Water Gateshead & Newcastle Councils January 2012
NewcastleGateshead Surface Water Management Plan
Surface Water Management Strategy Final Report
Prepared by: Daniel Alstead Checked by: Christian Lomax Consultant Principal Consultant
Approved by: Roy Lobley Associate Director
Rev No Comments Checked by Approved Date by 0 Draft Final Report CSL Aug 2011 1 Final Report CSL RAL Jan 2012
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Job No 60198244 Reference SWMP SW & SuDS Date Created January 2011
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Table of Contents
Abbreviations ...... 2 Glossary ...... 3 Glossary ...... 3 1 Introduction...... 6 1.1 Surface Water Management Strategy ...... 6 2 Surface Water & Howdon Sewage Treatment Works ...... 9 2.1 Introduction ...... 9 2.2 Howdon STW ...... 9 2.3 Overcoming the Lack of Headroom at Howdon STW ...... 10 3 Sustainable Drainage Systems ...... 18 3.1 Introduction ...... 18 3.2 SuDS Implementation Policy ...... 18 3.3 Sustainable Drainage Systems ...... 19 3.4 The SuDS Management Train ...... 19 3.5 Water Quality Capabilities ...... 21 3.6 Environmental Benefits...... 21 3.7 Other Benefits ...... 21 3.8 Different Types of SuDS ...... 22 3.9 SuDS Summary ...... 25 4 NewcastleGateshead SuDS Requirements ...... 28 4.1 Introduction ...... 28 4.2 Baseline Data ...... 28 4.3 SuDS Selection ...... 30 4.4 Soils across Gateshead and Newcastle ...... 39 5 Multifunctional Green Space ...... 41 5.1 Introduction ...... 41 5.2 Multifunctional Green Space ...... 41 5.3 Practical Examples of Utilising Green Space from around the UK ...... 42 5.4 Potential Opportunities in Gateshead and Newcastle ...... 44
Appendix A – SuDS Synopsis ...... 47
List of Figures
Figure 2.1: Howdon STW Catchment Area...... 9 Figure 3.1: Guide to the design of a SuDS management train ...... 20 Figure 3.2: Exceedance Flow Routes ...... 25 Figure 4.1: Selection Process ...... 30 Figure 5.1: Multifunctional Green Space across Gateshead and Newcastle...... 41 Figure 5.2: Manor Parks ...... 42 Figure 5.3: Elvetham Heath Village Pond ...... 43 Figure 5.4: Detention Pond and Swale within Bristol Business Park ...... 44 Figure 5.5: Pond and Wetland at Hopwood Services ...... 44 Figure 5.6: Walker Park...... 46
List of Tables
Table 2.1: Key Actions for Surface Water Removal ...... 12 Table 2.2: Actions for Partners – Headroom at Howdon STW ...... 16 Table 3.1: Number of treatment train components (assuming effective pre-treatment is in place) ...... 21 Table 3.2: Common SuDS...... 22 Table 3.3: Capability of different SuDS techniques ...... 26 Table 4.1: Required Information – to be provided by developers ...... 29 Table 4.2: Land Use Selection Matrix ...... 31 Table 4.3: Site characteristics selection matrix ...... 32 Table 4.4: Quantity and quality performance selection matrix ...... 34 Table 4.5: Community and environmental factors selection matrix ...... 36 Table 4.6: Summary Table for SuDS Selection ...... 38 Table 4.7: Soils found across Gateshead and Newcastle ...... 39
Abbreviations
AMP Asset Management Plan
Defra Department Environment, Food and Rural Affairs
ELR Employment Land Review
FWMA Flood and Water Management Act
GI Green Infrastructure
GIS Geographical Information System
LDF Local Development Framework
MCA Multi Criteria Analysis
OFWAT Economic Regulator for the Water and Sewerage Industry in England & Wales
PFRA Preliminary Flood Risk Assessments
SFRA Strategic Flood Risk Assessment
SHLAA Strategic Housing Land Availability Assessment
SuDS Sustainable Drainage Systems
SWMP Surface Water Management Plan
WCS Water Cycle Study
WFD Water Framework Directive
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Glossary
Annual Exceedance Probability (AEP) - The probability (%) that a given rainfall total accumulated over a given duration will be exceeded in any one year. I.e. the ‘1 in 100 year event’ has a 1% probability of occurring each year. Areas Susceptible to Surface Water Flooding – National mapping prepared by the Environment Agency in 2008 illustrating the flooding that takes place from the 'surface runoff' generated by rainwater (including snow and other precipitation) which: (a) is on the surface of the ground (whether or not it is moving), and (b) has not yet entered a watercourse, drainage system or public sewer. Average recurrence interval (ARI) ‘1 in 100 year event’- The average or expected value of the periods between exceedances of a given rainfall total accumulated over a given duration. It is implicit in this definition that the periods between exceedances are generally random. Boundary Flow Interception – Capturing or addressing surface water flows before they reach the boundary of a hotspot. Core Strategy – A Development Plan Document setting out the spatial vision and strategic objectives of the planning framework for Gateshead and Newcastle. Department for Environment, Food and Rural Affairs (Defra) – Department that brings together the interests of farmers and the countryside; the environment and the rural economy; the food we eat, the air we breathe and the water we drink. Development Plan Document (DPD) – Local Development Framework document that contain policies and are subject to external examination by an Inspector. Employment Land Review (ELR) - provides an evidence base that will be used to inform the preparation of employment land policies and allocations in the Local Development Framework (LDF). The ELR ensures that sites and buildings that are important to the future prosperity of an area are retained in employment use and to enable the release of sites that could sensibly be used for other purposes. Environment Agency (EA) – The Environment Agency was established under the Environment Act 1995, and is a Non- Departmental Public Body of Defra. The Environment Agency is the leading public body for protecting and improving the environment in England and Wales today and for future generations. The organisation is responsible for wide ranging matters, including the management of all forms of flood risk, water resources, water quality, waste regulation, pollution control, inland fisheries, recreation, conservation and navigation of inland waterways. It will also have a new strategic overview for all forms of inland flooding Flood Map for Surface Water – National mapping prepared by the Environment Agency in 2009 to improve the Areas Susceptible to Surface Water Flooding using improved model data. Flood Risk Assessment (FRA) – a FRA is required under PPS25 at the planning application stage for new developments. An FRA will demonstrate how flood risk from all sources to the development itself and flood risk to others will be managed now and in the future (including climate change). Floods and Water Management Act (2010) – Act of Parliament to clarify the legislative framework for managing surface water flood risk in England. Green Infrastructure –is the network of multifunctional green and undeveloped land, urban and rural, which supports the activity, health and well being of local people and wildlife. Growth Point – communities that are pursuing large-scale, sustainable housing growth through a partnership between local organisations and central government. Local Authority or Local Planning Authority (LA or LPA) – the Local Authority or Council that is empowered by law to exercise planning functions. Often the Local Borough or District Council, National Parks and the Broads Authority are also considered to be local planning authorities. County Councils are the authority for waste and minerals matters.
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Local Development Framework (LDF) – a folder of local development documents that outlines how planning will be managed in the area. Main River – Generally main rivers are larger streams or rivers, but can be smaller watercourses. Main Rivers are determined by Defra in England, and the Environment Agency has legal responsibility for them. Ofwat – The Water Services Regulation Authority (Ofwat) is the body responsible for economic regulation of the privatised water and sewerage industry in England and Wales. Ofwat is primarily responsible for setting limits on the prices charged for water and sewerage services, taking into account proposed capital investment schemes (such as building new wastewater treatment works) and expected operational efficiency gains. Ordinary watercourse - An ordinary watercourse is any other river, stream, ditch, cut, sluice, dyke or non-public sewer which is not a Main River. The Local Authority has powers for such watercourses. Pitt Review - An independent review of the 2007 summer floods by Sir Michael Pitt, which provided recommendations to improve flood risk management in England. Planning Policy Statements (PPS) and Planning Policy Guidance (PPG) – these documents set out the Government’s national policies on different aspect of planning. The policies in these statements apply throughout England and focus on procedural policy and the process of preparing local development documents. Strategic Flood Risk Assessment (SFRA) – an assessment of flood risk from all sources which is used to inform the planning process of flood risk and provides information on future risk over a wide spatial area. It is also used as a planning tool to examine the sustainability of the proposed development allocations. SFRAs form the basis of flood risk management in England and are a requirement of PPS25. Supplementary Planning Documents (SPD) - supplementary planning documents can give further context and detail to local development plan policies. It is not part of the statutory development plan. Therefore, it does not have the same weight when local planning authorities are considering planning applications. Surface Water Flooding - surface water flooding describes flooding from sewers, drains, groundwater, and runoff from land, small water courses and ditches that occurs as a result of heavy rainfall. Surface Water Management Plan (SWMP) - is a plan which outlines the preferred surface water management strategy in a given location. Sustainable Drainage Systems (SuDS) – Sustainable drainage systems (previously referred to as sustainable urban drainage systems): a sequence of source control, management practices and control structures designed to drain surface water in a more sustainable fashion than some conventional techniques (may also be referred to as SuDS or SDS). Water Cycle Study (WCS) – The purpose of a WCS is to strategically plan the most sustainable water infrastructure in a timely manner, across all of the water cycle from water supply and water resources, flood risk and surface water drainage, and wastewater and biodiversity. Water Framework Directive (WFD) – A European Community Directive (2000/60/EC) of the European Parliament and Council designed to integrate the way water bodies are managed across Europe. It requires all inland and coastal waters to reach “good status” by 2015 through a catchment-based system of River Basin Management Plans, incorporating a programme of measures to improve the status of all natural water bodies.
Introduction
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1 Introduction
This document is part of the NewcastleGateshead Surface Water Management Plan (SWMP). The primary purpose of the SWMP is to contribute robust evidence to support the Local Development Framework (LDF) including the Core Strategy being developed by Gateshead Council and Newcastle City Council. The SWMP Technical Guidance 1 states that a SWMP is a plan which outlines the preferred surface water management strategy in a given location.
In the context of the NewcastleGateshead SWMP this document outlines the surface water management strategy to facilitate future development across Gateshead and Newcastle.
In addition the guidance stresses the need for local partners with responsibility for surface water and drainage to work together to understand the issues associated with surface water and agree the most cost effective way of managing those issues. It further states the need to make surface water management decisions that are evidence based, risk based, future proofed and are inclusive of stakeholder views and preferences. From a planning perspective SWMPs can provide a framework to deal with surface water for new developments, whilst contributing to improving the water quality of our water networks and achieving the requirements of the Water Framework Directive (WFD). Four Partners have been involved with the NewcastleGateshead SWMP; two Local Authorities (Gateshead Council & Newcastle City Council, referred to as ‘the Councils’), Northumbrian Water and the Environment Agency. Each of the four Partners has an important role to play concerning the management of surface water. The NewcastleGateshead Water Cycle Study (WCS) identified that Howdon Sewage Treatment Works (STW) does not have sufficient headroom (spare capacity) to cater for all of the growth planned for the region. This document sets out the surface water management strategy by which the Partners and developers will ensure that drainage systems to Howdon STW do not constrain growth and that growth does not detrimentally impact on the environment. In order to achieve this there is a need for all five Local Authorities affected by Howdon STW to co-ordinate their actions to ensure that there is a consistent approach adopted across the catchment area. 1.1 Surface Water Management Strategy
The strategy is to remove and reduce the volume of surface water entering the combined sewer system that serves Howdon STW. Removing surface water from the system will free up headroom to receive foul water from new development.
This report has the following format: - Chapter 2 sets the scene concerning the available headroom at Howdon STW and the role that surface water can play in freeing up headroom at the STW, - Chapter 3 provides an introduction to Sustainable Drainage Systems (SuDS), which offer an alternative means of managing surface water to the combined sewers and will need to be implemented by developers as part of the Strategy, - Chapter 4 presents one means by which developers and the Councils can demonstrate why different SuDS schemes are, or are not, appropriate for a specific development site, and - Chapter 5 provides high level information concerning how development sites can utilise multifunctional space to manage surface water rather than disposing of it to the combined sewer system.
1 Defra (March 2010). Surface Water Management Plan Technical Guidance.
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The SWMP is a high level, strategic document which serves as a starting point for the Partners to address surface water flood risk. It should be noted that the SWMP ought to remain a living document, to be up dated as further data as information becomes available. An action to come out of the SWMP is for the Councils to take the lead in ensuring that the Partnership continues to work together to address surface water.
Surface Water & Howdon Sewage Treatment Works
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2 Surface Water & Howdon Sewage Treatment Works
2.1 Introduction Gateshead Council and Newcastle City Council have aspirations for significant growth across their authority areas however the NewcastleGateshead WCS identified that Howdon STW does not have sufficient headroom (spare capacity) to accommodate all of the planned growth across Gateshead and Newcastle. The volume of surface water arriving at Howdon STW is perceived to be both the issue and the solution; removing surface water from the combined sewers serving Howdon STW will free up headroom. As such the NewcastleGateshead SWMP has endeavoured to provide some background to this issue and outline the steps that the Councils, Northumbrian Water and the Environment Agency are taking to ensure that Howdon STW does not constrain future growth and that development does not detrimentally impact upon the environment. 2.2 Howdon STW Howdon STW treats a combination of domestic, trade and surface water discharges from the Local Authority areas of Gateshead, Newcastle, North Tyneside, South Tyneside and parts of south Northumberland (Figure 2.1; the purple line illustrates the catchment boundary). Across these five Local Authority areas Howdon STW serves a current population equivalent of 912,000. Figure 2.1: Howdon STW Catchment Area
Howdon STW was designed in the early 1970’s with the intention that it would predominantly treat foul sewage. However, the bulk of Howdon’s catchment area is served by combined sewers which transport both foul flows and surface water to the STW. The presence of surface water limits the ability of the STW to accept additional foul flows from new developments.
