- Channel fall: Water that falls directly into the river. (Approx. 5% does)
- Evaporation: Water in the river becomes gas due to heat.
- Channel flow: The volume of water in the river at any time.
- Percolation: The downward movement of water within the soil.
- Interception: Precipitation is prevented from hitting the ground due to landing on vegetation (E.g. leaves)
- Throughflow: The lateral (Sideways) movement of water through soil.
- Groundwater flow: Water in pore spaces of rock and soil underground.
- Surface runoff: Water that runs over the land to reach the river.
- Precipitation: Water falling onto the Earth as rain, snow, sleet or hail.
- Infiltration: Water soaks into the soil.
- Vegetation store: The volume of water stored in vegetation at a given time.
- Evapotranspiration: The process of evaporation and transpiration.
- Surface store: Water stored on the surface (E.g. puddles)
- Throughfall/leaf drip: Water that drips from leaes to reach the ground.
- Stemflow: Warer that runs from stems/trunks/branches to reach the round.
- Transpiration: The process whereby plants lose their water.
- Soil moisture: The volume of the water within the soil at any given time.
The water balance shows how the water in a drainage basin is balanced, relating to the balance between inputs, outputs & stores. Formula for water balance: discharge = precipitation - evapotranspiration +/- changes in store.
Factors affecting water balance:
- Leaves intercept precipitation, delaying runoff into the river.
- Vegetation transpires, which reduces the amount of water that reaches the river.
- Roots take in water, reducing what enters the river.
- Roots slow down throughflow to the river.
- There are fewer trees so reduced interception (more water flows to river)
- Sewage systems create artifical throughflow, which is where water flows very quickly through pipes to the river.
- Urban areas increase the amount of impermeable surfaces, increasing surface runoff to the river which is very fast.
- Surface store is increased due to the lack of infiltration, meaning there are puddles which are essentially flooding.
A storm hydrograph shows how a rivers discharge at a given point over a period of time responds to one particular storm. This is different to a river regime because that shows changes in the discharge over a longer period of time (Annual variations) whereas a storm hydrograph just shows changes after 1 rainfall event.
Definitions of the labels:
- Approach segment: The discharge before a rain event.
- Peak rainfall: The highest amount of rainfall.
- Rising limb: An increase in the rivers discharge.
- Falling limb: A decrease in the rivers discharge.
- Peak discharge: The highest amount of water/discharge in a river.
- Lag time: The time difference between the peak rainfall & peak discharge.
- Storm flow: The increase in a rivers discharge due to surface runoff & throughflow.
- Base flow: The usual amount of discharge/water in a river from groundwater flow.
Two main types of storm hydrographs: FLASH & NON-FLASHY.
Factors affecting shape of flashy hydrograph
Have a SHORT lag time, STEEP rising limb & HIGH peak discharge.
- Antecedent soil moisture: High soil moisture reduces infiltration & increases surface runoff.
- Drainage basin: Water hasn't got far to travel & channel fall is more likely in a small, circular drainage basin.
- Relief: Steep slopes means more quick surface runoff so a shorter lag time.
- Drainage density: Channel fall is more likely if there's more surface streams.
- Rainfall: Long/heavy rainfall saturates the ground so infiltration is replaced by surface runoff.
- Natural vegetation: Less interception & more channel fall if there's little vegetation.
- Rock type: Water can travel quickly by surface runoff without infiltrating if the rocks impermeable.
- Land use: Urban areas have alot of impermeable surfaces so water travels quickly by surface runoff.
- Use of river: Limited use of the river.
Factors affecting shape of non flashy hydrograph
Have a LONG lag time, GENTLE rising limb, LOW peak discharge.
- Antecedent soil moisture: Low soil moisture makes the land unsaturated so infiltration increases.
- Drainage basin: Large, elongated so water has far to travel and channel fall is less likely.
- Relief: Gentler slopes give time for a slower flow & more infiltration & percolation.
- Drainage density: Less means fewer streams to collect water and longer lag time.
- Rainfall: Unsaturated ground if there's less precipitation so infiltration increases.
- Natural vegetation: More vegetation increases interception, evapotranspiration, stemflow & throughflow which increases lag time.
- Rock type: Permeable rock allows infiltration so lag time will be longer.
- Land use: In rural areas more vegetation means more interception, evapotranspiration, stemflow and throughfall occurs which increases lag time.
- Use of river: If water is extracted or stored elsewhere it will take longer for the river to reach bankfull discharge and flood.
Causes of Shrewsbury flood in 1998
- Land in the drainage basin is often underlain by impermeable rock which reduces infiltration/percolation & creates surface runoff that flows quickly to the river.
- The river passes through steep upland areas which contribute to high levels of surface runoff.
- The R. Severn drainage basin is circular so many short tributaries feed into the river & produce a flashy hydrograph.
