Geography > river landscapes > Flashcards
river landscapes Flashcards
What is a drainage basin
an area of land drained by a river and its tributaries
what is a tributary
a small stream that joins a large river
what is a confluence
where a tributary joins a larger river
what is a watershed
edge of the/a river basin
what is a source?
the start of a river
what is the mouth?
the end of a river, usually where a river joins the sea
Cross profile?
imaginery ‘slice’ across a river chanel and its valley at a particular point
v shaped valley?
Valley: steep sided, v shapes
River: narrow, shallow, turbulent
floodplain?
valley: wider, flat floor
River: wider and deeper
levees?
Valley: very wide and flat
River: wide, deep, with large sediment load
verticle erosion?
downwards
lateral erosion?
sideways
upper course?
mostly erosion landforms, e.g. waterfalls
- steep gradient
- larger rocks deposited, only transported for a short distance, mostly by traction, during periods of high flow
middle course?
mostly erosion and deposition landforms, e.g. meanders and transportation
gentle gradient
- finer sediment deposited, mostly held in suspension. This material will be deposited on the river bed or banks, where velocity is slowed by friction
lower course?
mostly deposition landforms e.g. levees
- very gentle gradient
- a large amount of deposition occurs at the river mouth, where the interaction with the tides, along with the very gentle gradient, gentl reduces the river’s velocity.
solution?
dissolved load
or
when the river flows over limestone or chalk, the rock is slowly dissolved. This is because it is soluble in midly acidic river water
suspension?
small sediment held in the river
traction?
large particles rolled on the riverbed
saltation?
‘bouncing’ of particles too heavy to be suspended
hydraulic action?
force of water hitting the river bed and banks. this is most effective when the water is moving fast and when there is a lot of it.
abrasion?
when the load carried by the river repeatedly hits the bed or banks dislodging particles into the flow of the river.
attrition?
when stones carried by the river knock against each other, gradually making the stones smaller and less rounded
deposition?
- when the river loses velocity, it drops the sand, rock particles and pebbles it has been carrying
River velocity (surface and internal) ?
surface velocity is smaller than internal-
–friction affects the velocity
distinctive river landforms?
upper course- erosion dominates to form landforms such as interlocking spurs, waterfalls and gorges
middle course- erosion and deposition combine form meanders and ox-bow lakes
lower course- deposition domintes as the river nears the sea to form a floodplain, levees and the river estuary.
interlocking spurs
upper course
- fingers of land that jut out
- river is near its source so its not powerful enough to cut through the ‘spurs’ of land so has to flow around them
Waterfalls
- as a river makes it way from its source to the mouth of the river, it flows over a variety of different rock types
- more resistant rocks aare less easily eroded than weaker rocks and they will form ‘steps’ in the long profile of a river
- these steps form a waterfall
- most commonly formed when a river flows over a relatively resistant band of hard rock
- when the river plunges over a waterfall it forms a deep and turbulent plunge pool
- here processes of erosion, particularly hydraulic action and abradion, are active and they combine to undercut the waterfall in the softer rock
- eventually the overhanging rock collapses and the waterfall retreats upstream
- the retreating waterfall over many years will leave behind a steep-sided gorge
- they can also form when a drop in sea level causes a river to cut down into its bed creating a step in the LP ofa river
- this step being a knick point, marked by a presence of a waterfall
- can also be found in glacial hanging valleys
Gorges
- narrow steep sided valley usually found immediately downstream of a waterfall
- formed by the gradual retreat of a waterfall over hundreds or even thousands of years
- can sometimes form in other ways, at the end of the glacial period (around 8000 years ago), huge quantities of water from melting glaciers poured off upland areas to form gorges such as CHEDDAR GORGE, SOMERSET
- more rarely they can form on limestone as a result of the collapse of underground caverns
Meanders
- wide bends of a river found mainly in lowland areas
- mist efficient channel for a heavy laden river as it flows over fine sediment on very gentle slopes
- constantly change their shape and position
- this is a result of processes of lateral (sideways) erosion and deposition in the river channels
- thalweg= fastest line of flow (velocity) within the river
- swings from side to side causing deposition on the inside bend
- over time erosion and deposition causes meanders to migrate across the valley floor
pools and riffles
- where streams carry coarse sediment they may develop an alternating sequence of shallow, fast-flowing sections (called rifflles) and deeper slower-moving sections (called pools)
- clearly shown in low flw conditions
- the faster flowing riffles have deposits of coarser gravel
- calmer pools have finer deposits
- pools develop close to the outside bend of the meander, in high flow conditions this is the pathof the fattest flow- thalweg- and where the most erosion takes place
- riffles develop between meander bends where a drop in velocity, has led to deposition of coarser sediment.