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Howdon STW is consented by the Environment Agency both in terms of treatment performance standards (i.e. the quality of effluent that is discharged into the River Tyne), which it complies extremely well with, and volumetric flows received at the works. Howdon is currently consented to treat sewage with a dry-weather flow 2 of not more than 229,720 m 3/day to the standards specified in the EC Urban Waste Water Treatment Directive. There are also consented overflows both at Howdon and further up the sewerage network, from which discharges are permitted of storm sewage. These are regulated not by what is allowed to be discharged but by specifying the flow rate that must be retained within the sewer before a discharge occurs. Increasing sewage flows within the combined sewers (through development) have the potential to increase the volumes and frequency of storm sewage discharges (via combined sewer overflows) which could cause pollution of the water environment. Therefore Northumbrian Water and the Environment Agency are required to work together to ensure that environmental standards are maintained. It is impossible to know precisely how much headroom is available at Howdon STW because the actual flows vary randomly in response to rainfall. This random variation limits the ability of the Environment Agency and Northumbrian Water to detect changes, whether step or gradual, in the flow received at Howdon STW. This random variation is accounted for by using different definitions of dry-weather flow for planning and compliance assessment so that a STW is not considered to be in breach of its consent limit if it’s within this random noise. At present, the Environment Agency and Northumbrian Water have agreed that based upon data from the last five years, the best estimate of the available headroom is equivalent to around 3% of the consented flow, which is similar to the random variation. In terms of the volumetric flows this is equivalent to approximately 13,000 additional houses (this assumes that each additional house generates around 0.5m 3 of foul sewage per day). However the figure of 13,000 houses is not exact and will be subject to change as more data becomes available in the future. The planning horizon for Gateshead and Newcastle extends to 2030 during which time they intend for significantly more than 13,000 houses to be built. In addition to this, all of the Local Authorities served by Howdon STW expect to see significant growth over the planning horizon, not just Gateshead and Newcastle. In light of this; Northumbrian Water, the Environment Agency, Gateshead Council and Newcastle City Council have taken the opportunity that the SWMP presents to set out their joint working strategy, which is to ensure that the development aspirations of the five Local Authorities served by Howdon STW can be delivered in a timely manner without increasing the frequency of combined sewer overflows which could detrimentally impact upon the environment. 2.3 Overcoming the Lack of Headroom at Howdon STW The limited headroom at Howdon STW has been recognised and Northumbrian Water has co-ordinated senior level meetings with representatives from each of the five Local Authorities draining to Howdon STW. The output is that each of the Councils has agreed to the development of a statement of commonality for inclusion in their Core Strategies. Northumbrian Water is confident that a strategy of removing surface water from the combined sewer network serving Howdon STW, will free up sufficient headroom at the STW for the proposed housing within Gateshead and Newcastle and neighbouring Councils across the planning period. There are a significant number of feasible opportunities which will allow headroom to be freed up either through formal separation schemes or as a consequence of redevelopment. It is not possible to quantify in advance how much headroom can be freed up by each intervention to remove surface water as this would require advance knowledge of the flow rates involved, which Northumbrian Water does not currently have and is dependent on a number of spatial and topographical factors. For example the availability of a suitable watercourse to which surface water can be drained when it is removed from the combined sewers is a key factor as it the ability to retrofit SuDS on site. Northumbrian Water are currently identifying candidate brownfield sites which appear to be suitable for surface water separation and have also launched a project to build a number of detailed hydraulic models in areas where high levels of growth are forecast to identify further suitable sites.
2 Dry-weather flow is the flow that is expected to be exceeded 80% of the time.
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It is likely that Northumbrian Water should, in some circumstances, be able to measure and quantify the success of surface water removal once schemes have been implement, by reduced “run-time” at pumping stations or where there are adjacent flow meters. The overall success of the strategy of surface water removal will ultimately be measured by a reduction in the baseline flows observed at Howdon STW. The following sections set out the roles that each of the Partners will play to deliver a strategy to remove and reduce the volume of surface water entering the combined sewer system that serves Howdon STW to free up headroom. 2.3.1 Northumbrian Water Northumbrian Water has launched a Howdon STW and Tyneside Interceptor Asset Management Plan (H&TI AMP) which is a long-term (25 year) plan for the management of the network and treatment capacity in the Tyneside catchment. It is designed to facilitate a number of needs including: - Identifying and prioritising the removal of excess surface water from the system - Accommodating future growth for the Howdon catchment, - Managing long-term compliance for Howdon and the network assets, - Addressing environmental protection, - Identifying and managing future flood risk, - Ensuring operational efficiency, - Identifying long-term investment needs, and - Enabling robust business planning for future asset management plans. In addition to this, Northumbrian Water will complete the Tyneside Sustainable Sewerage Project which will identify tools and techniques for surface water removal from the combined sewer system. It will also identify candidate projects for inclusion in their Business Plan submission for AMP6. The study has a three stage approach and is aligned with the principles of Surface Water Management Planning. Stage 1 Focus on the collection, collation and analysis of detailed information to identify (Complete) locations of potential dependency and interaction between drainage systems. Stage 2 Using the outputs from Stage 1 focus on the application of a risk based approach to identify and assess sustainable drainage options. Stage 3 Investment planning. The Stage 1 reports have identified a number of opportunities for source control, reduced infiltration and surface water separation. The Stage 2 studies will further investigate these opportunities to understand the wider impacts of such approaches, quantifying benefits and promoting appropriate opportunities. Table 2.1 details at a high level, Northumbrian Water’s key actions for surface water removal within the catchment of Howdon STW.
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Table 2.1: Key Actions for Surface Water Removal Action Date Due Completion Date
H&TI AMP Launch Meeting. August 2011 August 2011
H&TI AMP Surface Water Removal Workshop to identify scope of project and priorities for four key surface water types; Ingress, Inflow, Infiltration October 2011 October 2011 and Inundation.
Identify quick wins for ingress, agree requirements for monitoring and Identified at workshop. implement solutions. Solutions currently on site
Appoint Project Manager to identify and map all current areas of Ingress, March 2012 Inflow, Infiltration and Inundation.
Agree long term roles and responsibilities for maintaining log of surface April 2012 water interventions.
H&TI AMP Surface Water Removal Workshop 2 to review surface water June 2012 mapping exercise and agree strategy for each surface water type.
Annual review of baseline flows at Howdon STW and revise strategy as Annually commencing required. April 2012
Completion of the Tyneside Sustainable Sewerage Study and outputs 2013 incorporated in Business Plan submission for AMP6.
Dependent upon AMP6 funding being agreed with Ofwat, begin to deliver 2015 outputs from Tyneside Sustainable Sewerage project.
Formal Review of the success of the surface water strategy in advance of 2017 preparation for AMP7 Business Plan.
The four types of surface water referred to in Table 2.1 are defined as; Inflow – Point flow connections to the network that are designed and meant to be there e.g. remove connections of combined sewers to balancing ponds/watercourses and remove surface water inflows to the sewers through redevelopment. Ingress – Point flow connection to the network that is not designed or meant to be there e.g. restricting entry of river water into combined sewer system e.g. tidal flaps. Northumbrian Water has identified a number of tidal flaps which allow river ingress and are investing in their refurbishment so that this can be prevented. It is likely that the success of this can be measured and assessed at the completion of the project and is expected to have an immediate impact on the flows at Howdon STW, freeing up headroom. Infiltration – Flows entering the system through the fabric of the assets. Identify where water is entering system, including mains water leakages (drippers, seepers, runner and gushers) and prioritise investment to prevent entry into combined sewer system e.g. Ouseburn brick sewer. Inundation – Flood waters coming into the system and excessive surface water entering combined sewer system from runoff from fields. The success of the actions in removing surface water will be assessed as each intervention is carried out, if there is an appropriate means of doing so, or annually as part of the H&TI AMP review and in 2017 when Northumbrian Water prepare the
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AMP7 Business Plan. Northumbrian Water and the Environment Agency agree that 2017 is a sensible date for a formal review (although informal reviews will be ongoing before this) as it allows time for progress to be made but is early enough to inform planning for Northumbrian Water’s investment round starting 2020 (AMP7). The annual reviews starting in April 2012, will allow Northumbrian Water to monitor the success of the strategy to free up headroom at Howdon STW to support housing development. Annual monitoring will allow the strategy to be revised as necessary. Additionally, the extra data that will be available each year concerning headroom at Howdon STW will inform the need for an alternative longer term approach with regard to sewage disposal and treatment for Gateshead, Newcastle and the wider Tyneside catchment. Northumbrian Water would give an annual update on progress to the Tyne and Wear Flood Risk Management Group meeting where both Gateshead and Newcastle Councils are represented. This approach has been agreed between Northumbrian Water and the Environment Agency; senior management at the Environment Agency have discussed headroom issues at Howdon STW and elsewhere with Northumbrian Water directors in order to ensure that the two organisations have a joined-up approach on responding to planning matters. Environment Agency environment planning managers, planning officers and water quality planners meet regularly to discuss matters relating to planning and growth, including those at Howdon. Any significant decisions around growth would be made by the Environment Agency and Northumbrian Water management at the appropriate level. If the annual reviews indicate that the strategy of surface water removal will not free up sufficient headroom at Howdon STW to support development, the alternative longer term approaches referred to above, could include; increasing the capacity at Howdon STW, provision of alternative STWs or transferring parts of the sewer network to other STWs where there is sufficient headroom available. However, Northumbrian Water would exhaust all of the actions identified above in preference to these alternative options which are longer term and if required would take over ten years to plan and deliver. Should Northumbrian Water propose one of these alternative options, the role of the Environment Agency would be to identify consent conditions to protect water quality, with the aim of ensuring Good Ecological Status in receiving water bodies and preventing any deterioration in existing environmental quality. It is likely that there would be dialogue between the Environment Agency and Northumbrian Water around a number of options which would lead to one (or a combination) that produced an acceptable environmental outcome at an acceptable cost. 2.3.1.1 Funding the Strategy Northumbrian Water is funded in five year Asset Management Plan cycles and secures investment funds through their Business Planning Process with Ofwat, the industry regulator. Ofwat set the framework within which water companies fund their obligations, of which complying with consents is one, either directly through charging or by being sufficiently financially viable to borrow at sensible rates. The outputs from the Tyneside Sustainable Sewerage Study will help to inform the Business Plan for AMP6 (2015 – 2020) and Northumbrian Water hope to secure investment funds to reduce the impact of surface water on the combined sewers and at Howdon STW. The purpose of the H&TI AMP is to inform Northumbrian Water’s Business Plan for AMP7 (2020-2025) and the 2017 review meeting will provide the data required to make the longer term investment proposals for Howdon STW and its associated sewerage system. Should there be a requirement for additional sewage treatment capacity for the Tyneside catchment Northumbrian Water would seek to build this into their Business Plan for AMP7. 2.3.1.2 Resisting Development Northumbrian Water has a number of key duties to provide sewerage infrastructure for new development. Section 94 of The Water Industry Act 1991 imposes a duty upon Northumbrian Water to “provide, improve and extend a system of public sewers” and to “cleanse and maintain”. It also imposes a duty to provide an outfall for those public sewers to sewage treatment facilities. It is important to note though that whilst Northumbrian Water has this duty, there can be a delay between
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development being proposed and Northumbrian Water securing the necessary funding to enable it to improve and extend the sewerage network or sewage treatment facilities. In such cases they would seek to use the planning process to seek deferral until such time as the capacity of the network and treatment facilities could be increased. This emphasises the importance of early engagement with Local Planning Authorities and the need for them to provide detailed and realistic housing locations and projections. Under Section 98 of the Water Industry Act 1991, Northumbrian Water has a duty to comply with a sewer requisition when a developer asks them to provide a sewer for domestic purposes. Typically developers tend to only requisition sewers where they are being held to ransom over their inability to lay a pipe across third party land. Many sewer requisitions are to provide an off– site surface water outfall to a watercourse. This may be a useful mechanism for surface water separation to assist developers in gaining access to new surface water outfalls on previously developed land which drains to the combined sewer system. Section 106 of the Water Industry Act 1991 gives the developer an automatic right to connect for surface water into sewerage system although it is possible that this right may be removed in the near future. Northumbrian Water cannot resist development using the Water Industry Act 1991. They can only seek to resist inappropriate development on the grounds of sewerage infrastructure capacity using the planning process. Water and Sewerage companies are statutory consultees in the development of the LDF and are able to identify areas of incapacity as Northumbrian Water has done within the NewcastleGateshead WCS and SWMP. Water and Sewerage Companies are not statutory consultees in the planning application process, albeit Local Planning Authorities have tended to treat them as such since the implementation of Planning Policy Statement No.25 “Development and Flood Risk”. This is because sewer flooding has to be considered as part of the Flood Risk Appraisal. It is at this time that Northumbrian Water would seek to object to development on the grounds of incapacity or secure suitable planning conditions to protect its interests and those of its customers who may become at risk of flooding. Should a Local Planning Authority choose to disregard the Water and Sewerage Company and grant planning permission without the necessary planning conditions; the developer then has the right to connect to the public sewerage system under Section 106 Water Industry Act 1991. (This may however lead to potential investigation via the Local Authority Ombudsman should the development lead to flooding). 2.3.2 Environment Agency As regulator, the Environment Agency will review and ensure that Howdon STW continues to comply with its consent standards. Failure to comply with a consent (or environmental permit) leaves any discharger open to enforcement action by the Environment Agency. The enforcement action will be proportional to the offence and decisions will be taken in accordance with Environment Agency Enforcement and Sanctions Guidance. The environmental impact of a discharge depends upon the amount of pollutant it contains, which is a product of the volume of the discharge and the concentration of the pollutant in it. The Environment Agency determine consent limits based on the amount of pollutant but set them as concentration and flow. Previously, conditions relating to the quality of discharges were enforced but there was no evidence to enforce those relating to flow. This was recognised as a weakness in controlling environmental impacts and water companies were funded between 2000 and 2005 to install flow measurement equipment and are now required to measure it to agreed standards and report to the Environment Agency. Based on this reported information, funding has been allocated in AMP5 to increase treatment capacity (if needed) at sewage works where consented volume has been identified as being exceeded. What happens as other works become non-compliant (such as is the case at Howdon) is still evolving nationally. In addition, the Environment Agency report water company performance to Ofwat who take this into account in determining their financial settlements. In the past, Ofwat penalties have been a significantly greater incentive than other consequences of Environment Agency enforcement.
