Planning for floods

Revision as of 16:56, 14 December 2012

Floods

Irrespective of human development floods occur naturally all across the world due to the hydrological cycle, ranging from annual occurrences to random natural disasters. More often than not the term disaster is denoted by a response to the built environment upon areas susceptible to flooding. As such flooding will be considered as a human problem – posing a risk to life, homes and livelihood. By studying such events it is thought that in conjunction with the correct planning the adverse effects and vulnerability to human society can be reduced.

The characteristics of flooding must first be analysed by understanding;

• cause of floods
• when and where it will flood
• effects of climate change

Quantifying these factors will determine the investment of resource, expenditure and management needed to mitigate the effects of flooding

The physical causes of floods

It is generally assumed that socioeconomic activities such as urbanisation and deforestation are accountable for the worsening of floods. Understanding natural processes will help to ensure options are appraised effectively and working with nature. Factors that initiate flooding worldwide can differ greatly b

A sudden input of water on topography with varying hydraulic properties dramatically influences infiltration rates. Typically urban stratums are impervious inhibiting infiltration and increasing the problem. Drainage networks are susceptible to blockages and discharge limitations. Sewer flooding is particularly hazardous and can compound the problem.

Inundation prediction

Real-time environmental monitoring is invaluable to gauge meteorological conditions worldwide. By identifying severe weather patterns an indication of flooding can be given days in advance. The continuing improvements in resolution and timeliness of Met Office forecasts will provide better warning of the situation.

The Environment Agency (EA) already conducts multiple surveys to evaluate flood risk across the nation. With greater resolution flood risk becomes more defined, especially regarding surface water in urban catchments.

The principles of climate change

Understanding climate change will help informed planning decisions to be made accommodating future needs.

The Natural variations in weather patterns are being exacerbated by industrial activity, particularly the release of green house gases which are trapping solar energy with an unpredictable effect on precipitation.

UK Climate Projections (UKCP) is the methodology developed by the Met Office to reflect 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.

This information has importance in applications worldwide as climate change is inevitable for the foreseeable future – Civil Engineers can plan to mitigate the consequences. Investment in flood management schemes based on logical decisions will have a significant impact that will protect life, industry and the natural environment.

Review

Urban flooding is complex issue. Catchments in close proximity can have dramatic variation so must be considered independently. A case study is considered to identify the critical factors.

In some circumstances time is the key to mitigation procedures. Communications are known to break down in emergencies and the time between acknowledging a serious event and publicising warnings cannot always be achieved.

Predicting the outcome of climate change is an uncertain and iterative process, no one model can provide a definitive answer. Resolution is improved by refining models with more accurate data and greater computational power.

Climate change is more complex than global warming. Trends in precipitation are harder to identify with no statistically significant results. This study will focus on fluvial flooding.

It is clear that the storm profiles and frequencies are increasing, but the methods of estimating extremes are not so well established.

The procedure of managing floods will be best demonstrated by a specific case study, demonstrating how these aspects are considered.

Managing floods

The need for socioeconomic growth continues to increase, exposing more individuals to the consequences of flooding on a global scale. The opportunity to lessen the impact of flooding is greatest during the planning stages, improving the communities overall resilience. Infrastructure asset managers undertake these decisions, ensuring adequate protection is given while minimising whole-life costs and maximising environmental gain. This requires a comprehensive overview of the catchment.

Ottery St. Mary will be a case study, examining how precipitation affects the catchment and what planned measures have been derived to manage flooding. The problem will be outlined under these considerations;

• factors affecting planning
• incorporating flood data
  
• accommodating climate change
• alternatives

These points are essential to determine an effective solution of a physical flood defence from the planning stage.

Ottery St. Mary

Is a small market town in East Devon that has historic problems of flooding due to the steep local topography. Homes and businesses in the town are susceptible to persistent flooding and have been inundated by the extremes associated with climate change.

In October 2008 a hailstorm lasting an exceptional 2 hours fell upon the catchment. The Met Office, outlines the build up to the event where 160mm of precipitation fell within a 3 hour period leading to a return period of greater than 200 years – making flooding inevitable. An estimated £1 million of damage with flood depths of 1.5m in places.

Outline of Problem

An overview of the situation can be seen in figure 3.1

Furzebrook casuses the most flooding, flowing through a steep catchment of grade 2 agricultural lands in a small open channel. Surface runoff ensues and the channel overflows onto roads that 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, at most 1 in 5 years. Farming practices are also a factor, however land management alone based on the hydrology report won’t improve conditions.

Factors affecting planning

Many factors are considered to a degree during planning. A summary of the appraisals findings can be seen in table 3.1.

Table 3.1- Prioritising factors that affect planning.

Although Q is the more economical it still needs to provide the most benefit for the cost. 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.

Proposed scheme

The total life cost of the scheme is then divided by the aforementioned benefit giving a ratio, if >1 the scheme is an asset to the community. Option Q satisfies these conditions most efficiently and is the proposed scheme. Issued by the EA resulting from the Governments objective to provide adequate, economical and sustainable flood defences under section 147 of the Water Resources Act, 1991. To alleviate flooding from the 3 interconnected watercourses discharging into the River Otter.

Review

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 to the possible impacts but these are inconclusive and measures should be taken to provide solid information on expected affects.

