Floodplain management


Erosion: Erosion is the wearing away of something. When talking about rivers it normally means the wearing away of the bed, banks and its load.

Attrition: This when load in a rivers flow crash into each other, causing pieces to break off.

Hydraulic Action: This is when air and water gets trapped in cracks on a rivers beds and banks. The build up of pressure within the cracks causes bits of the bed and banks to break off and the cracks to get bigger.

Corrosion (solution): When the slight acidity of water cause bits of load and the bed and the banks to dissolve.

Corrasion (abrasion): When bits of load crash into the bed and banks. This process causes the load, bed and banks to wear away.

Factors Affecting the Rate of Erosion


Load: If the river has large angular load, then it is more likely to erode.

Velocity and Discharge: The faster the river is travelling and the greater its discharge the more likely the river is to erode.

Gradient: The steeper a river is the more likely it is to erode.

Geology: Softer rock and particularly unconsolidated rock like sand are going to be more vulnerable to erosion. Sedimentary rock with lots of cracks is going to be more vulnerable to hydraulic action.

pH: If water is more acidic it is obviously going to increase the rate of erosion (corrosion).

Human Impact: Humans can affect erosion in many ways; deforestation can increase run-off and therefore the discharge of the river, pollution can increase the acidity of the river, dams can remove load and increase clear water erosion, irrigation may remove water and therefore reduce velocity.

Clear water erosion: This happens in rivers with little load. It is simply the power of the water that is causing the banks and the bed to erode.

Vertical Erosion: In a rivers upper course there tends to be more vertical erosion. The main reason for this is the river is above its base level and most of its load is big. Because the load is big, most is bedload which means erosion is mainly concentrated on the bed, therefore eroding downwards.

Horizontal (lateral) Erosion: In the middle and lower course, more of the rivers load is smaller and the river is near its base level. Because the load is lighter, more is suspended load so there is greater erosion on the river banks.

Base level: This is the lowest level that a river can erode to. You can have artificial base levels e.g. a dam, but normally the base level of the river is sea level. It is impossible for a river to erode below its base level because water can not travel up hill.
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Transportation: When a river has surplus energy it may carry some of the material that it has eroded.

Traction: Load that is rolled along the bed of the river.

Saltation: Load that is bounced along the bed of the river.

Suspension: Load that is transported in a rivers' flow (current).

Solution: Load that is dissolved by a river and then transported by it.

Flotation: Material transported on the surface of a river.

The larger pieces of material tend to be transported along a rivers' bed. As they get smaller they can they be transported in the current (flow). Only the smallest bits may be dissolved.

The processes of erosion and transportation tend to make a rivers' load smoother and rounder as you move from the source to the mouth.
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Load: Any material that is transported by a river. A rivers' load can be divided into bed load, suspended load and dissolved load.

Deposition: When the velocity of a river falls causing its energy to fall. Because the energy of the river is falling so does its capacity and competence, causing to put down its load. This process of putting down load is deposition.
Hjulstrom Curve: A graph that shows the relationship between river velocity and particle size when looking at a rivers' ability to erode. transport and deposit.

Competence: The maximum diameter of a piece of load that a river can transport.

Capacity: The maximum amount of load that a river can transport.

Critical Erosion Velocity: The minimum velocity that a river needs to be traveling for it to start eroding and then transporting material.

Settling (or fall) Velocity: The velocity that a river needs to fall below to start depositing its load.

What apparent anomaly with the Hjulstrom curve is that it can erode sand at a much lower velocity than it can erode clay and silt. This is because that clay and silt are very cohesive (they stick together). This means that even though the particles sizes are small they have a very strong bond between them.
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River Landforms

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A rivers course is often divided into the upper course, the middle course and the lower course.

Upper Course: This is the section of the river nearest the source. This is where load is biggest and most erosion is vertical. Most landforms are made by erosion and include; waterfalls, gorges, rapids, v-shaped valleys and interlocking spurs.