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The Environment Agency does not have powers to stop development but could object to planning applications if the drainage system was believed to be unsatisfactory. If development would lead to Howdon STW breaching its consent limits, the Environment Agency would look for evidence that plans were in place redress this, their remit is to protect the environment and would generally only object to an application where sufficient measures had not been put in place to prevent a deterioration in environmental quality or prevent a breach of a relevant environmental directive or regulation. The surface water strategy set out above indicates that Northumbrian Water have recognised that a breach is possible subject to future development and plans are in place to address this. At the site allocation stage, Environment Agency would expect allocations to take account of constraints identified in WCS’ and phasing to take account of planned solutions to them. At the individual application level, they could object to proposals that were either not incorporating appropriate technically feasible means of preventing environmental impact or were proposed to occur before planned measures had been implemented. It is likely that the Environment Agency would object to a development only if sufficient measures to prevent deterioration or ecological status were not proposed. The aim the Environment Agency is to have no need to object because capacity has been provided as needed so that development can be accommodated without causing environmental detriment. 2.3.3 Gateshead Council & Newcastle City Council (& North Tyneside, South Tyneside and Northumberland)
All five Local Authorities that drain to Howdon STW need to take unilateral action to ensure that the limited headroom at Howdon STW is holistically addressed through policies to remove and reduce the volume of surface water reduction entering the combined sewers. It is important not to underplay the vital role that each of the Authorities can play in helping to deliver some of the development-led surface water separation opportunities. This will require robust enforcement of the hierarchy of preference for the connection of surface water contained within Part H of the Building Regulations and Planning Policy 25 (PPS25) “Development and Flood Risk”. As PPS25 may be superseded by the new National Planning Policy Framework it is essential that Local Planning Authorities have sufficient policies built into their LDF documents to ensure that developers must fully investigate all sustainable options for surface water management. This is particularly important for brownfield sites which are currently connected to the public sewerage system; developers should not disregard the opportunity to drain to watercourses with the appropriate SuDS in place. The Local Authorities will adopt policies of surface water reduction and separation for new developments. All brownfield development sites occurring in areas served by combined sewer systems present the opportunity to separate the combined flows so that only foul flows enter the combined sewers and surface water is removed from the system. At the very least the Local Authorities will be expecting developers to reduce the volume of surface water entering the combined sewer system. The policies, to be incorporated into Development Plan Documents, will require the developer to remove as much surface water from the combined sewer systems as possible, managing the water on site, disposing of it to a watercourse or only as a last resort utilising public surface water sewers or the combined system. One means by which this could be encouraged is through the creation of “Drainage Groups” in each of the Authority areas. Such a group was set up in Lincoln, where when the local Planning Officers receive proposals for significant development they refer the application to the Drainage Group. The Group subsequently review the drainage proposals and made their recommendations. The Group comprised representatives from the Local Authority Planning and Highway departments, the Environment Agency, Anglian Water and a number of Internal Drainage Boards who meet to discuss the drainage proposals collectively. The Lincoln Drainage Group has subsequently been subsumed into the remit of the Lead Local Flood Authority; Lincolnshire County Council and the membership expanded significantly. Such Drainage Groups could provide additional encouragement to developers to manage surface water in a means other than disposing of it in the combined sewer system simply because it is there and has been used historically. The Highway departments for each of the Local Authorities have a particularly valuable role to play since roads usually only get adopted by the Authority if their surface water runoff drains to a public sewer or watercourse. If they were to review their adoption procedures
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there could be scope to reduce surface water entering combined sewers through brownfield redevelopment. In addition Gateshead Council and Newcastle City Council intend to work with the other Local Authorities, (and Northumbrian Water and the Environment Agency), to monitor the level of development (housing and employment) and where it is occurring. In doing so all parties will be able to regularly review the headroom (available capacity) that currently exists at Howdon STW utilising the Tyne and Wear Flood Management Group. Table 2.2 presents a summary of the actions that each of the Partners are to undertake to address the limited headroom available at Howdon STW so that it does not constrain development and the development does not detrimentally impact the water environment. Table 2.2: Actions for Partners – Headroom at Howdon STW Action Partner Timescale
Continue the commitment to collaborative working to All Partners Ongoing address the situation.
Develop tools & techniques to manage surface water Northumbrian Water 2010 – 15 through the Tyneside Sustainable Sewerage Study
Implement tools and techniques developed to remove Northumbrian Water 2015 – 20 surface water from the combined sewer networks.
Continue to monitor flow data for Howdon STW and review Northumbrian Water & Environment Agency 2010 – 2020 the spare capacity.
Gateshead Council, Newcastle City Council, Monitor the level and location of development across the North Tyneside Council, South Tyneside 2010 – 2020 five Local Authorities served by Howdon STW Council & Northumberland County Council
Develop and implement policies to remove and reduce the Gateshead Council, Newcastle City Council, volume of surface water entering the combined sewer North Tyneside Council, South Tyneside 2010 – 2020 system. Council & Northumberland County Council
Review if the strategy of removing surface water from the All Partners 2017 combine sewers is freeing up capacity at Howdon STW.
Develop actions in light of the findings of the review. All Partners 2017
Inform/Review Infrastructure Delivery Plan. Gateshead Council & Newcastle City Council Ongoing - 2030
Sustainable Drainage Systems
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3 Sustainable Drainage Systems
3.1 Introduction Whilst the Councils, Northumbrian Water and the Environment Agency will collectively take the lead in removing surface water from the combined sewer system, developers also have a role to play in designing their surface water drainage schemes in such a way to avoid disposing of surface water to the combined sewer system, even if the site has historically had a connection to the sewer system. The following chapter provides an introduction to SuDS for both developers and the Councils’ Development Management officers. It endeavours to ensure that development proposals maximise SuDS implementation and minimises surface water entering the combined sewer system.
In light of the situation at Howdon STW; the Councils, Northumbrian Water and the Environment Agency are adopting a joint approach and will require developers attempt to manage surface water rather than dispose of it to the sewer system. Free discharge to the sewer system will not be granted unless it is demonstrated that there is no alternative.
The implementation of SuDS for both existing and proposed developments will be more difficult for some sites than others, particularly smaller sites where it is often assumed that the site is too small for SuDS due to the perception that all systems have a large footprint e.g. ponds. However, it is very unlikely that a site, no matter what size, would not be able to implement some type of SuDS scheme. SuDS are usually feasible for small sites and in the case of Gateshead and Newcastle could play a very important role in removing surface water from the sewer system and contributing to sustainable development in the area. The information set out here is relevant for all types and scales of development as hard surfaces will create surface water runoff no matter how small the development site is, or whether it is a brownfield site within a highly urbanised area such as the Urban Core. This is particularly relevant for sites less than one hectare, in Flood Zone 1 where SuDS are often not considered because a Flood Risk Assessment is not a requirement. The SWMP is not intended to provide detailed technical design guidance but rather provide an overview of what SuDS are available. The feasibility of different SuDS schemes will need to be determined on a site by site basis and is given further consideration in Chapter 5. Detailed information concerning SuDS can be found in a variety of publications including The SuDS Manual, CIRIA 697.
If in doubt, developers should contact the Partners for advice on what SuDS types are possible for their site.
3.2 SuDS Implementation Policy An increasing number of documents are calling for the inclusion of SuDS schemes within developments, which includes, but is not limited to: - The Flood and Water Management Act addresses flood risk by clarifying responsibilities and the encouragement of SuDS by ending the automatic right to connect to sewers 3. This makes it a requirement for developers to include SuDS within new developments. - Planning Policy Statement 1 advises that the planning system should facilitate and promote sustainable patterns of development, avoiding flood risk and accommodating the impacts of climate change. The use of SuDS can be used at a local and regional scale to promote sustainable drainage which can; accommodate an increase in rainfall as a result of climate change, slow the rate at which runoff reaches watercourses, improve the environment through improvements in wildlife habitat, encourage biodiversity and improve the quality of surface water runoff.
3 Automatic Right to Connect – Once a drainage network is built it is normally ‘adopted’ by the relevant water company who takes responsibility for its maintenance and performance. Under the Water Industry Act 1991, developers have a limited right to connect developments to the drainage network.
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- Planning Policy Statement 25 promotes the incorporation of SuDS within developments with the aim of achieving greenfield runoff rates from greenfield development, or, a reduction in the runoff from a brownfield site (both are subject to an increase in rainfall intensity as a result of climate change). - Paragraph 42 of Planning and Climate Change (Supplement to PPS1) confirms that Local Authorities should give priority to the use of SuDS paying attention to the contribution gained from rainwater harvesting from impermeable surfaces and encouraging site layouts which accommodate recycling of water. - The Strategic Flood Risk Assessments for Gateshead and Newcastle recommend that SuDS are integrated into the development management process. - The NewcastleGateshead Green Infrastructure Strategy recognises that green space is an essential component of good planning within the urban and rural environment particularly for use with SuDS in the face of climate change. The Flood and Water Management Act also sets out that the Councils will become the SuDS Approving Body (SAB) to review and approve SuDS drainage designs and check that schemes have been constructed in line with the approved design. The skills required to assess the majority of schemes will be within the capability of the Councils; however some large, complex schemes (or those with specific technical challenges) may require specialist advice from technical experts.
As part of all planning applications, the developer will be expected to demonstrate an awareness of the local site conditions (e.g. contamination, geological, topographical, hydrological and site orientation factors) and how they have been considered in their proposals for site drainage and why the different types of SuDS are, or are not, considered to be appropriate for the site.
3.3 Sustainable Drainage Systems SuDS provide an alternative solution to conventional piped drainage methods whereby the SuDS system will mitigate the adverse effects of urban surface water runoff by mimicking natural processes. This can be achieved through; - a reduction of runoff volumes entering a drainage system or watercourse through infiltration, evaporation, plant uptake and recycling of stored runoff; - a reduction in runoff rates through storage and attenuated flows as surface water is discharged to a drainage system or watercourse; and - a reduction and removal of pollutant concentrations entering the receiving watercourse. SuDS also include the additional benefits of wildlife habitats within urban areas and biodiversity enhancements. 3.4 The SuDS Management Train To mimic natural catchment processes as closely as possible, a “management train” is required. The concept is fundamental to designing a successful SuDS scheme; it uses different SuDS in series to incrementally reduce pollution, flow rates and volumes. The hierarchy of techniques that should be considered in developing the management train are as follows: I. Prevention – the use of good site design and site housekeeping measures to prevent runoff and pollution. II. Source Control – control runoff at or very near to its source. III. Site Control – control runoff within the local area or site boundary. IV. Regional Control – control runoff from a site or several sites. The management train is summarised in Figure 3.1. Wherever possible, surface water should be managed in small, cost- effective landscape features rather than being conveyed to and managed in large systems at the bottom of drainage areas (end
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of pipe solutions). The techniques that are higher in the hierarchy are preferred to those further down so that prevention and control of water at source should always be considered before site or regional controls. However, where upstream control opportunities are restricted, a number of lower hierarchy options should be used in series. Water should be conveyed elsewhere only if it cannot be dealt with on site. Figure 3.1: Guide to the design of a SuDS management train 4 .
