International approaches to the hydraulic control of surface water runoff in mitigating flood and environmental risks

This paper compares and contrasts a number of international approaches to the hydraulic control of surface water runoff from new development and redevelopment, known as sustainable drainage systems (SuDS) or low impact development (LID). The paper provides a commentary on the progress and current status of national standards for SuDS in the UK to control the frequency, flow rate and volume of runoff from both frequent and extreme rainfall events, and the best practice design criteria presented in the revised UK CIRIA SuDS Manual, published in November 2015. The paper then compares these design criteria and standards with those developed and applied in China, USA, France and Germany and also looks at the drivers behind their development. The benefits of these different approaches are assessed in the context of flood risk mitigation, climate resilience and wider environmental protection objectives, including water quality, morphology and ecology. The paper also reviews the design approaches promoted by the new SuDS Manual and internationally for delivering additional benefits for urban spaces (such as recreation, visual character, education and economic growth) through multi-functional urban design.


Introduction
Approaches to the management of surface water runoff from urban development are undergoing a period of significant change.The efficacy and sustainability of traditional piped solutions is being questioned as climate, population and environmental pressures put the economics of such an approach in question.The need to take a more integrated approach to water management -maximising its value through all stages in the water cycle -has never been more important and surface water runoff is a crucial element of this.
Global water shortages mean that the intrinsic 'value' of runoff from rainfall is becoming increasingly understood and recognised in decision-making.Drivers for the use of runoff to meet non-potable supply needs (including toilet flushing, landscape and horticulture, building cooling water, vehicle cleansing etc) increase as the availability and security of water resources decrease and the costs rise.The heat stress associated with dense urban areas is being exacerbated by rising global temperatures and, with the density of urban populations rising rapidly, urban designers are looking to vegetated surfaces and water storage zones to provide essential cooling to ensure urban 'liveability' and community health under future climatic conditions.In many places, more intense rainfall is another outcome of observed climatic changes and, combined with increasing urban impermeability levels (often described as 'urban creep'), leads to the design standard of service of existing sewerage infrastructure often being lower than expected and the risk of surface water flooding therefore higher.The cost of enhancing capacity of this infrastructure is often prohibitive, so new solutions to reducing surface water loadings from existing areas and minimising loadings from new development zones are needed.The adaptability of systems, in the face of climate and urbanisation change, is also a key attribute (Ashley et al, 2015).
International environmental protection regulations (eg EU Water Framework Directive) have developed in response to recognition of the urgent need to reverse the deterioration in quality of surface waters and protect their functionality now and for future generations.Urban diffuse pollution has been identified as a significant influence on quality and morphologyprincipally resulting from the uncontrolled discharge of surface runoff (normally contaminated with a suite of urban surface pollutants) and spills from combined sewer overflows.
These drivers exist for both developed and developing countries and have triggered the need to The guidanc enefits that c ependent on evelopment c roadly into f uality, amenit n the Manual ach of these pi n Figure 1.It is emphasized that many of these criteria are interdependent or cross-cutting, and that in order to maximise opportunities and the associated benefits, the criteria should be considered at an early stage and fully integrated into the surface water management and urban design process.In so doing, it is then possible to ensure that the scheme is truly multi-functional and delivers the highest return for the developer and for the community who will live there.

England: Drivers and Policy
The independent review into the causes of the 2007 floods (Pitt, 2008) concluded SuDS were an effective way to reduce the risk of surface water flooding and recommended that the government implement measures to increase uptake of SuDS alongside the removal of the automatic right of connection of developers to the local sewerage network.Schedule 3 of the Flood and Water Management Act 2010 made the delivery of SuDS to meet a set of statutory National Standards a requirement for all new development, and established SuDS Approving Bodies within Lead Local Flood Authorities to approve and adopt the systems post construction.After two phases of consultation, this approach raised too many challenges for the English Government and a decision was made instead to incorporate SuDS delivery within the planning process (DCLG, 2014) and to include non-statutory policy standards (Defra, 2015) within the National Planning Practice Guidance (DCLG, 2015).The proposed benefits of the revised approach are: • the strengthening of the planning regime -enabling decision-makers to give increased weight to the provision and maintenance of SuDS as a material planning consideration alongside e.g.transport infrastructure, density of development etc; • the alignment of drainage delivery with development design facilitating and promoting joined up approaches to the management and use of surface water on developments -between urban and landscape designers, architects and building engineers, ecologists and drainage designers; • improved efficiency and reduced delivery risks associated with approvals from multiple bodies within the local authority system.However, there are a significant number of risks to widespread SuDS implementation, yet to be addressed: • there is now no standard body responsible for taking ownership of SuDS in perpetuity.This is contrary to Pitt s recommendations regarding the benefits of ownership by either the local authorities or water companies both offering integrated surface water management benefits above and beyond service management; • the automatic right to connect for developers remains (although is being re-debated at the time of publication); • the set of non-statutory standards within the planning guidance address only hydraulic control of surface water discharges (peak flow control for 1 and 100 year return period events; volume control for the 100 year, 6 hour event; pumping only in exceptional circumstances); the standard of service and management of risk of flooding from the drainage system itself; and structural integrity of the system and adjacent or connected infrastructure -leaving the planning authority to make any requirements for water quality protection, amenity or biodiversity a site-specific condition of planning; • the associated guidance (Lasoo, 2015) requires SuDS only where they are not more expensive than meeting current (i.e.traditional) drainage requirements.