Middle Course: This is the section when the river leaves the mountains and enters are more hilly environment. The valley floors starts to widen as you get more horizontal erosion. The landforms that you get in the middle course include alluvial fans and meanders.

Lower Course: This is the section closest to the mouth. Here the river is travelling over much flatter land and the load is much smaller and smoother. This is more horizontal erosion here as the river nears its base level. The landforms you find in the lower course include meanders, oxbow lakes, braided rivers, levees and deltas.

Classification of Landforms: As well as classifying landforms as upper, middle and lower course landforms, it is also possible to classify them as erosional, depositional and erosional and depositional.

Alluvial River: An alluvial river is any river that carries load. Nearly all rivers (except some rivers flowing over ice shelves and glaciers) carry load.

Fluvial: Anything found on or made by a river. This includes all landforms.

Meanders: Very simply a meander is a bend or a curve in a river. Why a river starts to bend is not fully understood, but one theory believes it is to do with riffles (shallow sections) and pools (deep sections). The river flows through the pools and around the riffles starting a meander.

Slip-off Slope (point bar): A slip-off slope is found on the inside of a meander. This is where the flow is slowest and deposition takes place.

River Cliff: A river cliff is found on the outside of a meander where the flow is fastest and erosion takes place.

Thalweg: Simply the fastest section of a river. Normally the section with least friction.

Sinuous: This means bend, if a river is sinuous then it is very bendy.
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Levees: Levees are embankments found on the sides of a river channel. Levees can be made by or enlarged by humans, but we are only interested in levees that are made naturally. Levees are made when a river exceeds bankfull discharge i.e. it is in flood.

When a river floods onto its floodplain then the river immediately experiences greater friction. The friction causes the velocity of the river to fall. As the rivers velocity and energy falls is starts to deposit its load. The heaviest load is deposited first. The lighter load (alluvium) will be deposited further away from the river. As more and more load is deposited by the banks of the river levees start to develop.

Levees actually increase the size of the river channel and can reduce the risk of flooding in the future.

Alluvium: This is basically the rivers load that is deposited on floodplains in times of flood. Alluvium has a lot of minerals and nutrients so it help fertilise the floodplain and benefits agriculture.
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River Terraces: A river terrace is an old floodplain separated from the new floodplain by a steep slope.It can be created by changes in a rivers load, changes in sea level, or human activity.

Changes in sea level: If the sea levels drop then so does the rivers base level. This means that a river can now carry out more vertical erosion, creating a lower floodplain.

Rivers load: If the size or quantity of a rivers load increases (possibly because the river is traveling over different rock or deforestation has lead to more weathering) then there might more bedload and more vertical erosion.

Human Activity: The building of a dam can cause more clear water vertical downstream, or deforestation can increase surface run-off and erosion increasing the amount of load in a river. Both human actions can cause the river and the floodplain to lower, leaving behind river terraces.
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Braided River: A braided river is a river with a number of smaller channels, separated by small and often temporary islands called eyots. Braided rivers usually form on rivers with variable flow (wet and dry season or snow melt season) and high quantities of load. When a river is at maximum discharge it is able to transport most of its load. However, when the discharge falls along with the velocity an energy of the river, deposition starts to take place, creating eyots.
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Oxbow Lake: An oxbow lake is a meander that has become cut off from the main river channel. If you have the outside of two meanders near each other they will eventually connect. They connect because erosion is at its maximum on the outside of the meander. When they eventually connect the thalweg (fastest flow) will no longer go around the old meander, but actually go in a straight line. This meas that the outside of the river channel now has a slower flow so deposition takes place cutting off the old meander.
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Floodplain: The floor of the valley floor that gets flooded when a river exceeds bankfull discharge. Floodplains tend to be much wider in a rivers' lower course where horizontal erosion has had a greater effect.

Bluff line: The outer limits of the floodplain. The bluff line is basically the edge of the valley floor.

Strand line: A line of load (usually sticks and litter) that is deposited at the limit of a flood.

River Valley: A long depression following the course of a river. It is made by the erosive power of the river. In a rivers' upper course a valley tends to be quite narrow, but very deep and often formed in a v-shape. In the lower course valley's tend to be much wider, but less deep. The outer limit of a river valley is basically its watershed.

