 saw prolonged flooding, worst in over century.
Villages cut off as roads became impassible and animals moved from some of 6880 hectares of flooded farmland.
What are the physical causes of the Somerset Levels flooding?
Combination of impermeable bed rock and low interception levels means area naturally at risk of flooding.
Series of severe winter storms in December and January in southern England during 2013-14. One of windiest Decembers on record and highest January rainfall since records began in 1910.
High tides cause floodwater to back up along rivers across Levels and Moors. Due to rivers levels being higher because of recent rainfall and had not been dredged.
What are the human causes of the Somerset Levels flooding?
River Tone and Parrett not dredged properly in 20 years, leaving farmland and homes without proper defence from floods. Resulted in hectares of land left underwater from storms which began in December.
Extra water pumped away from Taunton and Bridgewater into Levels to protect new homes built on former floodplains.
Sample answer for Somerset Levels.
The Somerset Levels is a low-lying region in the South West of England. During the winter of 2013-14, the River Tone and River Parrett experienced prolonged flooding which was its worst in over a century.
A physical cause for this flooding was a series of severe winter storms in December and January in southern England during 2013-14. December 2013 was one of the stormiest Decembers on record, and one of the windiest since 1993. Figures released in 30 January showed that southern England and parts of the midlands had experienced their highest January rainfall since records began in 1910.
A human cause is that the River’s Tone and Parrett had not been dredged properly in 20 years, leaving farmland and homes without proper defence from flooding. This resulted in hectares of land being left flooded from the storms which began in December. Locals believe if the river had been dredged, this would have cleared them of silt, making them wider, deeper and easier to maintain. It would have also helped the rivers to carry away flood waters through surface runoff, draining the floodplains quicker.
What are the positive impacts of flooding on people?
Replenishes drinking water supplies, especially wells.
Provides sediment (silt or alluvium) that naturally fertilises soils of floodplain.
Countries such as Bangladesh and Egypt rely on floods to help crops like rice grow.
Can encourage innovative solutions in future buildings design, such as building houses on stilts to avoid floodwater, or tiling ground floor walls and floors to facilitate easy clean-up after flooding.
What are the negative impacts of flooding on people?
Spreads waterborne diseases (e.g. cholera).
People and animals made homeless or drown.
Buildings and infrastructure (roads and railways) can be damaged or destroyed.
Crops grown on fertile floodplains washed away in flash flood.
Can increase house insurance costs for householders, or make it impossible to insure their home at all.
What are the positive impacts of flooding on the environment?
Fish benefit as can breed in standing floodwater.
In dry areas, floods bring relief from drought, providing drinking water for wild animals.
What are the negative impacts of flooding on the environment?
Flooding can wash chemicals or sewage into local rivers and so pollute them.
Wild animals may drown or lose habitat during flood.
What are flood management schemes?
Used to try to reduce flooding.
Hard engineering strategies work against natural flow of river.
Soft engineering strategies work with river and more environmentally friendly.
What are hard engineering methods?
Involve using large artificial structures to control river, breaking natural cycle of flood and subsidence. Not sustainable long term.
What are examples of hard engineering methods?
Building dam or reservoir (upper course).
Deepening and widening river channel.
Straightening river.
Building high embankments along sides of river.
Building flood wall.
Creating storage areas.
How does building a dam or reservoir help river management?
Built in upper course.
Can be used for leisure and hydroelectricity.
But can flood farmland, displace local people and destroy habitats.
How does deepening and widening the river channel help river management?
Increase cross-sectional area, allowing it to contain more water.
Discharge has to be much greater to create flood.
How does straightening a river help river management?
Cutting off loop of meander to speed up flow of river and therefore reduce flood risk.
How does building high embankments help river management?
Along sides of river to contain any floodwater.
Might be done by raising natural levees.
How does building a flood wall help river management?
Along course of river where flows through settlements.
Expensive and do not look natural.
Effective if maintained properly and built high enough.
How does creating storage areas help river management?
Like temporary lakes.
As rivers rise, extra water can be pumped or diverted into them, then emptied back into main river when risk of flooding is over.
What are soft engineering methods?
Generally sympathetic to natural landscape, tend not to damage river for future generations.
More sustainable than hard engineering methods.
What are examples of soft engineering methods?
Planting trees (afforestation), (upper course).
Land use zoning.
Washlands.
How does afforestation help river management?
Upper course of river in order to increase interception and reduce risk of flash flooding.
As trees absorb water from deep within soil, also reduces flow of water to river from soil moisture.
This reduces percolation and also water within groundwater store.
How does land use zoning help river management?
Areas most likely to be flooded are protected from urban development.
High risk flood areas left as pasture for grazing while low risk zones become playing fields or parkland.
Housing and important buildings (hospitals) only built on land least at risk.
How do washlands help river management?
Parts of river floodplain (lower course) which river can flood into temporarily.
Are a kind of flood storage area.
Can prevent flooding of more valuable land further downstream.
What is the context to Mississippi River management scheme cast study?
Mississippi fourth lap longest river in world.
Essential river for USA, providing 18 million people with water supply.
Flooding almost annual event.
Severe floods in 2001 caused $13 million of damage.
4400 people had to move.
What are the hard engineering methods and their description for the Mississippi River?
Raised embankments (levees) - built 15m high, for 3000km along river.
Dams - over 100 dams built on tributaries.
Cut meanders - engineers cut through meanders to straighten 1750km of channel, make river flow faster.
What is the evaluation of the raised embankments (levees) for the Mississippi River?
Levees partly blamed for 2001 floods - they protect area where built but push problem downstream.
After flood, silt deposited on channel bed instead of floodplain, so water level rises higher than floodplain.
Parts of New Orleans lie 4.3m below river level, increasing risk of flood damage.
What is the evaluation of dams for the Mississippi River?
Dams trap silt, preventing reaching delta (so birds like heron endangered there) or enriching farmland (so more fertiliser used).
What is the evaluation of cutting through meanders for the Mississippi River?
Straightforward river loses variety of habitats for plants, fish and insects.
River erodes banks to resume natural meandering course, so money and effort been wasted.
What are the soft engineering methods and their descriptions for the Mississippi River?
Afforestation (tree planting) - Tennesse Valley, trees absorb water.
Safe flooding zones - houses near river bought and demolished (e.g. Rock Island, Illinois) and areas of floodplain turned into green spaces.
What is the evaluation for afforestation for the Mississippi River?
As well as preventing water from reaching river, tree planting helps reduce soil erosion and provides wildlife habitats and opportunities for recreation.
What is the evaluation for safe flooding zones for the Mississippi River?
Cheaper in long term to prevent property damage than compensate owners when damage has happened.
Wetland habitats close to rivers can be preserved.
