Flooding
There are both primary and secondary causes of flooding:
Primary causes are usually the
result of climatic factors, such as
heavy autumn and winter rains in
the UK in 2007 due to low
pressure weather systems, or
monsoon related rains in countries
like Pakistan.
Secondary causes tend to
be drainage basin specific.
These factors are
dependent on geology, soil,
topography and vegetation.
Natural causes of floods
1. Rock Type – permeable rocks allow infiltration and greater
amounts of ground water storage. Less water runs off, so lower
river discharge.
2. Land topography – steeper slopes mean that less water infiltrates
and more runs off, giving a higher discharge and a greater risk of
flooding.
3. Drainage density – this is the ratio of the length of streams to
the total drainage basin area. A high drainage density gives a short
lag time and a greater risk of flooding.
4. Soil type and depth – deeper soils have a greater capacity for
water absorption and so more infiltration and less run off results,
giving a reduced risk of flooding. Clay soils allow no infiltration.
5. Rainfall – heavy, prolonged rainfall causes soil to become
saturated, and unable to absorb more water. All rain will then run
off to the river channel, creating a high discharge and increased
risk of flooding.
6. Snowmelt – water from the mountains in Spring adds to discharge
Human Causes of Flooding
1. Agriculture – ploughing compacts soil making infiltration difficult.
Streams channelled into culverts to aid rapid drainage of farmland
allow faster transfer of water from land to channel, giving a
shorter lag time.
2. Sewers feed water into the channel, increasing discharge further
3. Deforestation – reducing the number of trees stops interception
from occurring. This means less evaporation and more run off.
Urbanisation increases the frequency and magnitude of flooding:
o Impermeable surfaces like concrete and tarmac on roads and
pavements means there is no infiltration
o Drainage of water is speeded up by artificial conduits like drains
and sewers
o Channel flow is impeded by building alongside or in the river like
bridge supports
o Straightening of channels to increase speed of flow results in
flooding downstream
o Development changes land use – deforestation, ploughing and
overgrazing result in more run off and more sediment being
washed into streams, blocking the channel.
This is an urban area – lots
of impermeable surfaces,
few trees, drains and
sewers. The lag time is
short because water is
transferred quickly off the
land and into the channel.
The peak discharge is high,
because all of the
precipitation reaches the
channel (there is little soil
or rock to store the water)
This is a rural area with lots of trees, grassy areas, few buildings, and
few artificial drains. The lag time is long, because water must travel
via overland flow and through flow to get to the channel. The peak
discharge is low because water can be stored as ground water, so
not all precipitation reaches the channel.
Flood Management Strategies
Flood management or flood alleviation strategies seek to reduce the
effects of flooding on the human environment.
Risk Assessment - The process of establishing the probability that a
hazardous event of a particular magnitude will occur within a given period;
Estimating its impact, taking into account the locations of the buildings,
facilities, and emergency systems in the community; estimating the
potential exposure to the physical effects of the hazardous situation or
event, and the community’s vulnerability when subjected to those physical
effects
In order to minimise the
damage from floods it is useful
to know how often a flood of a
certain size might occur. The
time between flood events of
the same size is called the
recurrence interval. This can allow planners to zone the flood plain and
plan land use. They can also be used to predict the occurrence of low
flow.
Structural Methods – Hard Engineering
Flood Walls are designed to increase the
height of the channel to stop water spilling out
onto the floodplain. They restrict access to the
riverside and offer little floodwater storage
capacity. Case Study: Uckfield Flood Wall,
East Sussex.
Embankments are often made from earth and
rubble fill. If embankments are set back from
the channel they can provide storage for
excess flood waters so that inhabited areas can
remain unaffected. Case Study: River Irwell
Embankments, Salford
Levees may be artificially enhanced or introduced in order to raise the
level of the river banks.
Channel Improvements attempt to restrict floods by either creating a
smoother channel for faster flow, so that water can get out of an area
quickly, or by deepening and widening the channel. A smoother channel is
created by lining the channel with concrete, then dredging it to increase
capacity.
Relief Channels are constructed to redirect excess water
upstream of a settlement via an alternative route. Water can
then re-enter the main channel further downstream, reducing
flood risk. The bypass is only accessed at high discharge
levels, so that the peak flow of the main channel is reduced.
Case study: River Lee relief channel, Stratford
Flood Storage Reservoirs aim to store excess water in the upper
reaches of catchment areas. They are however very expensive to build
and require lots of land.
Flood Interception Schemes include rerouting rivers, using new channels
and embankments.