Alluvial deposits (alluvium): Load that is deposited by a river in time of flood.

Human Modifications (Urban Hydrology): Humans can change the hydrology of rivers in many ways. Below is a summary of some of the most common.

  • Urbanisation: Urbanisation tends to cause deforestation reducing interception and transpiration. Sewers also reduce surface stores and therefore evaporation. Urban areas usually create large impermeable surfaces which can lead to greater surface run-off.
  • Sewer Systems: Generally sewer systems create create artificial channels, which often reduces a rivers' lag time and can lead to increased flooding downstream.
  • Pollution: Transport, industry and housing all create pollution which works its way into the water system. Areas that don't have proper sewers and water treatment tend to be effected more. Metals and chemicals are particularly polluting.
  • Water table (groundwater depletion): Unsustainable use of groundwater can cause subsidence. Mexico City has experienced subsidence because of aquifer depletion underneath the city. On the scale, London has actually seen its water table rise since deindustrialisation has meant the demand for water has fallen.
  • Deforestation: Deforestation reduces interception and transpiration. Removal of trees can also increase the risk of mudslide by reducing slope stability and stops root uptake. Less interception speeds up the rate the ground become saturated and therefore increases the risk of flooding
  • Micro-climate: Urban areas create heat islands which can increase convectional rainfall. Particulates released by industry and transport also make excellent condensation nuclei.
  • Channelisation: Artificially smoothing channels may remove river discharge from one area, but areas down stream that haven't been smoothed are likely to experience an increase risk of flooding.
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Subsidence - USGS

Rising water threatens London - BBC article

Flood Management Strategies


Hard engineering: Physically building structures to try and prevent flooding. Hard engineering usually involves the use of concrete, steel, rock and wood.

Soft engineering: This is working more with nature to provide flooding. This might involve planting trees or restoring the natural course of a river.

