Rivers Flashcards
What are the 4 types of erosion?
Hydraulic action-Water smashes into the river bank and air gets trapped and breaks the rocks apart
Abrasion-Pebbles grind along the river bed and bank
Attrition-River rocks collide in with each and break
Solution-The water dissolving certain chemicals in rocks
What are the 4 types of transportation
Traction - large, heavy pebbles are rolled along the river bed. This is most common near the source of a river, as here the load is larger
Saltation - pebbles are bounced along the river bed, most commonly near the source
Suspension - lighter sediment is suspended (carried) within the water, most commonly near the mouth of the river.
Solution - the transport of dissolved chemicals. This varies along the river depending on the presence of soluble rocks
Define deposition.
When the river loses energy, it drops any of the material it has been carrying.
What are the type of features in a drainage basin?
Watershed - the area of high land forming the edge of a river basin
Source - where a river begins
Mouth - where a river meets the sea
Confluence - the point at which two rivers meet
Tributary - a small river or stream that joins a larger river
Channel - this is where the river flows
Explain upper course and lower course
Upper course - in the upper course, where the river starts, there is often an upland area. The river’s load is large in the upper course, as it hasn’t been broken down by erosion yet. The river erodes vertically.
Lower course - in the lower course, the land is a lot flatter. The river’s load is fine sediment, as erosion has broken down the rocks. The river is much more deeper and the river erodes laterally.
What is a waterfall?
A waterfall is a sudden drop along the river course. It forms when there are horizontal bands of resistant rock (hard rock) positioned over exposed, less resistant rock (soft rock).
Explain the process of making a waterfall.
The soft rock is eroded quicker than the hard rock and this creates a step.
As erosion continues, the hard rock is undercut forming an overhang.
Abrasion and hydraulic action erode to create a plunge pool.
Over time this gets bigger, increasing the size of the overhang until the hard rock is no longer supported and it collapses.
This process continues and the waterfall retreats upstream.
A steep-sided valley is left where the waterfall once was. This is called a gorge.
What is an interlocking spur?
In the upper course there is more vertical erosion. The river cuts down into the valley. If there are areas of hard rock which are harder to erode, the river will bend around it. This creates interlocking spurs of land which link together like the teeth of a zip.
What is a meander?
As the river makes its way to the middle course, it gains more water and therefore more energy. Lateral erosion starts to widen the river. When the river flows over flatter land they develop large bends called meanders.
Explain the features of a meander.
As a river goes around a bend, most of the water is pushed towards the outside. This causes increased speed and therefore increased erosion (through hydraulic action and abrasion).
The lateral erosion on the outside bend causes undercutting of the bank to form a river cliff.
Water on the inner bend is slower, causing the water to slow down and deposit the eroded material, creating a gentle slope of sand and shingle.
The build-up of deposited sediment is known as a slip-off slope (or sometimes river beach).
How is an oxbow lake formed?
Due to erosion on the outside of a bend and deposition on the inside, the shape of a meander will change over a period of time. Erosion narrows the neck of the land within the meander and as the process continues, the meanders move closer together. When there is a very high discharge (usually during a flood), the river cuts across the neck, taking a new, straighter and shorter route. Deposition will occur to cut off the original meander, leaving a horseshoe-shaped oxbow lake.
What is a floodplain?
A floodplain is an area of land which is covered in water when a river bursts its banks
What are floodplains used for?
Floodplains are often agricultural land, as the area is very fertile because it’s made up of alluvium (deposited silt from a river flood). The floodplain is often a wide, flat area caused by meanders shifting along the valley.
How are levees made?
Levees occur in the lower course of a river when there is an increase in the volume of water flowing downstream and flooding occurs.
Sediment that has been eroded further upstream is transported downstream.
When the river floods, the sediment spreads out across the floodplain.
When a flood occurs, the river loses energy. The largest material is deposited first on the sides of the river banks and smaller material further away.
After many floods, the sediment builds up to increase the height of the river banks, meaning that the channel can carry more water (a greater discharge) and flooding is less likely to occur in the future.
What is an estuary?
An estuary is where the river meets the sea. The river here is tidal and when the sea retreats the volume of the water in the estuary is reduced. When there is less water, the river deposits silt to form mudflats which are an important habitat for wildlife.
What are the causes of flooding?
Prolonged rainfall
Heavy rainfall
Relief
Geology
Vegetation
Urban land use
Why can lag time be different?
The lag time can be short or long depending on different factors. For example, if there is no vegetation in an area, the water runs off into the river quicker, therefore it would have a short lag time.
What are the factors affecting lag time?
Factors influencing lag time include:
Size of drainage basin
Vegetation
Valley side steepness
Soil type
What is hard and soft engineering?
Hard engineering involves building artificial structures which try to control rivers. They tend to be more expensive.
Soft engineering does not involve building artificial structures, but takes a more sustainable and natural approach to managing the potential for river flooding.
What is a dam?
The dam traps water, which builds up behind it, forming a reservoir. Water can be released in a controlled way.an be used to produce electricity by passing the water through a turbine within the dam.
How does flood warnings and preparation defend against flooding?