- Prevention – preventing an increase above the natural runoff from occurring by using soft landscaping and incorporating appropriate surface types; - Source control – limiting direct discharge rates to the system at the point of origin through utilising elements that include permeable paving, green roofs and water butts within the development; - Conveyance – providing clearly defined drainage paths (swales and trenches) which also provide ‘holding’ points where water will be contained during extreme events, using these routes to create exceedance routes where water will be anticipated to flow during extreme events including those which exceed the design standards for the drainage systems, clearly directing water to the least vulnerable areas; and - Attenuation/Retention – wetlands, ponds or detention basins will provide treatment volume storage and the remaining amount of the attenuation which is not being contained within the upper elements of the system.
4 Source: CIRIA (2010). Planning for SuDs – making it happen. CIRIA C687.
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The aspiration of a proposed strategy is to change the current practice of incorporating the minimum requirements in achieving compliance with regulations and guidance and introduce a more holistic approach to surface water management. By looking at a more comprehensive suite of SuDS tools and considering these as not only surface water management tools, but also as assets with greater value to the place in which they are built, environmental and urban interactions, potential for incorporating water elements throughout the development, thereby creating more diverse urban and natural environments. Table 3.1 sets out the number of SuDS components required in the management train depending on the sensitivity of the receiving water body and the areas which are generating surface water runoff. It indicates that if the only source of runoff is from roofs then only one SuDS component would be required in the management train. However, if runoff was being generated from roads as well as roofs then at least two SuDS components would comprise the management train, unless the receiving watercourse was considered to be highly sensitive in which case at least three SuDS components would be required. Generally the more sources of runoff there are, the more SuDS components will be required in the management train. Table 3.1: Number of treatment train components (assuming effective pre-treatment is in place) 5 Receiving water sensitivity
Runoff catchment Characteristics Low Medium High
Roofs only 1 1 1
Residential roads, parking areas, 2 2 3 commercial zones
Refuse collection/ industrial areas/ loading 3 3 4 bays/ lorry parks/ highways
3.5 Water Quality Capabilities Numerous pollutants including oil, sediments, fertilisers, pesticides, animal waste and litter can be sourced from urban environments which can cause diffuse pollution or get washed into sewers and watercourses through surface water runoff. Some SuDS designs can provide various levels of water quality improvements by reducing and capturing sediments and contaminants from urban runoff, which can then be removed after settlement or allowed to break down through biological actions. The effectiveness of SuDS design abilities to incorporate water quality improvements is reviewed in Table 3.3. 3.6 Environmental Benefits Local and national policies provide significant support for the protection, enhancement and creation of green infrastructure. The use of SuDS can meet the challenges of surface water flooding problems as well as provide environmental benefits by creating habitats which improve biodiversity, improves the amenity of the environment as well as adding an aesthetic quality. The effectiveness of SuDS design abilities to add environmental benefits to an area is reviewed in Table 3.3. 3.7 Other Benefits The use of SuDS can provide the additional benefits of: - Water Resources – some SuDS designs can recharge underground aquifers whilst some can allow rainwater to be harvested to be used for non potable domestic use and for garden irrigation. - Community and Recreation – SuDS can improve the quality of life within a community by providing an attractive environment and provide areas for recreation.
5 Source: The SuDS Manual. Ciria C697.
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- Education – SuDS can provide an attractive educational environment but can also be used to improve the understanding of water and the natural environment. - Benefits for the Developers – SuDS can provide the opportunity to reduce construction and maintenance costs as well as increase property values whereby attractive, open green space provides a greater amenity value to an area. 3.8 Different Types of SuDS There are a wide variety of different types of SuDS that can be applied to different sites in different settings. Combinations of the different types should be used to make up the management train for each site. For brevity the most common SuDS are introduced in Table 3.2 by way of an introduction to what is available. More information on each of these can be found in Appendix A and the SuDS Manual, CIRIA C697. Table 3.2: Common SuDS SuDS Type Photo
Source Control
Green Roofs Green roofs comprise a multilayered system that covers the roof (or walls) of a building with vegetation over a drainage layer. Green roofs are suitable for most developments.
Rainwater Harvesting Rainwater harvesting is the collection and storage of rainwater from roofs and other hard surfaces. Rainwater harvesting systems can be used for residential, commercial and industrial developments.
Water Butts Water butts are a common means of harvesting rainwater for garden use via an inlet connected to roof downpipes. Water butts are best suited to low and medium residential development where the catchment area is limited to the property and ancillary building roof area.
Permeable Pavements Permeable pavements allow rainwater to infiltrate through the surface and into under-layers where it is temporarily stored before infiltrating into the ground, or released to a watercourse or drainage system. Permeable pavements can be used for a wide variety of developments.
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SuDS Type Photo
Soakaways Soakaways store rapid runoff from a single development and allow it to infiltrate into the surrounding soil. Soakaways are not suitable where there is a risk of contamination, where there are unstable ground conditions and where there are poor draining soils. Field investigations are required to determine infiltration rates. Site Control
Filter Strips Filter strips are vegetated strips of land designed to accept runoff and allow it to infiltrate or be filtered by vegetation before being received by a stream or surface water collection system. Filter strips are considered to have a large land requirement, and are not suitable for significant attenuation or if there is risk of ground contamination.
Trenches Trenches are shallow excavations filled with rubble, stone or other void media that create temporary subsurface storage for runoff. There are two types of trenches; filtration trenches are used where soils are impermeable or where the groundwater is vulnerable to pollution and infiltration trenches filter runoff through the stone media and infiltrate it into permeable soils.
Swales Swales are linear vegetated drainage systems where surface water can be stored to allow infiltration, and/or conveyed to other SuDS components, a stream or river. This type of SuDS design can be used in a wide variety of situations where catchments have small impermeable areas.
Bioretention Bioretention areas, filters or rain gardens are shallow landscaped depressions designed to capture, filter and treat surface water. Bioretention areas are suitable for various development types including residential plots, car parks, along highways and roads, commercial, and industrial sites and can be retrofitted into existing developments and used where the groundwater is vulnerable. Geocellular / Modular Systems Geocellular systems are high void structures which are below ground and used to infiltrate or store runoff before it is discharged to a downstream drainage system. They can be used for a variety of development types including residential, commercial and industrial developments. This type of system can be used where there are contaminated sites.
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SuDS Type Photo
Regional Control
Infiltrations Basins Infiltration basins are vegetated depressions that store runoff for infiltration into the subsurface soil. The suitability of a site must be confirmed by geotechnical investigations.
Detention Basins Detention basins are dry basins which temporarily store runoff by use of a controlled release which attenuates flow. Detention basins are suitable for use on at a variety of development types including residential, commercial, industrial, contaminated sites and where there is high density infrastructure.
Ponds Ponds are basins which have a permanent pool of water. Ponds can generally be used for most types of developments and redevelopments for both residential and non residential areas.
Wetlands Wetlands are constructed shallow marsh systems covered almost entirely by aquatic vegetation. Wetlands are suitable for residential, commercial and industrial developments.
3.8.1 Drainage Solutions for Existing Developments Retro-fitting SuDS designs into existing developments is largely limited by the available space as well as the sites geometry, underlying geology, and urban catchment size. Swales, infiltration basins, wet ponds, detention basins, wetlands and bioretention areas have large footprints and can be incorporated into development which can set aside space or where a site boundary is located next to or partially within areas of green space. Pollution controls can often be incorporated into these types of SuDS design in order to improve the water quality of urban runoff and protect watercourses. The type of water quality improvements from residential and urban development includes sediments, oil and hydrocarbons from roads and highways. Due to the availability and locality of large green infrastructure in existing developed areas, surface water can be management at source, on a property by property basis. SuDS designs in development areas not local to green infrastructure may include the use of trenches, green roofs, water butts, soakaway, rainwater harvesting and permeable paving. The advantage of these types of SuDS techniques is that they can be retrofitted into existing development and help to reduce the existing runoff from many small catchments where additional space for water is limited by dense development. These types of SuDS can help to reduce
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the amount of runoff entering the existing drainage system but can also be used alongside the larger SuDS designs to reduce the volume of runoff from the catchment areas. Refer to CIRIA report 2441 for further information on retro-fitting surface water management infrastructure. 3.8.2 Drainage Solutions for New Developments Development of greenfield sites must be safe from flooding over the lifetime of the development (with the additional impacts of climate change) but also reduce and mitigate flood risk from the development. This can be achieved through the integration of SuDS which can be used to keep surface water from a new development at or below the existing greenfield runoff rate. Additionally, SuDS designs can provide the additional benefit of water quality treatment of urban runoff. Greenfield sites are generally less restricted to the type of SuDS, than brownfield sites, since they can be designed into a development during conception and make arrangements for their maintenance. This means that SuDS designs can be tailored specifically to a development site and include a train of SuDS to meet the require attenuation/storage requirements, pollution control and water quality treatment. Brownfield sites are more constrained by the type of SuDS that will be appropriate, for example infiltration techniques are often not appropriate since they can mobilise contaminants that could be present as a result of the former land use. 3.8.3 Designing for Exceedance Exceedance of drainage systems (including SuDS) during extreme events is inevitable; how the designer chooses to manage it will determine whether there is a risk associated with this flooding. If exceedance is not designed into urban developments, pathways will be formed during extreme events, which may flow to receptors which are vulnerable and create unacceptable risks. To prevent this, exceedance pathways must be incorporated into designs. Figure 3.2 highlights how sewers and exceedance routes interact during extreme events. Figure 3.2: Exceedance Flow Routes
3.9 SuDS Summary A summary of the commonly used SuDS designs and their capabilities is included in Table 3.3. The SuDS capabilities are split into three categories and include; management train suitability, water quantity, water quality, and environmental benefits. This table will act as a quick reference guide for planners and developers when designing the SuDS treatment train.
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Capabilities on project: Water Table 3.3: Capability of different SuDS techniques
Environmental Management Train Suitability Water Quantity Water Quality Benefits
Technique Description
ration -treatment Prevention Conveyance Pre Source control Control Site RegionalControl Conveyance Detention Infilt WaterHarvesting Sedimentation Filtration Adsorption Biodegradation Volatilisation Precipitation Uptake byplants Nitrification Aesthetics Amenity Ecology
Green roofs Vegetated roofs that reduce runoff volume and rate. ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■
Water butts, site layout and Good housekeeping and design practices. ■ ▲ ■ ▲ ▲ ■ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ management
Pervious Surfaces Allow infiltration of rainwater into underlying construction/soil. ■ ■ ▲ ■ ■ ▲ ■ ■ ■ ■ ■ ▲ ▲ ▲
Soakaways Sub-surface structures that store and dispose of water via infiltration. ■ ■ ■ ■ ■
Filter Strips Vegetated strips of gently sloping ground designed to drain water from impermeable areas and filter out silt and other particulates. ■ ■ ▲ ▲ ▲ ■ ■ ■ ■ ▲ ▲ ▲
Filter drains and Perforated Pipes Linear drains/ trenches filled with a permeable, often with a perforated pipe at the base of the trench. ■ ■ ▲ ■ ■ ■ ■ ■ ■
Infiltration Trenches As filter drains, but allowing infiltration through trench base and sides. ▲ ■ ■ ▲ ■ ■ ■ ■ ■ ■
Bioretention areas Vegetated areas for collecting and treating water before discharge downstream, or to the ground via infiltration. ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■
Shallow vegetated channels that conduct and/or retain water (and can permit infiltration when underlined). The vegetation filters Swales ■ ■ ■ ■ ■ ▲ ■ ■ ■ ■ ▲ ▲ ▲ ▲ particulates.
Ponds Depressions used for storing and treating water. They have a permanent pool and bankside emergent and aquatic vegetation. ■ ■ ■ ▲ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■
Infiltration basins Depressions that store and dispose of water via infiltration. ■ ■ ■ ■ ■ ■ ■ ■ ▲ ▲ ▲
Detention Basin Dry depressions designed to store water for a specified retention time. ■ ■ ■ ■ ▲ ▲ ■ ■ ▲ ▲ ▲
As ponds, but the runoff flows slowly but continuously through aquatic vegetation that attenuates and filters the flow. Shallower Wetlands ▲ ■ ■ ▲ ■ ▲ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ than ponds.
Key to Symbols ■ High/primary process ▲Some opportunities, subject to design
NewcastleGateshead SuDS Requirements
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4 NewcastleGateshead SuDS Requirements
4.1 Introduction There are many different SuDS components that can be used on a site and each individual site will have unique characteristics that will influence the applicability of different types of SuDS. As such the characteristics of a development site should be used to guide the selection of the most appropriate set of SuDS techniques. Not all SuDS techniques will be suitable for all sites and therefore it is important that the opportunities and constraints are identified at an early stage in the design process. Given the wide variety of development sites ranging from a single house to hundreds of houses or commercial development it is not possible to specify guidance that will be applicable in every instance. Rather, the following chapter has tried to set out a procedure that can be used as a template and adjusted to fit the specific characteristics of the development in question. Using the following as a standard procedure ought to provide Development Control officers with confidence that a developer has selected the most appropriate SuDS for their site. In many instances there will be a number of different SuDS that will be appropriate in which case it may come down to preference or financial cost. 4.2 Baseline Data To aid Development Management Officers review planning applications it is recommended that developers provide the information contained within Table 4.1 as part of their application. The information would be essential to enable the developer to select appropriate SuDS for their development and will allow the Councils to review the data to determine if they agree with the SuDS selected. The information could be incorporated into the drainage strategy for the site thereby covering all sites irrespective of whether or not a Flood Risk Assessment is required.