Channel Enlargement(widening/deepening): Making the width and depth of the river wider and deeper to increase its cross-sectional area.
Channel Straightening: Removing meanders from a river to make the river straighter.
Flood Relief Channels: Building new artificial channels that are used when a river nears bankfull discharge.
Artificial Stores: Creating reservoirs or lakes that can store excess water in times of flood.
Advantages: By enlarging the cross-sectional area you are increasing the bankfull discharge of the river along with its hydraulic radius. This will increase the velocity of the river and reduce the chances of it flooding in the immediate area by moving the floodwater further on downstream.
Disadvantages: If buildings are built up to the river bank it might not be possible to enlarge the channel. Also the process can be expensive and can cause problems to areas downstream who are receiving more flood water quicker, but with an un-enlarged channel.
Advantages: By removing meanders the velocity of the water through a settlement will increase. This will stop a backlog of water and should reduce the risk of flooding. It also improves navigation.
Disadvantages: By changing the course of the river, you might remove flowing water from industries that depend on it. There might also be building that have to be demolished to allow straightening. Again it is expensive and may cause flooding problems downstream.
Advantages: They take the pressure off the main channels when floods are likely therefore reduce flood risk.
Disadvantages: It can be hard find land to build relief channels, they are expensive and when empty can become areas to dump rubbish, etc. If river levels rise significantly it is also possible for relief channels to flood as well.
Advantages: They can remove pressure of the main channel and can become new habitats and serve other purposes e.g. leisure, drinking water.
Disadvantages: Building dams, sluices, diversion channels are all expensive. They also involve flooding areas of land which may be hard to find near large vulnerable urban populations.
Flood Embankments (levees): Like levees these increase the channel depth of a river, raising its bankfull discharge and reducing the risk of flood.
Controlled Flooding: Allowing low value land e.g. farmland to flood, therefore protecting higher value areas.
Afforestation / Reforestation: Simply planting more trees in a drainage basin.
Flood Proofing: This is making property less vulnerable to flooding or flood damage. This might be temporary like using sandbags or design by removing carpets downstairs.
Advantages: They increase the cross-sectional area of the river and therefore its hydraulic radius. This should reduce the risk of flooding.
Disadvantages: Like in New Orleans under extreme conditions, embankments may fail causing even bigger problems. They are expensive to build and again may cause problems downstream.
Advantages: By allowing the river to flood naturally you are taking the pressure of high value areas, you are letting the river behave more naturally and it adds alluvium to the floodplain.
Disadvantages: You have to make the decision what is worth protecting which is always going to upset someone. You also have to protect areas that you don't want to flood which costs money (cost benefit analysis)
Advantages: This is a natural process, increasing the amount of interception, transpiration and root uptake. People would not normally protest against trees being planted.
Disadvantages: It is not possible to cover the whole drainage basin in trees, so if it rains in an area with no trees, then there is no reduction in flooding. Also most trees lose there leaves in autumn and winter reducing interception in those months.
Advantages: This can be done on an individual level and can be relatively cheap. Temporary protection can be removed under normal circumstances so it does not change the aesthetics of properties.
Disadvantages: Temporary defences can usually only protect against minor floods. Not everyone will be happy with having to redesign their houses.
Insurance: Although it doesn't prevent flooding, it can help individuals and industries to recover and protect against future flooding.
Land Use Planning (zoning): Mapping areas by looking at there likelihood to flood and then only building low value uses on areas with high flood risk.
Contour Ploughing and Strip Cultivation: Either ploughing with the contours creating temporary surface stores or leaving vegetation to increase interception and transpiration
Interception Channels: These are channels that divert a rivers' discharge around settlements. The old channel remains but with a smaller discharge.
Advantages: It helps individuals and settlements to recover after flood events and may help them protect property and be less vulnerable in the future.
Disadvantages: They do not actually prevent flooding. Not everyone can afford insurance and insurance companies may not insure high risk areas.
Advantages: Very good at removing high value areas and high density populations from hazardous areas.
Disadvantages: It is not always possible to change land uses that already exist in an area. You have to decide what size flood to map for e.g. a once in ten year flood or once in one hundred year flood. Often poor will still choose to live on marginal land.
Advantages: Contour ploughing is simply a cheap and easy change in existing farming methods, Keeping vegetation is natural and relatively cheap.
Disadvantages: Won't protect against big floods and farmers may not be happy giving up farmland, simply to grow trees.
Advantages: They remove pressure of the main river and areas of high land value. They may also develop into new habitats for plants and animals.
Disadvantages: They are expensive, may flood themselves in times of heavy floods and may restrict future urban growth.
Settlement Removal: Moving settlements from high risk flood areas to less vulnerable locations often on higher land.
Dams: Often built as part of a multipurpose scheme, they create artificial stores which can hold water in times of increased precipitation.
Wing Dykes: Barriers placed out into a river, these can be used to divert the cause of rivers by shifting the thalweg of rivers. This may move the channel away from high value areas.
Electronically Controlled Sewers: Advanced sewers which can control the flow of rain water to stop increased discharge into rivers and therefore flooding.
Advantages: Is probably the most effective because you remove high value property and humans from vulnerable areas.
Disadvantages: It is usually not practical to move whole settlements, because of the cost and the problems of finding alternative locations. Also many settlements depend on water for their survival.
Advantages: They can store large amounts of water and can be used for other purposes.
Disadvantages: If rain is downstream of the dam then they have no effect. In large flood events they are vulnerable to breaking and are expensive to build.
Advantages: They can move the main channel from vulnerable areas to protect high value areas.
Disadvantages: They are expensive to build and during big flood events the flood water may go over the wing dykes. Also if there is property on both sides of a river, which side do you protect.
Advantages: They can be very effective at controlling smaller floods. They are underground so do not cause any visual pollution.
Disadvantages: This involves a complete redesign of sewers. Sewers usually have to be increased in size and electronic sluices have to be added. They also have to be operated from a central command centre and with all electronically operated equipment can break. Also they might not be able to cope with large scale floods, so water has to be released into rivers anyway.
Channelisation: The concreting of beds and banks.
Dredging: The removal of material from the bed of the river deepening it.
River bank conservation: Protecting the banks and sides of the river to reduce erosion. This can be done through planting vegetation.
River restoration: Returning a river to its natural state before it had been managed. This might involve removing channelisation.
Advantages: Reduces friction and increases velocity of river, removing water from the channelised area quicker. Bank erosion is also reduced.
Disadvantages: It is expensive and is not natural so vegetation and animal life will find it harder to grow and live. Flooding maybe caused downstream of the channelised area.
Advantages: Channel cross-section is increased so the river can hold greater discharge. It can look more natural because no structures are built.
Disadvantages: Deposition can mean that dredging needs to happen regularly.
Advantages: It looks natural, promoted wildlife and is relatively cheap compared to hard-engineering.
Disadvantages: During large flash floods vegetation can be easily removed.
Advantages: This looks natural, is attractive and can attract wildlife. Can allow the floodplain to become more fertile.
Disadvantages: Can't protect aganist big floods and may have to coincide with zoning.
Indonesia dam bursts kills dozens - BBC article

