Local Plan Regulation 18 - Managing surface water run-off

5.74 Surface water flooding is a known risk across many parts of the borough and serious flood incidents have taken place in the past, most recently in summer 2021. Flooding can be caused during periods of heavy rainfall which causes ponding of water at the surface which is exacerbated by the sewer system reaching capacity and also causing flooding. The council's Strategic Flood Risk Assessment indicates that a high number of residential properties and non-residential properties are at risk of surface water flooding during a 1 in 100-year rainfall event.

5.75 As is the case for a number of central and inner London Boroughs, the sewer infrastructure that serves H&F is a combined rather than separate system which means that sewers not only convey foul water to the sewage treatment plants further downstream, but also all surface water that enters the system – i.e. water that drains from paved areas, roads, roofs etc when it rains. Under normal circumstances, there is capacity in the sewers for all foul and surface water to be accommodated without significant flood risk, however, during storm conditions when there can be high levels of rainfall in a short period of time, the volume of surface water and the rate at which it enters the sewers can overwhelm the system and cause sewers to surcharge and flow back into properties through drains, toilets, sinks etc flooding them internally.

5.76 The risk of exceedance of the drainage system and surface water flooding in the borough is likely to increase in the future unless steps are taken to manage and mitigate this form of flooding. In line with the council's duties as a Lead Local Flood Authority, surface water therefore needs to be properly managed in new developments, particularly major developments, for all storm events up to the 1 in 100 year event + climate change allowance.

London Plan drainage hierarchy

5.77 All development schemes, including minor proposals will be expected to show that they have managed surface water by utilising all available techniques in line with the London Plan Drainage Hierarchy to avoid increasing runoff and to reduce it as far as possible, down to the greenfield run off rate for the site in the case of major developments.

5.78 The London Plan Drainage Hierarchy emphasises the importance of maximising inclusion of SuDS measures which are capable of managing rainfall as close to its source as possible, helping to prevent or delay its entry into the sewer system. SuDS also have the capability to provide many additional benefits alongside flood risk reduction. Measures such as rainwater harvesting and blue roofs which collect rain to re-use for irrigation are given the highest priority and should be included where feasible. Infiltration into the ground where the soils are suitable is also prioritised alongside other landscaped based measures such as rain gardens and green roofs as well as general soft landscaping measures. Trees can also intercept significant amounts of rainfall which helps reduce surface water run-off.

5.79 For riverside developments, direct discharge into the Thames may be a suitable option to consider as this is also preferable to discharging into the sewer system so long as this can be done without causing adverse impacts on the river, including its habitat and associated biodiversity.

5.80 However, where it is necessary to discharge surface water into the sewers, this must be done in a controlled way, using attenuation storage tanks, if necessary, to reduce the rate of discharge to greenfield rates and minimise the risks of surface water flooding. The use of combined attenuation tanks which can also be used for rainwater harvesting and re-use along with "smart controls" which remotely monitor weather forecasts so they can be emptied before a major storm event when their storage is most in demand are recommended.

5.81 The proposed SuDS measures must clearly demonstrate how they will achieve the required attenuation of peak surface water run-off, in line with the drainage hierarchy.

A maintenance programme must also be included for implementation to ensure the effectiveness of the system for the lifetime of the development.

5.82 If a major development is not able to meet the greenfield run-off rate requirements and this has clearly been demonstrated and accepted by the council in the Surface Water Management Strategy, then the developer will be required to make a financial contribution to the council to contribute to the implementation of SuDS measures in the borough to achieve the required attenuation of surface water run-off into the sewer system.

Simple drainage measures

5.83 For minor schemes, simple SuDS measures such as soft landscaping, including rain gardens, permeable paving, rainwater harvesting in the form of water butts and living roofs should be integrated wherever possible. The council has produced a guide to small-scale SuDS measures called "Living with Rainwater" which provides further details on small SuDS measures such as these.

5.84 To help minimise run-off from new areas of hard standing, including car parks, these must be designed to be permeable and allow infiltration of surface water with no run-off being directed to the sewer system (unless there are practical reasons for this not being possible – i.e. unsuitable underlying soils). This includes front driveways accessed by crossovers.

5.85 Landscaping schemes associated with major and minor schemes will be expected to minimise the use of impermeable surfaces, maximising use of permeable materials, direct run off to soft landscaped areas including features such as swales and raingardens where there is space.

Consultation with Thames Water

5.86 Developers are recommended to consult with Thames Water on their proposals. They can provide advice on issues such as sewer capacity and connecting to their sewer network. Developers for major schemes in particular will need to demonstrate that there is adequate capacity in the sewer system both on and off site to serve their development and that it would not lead to capacity issues for existing users. In some circumstances, including all major developments impacting on surface or foul water drainage within the catchment of the Counters Creek sewer, this may make it necessary for developers to carry out appropriate studies to ascertain whether the proposed development would lead to overloading of existing infrastructure. Any necessary upgrades will need to be delivered ahead of the occupation of a development.

5.87 SuDS measures must be properly maintained in order to ensure they can function as intended when they are required to help manage run-off from a major storm. Developers or site owners/occupiers will usually be responsible for maintaining SuDS on their property. The council does not have a policy of adopting private SuDS measures for maintenance purposes.