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Table 4.1: Required Information – to be provided by developers
Site Characteristics Source
Site area (ha) Site survey/inspection
Topography Site survey
Soil Type Site investigation/FSR maps
Former land use Local Authority/OS maps/library
Infiltration potential of soil & seasonal groundwater Site investigation levels
Hydraulic
Flood risk Site inspection/observations/maps/EA
Rainfall data Meteorogical office/FSR maps
Design discharge criteria – Quantity EA/water engineering company
Design discharge criteria - Quality EA/water engineering company
Acceptable overland flow/surface flooding Owner/operator/EA
Current discharge points and rates for brownfield Northumbrian Water sites
Environmental
Contamination of ground below site Local authority/OS maps/library/site investigation
Details of receiving water/watercourse/aquifer EA/water engineering company
Groundwater vulnerability and source protection EA status
Further information
Proposed Land use
Development type/land use Proposed development plans
Potential areas for SuDs Proposed development plans
Further information
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4.3 SuDS Selection The flow chart in Figure 4.1 illustrates a process by which a developer or the Councils can select the most appropriate SuDS for their site. Figure 4.1 and the tables that follow are based upon information taken from the CIRIA C697 SuDS Manual. Figure 4.1: Selection Process
Step 1 Land Use Selection
Site Characteristics Step 2 Selection
Quality and Quantity Step 3 performance Selection
Environmental and community Step 4 performance Selection
Summary table and scoring Step 5
4.3.1 Step 1: Land use It is important to determine which SuDS techniques are best suited to the proposed land use of the area draining to the system. Table 4.2 can be used to assess whether the proposed SuDS element is suitable for the proposed land use.
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Table 4.2: Land Use Selection Matrix
SUDS Group Technique Low density density Low Residential roads Local Commercial Hospitals site Construction Brownfield land Contaminated
Retention pond Y Y Y1 Y2 Y2 Y3 Y Y2 Retention Subsurface storage Y Y Y Y Y Y3 Y Y Shallow wetland Y Y Y1 Y2 Y2 N Y Y2 Extended detention wetland Y Y Y1 Y2 Y2 N Y Y2 Pond/wetland Y Y Y1 Y2 Y2 N Y Y2 Wetland Pocket wetland Y Y Y1 Y2 Y2 N Y Y2 Submerged gravel wetland Y Y Y1 Y2 Y2 N Y Y2 Wetland channel Y Y Y1 Y2 Y2 N Y Y2 Infiltration trench Y Y Y1 Y2 N N Y Y4 Infiltration Infiltration basin Y Y Y1 Y2 N N Y Y4 Soakaway Y Y Y1 Y2 N N Y Y4 Surface sand filter N Y Y1 Y2 Y2 N Y Y2 Sub-surface sand filter N Y Y1 Y2 Y2 N Y Y2 Filtration Perimeter sand filter N N Y1 Y2 Y2 N Y Y2 Bioretention/filter strip Y Y Y1 Y2 Y2 N Y Y2 Filter trench Y Y Y1 Y2 Y2 N Y Y2 Detention Detention basin Y Y Y1 Y2 Y1/2 Y3 Y Y2 Conveyance swale Y Y Y1 Y2 Y2 Y3 Y Y2 Open channels Enhanced dry swale Y Y Y1 Y2 Y2 Y3 Y Y2 Enhanced wet swale Y Y Y1 Y2 Y1 Y3 Y Y2 Green roof Y Y N Y2 Y N Y Y Source control Rain water harvesting Y Y N Y2 N N Y Y Previous pavements Y Y N Y2 Y1 N Y Y2 Y: Yes N: No 1 may require two treatment train stages, depending on type and intensity of road use and receiving water sensitivity 2 may require three treatment train stages, depending on receiving watercourse sensitivity 3 will require draw-down and rehabilitation following construction activities, prior to use as a permanent drainage system 4 providing designs prevent mobilisation of contamination
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4.3.2 Step 2: Site Characteristics It is also important to determine whether there are any site characteristics that may restrict or preclude the use of a particular SuDS technique. Table 4.3 highlights the key site characteristics that influence the choice of SuDS elements and states whether the SuDS elements can be used on the selected site. Table 4.3: Site characteristics selection matrix
SUDS Group Technique Soils draining Area single to a SUDS component Minimum water to depth table slope Site head Available Available space Impermeable Impermeable Permeable 0–2 ha >2 ha m 0–1 >1 m % 0-5 >5 % m 0-1 1-2m Low High
Retention pond Y Y1 Y Y5 Y Y Y Y Y Y N Y Retention Subsurface storage Y Y Y Y5 Y Y Y Y Y Y Y Y Shallow wetland Y2 Y4 Y4 Y6 Y2 Y2 Y N Y Y N Y Extended detention wetland Y2 Y4 Y4 Y6 Y2 Y2 Y N Y Y N Y Pond/wetland Y2 Y4 Y4 Y6 Y2 Y2 Y N Y Y N Y Wetland Pocket wetland Y2 Y4 Y4 N Y2 Y2 Y N Y Y Y Y Submerged gravel wetland Y2 Y4 Y4 Y6 Y2 Y2 Y N Y Y N Y Wetland channel Y2 Y4 Y4 Y6 Y2 Y2 Y N Y Y N Y Infiltration trench N Y Y N N Y Y Y Y N Y Y Infiltration Infiltration basin N Y Y Y5 N Y Y Y Y N N Y Soakaway N Y Y N N Y Y Y Y N Y Y Surface sand filter Y Y Y Y5 N Y Y N N Y N Y Sub-surface sand filter Y Y Y N N Y Y N N Y Y Y Filtration Perimeter sand filter Y Y Y N N Y Y N Y Y Y Y Bioretention/filter strip Y Y Y N N Y Y N Y Y N Y Filter trench Y Y1 Y N N Y Y N Y Y Y Y Detention Detention basin Y Y1 Y Y5 N Y Y Y N Y N Y Conveyance swale Y Y Y N N Y Y N3 Y N N Y Open channels Enhanced dry swale Y Y Y N N Y Y N3 Y N N Y Enhanced wet swale Y2 Y4 Y N Y Y Y N3 Y N N Y Green roof Y Y Y N Y Y Y Y Y Y Y Y
Source control Rain water harvesting Y Y Y N Y Y Y Y Y
Previous pavements Y Y Y Y N Y Y N Y Y Y Y Y: Yes N: No 1 with liner 2 with surface baseflow 3 unless follows contours 4 with liner and constant surface baseflow, or high ground water table 5 possible, but not recommended (implies appropriate management train not in place) 6 where high flows are diverted around SUDS component
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4.3.3 Step 3: Quantity and quality performance selection The third step in the process is to determine which SuDS techniques are suited to meeting the hydraulic and water quality design criteria that are required for the site. The water quality treatment volume may be managed within a single SuDS technique or within a series of techniques, forming part of the treatment train. Each technique has different removal efficiencies for each pollutant of concern. Hydraulic criteria will require peak flow and volume control for a range of return periods (or probabilities). Flow rates are likely to be a function of the extent of structural hydraulic control at the system outfall, together with the size of attenuation storage provided in the system design. Significant volume control will be possible only through the use of infiltration systems. Table 4.4 (and 4.5) provides a weighted guidance on SuDS selection for the quantity and quality performance to assist in identifying SuDS that are appropriate for each site. CIRIA C627 – The SuDS Manual provides additional guidance for detailed SuDS selection. A variety of techniques can be applicable and designers should be encouraged to select those most suitable to be integrated into development proposals, using the most desirable techniques as a short list. There are a range of additional tools that can assist designers in selecting SuDS elements, to provide appropriate justification for the design proposals. SuDS should be include in all development sites, developers not including SUDS in their schemes will be required to provide a strong justification for their exclusion.
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Table 4.4: Quantity and quality performance selection matrix Water quality treatment potential Hydraulic control
SUDS Technique Group Suitability for flow rate control control rate flow for Suitability (probability) Total suspended solids removal removal solids suspended Total removal metals Heavy removal nitrogen) (phosphorus, Nutrient (*) removal Bacteria suspended fine treat to Capacity pollutants dissolved and sediments Score Quality Average reduction volume Runoff yr) (1/2 0.5 0.1-0.3 (10/30 yr) yr) (100 0.01 Score Quantity Average
Retention pond 3 2 2 2 3 2.4 1 3 3 3 2.5 Retention Subsurface storage 1 1 1 1 1 1 1 3 3 3 2.5 Shallow wetland 3 2 3 2 3 2.6 1 3 2 1 1.75 Extended detention 3 2 3 2 3 2.6 1 3 2 1 1.75 wetland Pond/wetland 3 2 3 2 3 2.6 1 3 2 1 1.75 Wetland Pocket wetland 3 2 3 2 3 2.6 1 3 2 1 1.75 Submerged gravel 3 2 3 2 3 2.6 1 3 2 1 1.75 wetland Wetland channel 3 2 3 2 3 2.6 1 3 2 1 1.75 Infiltration trench 3 3 3 2 3 2.8 3 3 3 1 2.5 Infiltration Infiltration basin 3 3 3 2 3 2.8 3 3 3 3 3 Soakaway 3 3 3 2 3 2.8 3 3 3 1 2.5 Surface sand filter 3 3 3 2 3 2.8 1 3 2 1 1.75 Subsurface sand filer 3 3 3 2 3 2.8 1 3 2 1 1.75 Filtration Perimeter sand filer 3 3 3 2 3 2.8 1 3 2 1 1.75 Bioretention/filter strips 3 3 3 2 3 2.8 1 3 2 1 1.75 Filter trench 3 3 3 2 3 2.8 1 3 3 1 2 Detention Detention basin 2 2 1 1 1 1.4 1 3 3 3 2.5 Conveyance swale 3 2 2 2 3 2.4 2 3 3 3 2.75 Open Enhanced dry swale 3 3 3 2 3 2.8 2 3 3 3 2.75 channels Enhanced wet swale 3 3 2 3 3 2.8 1 3 3 3 2.5 n/ Green roof n/a n/a n/a 3 0.6 3 3 3 1 2.5 a Source control Rain water harvesting 2 1 1 1 n/a 1 2 2 3 1 2 Permeable pavement 3 3 3 3 3 3 3 3 3 1 2.5 *limited data available n/a: not applicable 3 = high potential 2 = medium potential 1 = low potential
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4.3.4 Step 4: Environmental and Community factors selection
It is important to determine whether the proposed SuDS components meet all the community and environmental requirements at the site. Adaptations to the proposed solutions that may enhance the benefits of the system should also be considered. Table 4.5 below provides average scores for the Environmental and Community factors of the individual SuDS elements. Although it should be noted that Table 4.5 considers each of the factors equally. There will be many instances when certain factors will require preference over the others for a variety of reasons.
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Table 4.5: Community and environmental factors selection matrix
SUDS Group Technique Maintenance Maintenance Community acceptability Cost creation Habitat potential Score Average
Retention pond 2 3 2 3 2.5 Retention Subsurface storage 1 3 2 1 1.75
Shallow wetland 3 3 3 3 3
Extended detention wetland 3 3 3 3 3
Pond/wetland 3 3 3 3 3 Wetland Pocket wetland 3 2 3 3 2.75
Submerged gravel wetland 2 1 3 2 2
Wetland channel 3 3 3 3 3
Infiltration trench 1 2 1 1 1.25
Infiltration Infiltration basin 2 3 1 2 2
Soakaway 1 2 2 1 1.5
Surface sand filter 2 1 3 2 2
Sub-surface sand filter 2 1 3 1 1.75
Filtration Perimeter sand filter 2 1 3 1 1.75
Bioretention/filter strip 3 3 2 3 2.75
Filter trench 2 2 2 1 1.75
Detention Detention basin 1 3 1 2 1.75
Conveyance swale 1 2 1 2 1.5
Open channels Enhanced dry swale 1 2 2 2 1.75
Enhanced wet swale 2 2 2 3 2.25
Green roof 3 3 3 3 3
Rain water harvesting 3 2 3 1 2.25 Source control
Previous pavements 2 2 2 1 1.75
3 = high potential 2 = medium potential 1 = low potential *there may be some public safety concerns associated with open water that require addressing at design stage
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4.3.5 Step 5: Summary and Scoring Table 4.6 can be filled out following the above steps and the highest scores highlight the most suitable SuDS elements for the selected site. Two case studies are set out below to illustrate how the stepped process could be applied. The case studies set out basic information that can provide enough detail to support the initial selection of appropriate SuDS elements. 4.3.5.1 Case Study 1 - Site area - 0.8ha - Topography – gently sloping site - Ground conditions – permeable soils within groundwater outer zone - Current land use – greenfield site - Proposed development – commercial - Density/available space – high density - Receiving watercourse sensitivity – high - Groundwater – 3m depth The first step is to ascertain the number of treatment levels required from Table 3.1. The site is less than 1ha so does not fall under PPS25 but none the less SuDS should be considered to manage the increasing pressure on the surrounding sewer system and to protect groundwater. The site has permeable soil and is located within an outer groundwater protection zone which indicates that infiltration can be considered given the correct levels of treatment before infiltration. The commercial development is high density so finding space for SuDS elements could be challenging. Table 4.6 presents the results of the exercise, with two treatment levels being required and the preferred SuDS of permeable paving and infiltration/filtration trenches having been highlighted in green. 4.3.5.2 Case Study 2 - Site area - 20ha - Topography – steeply sloping >5% gradient - Ground conditions – permeable soils. - Current land use – brownfield site - Proposed development – residential - Density/available space – low density - Receiving watercourse sensitivity – low - Groundwater – 6m depth The first step is to ascertain the number of treatment levels required from Table 3.1. Case study 2 is a proposed residential development, on a brownfield site. The soil on site is permeable but with the potential for the brownfield site to contain contaminants a full site investigation should be carried out to determine if infiltration is possible. The residential site is proposed to be low density therefore space will not be an issue when designing the SuDS systems. Where possible water should be kept and conveyed on the surface providing dual benefits as community space and surface water drainage. Table 4.6 presents the results of the exercise, with three treatment levels being required and the preferred SuDS of permeable paving, wet swale and a retention pond having been highlighted in green.