- Shrewsbury is downstream from the Vrynwy, a major tributary that almost doubles the discharge of R. Severn.
- The river meanders across flat land in Shrewsbury. This slows the flow of water and leads to increased discharge & flooding.
Council in Shrewsbury turned down a £5 million flood defence package because it was an eye sore. So as a result the town was un prepared.
Causes of Shrewsbury flood in 1998
Antecedent rainfall levels had been very high prior to the floods.
Mid-Wales has high annual levels of rainfall. This increases the amount of discharge in the river and increases flood risk.
October 1998 was the wettest October on record. The ground saturated quickly so surface runoff reached the river quickly.
Over the past 1000 yrs, Shrewsbury town has expanded onto the river's floodplain which is very flat and prone to flooding.
Drains carry water from the impermeable surface to the river which creates a sudden increase in river discharge.
The Frankwell, English and Welsh bridges cross the R. Severn in Shrewsbury & lower the river's bankfull discharge.
Impacts of Shrewsbury floods
- Many people suffered trauma and stress if their valuable possessions were damaged or if they were injured during the floods.
- The river became polluted by sewage that was released from overrun sewage works.
- Polluted flood water entered homes and businesses, which could potentially have spread diseases if not for EA warnings.
- Water rose up to 5ft on some streets and cars washed away.
- The 2 main bridges into the town were closed so everyone had to use a privatley owned toll one
- People became trapped in their homes by flood water on the ground floor and had to be rescued.
- 12 people died in the R. Severn drainage basin (0 in Shrewsbury)
- Positive thing - there was a strong feeling of community cohesian as residents came together to help one another which made long term friendships.
Impacts of Shrewsbury floods
- Total damage costs were approx. £100 million.
- Local economy was affected as crops and land was flooded, reducing farmers income.
- Insurance rose after the floods because many residents had to make claims for lost or damaged personal belongings.
- 400 buildings were submerged in Shrewsbury.
- Companies had to pay large costs to increase staff numbers.
Flood management - Soft engineering
Soft engineering - Working with nature and managing the land use around the rivers drainage basin.
Hard engineering - controlled disruption of natural processes by using man-made structures.
Areas of controlled flooding
This has been done in Silk Meadow and Frankwell car park. Both are just NW of Shrewsbury. This reduces the discharge of the river before it reaches Shrewsbury, reducing the flood risk. Compensation offered to farmers and the EA will purchase the land if necessary.
By planting trees there is a greater interception of rainfall and roots of the tree will take in water from the soil and groundwater store. This increases the lag time and lowers the peak discharge of the river as less water will reach the river to due evapotranspiration.
Flood management - Hard engineering
Building design - Buildings on the riverside don't put anything valuable on the ground floor. This reduces damage and costs associated with it. In the theatre all important rooms are on higher floors, only the reception is on the ground floor. They're also on stilts to elevate them.
Floodwalls - Reinforced concrete flood walls were built along the riverside. They are clad with carefully selected bricks or stone so that they don't ruin the view. Floodgates seal access to the footpath during a flood. The walls increase bankfull discharge and protect buildings on the other side.
Demountable barriers - They can be put up rapidly before a flood and increase the bankfull discharge and works with a below ground wall which prevents underground flow during floods. They protect areas of high density areas and buildings on the floodplain. Only cause temp eyesore.
'One way' drain valves - The drains stop the backflow of floodwater and reduce the flooding in the town centre. An artificial throughflow is created and surface runoff & surface storage is reduced.
Temp flood defences - A mobile dam consists of 2 tubes fitted togeether & filled with floodwater to create a dam. Increase bankfull discharge.
In 1993 Shrewsbury turned down a £5million flood defencee scheme because it would be an eyesore. After 1998 & 2000 floods, they agreed that the defences were needed.
Areas of controlled flooding
Advange - Land of flow value is allowed to flood so that the risk to homes and businesses is reduced. Farmers are compensated for use of their land. Water infiltrates so takes longer to reach the river.
Disadvantage - A considerable amount of land has to be flooded for it to be effective.
Advantage - No valuable items are left on the ground floor in buildings so insurance claims are reduced.
Disadvantage - Elderly residents may have to climb to first floor of building for a service.
Advantage - Hold floodwater back so it can't overflow to steets. Can be taken down after so not a permanent eyesore.
Disadvantage - Council has to take time to erect them if a flood warning is issued (6-12 hrs)
Advantage - Trees intercept rainfall & absorb water in the soil Throughflow is delayed by tree roots.
Advantage - Reinforced concrete walls increase the bankfull discharge of the river.
Disadvantage - Eyesore
Advantage - Block water and slightly increase bankfull discharge. Not permanent.
Disadvantage - Only effective for about 1 metre raised water.
One way drain valves
Advantage - Stop water retreating back up the storm drains & flooding the town centre.