Flood warnings and preparation
The environmental agency monitors rivers and issues warnings via newspapers, TV, radio and the internet when they are likely to flood so people can prepare.
Why does the velocity change downstream?
he speed the water travels at is dependent on which course of the river it is travelling through. Friction occurs between the water and the riverbed which slows the water down. The more contact the water has with the floor, the more friction occurs which means the water travels slower.
What are the causes of flooding?
Physical causes of flooding:
heavy rainfall
long periods of rain
snowmelt
steep slopes
impermeable rock (doesn’t allow water through)
very wet, saturated soils
compacted or dry soil
Human factors increasing flood risk:
urbanisation, because towns and cities have more impermeable surfaces
deforestation, because removing trees reduces the amount of water
intercepted and increases
run-off
What is discharge?
Discharge = velocity x cross-sectional area
What is a long profile?
A section of the course of a river drawn from source to mouth.
Explain a long profile graph A-Upper course
B-Middle course
C-Lower Course
A - as the river flows downhill there is an increase in vertical erosion. The channel is shallow and narrow because there is not a lot of water in the channel.
B – as the river flows into the middle course, there is some vertical erosion but more lateral erosion. The channel is wider and deeper as a result.
C - in the lower course there is a lot less erosion, with only some lateral erosion. The channel is at its widest and deepest.
What is a stom hydrograph
A hydrograph shows how a river responds to a period of rainfall.
What are the parts of a hydrograph?
Peak discharge - maximum amount of water held in the channel.
Peak rainfall – maximum amount of rainfall (millimetres).
Lag time - the time taken between peak rainfall and peak discharge.
Rising limb - shows the increase in discharge on a hydrograph.
Falling limb - shows the return of discharge to normal/base flow on a hydrograph.
Base flow - the normal discharge of the river
Factors affecting Lag time:
Geology:
Slope
Drainage basin shape
Antecedent conditions
Vegetation
What is hard engineering?
involves building artificial structures which try to control rivers. They tend to be more expensive. Each hard engineering strategy has its advantages and disadvantages.
What is soft engineering?
does not involve building artificial structures, but takes a more
sustainable and natural approach to managing the potential for river flooding. Each approach has its advantages and disadvantages.
Advantages and Disadvantages of Dam building:
(hard)
The dam traps water, which builds up behind it, forming a reservoir. Water can be released in a controlled way.
Advantages
Can be used to produce electricity by passing the water through a turbine within the dam.
Reservoirs can attract tourists.
Disadvantages
Very expensive.
Dams trap sediment which means the reservoir can hold less water.
Habitats are flooded often leading to rotting vegetation. This releases methane which is a
greenhouse gas.
Settlements are lost leading to the displacement of people. In developing countries locals are not always consulted and can have little say in where they are relocated.
Advantages and Disadvantages of Channelisation:
(hard)
River straightening
Advantages:
Straightening the river speeds up the water so high volumes of water can pass through an area quickly.
Improves Navigation
Disadvantages:
Water taken downstream quickly as velocity is increased may harm areas downstream
Advantages and Disadvantages of Embankments:
(hard)
Raising the banks of a river means that it can hold more water.
Advantages
Cheap with a one-off cost
Allow for flood water to be contained within the river.
Disadvantages
Look unnatural.
Water speeds up and can increase flood risk downstream.
Advantages and Disadvantages of Flood Relief channels:
(hard)
The floodwater flows into the relief channel and is taken either to an area where it can be absorbed, or re-enters the river further down its course.
Advantages
Removes excess water from the river channel to reduce flooding.
Disadvantages
Expensive to build.
If water levels continue to rise, the relief channel may also flood.
Advantages and Disadvantages of Flood Warnings:
(soft)
Advantages
People have time to try to protect their properties, eg with sandbags.
Many possessions can be saved, resulting in fewer insurance claims.
Disadvantages
Some people may not be able to access the warnings.
Flash floods may happen too quickly for a warning to be effective.
They do not stop land from flooding - they just warn people that a flood is likely.
Advantages and Disadvantages of Floodplain zoning:
(Soft)
Advantages
More expensive buildings and land uses are further away from the river, so have a reduced flood risk.
Less damage is caused, leading to fewer insurance claims.
Disadvantages
Not always possible to change existing land uses.
Planners have to decide what type of flood to plan for.
Advantages and Disadvantages of washlands:
(soft)
-Advantages:
slow floodwater
-efficient at storing water
as soil structure is restored
Disavantages:
-limits use of land
Advantages and Disadvantages of river restoration:
(soft)
Advantages
-slows river
-attractive
-wildlife habitats
Disadvantages:
-Changes in land
Explain one reason why river discharge changes along the course of a river.
As the river flows downstream more tributaries join it which provides
water from a larger part of the drainage basin
Why is there a lag time between peak rainfall and peak discharge following a storm?
The rain may infiltrate into the soil which means that it will take time to
reach the river .
The rain may be intercepted by vegetation which leads to surface run
off into the river .
What is floodplain zoning?
Floodplain zoning
Allowing only certain land uses on the floodplain reduces the risk of flooding to houses and important buildings
