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Last edited 27 Apr 2018
Planning for floods
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Floods occur naturally throughout the world as events ranging from annual occurrences to random natural disasters. More often than not the term disaster is denoted in response to flooding of the built environment where it poses a risk to life, homes and livelihood. With the correct understanding, and appropriate planning the adverse effects of flooding of the built environment can be reduced.
The characteristics of flooding that need to be understood include:
- The cause of floods.
- When and where it is likely to flood.
- The impact of climate change.
 The causes of floods
As population has increased, particularly in urban areas, and ‘soft’, permeable landscape has been replaced with hard surfaces, surface water runoff has reached traditional piped drainage faster and in larger quantities. This has been exacerbated by the loss of rural features that might once have slowed the passage of surface water, being replaced with large, intensively farmed fields that rapidly discharge surface water into piped drainage or waterways that make their way into urban areas.
 Flood prediction
Real-time environmental monitoring is invaluable in assessing meteorological conditions. By identifying severe weather patterns, flood warnings can be given days in advance. Continuing improvements in resolution and timeliness of forecasts will further improve warnings.
In addition, the Environment Agency (EA) undertakes surveys to evaluate the flood risk of particular areas. With greater resolution, this flood risk becomes better defined, in particular in relation to surface water flooding in urban areas. However, urban flooding is complex issue, and even catchments that are very close to one another can be affected in dramatically different ways.
Natural variations in weather patterns are being exacerbated by human activity, particularly the release of green house gases which are trapping solar energy, with an unpredictable effect on climate. Understanding climate change can help inform planning decisions.
UK Climate Projections (UKCP) is a methodology developed by the Met Office based on scientists understanding of the climate system and how it will change in future. UKCP09 summarises the most recent projections for the United Kingdom and is of particular relevance to planning flood defences as it enables investment in flood management schemes to be planned based on the best available information .
However, predicting the outcome of climate change is an uncertain and iterative process, no one model can provide a definitive answer. Furthermore, climate change is more complex than just global warming, and trends in precipitation for example are extremely difficult to predict. It is clear that the storm profiles and frequencies are increasing, but the methods of estimating extremes are not so well established.
Resolution is improved by refining models with more accurate data and greater computational power.
 Managing floods
The continued drive for global socioeconomic growth exposes more individuals to the consequences of flooding.
The opportunity to lessen the impact of flooding is greatest during the planning stages for new developments or infrastructure. Infrastructure asset managers need to ensure that adequate protection is given whilst at the same time minimising whole-life costs and maximising environmental gain. This requires a comprehensive overview of entire catchment areas.
This is best demonstrated through consideration of a case study, examining how precipitation affects a catchment and what planned measures can be used to manage flooding:
Ottery St. Mary is a small market town in East Devon with historic problems of flooding due to the steep local topography. Homes and businesses in the town are susceptible to persistent flooding.
In October 2008 a hailstorm lasting an exceptional 2 hours fell upon the catchment and there was 160mm of precipitation within a 3 hour period. Flooding reached 1.5m in some areas and there was an estimated £1 million of damage.
 Outline of problem
An overview of the situation can be seen in figure 3.1
Furzebrook is the cause of much of the flooding, flowing in a small open channel through a steep catchment of grade 2 agricultural land. Surface runoff ensues and the channel overflows onto roads that then act as watercourses. The flow is culverted for 545m under the town centre before it discharges into the Mill Leat. Due to cross sectional changes in the culvert it has insufficient capacity to cope with even moderate return period flows.
