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WELL FACTSHEET Go BackLiving with Floods Author: Brian Reed, April 2006 Quality Assurance: Sandy Cairncross For
diverse reasons, people will live in areas
that flood repeatedly. Heavy engineered
flood defences are expensive and may not be
the most sustainable solution, especially
for the poor. However there are steps that
can be taken to improve the lives of people
who face the problems of repeated inundation
and to safeguard water and wastewater
treatment plants that are often sited next
to rivers. Runoff occurs naturally and varies in quantity according to the frequency and intensity of rainfall, the impermeability of the land and the condition of the land when it begins to rain (e.g. saturated). Flooding can be divided into two categories;
External flooding is confined to the flood plain of the watercourse. The extent of the flood plain will vary from year to year with the quantity of water coming downstream. Often a regular seasonal pattern will be apparent. Long-term changes to the catchment can alter the size and the frequency of flooding. This type of flooding can be managed by:
Local flooding is not confined to the flood plain. It will vary considerably in terms of location, duration, frequency and intensity. It is made worse by the paving or compaction of the ground, reducing the capacity of the soil to absorb moisture and by actions upstream, such as inappropriate drainage provision. The division between external flooding and local flooding is not precise, as one person's local flooding will become somebody else's external flooding further downstream. External flooding will be on a greater scale than local flooding. Coping with both types of flooding and their impacts on people are the subject of this note.
Figure 1: Flooding Many
poor people in low-income countries live in areas
prone to flooding. They do this for very calculated
reasons. In rural areas, the alluvial land is most
productive; in urban areas, the rent will be cheap,
access to services and employment opportunities may
be easy, tenure may be secure – if there was an
easy way to stop the flooding it would be carried
out, the rent would rise and the poor be moved on.
Flooding may look dreadful, but predictable shallow
inundation may be preferable to the other options
available. Much
of the research on flooding has looked at flood
prevention and containment in the past, but now
flood warning and coping strategies are understood
to be much more practical responses. Before looking
at technical options to address the sanitary
problems caused by floods, is there a more
sustainable option that could be considered? Planning
where people live The adverse impacts of runoff can be mitigated by building housing away from areas that either (Figure 2) flood regularly Πor are on steep unstable slopes . This requires good planning regulations and enforcement. Poor and vulnerable people may not have any option but to live in areas that flood, as the benefits (such as cost of housing, location near to employment opportunities or lack of alternatives) outweigh the costs associated with flooding. Providing public transport or better employment prospects elsewhere may make living on a flood plain less attractive (and be cheaper than expensive engineered flood defences). Flood proofing infrastructure may not be economically viable, especially in informal settlements with poor land tenure.
Figure 2: Where not to build; areas that flood (or are affected by landslides) Flood
shelters People
may not be able to permanently avoid living in areas
that flood, but may be able to leave temporarily
when there is imminent danger. Where floods cannot
be avoided at a household level, communal flood
shelters can be provided and secure escape routes to
the shelter established. Emergency supplies and
water and sanitation facilities should be provided.
The fast flowing water can be dangerous to people,
so safe travel routes should allow individuals to
evacuate the area safely. Flood proofing rural roads
was identified as important in both Bangladesh (as a
refuge from the water) and across Sudan (as a means
of bringing in relief). This concerns both the level
of the road, the materials used to construct it and
the route - avoiding areas where it could be flushed
away. Drainage should be provided under the road
(culverts), to allow a route for water to drain
across the line of the road. Drainage and good
surfacing is also needed on the road, to get water
off the road as quickly as possible, preventing
lorries rutting the surface. Flood
warnings Communication
needs to be in place to warn of flooding and to
co-ordinate rescue operations. Giving people warning
of an impending flood can allow them to build bunds
from sand bags, move belongings and people to higher
ground and be prepared psychologically for the
disruption and danger. Flood forecasting can
increase the warning time. Warnings will have to fit
the local situation, such as radio broadcasts or
house-to-house visits. Flood
proofing houses Buildings and roads will suffer from two impacts of flooding;
Frequent
floods can be avoided if the ground level can be
raised (Figure 3) using earth platforms Πor stilts ,
but extensive earth filling will encroach on the
natural flood storage area of the flood plain and
may push the flood further downstream, increasing
its severity, or direct the flow elsewhere, with
unintended consequences. Pit latrines can be raised
to prevent them flooding and filling with silt, and
manholes sealed to prevent sewers overflowing. Bunds
(banks, dykes or levees) can be built to divert
flows away from housing Ž
and other vulnerable areas (such as wells or
latrines); but, as with earth filling, these may
worsen flooding elsewhere. Roads, paths and strong
walls can all be used to direct water away from
housing. Where there is a break in a wall, a solid
gate, stop logs or flood boards can be used to seal
the gap. Temporary bunds can be provided using sand
bags. Runoff often follows the route of roads, so
speed humps can help divert surface water,
especially where a side road is downhill from a main
road.