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Capabilities on project: Water Table 4.6: Summary Table for SuDS Selection Included Land use Site Quality Quantity Community / SuDs in Level 1 Selection 2 Characteristics 3 Scoring4 Scoring 5 Environmental Total Score Justification Element Design? (Y/N) (Y/N) (1-3) (1-3) Scoring 6 (1-3) (Y/N) CASE STUDY 1 – 2 Levels of treatment required (see Table 3.1)
Infiltration Site/ Y Y 2.8 2.5 1.25 6.55 Y trench source Y (with the Permeable exception Source Y 3 3.5 1.75 7.25 Y paving of local roads) Site/ Dry Swale Y N N Space constrictions Source Subsurface Site/ Y Y 1 2.5 1.75 5.25 N Storage Regional Filtration Site/ Y Y 2.8 2 1.75 6.55 Y trench Source CASE STUDY 2 – 3 levels of treatment required (see Table 3.1) Y (with the Y in flatter areas Permeable exception Source but N for areas 3 2.5 1.75 7.25 Y paving of local where slope >5% roads) Site/ Wet swale Y Y 2.8 2.5 2.25 7.55 Y Source Contamination means Infiltration Site/ Y N () N infiltration is not trench source advisable Site/ Wetland Y Y 2.6 1.75 2.75 7.1 N Regional Retention Site/ Y Y 2.4 2.5 2.5 7.4 Y pond Regional Detention Site/ Y Y 1.4 2.5 1.75 5.65 N Basin Regional Filtration Site/ Y Y 2.8 2 1.75 6.55 N trench Source 1. Source, site, regional 2. Taken from Table 4.2 3. Taken from Table 4.3 4. Average score taken from Table 4.4 5. Average score taken from Table 4.4 6. Average score taken from Table 4.5
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4.4 Soils across Gateshead and Newcastle There is a perception that the soils underlying Gateshead and Newcastle exhibit poor drainage properties and are therefore inappropriate for SuDS, particularly soakaways. Whilst the majority of soils found across Gateshead and Newcastle do exhibit poor drainage, it would be incorrect to say that this is the case for all soils; some of the soils across Gateshead and Newcastle have very good drainage. (Irrespective of the drainage properties of the soils it will always be possible to implement one form of SuDS). Table 4.7 presents an overview of soils found across Gateshead and Newcastle and includes an indication of their drainage properties. This table has been included to demonstrate that it should not be assumed that soakaways cannot be implemented in Gateshead and Newcastle due to poorly drained soils. Table 4.7 is intended to provide an indication of the soil types and their broad locations however it is essential that site specific investigations are undertaken to determine the drainage properties of the soils underlying a particular site since their distribution is highly variable and generalisations not appropriate. Table 4.7: Soils found across Gateshead and Newcastle Location Description Drainage Texture Habitats Gateshead & Slowly permeable seasonally wet Impeded Seasonally wet pastures and Newcastle – slightly acid but base-rich loamy Loamy drainage woodlands Widespread and clayey soils Gateshead & Slowly permeable seasonally wet Impeded Seasonally wet pastures and Newcastle – Loamy acid loamy and clayey soils drainage woodlands Widespread Gateshead & Newcastle – Freely Grassland; wet carr woodlands in Freely draining floodplain soils Loamy along draining old river meanders watercourses South & West Gateshead & Restored soils mostly from quarry Variable Loamy Variable Central & North and opencast spoil Newcastle Acid dry pastures; acid deciduous NW & SW Freely draining slightly acid sandy Freely Sandy and coniferous woodland; potential Gateshead soils draining for lowland heath Neutral and acid pastures and East & West Freely draining slightly acid loamy Freely deciduous woodlands; acid Loamy Gateshead soils draining communities such as bracken and gorse in the uplands North Newcastle Fen peat soils Naturally wet Peaty Wet fen and carr woodlands
Multifunctional Green Space
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5 Multifunctional Green Space
5.1 Introduction The purpose of this chapter is to provide information on how existing or new, multifunctional green space such as parks and recreational facilities can be used by developers, and the Councils to manage surface water runoff. Multifunctional green space has the potential to avoid disposing of surface water to the combined sewer system and manage the risk of extreme surface water flooding to the surrounding area, thereby contributing to sustainable development across Gateshead and Newcastle. 5.2 Multifunctional Green Space Multifunctional green space, which often forms part of an area’s green infrastructure, supports natural and ecological processes and is integral to the health and quality of life of sustainable communities. Multifunctional green space can support environmental functions such as biodiversity, climate change adaptation and flood mitigation, as well as providing quality of life benefits for the local community (including recreation, sustainable transport, wellbeing and sustainable quality of place). Of particular relevance to the NewcastleGateshead SWMP is the role that multifunctional green space can play towards the management of surface water. The NewcastleGateshead Green Infrastructure Strategy was developed to ensure that new development incorporates a sufficient standard of sustainability to mitigate the expected pressures on land use in Gateshead and Newcastle through the use of green infrastructure. The study identified a network of multifunctional green space across the two boroughs as shown in Figure 5.1. Figure 5.1: Multifunctional Green Space across Gateshead and Newcastle 6.
The majority of green infrastructure in Newcastle is located to the north and west in the form of open countryside. There is also a notable presence of green infrastructure concentrated along the Denes (Walbottle Dene, Denton Dene and in particular Jesmond Dene), along the lower Ouseburn valley and alongside the banks of the Tyne. Located within the urban form close to the City Centre are the Town Moor and Nuns’ Moor which together provide a large expanse of green infrastructure within the heart of the city. Further green infrastructure is generally well dispersed throughout the urban fabric through the presence of various types of open space (e.g. Leazes Park - a formal public park) which are interconnected via the public rights of way network. In Gateshead, large expanses of green infrastructure are located to the south and west, and similarly to Newcastle, these areas
6 Sourced: Entec (March 2011) Green Infrastructure Strategy – Final Report.
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are in the form of open countryside. Key green infrastructure designation sites within this area include Chopwell Wood (Forestry Commission woodland), Gibside (a National Trust property) and the Derwent Walk Country Park. For west Gateshead the provision of green infrastructure is strongly related to the topography and hydrology of the area. Large swathes of green infrastructure are associated with the river valleys (e.g. Blaydon Burn and the River Derwent) that separate the built-up areas of Ryton, Blaydon, and Whickham before joining the River Tyne. As in Newcastle the River Tyne has a corridor of green infrastructure alongside much of its bank. Within the urban area, additional green infrastructure is provided through various formal types of open space (e.g. Windy Nook Nature Park, Saltwell Park), many of which are linked through the public rights of way system. Given the prevalence of green space across the two authorities it is something that ought to be exploited to its full potential by developers and the Councils to both manage the risk of surface water flooding to a development site and the wider area, and avoid disposing of surface water to the combined sewer system. The creation of additional green space as part of a development will contribute to and support the green infrastructure networks of Gateshead and Newcastle. 5.3 Practical Examples of Utilising Green Space from around the UK 5.3.1 Manor Park, Sheffield Sheffield Wildlife Trust commissioned a scoping report into the integration of SuDS into urban regeneration schemes within south-east Sheffield. The study recognised the opportunity for a new park to manage urban runoff from a proposed 300 dwelling housing development through a partnership between Sheffield City Council and Green Estate (Manor and Castle Development Trust and Sheffield Wildlife Trust). The criteria for the design were to meet the storage requirements up to a 1% Annual Exceedance Probability (AEP) event (1 in 100 year return period) and improve water quality. This was achieved through a management train including a series of basins, low and high flow channels (including volume controls, overflow channels, basins for silt collection and pollution interception). Figure 5.2 illustrates Manor Parks during flood conditions in 2007. Figure 5.2: Manor Parks
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5.3.2 Elvetham Heath, Hampshire A mixed use development including 1,868 residential properties, a school, village centre, retail outlet, park and ride and sports pitches utilised public open space as part of its SuDS management train. The design criteria for the development was to limit discharge to the 2% AEP (1 in 50 year return period) greenfield rate of 7l/s/ha without flooding up to the 3.33% AEP (1 in 30 year return period), within the site boundary. The SuDS management train was designed to encourage infiltration of runoff and includes swales, retention ponds, and detention basins. The final retention pond is surrounded by a park area to form the village pond and is used for amenity purposes. It is estimated that property values have increased by 10% for those areas of the site in the vicinity of the SuDS components. Figure 5.3 shows the Elvetham Heath Village Pond within the multifunctional green space. Figure 5.3: Elvetham Heath Village Pond
Web Link: http://www.ciria.org.uk/suds/cs_elvetham%20health.htm 5.3.3 Bristol Business Park, Frenchay The Bristol Business Park is an 11.6 hectare site located in the Frenchay area of Bristol. The objective of the SuDS scheme was to minimise runoff and the risk of flooding up to and including the 1% AEP event. The SuDS management train included permeable paving, swales and a detention pond. The use of SuDS designs such as permeable paving and swales helped clean the water as well as slowing it down, which then allowed for a smaller pond which facilitated the development make best use of the site by maximising the amount of land that could be developed. The creation of green infrastructure through the use of swales and a detention pond created a valuable feature in the park for people and wildlife (Figure 5.4).
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Figure 5.4: Detention Pond and Swale within Bristol Business Park
5.3.4 Hopwood Services, M42 Junction 2 The motorway service area at Hopwood covers an area of 34 hectares, of which 25 hectares is a designated wildlife reserve. The design of the drainage system was to a 4% AEP event (1 in 25 year return period), attenuating to the greenfield runoff rate. The SuDS management train within the developed 9 hectares has been designed to improve the flow and quality of runoff, before being released to a watercourse. This is a good example of water treatment where pollution from an urban area is a high concern. The SuDS management train includes filter trenches, filter strips, wetlands, ponds, and other features such as bypass swales and silt/oil interceptors. The use of SuDS has helped to protect the existing nature reserve from pollution as well as provide additional green space connecting the urban environment to the existing green space as well as additional wildlife habitat (Figure 5.5). Figure 5.5: Pond and Wetland at Hopwood Services
Web Link: http://www.ciria.org.uk/suds/cs_bristol_business_park.htm 5.4 Potential Opportunities in Gateshead and Newcastle The NewcastleGateshead SWMP reviewed 18 hotspots with regard to the risk of surface water flooding. In doing so it considered a number of mitigation measures by which the risk could be managed. In some of the hotspots potential development sites (as identified in the Strategic Housing Land Availability Assessment (SHLAA) or Strategic Land Review (SLR)) are situated next to areas of open green space and present opportunities to mange surface water holistically for the development site and the wider area.
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Examples of where developers could utilise multifunctional green spaces as part of the surface water drainage strategy for development sites are presented below. 5.4.1 Gateshead The Shibdon Meadow Local Wildlife Site interacts with hotspot GH15, Blaydon, and provides an opportunity for multi-functional green infrastructure, combining flood alleviation with biodiversity enhancement. Increasing water storage here, undertaken sympathetically, would also extend the wetland habitat for wading birds. Limited, sensitively designed public access could also be introduced to enable the wildlife to be observed without causing disturbance. Similar benefits could be realised in the nearby MetroGreen area, hotspot GH21 Derwent Haugh. Surface water flood alleviation on the lower Derwent within and around the Tidal River Derwent Local Wildlife Site, and on the west bank of the river, should again be combined with biodiversity enhancement in the form of wetland habitat creation, and managed public access. Within the urban core, the enhancement of the green infrastructure network provides multi-functional opportunities, including the provision of living roofs and walls, as well as swales and water storage areas. On the upper River Team (GH27 and 28), multi-functional environmental improvements again combine flood alleviation with habitat creation and enhancement, extending the approach applied on the existing Lamesley Pastures Project. An SLR site is situated on the Whickham Hill Plantation (an area of strategic green infrastructure) to the north west of GH26 an identified surface water flooding hotspot near Lobley Hill. Due to the nature of the local topography it is possible that surface water from Whickham Hill contributes to the hotspot during extreme events. To ensure that any impact is managed it is recommended that the development of the SLR site implements source control measures to manage surface water runoff on site using the area of green infrastructure. 5.4.2 Newcastle – Hotspot NH24: Walker The green open space available within Walker Park (Figure 5.6), within hotspot NH24, provides the opportunity for attenuation storage if the SHLAA site bordering the park were to be developed. In addition to the SHLAA site bordering Walker Park, there are a number of other SHLAA sites in close proximity. A surface water management strategy could be holistically developed to cover the whole area so that all of the sites utilise Walker Park for storage of excess surface water.