|1)||The area of flood risk is essentially the valley floor of the Furze Brook tributary. 75 residential and small commercial properties are at risk and 180, including the fire station and health centre.||High|
|2)||A historic town in East Devon encompassing Conservation Areas and listed structures. The Mill Leat was constructed 800 years ago and is designated as a main river.||Medium|
|3)||The town centre provides typical and well used amenities with several car parks and recreation grounds.||Medium|
|4)||Outlying region primarily based on agriculture and not considered to be an area directly at risk with relatively dispersed communities therefore would not be economical to be considered further.||None|
|5)||Agricultural land is a habitat for several protected species, while the catchment itself includes archaeological remains||Medium|
|6)||The socio-political perspective where economical benefits of a flood defence will far outweigh the lifecycle cost of infrastructure.||Very high|
|7)||General guidance on climate change is available but the degree of application is uncertain. It is reasonable to suggest that peak flows will rise over a given threshold and become more frequent this is a subsidiary concern that can be managed by other means.||Low|
The proposal considers all aspects of the catchment, individually and as a whole. By isolating elements it may be possible to avoid over-engineering and provide flood defence for a lower cost. Some fo the key alternatives are presented below.
|A||Do nothing||Will not meet the primary objective of flood defence.|
|B||Maintain existing||A viable option if no justifiable alternative exists Further consideration|
|J||Channel and bank improvements at Butts Bridge||Minimal damages do not warrant substantial investment required. Rejected|
|O||Spillway on Mill Leat at Land of Canaan||Forms integral part of option Q below. Unlikely to provide sufficient benefits in its own right. Rejected|
|Q||Undertake major works of on-line conveyance improvement||Could provide sound economic proposition. Further consideration|
|R||Undertake major works of off-line conveyance improvement||Could provide sound economic proposition. Further consideration|
- Options A, B, Q and R are carried forward for detailed appraisal which will be considered on environmental economic and technical grounds. Total life cost has been calculated accounting for capital investment, maintenance, risk, site investigation and internal fees.
- Option A is the benchmark for all options, involving no investment, maintenance will cease immediately. A nominal 5 year period will dictate the decline of the culvert and properties.
- B is reactive option dealing with flood events and low-level maintenance. This routine maintenance is over a period of 50 years, subject to inflation.
- Options Q and R both improve conveyance and have a 100 year lifecycle.
A comprehensive damage model has been applied to every property under risk. The option of flood defence is said to act under three conditions; ideal, conservative and intermediate. Each with a different level of success. They can then be compared to the benchmark model, the damage sustained from no intervention. This gives the damage avoided due to the protection of the scheme i.e. the benefit.
Option Q satisfies these conditions and so is the proposed scheme.
There are a number of criticisms:
- The expansion of Ottery will increase pressure on the existing system. As a result more impervious surfaces will increase flood risk. Intense agriculture practices will add to the pressure.
- Flood defences tend to treat the symptoms and not the cause. Planning for an existing development focuses largely on economic factors and cost benefits.
- Although considered in the planning stage, climate change was not fully incorporated in design. Informed statements have been made about the possible impacts but these are inconclusive and measures should be taken to provide solid information on expected affects.
- Delivering resilient infrastructure is about more than just physical flood defences and adaptive management. Sub soiling has been mentioned as a long term approach to optimise the initial scheme for climate change. Effective flood resistant developments and other options should be evaluated in conjunction to solid defences.
 Recent forms of flood management
Globally, the trend has been to shift flood prevention to a more managerial role, reducing reliance on physical flood defence schemes; implementing strategies to mitigate flood prone areas and increase their resilience to flooding, rather than continually adapting, strengthening and extending conventional defences.
This is done by considering:
- Management flood plans.
- Legislation regarding new developments.
- Flood defence spending.
 Catchment Flood Management Plans (CMFPs)
|1)||If no flood risk is perceived no intervention will be taken.|
|2)||If risk is low no action can be justified.|
|3)||It is accepted that flood risk will increase over time due to climate change, however this does not necessarily mean increasing activities to deal with the rise in risk.|
|4)||Where future changes will have a significant impact further action will be taken to sustain the current scale of flood risk.|
|5)||Taking the necessary action to reduce flood risk where consequences are high.|
|6)||Increasing the frequency/magnitude of flooding in higher catchment areas, storing water upstream and lessening the effects downstream, as long as there is no increased risk to people property or habitats.|
CFMP’s are reviewed every 6 years and use river models to estimate increased flows by 2100.