Advantage - Hold water in a certain areas so flooding is confinsed to minimal places. Not permament.
Disadvantage - Water can spill over the top and once breached they're not effective.
Yangtze flood - China 1998
- Urbanisation - impermeable ground
- Building on the floodplain
- Levees not maintained
- Goverment blew up levees, killed the small towns to save the big city
- Deforrestation and farming on steep slodes led to soil erosion - less interception due to less trees
- Siltation - a thick layer of silt covered the land
- 3000 killed
- Farmland flooded and crops destroyed - social impact
- 30 million left homeless. 5.6 homes destroyed - social impact
- $24 billion worth of damage. Could increase taxes for goverment to fund for damage. - economic impact
Yangtze flood - China
Yangtze flood was the worst in 50 years. There was 1000mm of rain in 3 days.
- Cotton prices increased dramatically around the world.
- Built the Three Gorges Dam
- Afforestation - increased infiltration & interception, decreased surface runoff. Logging ban.
- Stopped farming on steep land.
- Heighten and reinforced all levees.
Yangtze flood - China. August - Oct 1998 (LEDC)
- Source from Tibetan Plateau - 6600m above sea level, steep valley sides meant high levels of surface runoff.
- Drainage basin covers 1/5th of the country (1/2 billion people live there)
- Extremely heavy snowfall in Tibetan Plateau combined with heavy rainfall - 1000m in 3 days.
- Low, flat land - the water flows down and has nowhere else to go.
- Big meanders - Slows down rate of flow into a river, which can lead to an increased discharge.
- High annual (seasonal) amounts of rain - high antecedent rainfall/soil moisture.
- Densely populated - Loss of land needed for flood control, used for housing/farmland.
- Deforrestation - Reduced interception.
- Soil erosion - Due to defforestation, more soil fell into the river reducing the channel capacity.
- Rapid urbanisation - Impermeabl surfaces reduced infiltration.
- Siltation - Raised river beds by nearly 2 metres in 30 yrs, loss of natural storage area.
- Farming on steep slopes - Land not terraced or contour ploughed.
- Poor maintenance of levees - Too poor. Could only withstand 1 in 10 yr flood.
Yangtze floods impacts
- 3000 deaths
- 5.6 million homes washed away
- 30 million left homeless
- After 7 months, still little food
- Psychological trauma of it happening again
- Community spirit increased
- $24 million in damage (£15 million)
- Cotton prices increased worldwide.
- 1/10 left without a job in cotton factory
- Huge areas of crops destroyed
Impacts on environment
- Worst flood in 50 yrs - rivers reached their highest recorded level, maintained for over 2 months
- Cities, like Yeuyang, had been submerged for over 30 days
- 50cm of silt left behind, which made it impossible for crops to grow (also socio-ecomic)
- Transport routes disrupted - Railways swept away, losing business & trade (also social-economic)
- Authories admitted that they were flooding smaller towns/villages to protect larger settlements, e.g. Wuhan.
- Suspicion that the govt blew up the levees, sacrificing lives to save the economy.
Hard engineering stratergies
Artificial levees - Walls built along the length of a river, protecting further downstream erosion.
Wing dykes - Tough stone walls that are built into the river channel. They catch sediment, creating new land. This narrows the river, making the flow of the water faster, increasing the erosion and decreasing the risk of flooding. The new land is also used as a floodplain, olding the water if it does burst its banks.
Straightening the river (river realignment) - Cut off all the meanders to realign the river, this means the water travels through the channel at a quicker pace, decreasing the risk of flooding as the water will erode the bottom of the channel, making it deeper and can therefore hold a larger capacity of water.
Bypass channels - Redirecting excess water upstream of a settlement via an alternative route, then the water can re-entre the channel further downstream, so reducing flood risk.
Weirs - Used to slow the river down. Steps in the river bed help slow down the pace of the discharge. e.g. in urban areas, weirs are used through the floodplain of undeveloped areas.
Hard engineering strategies
Dredging - Removing excess sediment on the river bed will enlarge the capacity of the river so that it can hold more water. However, the ecological balance of the river would be affected as habitats for plants, fish and animals are all affected by dredging. Dredging needs to be continual or the removed sediment would just slip back into the river channel. It could increase the risk of erosion at meanders, eventually causing an ox-bow lake.
Concreating the channel bed and banks - By lining the bed and banks with concrete, it reduces erosion (especially after the river allignment). It makes the river more efficient, decrease the chance of flooding and decreasing erosion but decreases the amount of water the channel can carry.
Hard engineering & the 3 gorges dam case study
The Three Gorges Dam
Location - Near the middle of the River Yangtze, China, upstream of Yichang. The Yangzte is the 3rd longest river in the world and it's drainage basin covers 1/5th of China. Its source is in the Tibetan Plateau, 6600 metres above sea level.