Channelisation is an attempt to alter the natural geometry of a watercourse. It
helps to prevent flooding by increasing the channel capacity and by preventing
bank erosion, both of which minimise the risk of a river bursting its banks in times
of high discharge. Resectioning a river involves widening and deepening a channel to
improve its hydraulic efficiency. This increases capacity and moves water out of
high risk areas quickly. Dredging removes surplus sediment from the river bed.
Realignment means straightening a river – shortening the
course by removing meanders. Revetments made of
concrete blocks, steel or wire mesh cubes filled with
boulders called gabions are used to strengthen banks. Wing
Dykes jut out from the sides of a channel to direct the
current to the centre of the channel and away from the banks. In urban areas,
rivers may be covered over and confined to concrete culverts. These remove the
increased amount of run off from impermeable surfaces and make it easier for
urban development to take place.
River basin management – Soft engineering
Flood Proofing may be temporary or permanent. Mew buildings can be
constructed with flood proof ground floor walls, or have temporary flood
gates installed at times of high risk. It may also be possible to put low
value land uses like car parks on ground level and important and vulnerable
facilities on a higher level, so that potential damage from floods is
reduced.
Flood Abatement aims to reduce the possibility of flooding by managing
land use upstream. This includes Afforestation to increase interception
and evapotranspiration, contour ploughing to reduce soil erosion and run
off and reducing the amount of bare earth to avoid excessive run off and
sediment problems.
Forecasts and warnings – In the UK, rivers are constantly monitored by
the Environment Agency, and records of floods and
river discharge are kept to help to predict events in the
future. Weather radars, rain gauges and river gauges
fee information to a central computer where a flood
warning officer interprets the information. Flood prediction software
helps to model likely outcomes and warnings can be issued as to the
potential severity of the flood risk. The Environment agency has a flood
mapping system which uses a method called Risk Assessment for
Strategic Planning. This is a probabilistic approach to risk assessment,
factoring in the influence of flood defences. It calculates the probability
that each 100m square will flood, and then places each into one of three
categories; low (<0.5% chance of flooding) moderate (0.5% <chance of
flooding <1.3%) or significant (>1.3%). Sometimes however, these
predictions and warnings come too late, and leave residents with no time
to prepare (like the Boscastle Flash Floods). Warnings were also
unsuccessful in the Workington and Cockermouth floods of 2009.
Land use management on floodplain – Flood Plain Zoning
The land around a river is categorised according to the relative risk of
flooding. Zone A is the prohibitive zone, and includes areas
near to the channel at a high risk from flooding. Future
development is unlikely to be allowed. Zone B is the
restrictive zone, and the little development which is
allowed must be flood proofed. Low intensity or low value
land uses are best here – pasture playing fields and car parks. Zone C is
the warning zone. These are areas situated on higher land which may be
used for residential developments or public buildings.
Wetland and river bank conservation – Wetlands are areas which are
deliberately allowed to flood at times of high discharge. They also
provide valuable wildlife habitats and help to maintain local biodiversity.
Case study: River Till, Lincoln. River bank conservation includes
restricting cattle access to rivers; trampling of river banks leads to
excessive erosion, but this can be minimised by using simple wooden rail
systems. Case study: River Cole, Oxfordshire. Afforestation on the
river bank, planting trees and shrubs, adds additional support to the
banks as well as reducing soil erosion.
Restoration of Peat Bogs in the Northern Uplands would slow down
water reaching lowland streams and rivers, reducing the threat of
flooding in towns and cities like Ripon, Hull and Sheffield.
River restoration and naturalisation intends to restore the river to a
more natural state, closer to its original course, by removing hard
engineering and other restrictive structures. The schemes are designed
to work alongside nature and be more sustainable than hard engineering.
Land alongside rivers is allowed to flood, and so less water will reach
downstream, reducing the risk of flooding. Two of the key advantages are
increasing biodiversity by providing a natural habitat for wildlife, and
being sustainable, with less need for maintenance. However, the flood risk
may be increased if a river is left to its natural course. Case study: River
Cherwell, Oxfordshire.
Case Study of Soft Engineering: River Quaggy, Greenwich
In 1968, there was extensive flooding in
Lewisham, and more recently, 50 properties
were flooded by more than a metre of
water. The estimated recurrence interval
for this type of flooding was 5 – 10 years.
The Quaggy River Flood Management
Scheme rejected plans to channelize the
River (this would replace the winding river
with straight artificial cuts) and instead came up with a solution which
stored water upstream and improved the channel downstream.
The river has been brought back to the surface, and it now runs the
route through Sutcliffe Park that it originally did in the 19th century.