Delivering resilient infrastructure encompasses more than just physical flood defences and adaptive management in the future. 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

A worldwide trend has seen the methods to combat flooding shift to a more managerial role, reducing the reliance on solid flood defence schemes. Implementing strategies to mitigate flood prone areas and increase their resilience to flooding, rather than continually adapt, strengthen and extend conventional defences. This continual development is unsustainable, environmentally unfriendly and increasingly unaffordable

By looking at;

• Management flood plans
• Legislation regarding new developments
• Flood defence spending

It will determine the most effective method of flood defence for a specific catchment.

Catchment Flood Management Plans (CMFPs)

Greater efforts in planning have been put into CMFPs to best achieve social, economic and environmental objectives across all of England and Wales breaking flood risk down into 6 policies. Produced by the EA these plans stem from flood risk and are featured in table 4.1

Table 4.1– Summary of CMFP Policies from (Environment Agency, 2007)

CFMP’s are reviewed every 6 years and use river models to estimate increased flows by 2100.

Incorporating urban development and land management modelling the presence of current flood defences has been ignored. The reduction in the time it takes flows to peak therefore increases and (Cresswell, 2012) states “flows increasing by up to 5% in the… Otter 'catchment” by 2100. Since the appraisal the number of properties at risk is expected to rise to the probability of 85 properties.

Ottery has been determined as a policy number 4) catchment requiring further action to keep pace with climate change. In addition the concern of land use and management changes within the catchment needs addressing.

Therefore the policy identifies the following measures in table 4.2

Table 4.2– Proposed Actions for the Ottery Catchment, taken from (Cresswell, 2012)

National planning policy framework

(Department for Communities and Local Governmnet, 2012) states “Local Plans should take account of climate change over the longer term, including factors such as flood risk”

Areas at risk from flooding of all sources are classed as inappropriate development. The EA will no longer fund defence schemes for developments that are placed in areas of high risk.

Local authorities should safeguard land and apply a Sequential, risk based approach to determine areas more susceptible. An Exception Test can alternatively be applied. Both are given in 4.3

Table 4.3 – Outline of Tests

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 surface water drainage to systems and front gardens must comply with sustainable urban drainage measures. These actions will help lessen the problem and provide clear accountability for those wishing to be compensated due to flooding.

A flood risk assessment can now be conducted in the form of sensitivity ranges these ranges are expressed in table 4.4.

Table 4.3 –Indicative sensitivity ranges (Department for Communities and Local Governmnet, 2012)

(Department for Environment, Food and Rural Affairs, 2006) contains a derivation of this information and supplementary notes on application in essence the precipitation is multiplied by a percentage.

Future investment

Flood management assets will need to cope with an expected average 20 per cent increase in river flows by 2050 where the degree of increase will vary between regions.

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 periods. From this year until 2015 the EA’s funding will be cut by 20%. While the agency is making substantial efficiency savings and strives for a benefit-to-cost ratio greater than two, a number of catchments are less. Ottery under certain flooding conditions falls below this ratio. However this significant investment ensures people are not exposed to substantial risk.

By 2035 investment will need to have almost doubled compared to the year 2010 -2011 (excluding inflation). Cost benefit and efficiency savings are not enough –alternative sources of funding will be needed. Local communities that benefit from flood defences are now 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 and consequentially reduce premiums. This could also apply to subscribers of flood warning services. Currently the availability of insurance is guaranteed due to an existing agreement between government and the insurance industry, however this ends in 2013. From the NPPF, developments built against Sequential Testing will now not guarantee insurance by the Association of British Insurers, leaving protection to the discretion of the developers.

Conclusion

Flood management will continue to be an on-going process. Openly acknowledging the mistakes and inconsistencies associated with flooding, allows issues to be dealt with as they develop.

It can be seen that there is no single solution to the problem of flooding. A full integration of partners, management processes and resource will optimise benefits.

The NPPFs have only just come into action and should have been adopted sooner. The legislation is clear and fair and could show exemplary results in the near future. The sensitivity ranges are imprecise; this will depend heavily on the catchment. By identifying the derivation of the measures, the range could be far more useful to planning.

The cost-benefit ratio provided by the EA can be deceiving. The ‘Do Nothing’ option is unrealistic as some maintenance would have to continue.

Local community funding for flood protection is problematic Affluent areas will become well protected where less wealthy areas will suffer decline in infrastructure, economy and health.

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 improving conveyance.

The benefits produced from the investment into CMFPs are not yet conclusive. If proven inadequate the tight financial constraints may be far better invested into physical defence schemes, but these schemes must stem from an inherently risk based design that better copes with uncertainty.

For a better understanding on the subject further reading can be conducted in the fields of;

• sustainable urban drainage.
• a risk based approach to design
• forecasting techniques.
  
• how the budget for flooding is allocated.

Developing the issues discussed in this report will have a significant impact on the way future infrastructure is designed. Incorporating these into the broader planning framework will help maintain efficiencies within the EA’s budget. This together with EU legislation provides good reason to expect this approach to be more universally adopted.

See also

• (UKCP09) http://ukclimateprojections.defra.gov.uk/22770
  
• Met Office Ottery st. Mary Hailstorm http://www.metoffice.gov.uk/education/teens/case-studies/otteryhail
• Investing for the future; Flood and coastal risk management in England - A long term investment strategy. http://a0768b4a8a31e106d8b0-50dc802554eb38a24458b98ff72d550b.r19.cf3.rackcdn.com/geho0609bqdf-e-e.pdf
• The Pitt Review http://www.webarchive.nationalarchives.gov.uk/20100807034701/http:/archive.cabinetoffice.gov.uk/pittreview/thepittreview/final_report.html