11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, Infiltration Blankets & CIRIA 697:The Bicester Experience.

Transcription

1 Infiltration Blankets & CIRIA 697:The Bicester Experience C J Tyler* WSP, Mountbatten House, Basing View, Basingstoke, Hampshire, RG21 4HJ, England, UK *Corresponding author, chris.tyler@wspgroup.com ABSTRACT The Kingsmere site in south west Bicester is sited on soils with varying permeability. Oxfordshire CC Highways have adopted a modern thinking approach to SUDS and recommended that their roads are constructed as permeable block paving with infiltration blankets below. Due to the proximity of housing and variable infiltration rates of the underlying Cornbrash and Kelleway Beds the engineering details have been a challenge. In order to meet the Government guidance on housing densities, combined with the need to comply with the latest highway guidance Manual for Streets there is little space left for soakaways or extensive swales. Within the development the Design Code sets out minimum storage volumes and maximum discharge rates per hectare. The combined capacity of the soakaways and infiltration blankets count towards the storage volume. To comply with CIRIA 697 a pipe is used to convey the excess flows to downstream detention basins. The presentation outlines the challenges, the way the storage and discharge rates were derived for the parcels to provide source control and how technical details have been developed to attenuate flows, protect utilities and overcome everyday practicalities such as construction traffic. KEY WORDS Infiltration blankets, permeable paving, SUDS, attenuation, maintenance, surface water drainage strategy INTRODUCTION In 2007 Countryside Properties obtained Outline Planning Consent for a major residential development on a 180 hectare site in Oxfordshire, Central England. The site forms an extension to Bicester Town. WSP were appointed as civil engineering consultants to plan and design the infrastructure and flood protection measures. In accordance with current environmental, flood protection and planning policy the preferred means of surface water disposal requires the use of Sustainable Drainage Systems (SUDS). As much runoff as possible shall be dealt with at source within each property boundary and potential infiltration systems should be employed where ever possible. The site is underlain with clays and limestone rocks with varying infiltration potential. The infiltration potential also varies at differing depths. Thus the surface water drainage strategy needed to optimise infiltration systems yet accommodate areas where infiltration was negligible. Chris Tyler 1

2 To compound the problem the underlying Cornbrash limestone is exceptionally hard and any deep sewers would be very expensive to construct. The depth constraints are also exacerbated by a high water table. Thus a shallow drainage system is required. The primary infrastructure comprising the spine roads, major service corridors and drainage on most major projects has to be planned such that individual development parcels can be served and phasing can be adjusted to meet cash flow and market demand. Thus a central spine containing the infrastructure with branches running off it is the ideal situation. In order for the system to be acceptable to Developers it has to be simple and costs predictable. A system had to be devised which could be applied across the whole site even where the infiltration rates were negligible. The permeable block paving system (see Fig 1) was adopted because the storage potential per hectare of development is relatively large and infiltration is not essential. Water can be conveyed within the subbase to areas where infiltration rates are more effective (see Fig 2). As the subbase is to be used as a storage zone flows from it had to be attenuated with discharge rates restricted to greenfield run off rates. The flows are relatively small and require a small orifice control. Thus a practical solution was required which is easily inspected and a maintained. The soakaways and permeable paving are designed in accordance with CIRIA Report 697 and provide protection for a 10 year storm. Overall flood protection is provided by dry detention basins at the lower parts of the site. Flows are attenuated to greenfield rates with storage volumes in the detention basins designed to accommodate the 100 year event plus climate change. Deductions have been made for the at source storage provided in the soakaways and permeable paving. The challenges have been to overcome conflicting requirements set out in policy guidance and design documents to deliver a simple, maintainable and commercially viable solution to all stakeholders. Many of the conflicts are well recognised by the parties experienced in this sort of development. This paper describes the solutions and aims to persuade influential parties to change the guidance on permeable paving exempting it from existing guidance on soakaway and infiltration systems. Figure 1 section through permeable block paving and infiltration blanket. Figure 2 Typical SUDS arrangement in housing parcel. 2 Infiltration Blankets & CIRIA 697:The Bicester Experience