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Figure 5.6: Walker Park
SHLAA Walker Sites Park
Appendix A – SuDS Synopsis
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Green Roofs
Description Green roofs comprise a multi-layered system that covers the roof of a building or podium structure with vegetation cover/landscaping, over a drainage layer. They are designed to intercept and retain precipitation, reducing the volume of runoff and attenuating peak flows. Key Design Criteria
- design for interception storage - minimum roof pitch of 1 in 80, maximum 1 in 3 (unless specific design features are included) - structural roof strength must provide for the full additional load of saturated green roof elements - hydraulic design should follow guidance BSEN 12056-3 (BSI, 2000) - multiple outlets to reduce risks from blockages - lightweight soil medium and appropriate vegetation
ADVANTAGES PERFORMANCE - Mimic predevelopment state of building footprint Peak flow reduction: Medium - Good removal capability of atmospherically deposited Volume reduction: Medium urban pollutants Water quality treatment: Good - Can sometimes be refitted Amenity potential: Good - Ecological, aesthetic and amenity benefits Ecology potential: Good - No additional land take - Improve air quality TREATMENT SUITABILITY: - Help retain higher humidity levels in city areas Source control: Yes - Insulates buildings against temperature extremes Conveyance: No - Reduces the expansion and contraction of roof Site system: No membranes Regional system: No - Sound absorption No SITE SUITABILITY DISADVANTAGES Residential: Yes - Cost (compared to conventional runoff) Commercial/industrial: Yes - Not appropriate for steep roofs High Density: Yes - Opportunities for retrofitting may be limited by roof Retrofit: Yes structure (strength, pitch etc) Contaminated sites/sites above Yes - Maintenance of roof vegetation vulnerable groundwater: - Any damage to waterproof membrane likely to be more critical since water is encouraged to remain on the roof COST IMPLICATIONS: Land-take: None Capital cost (depending on roof type and Low-High capacity): Maintenance burden: Medium
KEY MAINTENANCE REQUIREMENTS: POLLUTANT REMOVAL - Irrigation during establishment of vegetation Total suspended solids: High - Inspection for bare patches and replacement of plants Nutrients: Low - Litter removal (dependant on setting and use) Heavy metals: Medium
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Rainwater Harvesting
Description Rainwater from roofs and hard surfaces can be stored and used. If designed appropriately, the system can also be used to reduce the rates and volumes of runoff. Key Design Criteria
- Design dependant on demand requirements, contributing surface area, stormwater - First flush often diverted away from tank
ADVANTAGES PERFORMANCE - with careful design, can provide source control of Peak flow reduction: High stormwater runoff Volume reduction: High - reduces demand on mains water. Water quality treatment: Poor Amenity potential: Poor Ecology potential: Poor DISADVANTAGES - potential risks to public health - systems can be complex and costly to install TREATMENT SUITABILITY: - above ground tanks can be unsightly. Source control: Yes Conveyance: No Site system: No Regional system: No
SITE SUITABILITY Residential: Yes Commercial/industrial: Yes High Density: Yes Retrofit: Yes Contaminated sites/sites above Yes vulnerable groundwater:
COST IMPLICATIONS: Land-take: None Capital cost (depending on roof type and High capacity): Maintenance burden: Medium
POLLUTANT REMOVAL Total suspended solids: High Nutrients: Low Heavy metals: Medium
KEY MAINTENANCE REQUIREMENTS: inspection and cleaning of collection systems, filters, throttle and valves, pumps.
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Water Butts
Description Water butts are the most common means of harvesting rainwater for garden use. They are small, off-line storage devices that are designed to capture and store roof runoff. If stormwater management benefits are to be obtained, specific modification of these units is required. Key Design Criteria ADVANTAGES PERFORMANCE - easy to construct, install and operate Peak flow reduction: Low - easy to retrofit Volume reduction: Low - inexpensive Water quality treatment: Low - marginal stormwater management benefits Amenity potential: Poor - provides water for non potable water Ecology potential: Poor - uses, e.g. garden watering
DISADVANTAGES TREATMENT SUITABILITY: - high risk of blockage of small throttles Source control: Yes - very limited water quality treatment benefits Conveyance: No - property owner responsible for operation and Site system: No maintenance, therefore cannot be guaranteed Regional system: No
SITE SUITABILITY Residential: Yes Commercial/industrial: Yes High Density: Yes Retrofit: Yes Contaminated sites/sites above Yes vulnerable groundwater:
COST IMPLICATIONS: Land-take: None Capital cost (depending on roof type and Low capacity): Maintenance burden: Low
POLLUTANT REMOVAL Total suspended solids: Low Nutrients: Low Heavy metals: Low
KEY MAINTENANCE REQUIREMENTS: - inspection of inlet and outlet for blockages - silt and debris removal.
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Pervious Pavements
Description Pervious pavements provide a pavement suitable for pedestrian and/or vehicular traffic, while allowing rainwater to infiltrate through the surface and into the underlying layers. The water is temporarily stored before infiltration to the ground, reuse, or discharge to a watercourse or other drainage system. Pavements with aggregate sub-bases can provide good water quality treatment.
Key Design Criteria
- pervious surface and sub-base to be structurally designed for site purpose and design vehicular loading - surface infiltration rate should normally be an order of magnitude greater than the design rainfall intensity - temporary subsurface storage volume to meet requirements for infiltration and/or controlled discharge - geotextile may be specified as a filtration treatment component near the top of the structure - soil and other material must be prevented from contaminating the pavement surface and substructure
ADVANTAGES PERFORMANCE - effective in removing urban runoff pollutants Peak flow reduction: Good - lined systems can be used where infiltration is not Volume reduction: Good desirable, or where soil integrity would be compromised Water quality treatment: Good - significant reduction in volume and rate of surface runoff Amenity potential: Poor - low maintenance Ecology potential: Poor - removes need for gully pots and manholes - eliminates surface ponding and surface ice. TREATMENT SUITABILITY: - good community acceptability. Source control: Yes Conveyance: No Site system: Yes Regional system: No
DISADVANTAGES SITE SUITABILITY - cannot be used where large sediment loads may be Residential: Yes washed/carried onto the surface Commercial/industrial: Yes - in the UK, current practice is to use on highways with low High Density: Yes traffic volumes, low axle loads and speeds of less than 30 Retrofit: Yes mph Contaminated sites/sites above Yes - risk of long-term clogging and weed growth if poorly vulnerable groundwater: maintained. COST IMPLICATIONS: Land-take: None (Net capital: Low) Capital cost (depending on roof type and Low capacity): Maintenance burden: Low
KEY MAINTENANCE REQUIREMENTS: POLLUTANT REMOVAL - sweeping Total suspended solids High - regular brushing and vacuuming Nutrients: High Heavy metals: High
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Soakaways
Description Soakaways are square or circular excavations, either filled with rubble or lined with brickwork, pre-cast concrete or polyethylene rings/perforated storage structures surrounded by granular backfill. They can be grouped and linked together to drain large areas including highways. The supporting structure and backfill can be substituted by modular, geocellular units. Soakaways provide stormwater attenuation, stormwater treatment and groundwater recharge. Key Design Criteria
- design to meet site drainage standards – generally 1 in 10 or 1 in 30 year design event - site infiltration rate assumed for design should be based on appropriate site investigations and should include an appropriate factor of safety - appropriate pre-treatment is required - if used, fill material should provide >30 per cent void space - minimum distance of 1m from the base to the seasonally high groundwater table - minimum distance of 5m from foundations.
ADVANTAGES PERFORMANCE - minimal net land take Peak flow reduction: Good - provides groundwater recharge Volume reduction: Good - good volume reduction and peak flow attenuation Water quality treatment: Good - good community acceptability Amenity potential: Poor - easy to construct and operate Ecology potential: Poor - can be retrofitted TREATMENT SUITABILITY: DISADVANTAGES Source control: Yes - not suitable for poor draining soils Conveyance: No - field investigations required to confirm infiltration rates Site system: Yes - not suitable for locations where infiltrating water may put Regional system: No structural foundations at risk, or where infiltrating water may adversely affect existing drainage patterns - not appropriate for draining polluted runoff SITE SUITABILITY - increased risk of groundwater pollution Residential: Yes - some uncertainty over long-term performance Commercial/industrial: Yes - possible reduced performance during long wet periods High Density: Yes where property owner responsible for operation and Retrofit: Yes maintenance, performance difficult to guarantee. Contaminated sites/sites above No vulnerable groundwater:
COST IMPLICATIONS: Land-take: Low Capital cost (depending on roof type and Low capacity): Maintenance burden: Low
KEY MAINTENANCE REQUIREMENTS: POLLUTANT REMOVAL - removal of sediments/debris from pre-treatment device Total suspended solids: Medium - monitoring performance (using observation well) Nutrients: Low Heavy metals: Medium
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Filter Strips
Description Filter strips are vegetated strips of land designed to accept runoff as overland sheet flow from upstream development. They lie between a hard-surfaced area and a receiving stream, surface water collection, treatment or disposal system. They treat runoff by vegetative filtering, and promote settlement of particulate pollutants and infiltration.
Key Design Criteria
- recommended minimum width of 6m - runoff from an adjacent impervious area must be evenly distributed across the filter strip with a water depth <50 mm for the water quality treatment event - slopes not exceeding 1in 20, minimum 1in 50.
ADVANTAGES PERFORMANCE - well-suited to implementation adjacent to large impervious Peak flow reduction: Poor areas Volume reduction: Poor - encourages evaporation and can promote infiltration Water quality treatment: Medium - easy to construct and low construction cost Amenity potential: Medium - effective pre-treatment option Ecology potential: Medium - easily integrated into landscaping and can be designed to provide aesthetic benefits.
DISADVANTAGES TREATMENT SUITABILITY: - large land requirement Source control: Yes - not suitable for steep sites Conveyance: No - not suitable for draining hotspot runoff or for locations Site system: Yes where risk of groundwater contamination, unless infiltration Regional system: No is prevented - no significant attenuation or reduction of extreme event flows. SITE SUITABILITY Residential: Yes Commercial/industrial: Yes High Density: Yes Retrofit: Yes Contaminated sites/sites above No vulnerable groundwater:
COST IMPLICATIONS: Land-take: High Capital cost (depending on roof type and Low capacity): Maintenance burden: Low KEY MAINTENANCE REQUIREMENTS: - litter/debris removal POLLUTANT REMOVAL - mowing Total suspended solids: Medium - repair of eroded or damaged areas Nutrients: Low Heavy metals: Medium
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Trenches
Description Trenches are shallow excavations filled with rubble or stone that create temporary subsurface storage for either infiltration or filtration of stormwater runoff. Ideally they should receive lateral inflow from an adjacent impermeable surface, but point source inflows may be acceptable. Infiltration trenches allow water to exfiltrate into the surrounding soils from the bottom and sides of the trench. Filtration or filter trenches can be used to filter and convey stormwater to downstream SuDS components. Key Design Criteria
- excavated trench 1–2m depth filled with stone aggregate - effective upstream pre-treatment to remove sediment and fine silts - Infiltration should not be used where groundwater is vulnerable or to drain pollution hotspots - observation wells and/or access points for maintenance of perforated pipe components.
ADVANTAGES PERFORMANCE - infiltration can significantly reduce both runoff rates and Peak flow reduction: Medium volumes. It has been shown that important hydraulic Volume reduction: Low (FT) benefits are also achieved with filter trenches High (IT) - infiltration provides a significant reduction in the pollutant Water quality treatment: High load discharged to receiving body Amenity potential: Low - trenches can be incorporated easily into site landscaping Ecology potential: Low and fit well beside roads.
DISADVANTAGES TREATMENT SUITABILITY: Source control: Yes - high clogging potential without effective pre-treatment – Conveyance: No not for sites with fine particled soils (clays/silts) in Site system: Yes upstream catchment Regional system: No - build-up of pollution/blockages difficult to see - high historic failure rate due to poor maintenance, wrong SITE SUITABILITY siting or high debris input Residential: Yes - limited to relatively small catchments Commercial/industrial: Yes - high cost of replacing filter material should blockage High Density: Yes occur. Retrofit: Yes Contaminated sites/sites above No (IT)/ Yes vulnerable groundwater: (FT)
COST IMPLICATIONS: Land-take: High Capital cost (depending on roof type and Low (IT), capacity): Medium (if liner required) Maintenance burden: Medium
KEY MAINTENANCE REQUIREMENTS: POLLUTANT REMOVAL - Regular inspection for signs of clogging Total suspended solids: High - Removal of sediment from pre-treatment system Nutrients: Low/Medium - Removal and cleaning or replacement of stone. Heavy metals: High
AECOM Surface Water Management Plan 61
Capabilities on project: Water
AECOM Surface Water Management Plan 62
Capabilities on project: Water
Swales
Description Swales are linear vegetated drainage features in which surface water can be stored or conveyed. They can be designed to allow infiltration, where appropriate. They should promote low flow velocities to allow much of the suspended particulate load in the stormwater runoff to settle out, thus providing effective pollutant removal. Roadside swales can replace conventional gullies and drainage pipes. Key Design Criteria
- limit velocities during extreme events to 1–2m/s, depending on soil type, to prevent erosion - maintain flow height of water during frequent events below the top of the vegetation (typically 100mm) - maximum side slopes of 1 in 3 (where soil conditions allow) - minimum base width normally 0.5m.
