River management Flashcards
Define precipitation
Moisture falling from the atmosphere as rain, hail, sleet or snow
Define discharge
The quantity of water that passes a given point on a stream or river bank - within a given period of time
More precipitation = More ____
More precipitation = More discharge
What is a hydrograph
A hydrograph shows how a river responds to a rainfall event.
This helps us to understand discharge patterns of a particular drainage basin
What two graphs are hydrographs made up of
Bar chart = Rainfall
Line graph = Discharge
Features of a hydrograph
Peak rainfall
Lag time
Rising Limb
Peak discharge
Falling limb
Base flow (normal flow)
x - axis - time (days and hours)
y- axis - discharge in cumecs
What is peak discharge
The highest discharge in a given period of time
What is lag time
The delay between peak rainfall and peak discharge
What is rising limb
The increase in river discharge as rainwater flows into the river
Falling limb -
The decrease in the river discharge as the river returns to its normal level
What is the base flow
The normal flow of a river when its level is being sustained by groundwater
Label a hydrograph
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Two types of hydrographs
‘Slow response’ hydrograph
‘Flashy response’ hydrograph
Explain the features of a slow response hydrograph
Slow response = slow flow
Small amount of rainfall in a prolonged amount of time
There is a less steep rising limb as water is taking longer to enter the river channel
The lag time is long as the water is taking a long time to enter the river channel - the peak discharge is lower as water is lost to infiltration and interception
Conditions that would result in a slow hydrograph
Areas with permeable materials, vegetation, gently sloping land, unsaturated soil, good drainage
Describe the features of a flashy response hydrograph
flashy response aka storm hydrograph = quick flow
Large amount of rainfall in a short period of time
The rising limb is steep because rainfall has occurred in conditions that have caused a lot of surface run off (when the water flows on the surface instead of permeating in the ground). All rainwater rushes to the river at once
The lag time is short as water gets into the channel quickly due to a lot of surface runoff - therefore the time between rainfall and entering the river is short
The peak discharge is also high as the water has gotten into the river quickly and less water is lost -
High peak discharge as there’s lots of water
Conditions which may result in a flashy hydrograph
Prolonged rainfall, which results in saturated soil. Therefore more surface runoff -
Clay soil so water cannot infiltrate
Steep valley/hill so water runs down faster - reducing lag time
Little vegetation leads to more surface runoff (less interception)
Urban area with lots of impermeable materials
Poorly farmed land = increased surface runoff
What is flashy response
flashy response hydrograph is associated with sudden flooding called ‘flash floods’
What is a slow response
normal rainfall event (low flood risk)
State the physical and human factors that affect flood risk
Physical
Precipitation (Heavy rainfall, prolonged rainfall)
Geology (rock type)
Relief (changes in land height)
Human - land use
What is flooding
Flooding occurs when a river’s level rises so much that it spills over its banks
What does a shorter lag time result in
Shorter lag time = peak discharge is higher = flooding is more LIKELY TO OCCUR
Explain how heavy rainfall affects flood risk
Heavy rainfall means that water arrives too quickly to infiltrate, so there’s a lot of surface runoff, which increases the discharge (into river channels)
Explain how geology affects flood risk
Clay soils and some rocks e.g. granite and shale, are impermeable (they don’t allow infiltration), so surface runoff is increased (into river channels)
Explain how prolonged rainfall affects flood risk
Prolonged rainfall can saturate the soil
Any further rainfall can’t infiltrate, increasing surface runoff into river channels
Explain how relief affects flood risk
If a river is in a steep-sided valley, water will reach the river channel quicker because it can flow faster down the steep slopes - this rapidly increases discharge
Explain how land use affects flood risk
Buildings are often made from impermeable materials (e.g. concrete) and surrounded by rounds made from impermeable surfaces (e.g. tarmac), increasing surface runoff
Man-made drains then quickly transport runoff to rivers, increasing discharge
Trees intercept rainwater on their leaves, which then evaporates.
Trees also take up and store water from the ground. This means that curing down trees increases the volume of water that enters the river channel
State some types of different management strategies (FOR FLOODING)
Hard engineering
Soft engineering
What is hard engineering
Man-made structures built to control the flow of rivers and reduce flooding
Examples of hard engineering
dams and reservoirs,
straightening, embankments, flood relief
channels
Explain the method of dams and reservoirs
Dams are barriers built across the rivers, usually in the upper course
A reservoir (artificial lake) is formed behind the dam
Adv. of dams and reservoirs
Reservoirs store water (e.g. after heavy rain), control water flow and prevent flood downstream
They can also be used to generate hydroelectric power (HEP)
Disadv of dams and reservoirs
Dams are very expensive to build. Creating a reservoir can flood existing settlements
Material is deposited in the reservoir, not along the river’s course, so farmland downstream can become less fertile.
What is channel straightening
Meanders are removed by building straighter, artificial channels
Benefits of channel straightening
Water leaves the area more quickly rather than building up, so flood risk is lower
Disadvantages of channel straightening
Flooding may happen downstream instead.
Faster-moving water may cause more erosion downstream
What are embankments
Embankments are raised walls built along the river banks
Adv. of embankments
The river can hold more water, so floods are less frequent
Disadvantages to embankments
They are quite expensive
there is a risk of sever flooding, if the water rises above the level of the embankments or if they break
What are flood relief channels
Channels that are built to divert water around built-up areas or to divert excess water if the river level gets too high
Advantages of flood relief channels
Gates on the channels mean that the release of water can be controlled, reducing flood risk
Disadvantages of flood relief channels
There will be increased discharge where the relief channel rejoins the river (or joins another river), which could cause flooding in that area
What is soft engineering
Soft engineering is schemes set up using knowledge of a river and its processes to reduce the effects of flooding
What is flood warnings and preparation
The Environmental Agency issues flood warnings through various media (e.g. TV, radio, internet)
Buildings are modified to minimise flood damage.
Residents can prepare sandbags and flood boards prior to floods
Advantages/benefits to flood warnings and preparations
Warnings give people time to move possessions upstairs, put sandbags in place or evacuate.
This reduces the impact of flooding
Disadv. to flood warnings and preparation
Warnings don’t prevent floods
People may not have access to the warnings
Modifying buildings is expensive
Preparation doesn’t guarantee safety from a flood and it could give people a false sense of security
What is flood plain zoning
Flood plain zoning are restrictions that prevents building on parts of a flood plain that are likely to be affected by a flood
Adv of flood plain zoning
Flood risk is reduced since fewer impermeable surfaces are created (e.g. roads)
The impact of flooding is also reduced as there are no buildings to damage
Disadvantages of flood plain zoning
The expansion of an urban area is limited if there aren’t any other suitable building sites
It can’t help in areas with existing buildings
What are planting trees
Planting trees in the river valley increases the interception of rainwater (the lag time)
Adv. of planting trees
Discharge and flood risk decrease
This is because planting trees in the river valley increases the interception of rainwater (and lag time increases)
Vegetation reduces soil erosion in the valley and provides habitats for wildlife
Disadv. of planting trees
Less land is available for farming
What is river restoration
River restoration is making the river more natural. For example by removing man-made levees, so that the flood plain can flood more naturally
Adv. of river restoration
Discharge is reduced so there’s is less is less risk of flooding downstream
Little maintenance is needed and there are better habitats for wildlife
Disadv. of river restoration
Local flood risk can increase, especially if nothing is done to prevent major flooding.