3 BICESTER SITE CONSTRAINTS & CHALLENGES Site context The project known as Kingsmere, comprises the development by Countryside Properties of approximately 1600 homes with associated schools, health village, hotel, shops and employment areas on a site covering some 120 hectares of the 180 hectares available. Topography Catchments The site falls gently away from a central ridge to the Pingle Brook in the north and Whitelands Farm Ditch in the south. Level variation is about 15m. As with most sites the development parcels and sewer layouts within them do not precisely match the natural predevelopment subcatchment areas or shapes. The permitted rate of discharge is based on the greenfield rate of runoff from the natural subcatchment. Consequently despite attenuation of flow rates to the natural greenfield rates the Environment Agency (EA) has concerns that the volume of run off to watercourses may be significantly different. Consequently care was taken to ensure that the total areas draining to each catchment were similar to predevelopment areas. The spine road serving the development parcels is located along the central ridge. The developer s preferred location for primary sewers is along the spine road. Geology Site investigation Initial detailed site investigations were restricted to the spine road and primary sewer routes and the investigation report indicated that the underlying geology is predominantly Kelleway Clay over Cornbrash Limestone over Forest Marble. The Kelleway Clay is not present over the northern higher portion of the site. Geology Rockhead The Cornbrash was found to be very hard and could not be penetrated by normal excavators thus it was essential that all drainage systems were kept as shallow as possible. The rock head was encountered at about 3m below the surface in the southern Kelleway Clay areas and at about 1m below surface in the northern Cornbrash area. The Cornbrash is predicted to be approximately 5m in thickness, thus can be expected to be penetrated by drainage in the northern areas of the site. Geology Water Table The water table at the time of the site investigation, spring 2007, was between 2.5m and 1.5m below the surface Because of the water table, the shallow depth and the hardness of the rock, the primary drain runs have been repositioned to minimise depth. The sewers have been redirected to follow more closely the natural slopes with local drainage in the development parcels directed to two perimeter sewers located closer to the natural valley bottom. These replace the spine road sewer along the ridge. These routes are not ideal and restrict phasing flexibility, parcel release and primary infrastructure construction cash flow. However it delivers significant cost savings in both highway drain and sewer construction. Further site investigation work is planned to verify the viability of the proposed new routes. Geology Infiltration The in-situ tests identified a significant variation in infiltration rates from negligible to 2.0E-04m3/s/m2. Generally the weathered upper zone of the Cornbrash and where the Cornbrash is well jointed the rates were approximately 5.0E-05m3/s/m2. Building Regulations 5m rule - Section 3.25 of The Building Regulations 2000 (Part H Drainage and Waste Disposal) states that infiltration drainage should not be built within 5m of a building or road. 3 Infiltration Blankets & CIRIA 697:The Bicester Experience

4 DEVELOPMENT CONTROL & PLANNING POLICY CHALLENGES Urban Layout Housing Density UK planning policy (PPS3 Housing November 2006) calls for high density development thus leading to smaller gardens both to the front and the rear of properties. The recently published Manual for Streets promotes the use of buildings abutting the highway, thus eliminating front gardens altogether in many areas. These policies work against the provision of SUDS which generally require more space than conventional sewer systems. Swales require wider verges and open space areas while infiltration systems need to be at least 5m from buildings and not under roads. The combined constraints of water table and proximity of buildings restricts the areas in which infiltration can be deployed as the primary means of surface water disposal. SUDS ICOP Ownership & Maintenance Table 5.3 of The Interim Code of Practice for Sustainable Drainage (ICOP 2004) states that neither Filter Strips nor Pervious Surfaces will be adopted by the Sewerage Undertakers and that Swales are likely to require a high level of negotiation before the undertaker will adopt them. Surface water drainage excluded from Section 104 sewer adoptions - It quickly became evident that if SUDS were to be deployed then Thames Water, as the local sewerage undertaker, was unlikely to adopt the surface water system under Section 104 of the Water Industry Act At that point Oxfordshire County Council Highways (OCC) enthusiastically offered to adopt the surface water system as Highway Drainage using their powers under Section 38 of the Highways Act Oxfordshire Highways enlightened approach - Despite the Building Regulations advice against using infiltration systems under roads OCC are willing to accept infiltration drainage in the highway. Their forward thinking approach goes even further. Their experience in small developments is that permeable paving performs very well and they experienced no problems from them during the exceptional floods of Whereas adjacent developments utilising conventional gully and pipe drainage experienced urban sewer flooding. As a consequence OCC have been promoting the site wide use of permeable block paving throughout the site. Thus the challenge has been to develop a system which is; shallow to avoid the rock and high water table, flexible in performance to reflect the variable infiltration rates, meets the aspirations of the EA to balance their requirements for water quality improvements, source control and flood protection, satisfies the planner s requirements over density and adoption and which could be embraced by OCC as the highway authority to incorporate SUDS within the Highway. One of the challenges has been to contain OCC s enthusiasm and provide adequate risk controls more closely aligned with conventional design guidance for SUDS in CIRIA Report 697 the SUDS Manual (Woods-Ballard et al 2007) SOLUTIONS Design Code - As with most new developments, as part of the Planning Conditions, a Design Code has to be developed and agreed with the Planning Authority and stakeholders. This Code sets out the general guidance and design obligations on the overall site and individual site parcel developers. At the outset WSP worked very closely with Countryside Properties, the Environment Agency and Oxfordshire County Council Highway Department to agree the surface water drainage regime, work up the principles to prove the strategy was viable and summarise the system as a subsection within the Design Code. 4 Infiltration Blankets & CIRIA 697:The Bicester Experience