Figure 3: Avoiding runoff problems If houses are going to flood on a regular basis, stop logs (wooden planks) can be fixed in front of doors to prevent water from entering houses . A simple groove in the doorframe allows the stop logs to be fitted quickly. Because floods can also be physically damaging, eroding roads and undermining buildings, houses can be further flood proofed by improving their construction; providing a good foundation and a strong lower wall can reduce the likelihood of a low-cost house being destroyed. They should be sited out of the main flood flow routes to prevent damage by the force of the flowing water. Two or more storeys allow householders to more upstairs if there is a flood , as well as providing somewhere to store valued possessions. Even a high shelf can protect valued possessions.
Figure 4: A solid gate or board can prevent floodwater entering a walled area Flood
defences The
use of bunds and barriers can protect individual
houses, but larger scale dykes, levees and flood
barriers have been used in the past to protect whole
communities. Building large engineered flood
defences may give the impression of controlling a
flood, but although the flood frequency may reduce,
the impact of the flood, when it finally does
overtop the defences, is increased, due to the
increase in development in an area that is perceived
to be safe and the increase in the depth of
floodwater. Small embankments may allow more
frequent minor floods, but this limits floodplain
development (Figure 5).
Figure 5: Embankments can encourage inappropriate development The use of flood defences may protect land that normally floods, but this will stop floodwater being stored on the flood plain and attenuating the peak flows. This means that the water levels are increased downstream, increasing the flood risk to other areas. If water does overtop the embankment or there is heavy rain inside the protected areas, storm water will have problems draining away. Pumps cannot be relied upon to displace the water and natural drainage may take a long time as the routes are blocked by embankments. Flood routing General drainage paths should be considered. Directing flows away from vulnerable areas may not prevent them from being submerged but it may prevent physical damage and slow down flows so the silt load is reduced. Buildings and structurs can be sited away from the main force of the flow. Water should be allowed to drain freely from the area (but not so fast that the drained water causes flooding downstream). Concentrating the flow can increase speeds and force; spreading the flow with banks or filtering it through hedges will reduce the problems of erosion and also aid the early deposition of silt (Figure 6).
Figure 6: (a) Concentrating flows increases the depth and speed; (b) spreading it (eg with transverse bunds or hedges) reduces its force and helps remove solids Roads can be designed to act as a drain; routing the flow away from buildings to a safe discharge area. Having a camber on the road allows emergency access when most of the road is flooded Π. Roads can act as flow channels.
Figure 7: Roads can act as flow channels Preventing
pollution One of the main problems with areas that flood is not just the water, but also the pollution that it carries. The water will be contaminated by overflowing latrines and sewers and large amounts of silt. Contamination causing illness can be prevented in various ways (Figure 6);
Figure 8: Pollution Pathways Action
may be taken in all four areas, but some are easier
to address than others. For example, water supply
has a higher political priority than sanitation, and
drainage is only an issue when it floods – it has
a low political status but can attract large amounts
of funding as an emergency response – which is not
always spent in a sustainable manner. Education is
probably the most immediate response that can be
undertaken. When
flooding occurred in Sudan, previous experience
warned about increased levels of malaria and
cholera, so that preventive health and education
interventions were implemented. In the Mozambique
floods of 2000, such measures ensured that child
mortality in the temporary camps for displaced
people was less than it had previously been in the
local population. Where household goods have been
lost, the provision of water containers can begin to
improve hygiene practices.
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