ox- bow lakes
- over time, as meanders migrate across the valley floor they may start to erode towards each other
- diagram C on page 120
- gradually the neck of the meander narrows until it is completely broken through (usualy during a flood) to form a new straighter channel
- the old meander loop is cut off by deposition to form an ox-bow lake
floodplains
- wide, flat are of marshy land on either side of a river
- found in the middle and lower courses
- made of ALLUVIUM , a sediment (silt) deposited by a river when it floods
- floodplains are used for farming as the soils are very fertile
2 processes responsible for the formation of a floodplain:
- meanders migrate across the floodplain due to lateral erosion. when they reach the edge of the floodplain they erode the valley side (BLUFF) - explains why they are very wide
- when the river floods it deposits silt, creating a very flat floodplain. layer upon layer builds up over many years to form a thick deposit of fertile alluvium.
levees
- raised river bed found alongside a river in its lower coursse
- formed by flooding over many years
- ridge of sediment is deposted naturally to build up the levee
- during low flow conditions deposition takes place, raising the river bed and reducing the capacity of the channel
- when flooding occurs, water flows over the sides of the channel
- here the velocity of the river decreasses rapidly leading to deposition of sediment on the river banks
- coarser sands first are deposited and then the finer silt and mud
- gradually after many floods the height of the banks can be raised by as much as two metres.
estuaries
- in the Uk most river mouths form wide tidal estuaries, especially in areas where sea levels have risen
- estuaries are transitional zones between river and coastal environemnts and are affected by wave action as well as river processes
- main process operating in estuaries is deposition
- during a rising tide river water is unable to be discharged into the sea
- river’s velocity falls and sediment is deposited
- at low tide these fine deposits form extensive mudflats
- over time, mudflats develop into important natural habitats called saltmarshes
River tees
- North East of England
- source in the penines hill near cross feell (height 893m)
- from there it flows roughly east for around 128km to reach the Northh Sea at Middlesborough
High force waterfall and gorge
- located near forest-in-teesdale in the river’s upper course
- river drops 20m as a single sheet of water into the foaming and turbulent plunge pool below
- waterfall was formed due to a resistant band of igneous rock (cooled volcanic lava) called DOLERITE , which cuts across the river valley
- unable to erode this tougher band of rock, the river has formed a step in the long profile of the river- developed over hundreds of years to form highforce waterfall
- the underlying darker rock with horizontal layers (called beds) is the Carboniferous limestone.
- as the river plunges over the waterfall, it underuts the weaker limestone forming an overhang
- this eventually causes the waterfall to retreat upstream to form a gorge
what is flooding?
flooding is where the land that is not normallu underwater becomes inundated. A river flood occurs when a river channel can no longer hold the amount of water flowing in it. Water overtops the banks and floods the adjacent land- the floodplain
what causes flooding?
- river floods occur normally after a long period of rainfall often during the winter
- the volume of water steadily increases causing river levels to rise
- eventually the river may overtop its bank to cause a flood
- sudden floods can occur following torrential storms- flash floods, often more associated with heavy rainsotrms that occur in the summer
Physical factors - causes of flooding
- precipitation- torrential rainstorms can lead to sudden flash floods as river channels cannot contain the sheer volume of water flowing into them. Steady rainfall over several days can also lead to flooding in lowland river basins
- geology (rock type)- impermeable rocks (rocks that do not allow water to pass through them) such as shales and clays encourage water to flow overlandand into river channels- speeds up water flow and makes flooding more likely
- steep slopes- in mountain environents steep slopes encourage a rapid transfer of water towards river channels- increases risk of flooding
Human factors- causes of flooding
- urbanisation- building on a floodplain creates impermeable surfaces such as tarmac roads, concrete driveways and slate roofs. Water is transferred quickly to drains and sewers and then into urban river channels- this rapid movement of water makes flooding more likely
- deforestation- much of the water that falls on trees is evaporated or stored temporarily on leaves and branches. Trees also use up water as they grow. when trees are removed much more water is suddenly available and transferred rapidly to river channels, increasing the flood risk.
- Agriculture- in arable farming, soil is left unused and exposed to the elemnts for periods of time. This can lead to more surface runoff. This is increased if the land is ploughed up and down steep slopes, as water can flow quickly along the furrows.
what is a hydrograph?
- volume of water flowing along a river is its discharge
- measured in cumecs- cubic metres per second
- a hydrography is a graph that plots river discharfe adrer a storm (graph B page 125)
- shows how discharge rises after a storm, reaches its peak and then returns to the normal rate of flow
- one of the most important aspects of hydrography is the time lag- this is the time in hours between the highest rainfall and the highest (peak) discharge
- shows how quickly water is transfereed into a river channel and is a key factor in the flood risk
- the shorter the time lag the greater risk of flooding
what affects the shape of a hydrograph?
‘FLASHY” HYDROGRAPH WITH A SHORT LAG TIME AND HIGH PEAK
Basin size: small basins often lead to a rapid water transfer
Drainage density: a high density speeds up water transfer
Rock type: impermeable rocks encourage rapid overland flow
Land use: urbanisation encourages rapid water transfer
Relief: steep slopes lead to rapid water transfer
Soil moisture: saturated soil results in rapid overland flow
Rainfall intensity: heavy rain may exceed the infiltration capacity of vegetation, and leas to rapid overland flow
LOW, FLAT HYDROGRAPH WITH A LOW PEAK:
Basin size: large basins result in a relatively slow water transfer
Drainage density: a low density leads to a slower transfer
Rock type: permeable rocks encourafe a slow transfer by groundwater flow
Land use: forests slow down water transfer, because of interception
Relief: gentle slopes slow down water transfer
Soil moisture: dry soil soaks up water and slows down transfer
Rainfall intensity: light rain will transfer slowly and most will soak into soil
Hard engineering- what is it?
