The following tables summarise the problem or risk and show the policy we have selected and the reasons why. We have also described the actions; opportunities and constraints; and risks and uncertainties for each policy unit.
Summary of the Preferred Policy Policy Unit 1 covers the Rivers Cray and Shuttle from their sources (Orpington and Policy Unit 1: Eltham respectively) down to Crayford (where the Cray runs under the A226 road Shuttle and bridge) as shown on Figure 6.1. It includes the major conurbations of Bexley, Upper Cray Crayford, St Pauls and St Mary Cray, Blackfen, and Orpington. The main source of flooding is from a combination of overwhelmed surface drains, sewage flooding and high flows in the Shuttle and Cray themselves, possibly including breach of defences. There is also potential for flooding to occur from failure or breach of the storage reservoirs. Significant flooding has occurred in 1897, 1958, 1977, and 2005 to some of the major conurbations listed above. The scale of flood risk in the urbanised areas adjacent to the Cray and Shuttle is such that the estimated average annual damages (AADs) in the Upper Cray (St Pauls Cray and St Mary Cray), Shuttle (Blackfen and Old Bexley), Orpington, and Crayford are £3.5m which amounts to 29% of the total damages within the North Kent Rivers catchment. There is substantial damage in Crayford to commercial and industrial properties. It is thought that damages arise from the more extreme events that occur with above 3.3% annual probability every year. The flood with a 1% annual probability in a year would affect approx 1,781 properties, 2,709 people and some major trunk roads. One SSSI and 3 SNCIs are exposed to flooding. Problem / Risk The current flood depth range for the Shuttle is 0 to 2.7m with a medium velocity. The Upper Cray has a depth range of 0 to 2.0m and a medium velocity.
Flood defences in the unit include improved channels, embankments, Flood Storage Reservoirs at Lomorbey, Damson Park and Hall Place and regulation works at Ruxley Gravel Pits. Main defences within the policy unit provide the following standards of protection (S.O.P) are Crayford (Cray): 30yr, St Pauls and St Mary Cray (Cray): 30yr, Old Bexley (Shuttle): 30yr, Blackfen (Shuttle): 30yr and Orpington: 30yr. The current expenditure on maintenance by the Environment Agency is £0.2m/yr. Flood warning in the unit is provided but it is not always possible to meet the standard two hour warning in the steep, highly urbanised rivers Shuttle and Cray. The expected average annual damages could increase by 20% by 2100 as a result of the impacts of climate change. In total approximately 2,500 properties and 4,600 people may be at risk by 2100. Policy 5: Take further action to reduce flood risk (now and/or in the future) In this unit the flood risk is high with a 3.3% annual probability of flooding and substantial damages £3.5m AAD increasing to £6.2m with climate change by 2100. The flood depths range up to 2.7m with a medium velocity. There are no designated sites affected by flooding and the impacts on the environment of flooding are not significant. The economic impacts of flooding and the social impacts of risks to life from frequent flooding at fairly large depths and velocities justify taking further action.
By taking further action we would wish to see the urban watercourses opened up Justification providing more space for water and the environment. A joined up approach would be taken to tackling all sources of flooding.
This policy supports sustainability (economic, social and environmental) through reducing impacts of flooding on evacuation routes, reducing the impacts of flooding on human health, and reducing impacts of flooding on Ruxley Gravels Pits SSSI and three SNCIs. The gains and losses on the objectives for agriculture, flood warning, emergency dispersal points, heritage, and water resources are considered neutral.
Summary of the Preferred Policy Policy 1 – No active intervention. Ceasing all active intervention would allow a massive increase in risk 100 times present levels as the urban watercourses s become blocked, structures are not operated and flood storage systems fail. The resulting flood risk is unacceptable.
Policy 2 – reduce current level of flood risk management. This would lead to a doubling of flood risk which is not acceptable. Alternative policies Policy 3 – maintain current level of flood risk management. The current levels of considered flood risk are not acceptable and would rise by 20% with climate change.
Policy 4 – maintain the current level of flood risk into the future. Considering the balance of present actions (£0.2m maintenance) a year and the level of risk (£3.5m AADs) taking further action is warranted.
Policy 6 – increase flooding to reduce flood risk elsewhere. Due to its urban nature there are limited opportunities to transfer risk elsewhere. Flood risk management measures are likely to provide opportunities for improving the water quality through naturalisation of the river and incorporation of recreational features.