Wales: Drivers and Policy
On 5 January 2016 the Welsh Government published recommended non-statutory standards for SuDS in Wales (Welsh Government, 2016).In contrast to the English standards, the standards for Wales deal with the runoff destination hierarchy (recommending that rainwater harvesting is given early consideration), the control of runoff for frequent rainfall events to protect the quality and morphology of receiving surface waters, and the objectives of maximising amenity and biodiversity benefits for the development site and wider environment -as well as the objectives of hydraulic control and onsite flood risk management, constructability, maintainability and structural integrity.
The Welsh Government has recognised that a SuDS approach to surface water drainage will contribute to the realisation of the well-being goals within the framework of the Future Generations (Wales) Act -by supporting a more prosperous, resilient, healthier, cohesive and globally responsible Wales into the future.The Welsh Government has stated that their next steps will be to work with stakeholders to identify the best way to embed the SuDS approach in all new developments in Wales, as set out in their Water Strategy published in 2015possibly by commencing Schedule 3 of the Flood and Water Management Act 2010 for Wales independently of England.This would require new developments to manage surface water using the SuDS approach, with systems designed and built in accordance with a set of statutory standards and it would also provide a framework for their approval, adoption and ongoing maintenance.

Scotland: Drivers and Policy
In many ways, the development and application of SuDS in Scotland has led the way for the rest of the UK, with enabling legislation in place at an earlier stage than in England and Wales -although the primary focus in Scotland has been water quality protection.In 2003, the Water Environment and Water Services (Scotland) Act (WEWS) was implemented to deliver the requirements of the EU's Water Framework Directive.This was followed in 2005 by the Water Environment (Controlled Activities) Regulations under which there is a generalrequirement for new developments (except for single dwellings or discharges to coastal waters) with surface water drainage systems discharging to the water environment that such discharges will pass through SuDS with all reasonable steps being taken to ensure protection of the water environment.SEPA's guidance on designing drainage systems to meet the regulatory requirements for water guidance) and Wales (in their non-statutory Standards).A criterion for matching average annual runoff volumes for pre-and post-development states (Germany, China) reflects an awareness of the benefits of trying to replicate the natural hydrological response from sites across all scales of rainfall.The new UK SuDS Manual prioritises infiltration and use of runoff wherever appropriate, and sets as standard the retention of 5mm of the majority of rainfall events on site ('Interception') and limiting runoff volumes for the 1:100 year, 6 hour event to the predevelopment equivalent.A generic comparison of the criteria is provided in Figure 8.

Conclusions
Drainage criteria are continuing to evolve, with a greater focus on their integration within a more holistic approach to environmental management.If effectively implemented, regulated and monitored, these steps should bring about significant benefits in terms of mitigating the impact of development on the water environment, reducing urban flood risk, and securing greater social and environmental value from surface water -a valuable and an increasingly scarce resource for many.
Aiming for a more natural runoff response from our developed areas is laudable and legislation and regulation aimed at delivering this requirement will support urban planners in promoting development that has higher levels of 'permeability' The resultant intrinsic properties of such development should deliver drainage systems that are more adaptable to our changing climate, should help provide access to an alternative supply of non-potable water where required, and should help improve the resilience and liveability of our urban areas.Developing and supporting suitable economic drivers for these changes is, however, still highly challenging.Green space is not currently a financial commodity, and developing ways of aligning long-term positive societal benefits with short-term business interests are not often obvious.
However, from a technical perspective at least, learning from others is crucial.Reviewing the methods adopted by other countries provides us with an opportunity to explore how we can use more sustainable surface water management measures to most efficiently protect our natural environment, mitigate the impacts of development on natural hazards, and support future urban quality of life.In the UK, decision makers should consider the potential benefits of a more volumetric approach to runoff control.Lessons also need to be learnt from cities such as Copenhagen regarding future city design strategies to deliver resilience to the types of extreme intense rainfall events that have caused widespread damage (e.g.Hull, 2007, Copenhagen, 2011, Germany, 2013) and are likely to become increasingly common into the future.Integration is crucial, not only in delivering on environmental objectives, but across disciplines -with drainage design meshing seamlessly with the planning and design of the built environment, and the landscaping of our urban space.
Figure 1 Th

n of the criteria
FLOODrisk 2016 -3 rd European Conference on Flood Risk Management