ADVANTAGES PERFORMANCE - easy to incorporate into landscaping Peak flow reduction: Medium - good removal of urban pollutants Volume reduction: Medium - reduces runoff rates and volumes Water quality treatment: Good - low capital cost Amenity potential: Medium - maintenance can be incorporated into general landscape Ecology potential: Medium management - pollution and blockages are visible and easily dealt with.
DISADVANTAGES TREATMENT SUITABILITY: - not suitable for steep areas Source control: Yes - not suitable in areas with roadside parking Conveyance: Yes - limits opportunities to use trees for landscaping Site system: Yes - risks of blockages in connecting pipework. Regional system: No
SITE SUITABILITY Residential: Yes Commercial/industrial: Yes High Density: Limited Retrofit: Limited Contaminated sites/sites above Yes vulnerable groundwater:
COST IMPLICATIONS: Land-take: High Capital cost (depending on roof type and Low capacity): Maintenance burden: Medium KEY MAINTENANCE REQUIREMENTS: - litter/debris removal POLLUTANT REMOVAL - grass cutting and removal of cuttings Total suspended solids: High - clearing of inlets, culverts and outlets from debris and Nutrients: Low sediment Heavy metals: Medium - repair of eroded or damaged areas
AECOM Surface Water Management Plan 63
Capabilities on project: Water
AECOM Surface Water Management Plan 64
Capabilities on project: Water
Bioretention
Description Bioretention areas are shallow landscaped depressions which are typically under-drained and rely on engineered soils and enhanced vegetation and filtration to remove pollution and reduce runoff downstream. They are aimed at managing and treating runoff from frequent rainfall events. Key Design Criteria
- sufficient area to temporarily store the Water Quality Treatment Volume (Vt) at a depth <0.15 m on the surface - the water quality treatment event should half drain within 24hrs to provide adequate capacity for multi-event scenarios - minimum depth to groundwater of 1m, if unlined - overflow/bypass facilities for extreme events.
ADVANTAGES PERFORMANCE - can be planned as landscaping features Peak flow reduction: Medium - very effective in removing urban pollutants Volume reduction: Medium - can reduce volume and rate of runoff (High with infiltration) - flexible layout to fit into landscape - well-suited for installation in highly impervious areas, Water quality treatment: Good provided the system is well-engineered and adequate Amenity potential: Good space is made available Ecology potential: Medium - good retrofit capability
DISADVANTAGES TREATMENT SUITABILITY: - requires landscaping and management Source control: Yes - susceptible to clogging if surrounding landscape is poorly Conveyance: No managed Site system: Yes - not suitable for areas with steep slopes Regional system: No
SITE SUITABILITY Residential: Yes Commercial/industrial: Yes High Density: No Retrofit: Yes Contaminated sites/sites above Yes vulnerable groundwater:
COST IMPLICATIONS : Land-take: High Capital cost (depending on roof type and Low capacity): KEY MAINTENANCE REQUIREMENTS: Maintenance burden: Medium - regular inspection - litter/debris removal POLLUTANT REMOVAL - replacement of mulch layer Total suspended solids: High - vegetation management Nutrients: Low - soil spiking and scarifiying Heavy metals: High
AECOM Surface Water Management Plan 65
Capabilities on project: Water
AECOM Surface Water Management Plan 66
Capabilities on project: Water
Geocellular/modular systems
Description Modular plastic geocellular systems with a high void ratio that can be used to create a below ground infiltration (soakaway) or storage structure. Key Design Criteria
- standard storage design using limiting discharges to determine storage volumes - structural design to relevant standards for appropriate surface loadings - appropriate geotextile/geomembrane for wrapping
ADVANTAGES PERFORMANCE - modular and flexible Peak flow reduction: Good - high void ratios (up to 96%) providing high storage volume Volume reduction (storage only) Poor capacity Volume reduction: Good - can be installed beneath trafficked or non-trafficked areas Water quality treatment: Poor (providing structural performance is proven to be Amenity potential: Poor sufficient) Ecology potential: Poor - long-term physical and chemical stability can be installed beneath public open spaces, e.g. play areas TREATMENT SUITABILITY: Source control: Yes Conveyance: Possible DISADVANTAGES Site system: Yes - no water quality treatment Regional system: Yes
SITE SUITABILITY Residential: Yes Commercial/industrial: Yes High Density: Yes Retrofit: Yes Contaminated sites/sites above Yes vulnerable groundwater:
COST IMPLICATIONS: Land-take: Low Capital cost (depending on roof type and Low capacity): Maintenance burden: Low
POLLUTANT REMOVAL Total suspended solids: Low Nutrients: None Heavy metals: Low
KEY MAINTENANCE REQUIREMENTS: - regular inspection of silt traps, manholes, pipework and pre-treatment devices, with removal of sediment and debris as required.
AECOM Surface Water Management Plan 67
Capabilities on project: Water
AECOM Surface Water Management Plan 68
Capabilities on project: Water
Infiltration basins
Description Infiltration basins are vegetated depressions designed to store runoff and infiltrate it gradually into the ground. Key Design Criteria
- effective pre-treatment required to remove sediments and fine silts prior to infiltration - designed to infiltrate the water quality treatment volume, as a minimum - infiltration should not be used where groundwater is vulnerable or to drain pollution hotspots.
ADVANTAGES PERFORMANCE - reduces the volume of runoff from a drainage area Peak flow reduction: Average - can be very effective at pollutant removal via filtering Volume reduction (storage only) Poor through the soils Volume reduction: Good - contributes to groundwater recharge and baseflow Water quality treatment: Good augmentation Amenity potential: Good - simple and cost-effective to construct Ecology potential: Good - changes in performance easy to observe
DISADVANTAGES TREATMENT SUITABILITY: - Potentially high failure rates due to improper siting, poor Source control: No design and lack of maintenance, especially if appropriate Conveyance: No pre-treatment is not incorporated Site system: Yes - not appropriate for draining pollution hotspots where high Regional system: No pollution concentrations are possible - requires a large, flat area
SITE SUITABILITY Residential: Yes Commercial/industrial: Yes High Density: No Retrofit: No Contaminated sites/sites above No vulnerable groundwater:
COST IMPLICATIONS: Land-take: High Capital cost (depending on roof type and Low capacity): KEY MAINTENANCE REQUIREMENTS: Maintenance burden: Low - regular inspections for signs of deterioration in performance, clogging and other blockages - litter/trash removal POLLUTANT REMOVAL - inlet/outlet cleaning Total suspended solids: High - vegetation management Nutrients: Medium - regular removal of sediment from pre-treatment Heavy metals: High
AECOM Surface Water Management Plan 69
Capabilities on project: Water
AECOM Surface Water Management Plan 70
Capabilities on project: Water
Detention basins
Description Detention basins are surface storage basins or facilities that provide flow control through attenuation of stormwater runoff. They also facilitate some settling of particulate pollutants. Detention basins are normally dry and in certain situations the land may also function as a recreational facility. Key Design Criteria
- detention volume to manage design storms via constrained outflow - minimum length:width ratio of 2:1 - maximum side slopes of 1:4 for maintenance and safety reasons, unless the situation allows steeper slopes to be used - bioretention and/or wetland/micropools at outlets is desirable for enhanced pollution control.
ADVANTAGES PERFORMANCE - can cater for a wide range of rainfall events Peak flow reduction: Good - can be used where groundwater is vulnerable, if lined Volume reduction (storage only): Poor - simple to design and construct Water quality treatment: Medium - potential for dual land use Amenity potential: Good - easy to maintain Ecology potential: Medium - safe and visible capture of accidental spillages TREATMENT SUITABILITY: DISADVANTAGES Source control: No - little reduction in runoff volume Conveyance: No - detention depths may be constrained by system inlet and Site system: Yes outlet levels Regional system: Yes
SITE SUITABILITY Residential: Yes Commercial/industrial: Yes High Density: Yes Retrofit: Yes Contaminated sites/sites above Yes vulnerable groundwater:
COST IMPLICATIONS: Land-take: Medium Capital cost (depending on roof type and Low capacity): Maintenance burden: Low
POLLUTANT REMOVAL Total suspended solids: Medium Nutrients: Low Heavy metals: Medium KEY MAINTENANCE REQUIREMENTS: - litter/trash removal - inlet/outlet cleaning - vegetation management - sediment monitoring and removal when required
AECOM Surface Water Management Plan 71
Capabilities on project: Water
AECOM Surface Water Management Plan 72
Capabilities on project: Water
Ponds
Description Ponds can provide both stormwater attenuation and treatment. They are designed to support emergent and submerged aquatic vegetation along their shoreline. Runoff from each rain event is detained and treated in the pool. The retention time promotes pollutant removal through sedimentation and the opportunity for biological uptake mechanisms to reduce nutrient concentrations. Key Design Criteria
- permanent pool volume for water quality treatment - temporary storage volume for flow attenuation - sediment forebay or upstream pre-treatment - length:width ratio between 3:1 and 5:1 - minimum depth for open water areas of 1.2m - maximum depth of permanent pool of 2m - side slopes <3:1 for slopes
ADVANTAGES PERFORMANCE - can cater for all storms Peak flow reduction: Good - good removal capability of urban pollutants Volume reduction (storage only): Poor - can be used where groundwater is vulnerable, if lined Water quality treatment: Good - good community acceptability Amenity potential: Good - high potential ecological, aesthetic and amenity benefits Ecology potential: Good - may add value to local properties TREATMENT SUITABILITY: DISADVANTAGES Source control: No - no reduction in runoff volume Conveyance: No - anaerobic conditions can occur without regular inflow Site system: Yes - land take may limit use in high density sites Regional system: Yes - may not be suitable for steep sites, due to requirement for high embankments SITE SUITABILITY - colonisation by invasive species could increase Residential: Yes maintenance Commercial/industrial: Yes - perceived health & safety risks may result in fencing and High Density: Unlikely isolation of the pond. Retrofit: Unlikely Contaminated sites/sites above Yes vulnerable groundwater:
COST IMPLICATIONS: Land-take: High Capital cost (depending on roof type and Medium capacity): (High with liner) Maintenance burden: Medium KEY MAINTENANCE REQUIREMENTS: - litter/debris removal POLLUTANT REMOVAL - inlet/outlet cleaning Total suspended solids: High - vegetation management Nutrients: Medium - sediment monitoring and removal when required Heavy metals: High
AECOM Surface Water Management Plan 73
Capabilities on project: Water
AECOM Surface Water Management Plan 74
Capabilities on project: Water
Wetlands
Description Wetlands provide both stormwater attenuation and treatment. They comprise shallow ponds and marshy areas, covered almost entirely in aquatic vegetation. Wetlands detain flows for an extended period to allow sediments to settle, and to remove contaminants by facilitating adhesion to vegetation and aerobic decomposition. They also provide significant ecological benefits. Key Design Criteria
- water quality treatment volume, detained within or above the permanent water body - shallow, temporary storage volume for attenuation - sediment forebay or equivalent upstream pre-treatment - continuous baseflow - combination of deep and shallow areas (maximum depth <2 m) - length:width ratio of greater than 3:1 - shallow side slopes
ADVANTAGES PERFORMANCE - good removal capability of urban pollutants Peak flow reduction: - if lined, can be used where groundwater is vulnerable - Low frequency events Good - good community acceptability - Extreme events (if large wetland area Good - high potential ecological, aesthetic and amenity benefits available) - may add value to local property Volume reduction (storage only): Poor Water quality treatment: Good Amenity potential: Good Ecology potential: Good
DISADVANTAGES TREATMENT SUITABILITY: - land take is high Source control: No - requires baseflow Conveyance: Yes - limited depth range for flow attenuation Site system: Yes - may release nutrients during non-growing season Regional system: Yes - little reduction in runoff volume - not suitable for steep sites SITE SUITABILITY - colonisation by invasive species would increase Residential: Yes maintenance Commercial/industrial: Yes - performance vulnerable to high sediment inflows High Density: Unlikely - perceived health and safety risks may result in fencing Retrofit: Unlikely and isolation of wetland Contaminated sites/sites above Yes vulnerable groundwater:
COST IMPLICATIONS: Land-take: High Capital cost (depending on roof type and High capacity): KEY MAINTENANCE REQUIREMENTS: Maintenance burden: Medium/Low - litter/trash/debris removal - inlet/outlet cleaning POLLUTANT REMOVAL - vegetation management to retain high vegetation Total suspended solids: High coverage, possibly requiring specialist equipment Nutrients: Medium sediment monitoring and removal when required Heavy metals: High
AECOM Surface Water Management Plan 75
Capabilities on project: Water