- It involves using man-made structures to prevent or control natural processes from takin place.
- This form of flood management is usually very expensive - individual projects can cost several million pounds
- preferred option for protecting expensive property or land, such as housing estates, railways and water treatment works . The costs have to be weighed up against the benefits:
Costs- the financial cost of the scheme, and any negative impacts on the environment and people’s lives
Benefits- financial saving made by preventing flooding, along with any environmental improvements
dams and resevoirs
Widely used around the worldto regulate river flow and reduce the risk of flooding.
Most dam projects are multi-purpose, having several functions, for example:
- Flood prevention
- irrrigation
- water supply
- hydro-electric power generation
- recreation
Dams can be very effective in regulating water flow. During periods of high rainfall, water can be stored in the resevoir. It then can then be released when rainfall is low. BUt the construction of dams can be controversial. They cost huge amounts of money and the reservoir often floods large areas of land. Many people may have to be moved from their homes.
Clywedog reservoir, Llanidloes, Wales
- constructed in the 1960s to help prevent flooding of the River Severn
- over 70m high and 239m wide and the reservoir stretches for nearly 10km
- been in continous use since 1967, filling up in the winter and gradually releasing water in the summer to retain a constant flow.
- Although some flooding has occured and continue to affect settlements further downstream, it has undoubtedly stopped many castrophic flooding events.
channel straightening
- river straightening involves cutting through meanders to create a straight channel.
- speeds up flow of water along the river
- whilst it may protect a vunerable location from flooding, it may increase flood risk further downstream
- the problem is not really solved rather shifted somewhere elsewhere
- in some places straightened sections of river are lined with concrete- speeds up flow and prevents banks from collpasing, cause the channel to silt up
- however this gives an unnatural and a very unatractive river environment and can damage widlife habitats
embankments
- it is a raised riverbank
- raising the level of the riverbank allows the river channel to hold more water before flooding occurs
- hard engineering sructures involving concrete walls or blocks of stone are frequently used in towns or cities to prevent flooding of valuable property.
- sometimes mud dredged from the river may be used- this is cheaper and more sustainable and looks more natural
flood relief channels
- it is a man made river channel constructed to by-pass an urban area
- during times of high flow, sluice gates can be opened to allow excess water to flow away into the flood relief channel and reduce the threat of flooding
The Jubilee river, Maidenhead
- In the UK, a flood relief channel, named the Jubilee River, has been constructed on the River Thames near Maidengead in Berkshire
- the 11km channel was opened in 2002, costing around £110 million to construct and with a length of nearly 12km is the longest man-made channel in the UK
- As well as reducing the flood risk for over 3000 properties, the Jubilee River has had a postive impact on the environment by creating new wetlands.
- It is also popular for recreational activities such as walking and fishing.
soft engineering- what is it?
involes working with natural river processes to manage the flood risk
- it does not involve buidling artificial structures or trying to stop natural processes
- aims to reduce and slow the movement of water into a river channel to help prevent flooding
- in common with all forms of management there ar costs and benefits.
- planting trees to establish a woodland or forest is called afforestation. Trees obstruct the flow of water and slow down the transfer to river channels. Water is soaked up by the trees or evaporated from leaves and branches. Tree planting is relatively cheap and has environmental benefits
Examples of soft enginerring
Wetlands and flood sotrage areas:
- very efficient at storing water
- deliberately allowed to flood to form flood storage areas
- water can be stored to the reduce the risk of flooding further downstream
FLoodplain zoning
- reistricts different land uses to certain locations on the floodplain
- areas close to the river and at risk from flooding can be kept clear of high-walue and uses such as housing and industry
- instead these areas can be used for pasture, parkland or playing fields
- this can help reduce overall losses caused by flood damage
- difficult to implement on floodplains that have already been developed and can cause land prices to fall
River Restoration
- where the course of a river has been changed artificially , river restoration can return it to its original course
- it uses natural processes and features of a river, such as meanders and wetlands to slow down river flow and reduce the likelihood of a major flood downstream
preparing for floods
- Flood watch- flooding of low-lying land and roads is expected, people should be prepared and watch river levels
- Flood warning- there is a threat to homes and businesses. people should move items of value to upper floors and to turn off electricity and water
- Severe flood warning- extreme danger to life and property is expected. people should stay in an upper level of their home or leave the property.
Plan for people living in areas of risk of flooding:
- planning what to do if there is a flood warning (e.g. moving valuable items upstairs
- using flood gates to prevent floodwater from damaging property
- using sandbags to keep floodwater away from buildings