Why was it built?
- To reduce the effects of flooding of the R. Yangtze.
The rivers upper course is high in the Tibetan Plateau, an area of steep slopes and inputs of glaciers, snow melt and summer monsoons. The middle and lower course flatten out and the river flows in enormous meanders to the East China Sea, North of Shanghai.
The middle and lower reaches of the river valley are densely populated and is therefore highly industrialised and urbanised-account for 40% of China's output.
Three Gorges Dam
- 2km long
- Regulated by a sluice gate in the dam well
- 185m high
- Can hold back water up to 130m & room for excess
- Generates 18,000 MW of electricity (equivalent to 5 nuclear power stations)
- Ship lock to take large ships up and down past the dam
Advantages of the gorge
- 18,000 megawatts of HEP - reducing co2 emissions
- 20,000 jobs on dam site alone
- More navigable for ships - they can now reach the major city Chongquing
- Growth pole for economic development (e.g. Chongquing) therefore creating the multiplier effect
- Prevents 1 in 100 yr flooding or urban areas- saving lives
- Businesses received compensation from govt for relocation
- Warnings can now be issued as they are marked on the flood level
- Floodwater can be released in a controlled way out of sluice gates on the dam wall
The Three Gorges Dam
- Jobs not sustainable
- Massive scale damage to landscape and ecosystems
- 1.3 million people relocated - 1/2 of which to different regions
- 13 cities (inc. Fengdu),140 towns & 1350 villages moved
- Time pressure lowers construction standard
- Taxes will be paying for new cities for over a decade
- Farmland destroyed - only enough field room to feed 1 in 3 villages
- $15 billion cost
- Due to being 2km long and 185m high, it's unsightly
- Dams of this scale can cause earthquakes
Was it successful? - Could be a failure because it's costly and the lifestyle of residents was changed. Although the deaths caused by flooding have dropped from several 1000's to 100's. The dam was stimulated enormous economic growth, especially for Chingquong which is now the main base for Ford Mondeo as major ships can reach it through the ship lock. This increases China's overall income so it can develop economically. The 18,000 MW of HEP will help clean up emissions and make China more environmentally friendly.
Soft engineering. River Skerne case study
The use of the environment and the land around the drainage basin to manage flooding. E.g. afforestation & controlled flooding in certain areas.
River Skern restoration.
Aim - To improve water quality and ensure soft engineering is in place for flood control
- High sediment load = silt
- Had been straightened and deepened to carry water away from Darlington
- Water pollution from industry and sewage works discharge into the river
- No floodplain due to soil heaps
- Housing developments affected
- Gas, sewer pipes and electricity cables in the way
- Only 2km restored (benefits limited
River Skern case study
What they did
- Restored 2km of river
- New wetland for seasonal flooding
- Removal of spoil
- 4 new meanders - Look more natural, seasonal flooding
- Willow trees on the North bank - protect the bank from erosion
- Underground collection chambers - reduce pollution from factories and drains into the river
- Lowered floodplain
- Used some spoil to build embankments to screen the factories
- Improved water quality
- Flood management - Reduced flood risk, gentle hydrograph
- Landscape and access for the community - more attractive environment
- Wildlife moved into area - birds
- Environmentally friendly as living strucutures (e.g. trees) used for construstion
- The surrounding wildlife and habitats are less likely to be affected, and rather, protected instead
- Cheaper to construct compared to hard engineering as it makes use of available resources
- Less long term maintenance needed due to the fact that the living strutures mature and stabilise over time
- Can't be used in urban areas due to the use of living structures that don't go particulary well with concrete.
- Restricts development such as housing because the living structures cannot support hard structures above it, unlike hard engineering methods.
- If controlled flooding is used, that land is lost to the river so any property or landscape will be affected.
- Less likely to be effective against extreme storm events.
Flood forecasting and warning systems
Environment agency (EA)
- Govt body in charge of building and maintaining flood defences.
- Responsible for increasing public awareness of flood risk, forecasting & warning.
- During a flood event, the EA will work with the Borough Council and emergency services to help alleviate the situation.
- The EA website provies info about the flood risk for any postcode.
- Flood warnings are issues throughout a flood event - direct to residents by phone, fax, text, email etc.
Flood forecasting and warning
Reccurence interval graphs are used to predict when the next flood of a particular magnitude will occur and can be used to plan flood defences.
Magnitude frequence analysis
- Local authorities look at the reccurence interval to help them manage flood events - Shows the period of years within which a particular level of flooding is likely to occur. And is based on the magnitude and frequency of the floods.
- Cannot provide an accurate forecast. 1980 Skipton 1 in 100 yr flood followed by another in 1981. (Shouldn't have been for another 100 yrs)
- Hydrologists try to forecast the likelihood of future floods using historical records. - Shows the average time interval between similar sized floods.