Ottery has been determined as a policy number 4) catchment requiring further action to keep pace with climate change. In addition, land use and management changes within the catchment need to be addressed.
|Proposed Actions for the Ottery Catchment, taken from (Cresswell, 2012)|
· Promote self-help opportunities to provide protection from flooding in Ottery.
· Ensure all new developments conform with ‘Planning Policy Statement 25: Development and Flood Risk’. In particular Sustainable Drainage Systems should be incorporated into development to restrict surface water run-off.
· Carry out a study in the catchments of the Furze Brook, working with the agricultural industry, to try and change land management practices to reduce the direct run-off from agricultural land.
Areas at risk from flooding of any sources are classed as inappropriate development, and the Environment Agency (EA) will no longer fund defence schemes for developments that are placed in areas of high risk.
Steer development towards low risk flood zones.
If the Sequential test is not possible to allocate low risk flood zones apply the Exception Test.
Sites should be prioritised where there is; the lowest probability of flooding, availability and appropriateness for development.
On the basis of the Strategic Flood Risk Assessment, the site will not be accepted if a better alternative exists.
Demonstrating the safeness of the development for its lifecycle and the vulnerability of its users. It will not create flood risk elsewhere and will reduce overall risk. Based on a site specific flood risk assessment.
Following both tests, the most vulnerable development is in an area of lowest risk
Development is resistant and flood resilient
And gives priority to the use of sustainable drainage systems.
The opportunity offered by new developments to reduce the impact of flooding should not be neglected. It will be assumed that the developer must defend new developments and be accountable. This has further implications where new developments will no longer be allowed to connect to surface water drainage systems and front gardens must comply with sustainable urban drainage measures. These actions will help lessen flood risk and provide clear accountability for those wishing to be compensated due to flooding.
1990 to 2025
2025 to 2055
2055 to 2085
2085 to 2115
Peak rainfall intensity
(preferably for smaller catchments)
5% 10% 20% 30%
Peak river flow
(preferably for larger catchments)
 Future investment
Flood management assets will need to cope with an expected average 20 per cent increase in river flows by 2050.
Most of the available funding at the disposal of the EA is spent on improving existing flood defences and maintenance. This funding comes from central government and is allocated in 3 year cycles. During the period to 2015 the EA’s funding will be cut by 20%.
By 2035 investment will need to have almost doubled compared to the year 2010 -2011 (excluding inflation). It is likely that alternative sources of funding will be needed. Local communities that benefit from flood defences may be expected to contribute towards them.
By working with industry partners an integrated approach can help mitigate flooding. Flood effected homes can be restored to a higher standard of resilience by insurance companies in order to reduce premiums. Currently the availability of insurance is guaranteed due to an existing agreement between government and the insurance industry, however this ends in 2013. Developments built against Sequential Testing may then not be guaranteed insurance by the Association of British Insurers.
The issues surrounding flooding will have a significant impact on the way future infrastructure is designed. Defence against flooding can be increased, but floods will still happen, and not all properties can be defended. In the future too much may be expected of CMFPs and reliance may shift back and forth between active management and physical defences
Local community funding for flood protection may be problematic, as affluent areas will tend to protect themselves whereas less wealthy areas may simply decline.
The benefits produced from the investment into CMFPs are not yet conclusive. If proven inadequate the resource may be better invested in physical defence schemes, but these schemes must stem from an inherently risk-based, resilient design.
 Related articles on Designing Buildings Wiki
- BREEAM Flood risk management.
- Catchment flood management plans.
- Coastal defences.
- Flood and Water Management Act.
- Flood defences.
- Flood risk.
- Flood risk management plan.
- Future flood prevention.
- Highway drainage.
- Pitt review.
- Property flood resilience.
- Rainwater harvesting.
- River engineering.
- Sustainable urban drainage systems.
- Water engineering.
 External references
- DEFRA (UKCP09) Climate Projections.
- Met Office Ottery st. Mary Hailstorm.
- Investing for the future; Flood and coastal risk management in England - A long term investment strategy.
- The Pitt Review.
- The Pitt Review - Lessons learned from the 2007 summer floods.
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