Flood defence spending to low - BBC article

Questions and answers on flooding and insurance - BBC article

US breeches Mississippi River levee to ease flood risk - BBC article

Mississippi Floods: Thousands flee as floodgates open - BBC article

Sluices: Gates that can be opened and closed to control the flow of water.
Cost Benefit Analysis: Cost benefit analysis can be done on almost anything. However, in geographical terms, it is usually done to decide whether a location should be protected or not. The actual analysis can be very complicated, but put simply it compares the cost of protecting an area compared to the benefits that an ares creates.

EXAMPLES:

If it would cost $1 million to protect an area, but it brings $2 million in revenue through jobs, taxes, etc. then the area is worth protecting.

However, if it would cost $2 million to protect an area, bit the area only brings $1 million in revenue, then it probably is not worth protecting the area.
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River Conwy, North Wales - Floodplain Management


The River Conwy is located in North Wales. It has its source in Snowdonia and its mouth in the Irish Sea, east of the island of Anglessey. The river is only 27 miles long, but regularly floods causing problems to settlements and businesses in its floodplain. The river has a steep gradient and mainly sits on impermeable metamorphic slates. This means there is little infiltration and high rates of surface run-off. The peat soils lying on top of the slates become saturated very quickly. The weather near the source is very wet and received up to 4 metres of rainfall a year. During spring the river is also fed by snow melt. Deforestation and the fact that river is tidal 16km in land make it very prone to flash floods. The river suffers some form of flooding most years, but 2004 and 2005 were both bad. In 2005, 360mm rain fell in the first 3 days of February causing the river to flood. The flood damaged the railways, roads, farmland, parkland, houses and businesses. To try and reduce flooding, a number of management techniques have been used:
  • River wall: A 3 metre concrete wall has been built to protect the village of Llanrwst.
  • River training: Near Llanrwst rocks have been placed in the river channel. The idea of the rocks is to slow the river near the village and cause deposition. The hope is that the river will then be redirected further away from the village.
  • Channelisation: Little tributaries that flow through Llanrwst have been lined with concrete. The aim is to get water through the villages quicker by reducing friction.
  • Embankments/levees: Raised banks have been built a long sections of the river to try and increase the river's cross-sectional area and reduce the risk of flooding.
  • Raised buildings and pathways: Some buildings and some pathways have been built on stilts so that they don't get damaged if the river bursts its banks.
  • Controlled flooding: Some low value farmland is allowed to flood in order to try and protect higher value settlements.
  • Temporary defences: Sandbags and temporary flood barriers are used if the river does break its banks.
  • Flood proofing of houses: Houses are often designed with no carpets and removable furniture on lower floors so they are not damaged during floods.
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Environment Agency: The government agency in the UK that is in charge of flood control. (http://www.environment-agency.gov.uk/)

Flood Risk With New Rain Warning - BBC article

Anger Over Flood Warnings - BBC article