There is an opportunity to improve inter-agency cooperation for projects in this policy unit for example at Maiden Lane. Working closely together is particularly useful for flooding caused by a combination of surface water drainage and flooding from Main Rivers.
Catchment-wide Reduce significantly the number of properties at risk of unacceptably frequent Opportunities flooding and reduce the risks to life. and Constraints Improved gauging in this policy unit would help provide a better flood warning service for residents.
This unit is highly urbanized and the existing infrastructure and development is a significant constraint to managing flood risk.
There is a strategy and action plan for Crayford developed by the London Borough of Bexley. By working with Bexley the Environment Agency can work to ensuring flood risk is reduced, developments are appropriate and the river habitat improved. The modeling and flood risk analysis accuracy is by nature high level for the CFMP. Significant uncertainty surrounds the effect of climate change, and the impact of urban development on flood risk.
Risks, The estimated AADs for the policy unit are considered sufficiently accurate to justify Uncertainties and the cost of further investigations included in the action plan. Dependencies The flood risk from the Cray and Shuttle and from surface water systems is not well defined.
Predictions on future flood risk are dependent on a continued policy of flood risk assessment and development control.
Table 6.4 Shuttle and Cray summary table
Table 7.1 Action Plan
Shuttle & Cray Policy Unit
Action Success Criteria Lead Organisation Timeframe Priority Objectives Indicators Encourage the take up of flood resilience • Local people taking action Environment Ongoing High • Reduce risk of flooding to property, • Residual Annual Average measures by people living within the floodplain to reduce their risk of Agency infrastructure and services Damage flooding • To reduce risk of flooding to community • Onset of flooding assets where it is cost effective to do so • Numbers of properties at • To reduce risk of direct impacts of flooding risk of flooding with a 1% on human health and 4% annual probability Provide development control advice to ensure • No increase in surface Environment Ongoing High • To encourage sustainable developments • Before and after River no increase in run-off from new developments water run off from new Agency • To reduce the risk of flooding to property, Corridor Surveys and seek opportunities to reduce current run off developments Local Authorities infrastructure and services • Biological and chemical rates where possible • Reduced run off from water quality existing developments • Critical infrastructure at risk Use improved data from upper Cray telemetry • Improved modelling Environment 0-5 years High • To encourage sustainable developments of flooding with 0.1% annual gauge to remodel Shuttle & Cray for flood Agency • To reduce risk of direct impacts of flooding probability mapping and warning on human health • River Morphological Investigate other flood warning techniques such • Improved flood warning Environment 0-5 years Medium • To reduce risk of direct impacts of flooding Surveys as rainfall forecast or radar triggered to improve lead times Agency on human health • Number of objections to flood warning lead times development on grounds of Undertaken a integrated urban drainage study • Joined up study of surface London Borough of 0-5 years Medium • To reduce the risk of flooding to property, flood risk to investigate and reduce flooding on the River water drainage with Bexley infrastructure and services Cray between Crayford Riverside Gardens to Bexley, Thames Water • To encourage sustainable developments Maiden Lane, taking into account any and the Environment downstream effects. Agency Install high flow telemetry gauge on upper Cray • Additional gauge installed Environment 0-5 years High • To reduce risk of direct impacts of flooding to improve flood warning • Improved flood warning Agency on human health service Investigate non-Main River flooding in • Understanding of how we London Borough of 0-5 years Medium • To reduce the risk of flooding to property, Orpington. can address this flooding Bromley infrastructure and services
• To encourage sustainable developments
Prepare a joint urban drainage strategy to Environment 0-5 years Medium • A plan that reduces flood • To reduce the risk of flooding to property, ensure that flooding from urban watercourses, Agency/London risks from all sources. infrastructure and services drainage and main rivers are considered Borough of Bexley • To encourage sustainable developments together. • To protect (and enhance) ecological
resources, landscape and recreational features particularly the creation of wetland, wet wood land and reedbed habitat improvement for water vole Support the Crayford strategy in renaturalising • Renaturalise river Cray London Borough of 0-5 years High • To protect (and enhance) ecological the river Cray and river Wantsunt and reducing and reduce flood risk Bexley resources, landscape and recreational flood risk in the redevelopment of the area. features particularly the creation of wetland, wet wood land and reedbed habitat improvement for water vole. • To reduce the risk of flooding to property, infrastructure and services Develop a System Asset Management Plan • SAMP completed Environment 0-5 years High • To reduce the risk of flooding to property, (SAMP). This plan should aim to reduce flood Agency infrastructure and services risk, now or in the future taking account of these actions.