During periods of high rainfall, the park is allowed to flood, and further
flood storage has been created in local sports fields.
The Quaggy river restoration project has protected 600 homes and
businesses, and 2,500 people. Sutcliffe Park now has lakes and wildflower
meadows, which has created an environment for wildlife, promoting
biodiversity. The Quaggy scheme has won many awards; 2007 RSPB Living
Wetlands Award and a Green Flag Award in 2008. Surveys have shown
that visits to Sutcliffe Park are up by 73%, so the scheme has brought
more visitors to the area, as well as protecting locals from flooding.
Case study of flooding in an MEDC – Carlisle
o Flooding on 8-9th January 2005
o Human causes: extensive building on the
flood plains of the River Eden –
impermeable surfaces increase run off
o Natural Causes: 175mm rain in 36 hours
o Short term impacts: 3 people killed, 120
people injured, communications damaged,
roads including the M6 motorway impassable; 1865 residential
properties flooded, 300 business premises flooded; schools, hospitals
and police stations closed.
o Long term impacts: houses had to be rebuilt on the ground floor –
lengthy drying out period and extensive redecoration required; homes
became uninsurable and market value dropped; businesses lost trade;
public perception shattered – Carlisle seemed like an unreliable place
to work and live; £250 million total repair bill, with £8 million alone to
repair the damage caused to electricity and water supplies.
o Flood defences put into place: the Lower Eden Strategic and Planning
Appraisal Report (SPAR) was prepared; The existing defences at
Botcherby Bridge on the River Petteril were set back to create a
larger storage area on the flood plain. In July 2005, council agreed to
£1.5 million government grant for patrols of affected areas, spring
cleaning of affected areas (including street sweeping, gully repairs and
weed removal) and funding to repair flood damaged homes.
o Benefits of defences: double the existing standard of protection to a
1 in 200 year standard; environmentally acceptable with opportunity
for environmental enhancement.
Case Study o flooding in an LEDC – Bangladesh
o Flooding during the Monsoon Season of
1998; flood lasted 65 days, with 75% of
the country affected and 50% of the
capital city, Dhaka, covered. 1 million
km square of land was submerged.
o Human causes: deforestation in the head
water areas due to the increasing
populations of Nepal and Tibet, as well as the need for trees for fuel
and to clear land for grazing. This has lead to less evapotranspiration,
more run off, and a more rapid increase in River discharge.
o Natural Causes: Flash flooding is caused by heavy rainfall in
surrounding upland areas running off into the channel; River floods are
caused by melt water form the Himalayas. At the confluence of the
Ganges and the Brahmaputra, high discharge breaches embankments
and flooding ensues. Rainwater floods are caused by the monsoon
climate – run off accumulates in depressions and the water table rises
above land. Storm surges are caused by strong winds and low
pressures over Bangladesh. Seas can rise by 2m, and with the funeling
effect of the Bay of Bengal, a 7m rise can be seen. Tropical cyclones
also increase rainfall.
o Short term impacts: 31 million people affected; 980,000 homes
damaged; 1050 lives lost; 36,500 livestock lost; 2.2 million tonnes of
rice lost; 14,000 schools flooded; 4528km of flood embankment
damaged; starving and diseases such as cholera and typhoid spread
through contaminated water supplies.
o Long term impacts: Rice production lost for many months because silt
deposits bury land. 90% of people in Bangladesh are landless peasants
who love by subsistence rice farming on the delta – they will have no
income. Emotional impacts – the trauma of losing family and friends.
o Flood Defences: Flood action plan devised, which comprises 26
projects, including the Jamalpur Priority Project and the Meghna Left
Bank Protection Project. The key part is constructing new
embankments along side the Brahmaputra and Ganges in Bangladesh.
Behind the embankments, compartments of land are created by
building internal walls to link up riverside embankments. A flood
forecasting system was put into place. Boats were provided so that in
future, people could escape to emergency shelters on higher land.
Young mangrove trees were planted on the coast to break the force of
the tidal waves caused by cyclones.
o Benefits of the defences: reduce the loss of life; reduce economic
impact by minimising the amount of crops lost; local inhabitants are
warned before the floods so have time to prepare and minimise loss.
o Costs of defences: can be expensive for an LEDC like Bangladesh;
issues with embankment positioning – they were built further away
from the river as research shows this is most effective, but an
additional 5 million people now live in the flood zone;
compartmentalisation has impacts for human health – spread of water
borne diseases; studies from the Mississippi and the Maas suggest
that trying to control a narrow river with higher embankments tends
to raise the channel floor through deposition which in turn requires
higher embankments.