5 The SUDS Hierarchy is; Harvesting rainwater utilising water butts On plot soakaways (individual or linear trench through gardens with overflow to highway drain) Permeable surfacing with infiltration / storage blankets to driveways, minor estate roads and parking areas Swales to be used within open space areas as both conveyance and storage mechanism Dry detention basins as final flood protection and water quality facility Permeable Paving Principle Although permeable paving is designed primarily as a form of soakaway it can also function as a storage medium and a conveyance device. Rainfall percolates through the block paving joints, through the coarse sand bedding and into the open subbase material below. Water then percolates into the substrate. The capacity of the system is relatively large and the catchment area can exceed the area of permeable paving. The volume of water generated by this large catchment is focussed on a relatively small area of subgrade through which infiltration occurs. Thus the rate of infiltration under the permeable paving per square metre can significantly exceed the natural rate of infiltration of an open area, for example a gravel drive. Thus the potential to develop scour or solution features and put foundations at risk is greatly increased. Hence the 5m rule is applied to soakaways and other infiltration systems. This hard and fast application of the 5m rule is challenged by various parties and is discussed later in this paper. The subbase grading is open with an approximate void content of 33%. The bearing capacity of this subbase is also reduced. Consequently the thickness of the permeable subbase is much greater than a conventional subbase. The thickness can be reduced by increasing the bearing capacity of the sub-grade by using a geotextile. The thickness, width, length and slope of the subgrade dictates the volume of water contained within the subbase blanket before surface flooding will occur. Mini reservoirs within the subbase can be created to control the depth and volume. Design guidance can be found in chapter 12 of CIRIA Report 697 The SUDS Manual (2007) and the Interpave Design Guide Permeable Pavements edition 4 (2006a). The conservative approach set out in CIRIA 697 is to collect excess flow in a pipe network The pipes being designed for 30 year storms without surcharging. Thus the SUDS system is duplicated by a piped network and costs become significantly greater than the conventional gully and pipe system. The hard and fast application of this advice is challenged and is discussed later in this paper. OCC s approach to permeable paving - With negligible infiltration rates the subbase acts as a storage blanket. OCC recommended that the subgrade is not protected by an impermeable liner but that it is left exposed to allow maximum, albeit small, rates of infiltration. The natural attenuation of flow rates within the subbase means that the hydraulic head decreases and the surface of the water in the subbase falls as it flows in the direction of the slope of the subgrade. Consequently OCC promoted the elimination of collector pipes for excess flows. They recommend directing the flows within the subbase to open spaces or wide verge areas where the water can discharge into swales or ditches. It is likely that the long term performance as an infiltration system will deteriorate. Consequently the length of the conveyance route within the subbase has been limited and 5 Infiltration Blankets & CIRIA 697:The Bicester Experience

6 controls with overflow collector pipes introduced at intervals. Where possible these overflow pipes are kept shallow and discharge to swales or ditches. Elsewhere they continue as shallow surface water highway drains. The 5m rule - Both the Building Regulations (Sections 2.9 and 3.25) and CIRIA 697 (section 6.5 Key Design Criteria) advise against the construction of soakaways or other infiltration systems within 5m of a building structure. There is a risk that concentrated infiltration may cause settlement of foundations. The regulations appear to be over conservative for permeable paving particularly in situations where the permeable paving receives rainfall only from the identical plan area or slightly larger area. In such circumstances the volume of water infiltrating is the same or similar to the infiltration if the surface was not paved. Only for very short periods during excessive rainfall events are concentrations likely to be of concern. The overflow and drawdown details promoted at Bicester should be adequate to deal with this limited risk. Thus the risk to structures is limited and the verbal advice from many experts in the field of permeable paving is that the 5m rule does not apply to permeable paving unless it serves a significantly larger catchment. One of the objectives of this paper and the evidence gained from site is to seek to relax the 5m rule for permeable paving. There will be cases where ground conditions dictate that caution prevails and infiltration may cause damage. Where buildings are in close proximity to areas served by permeable paving which receive or convey water from a large area then it will be necessary to line the subbase zone with an impermeable membrane and adhere to the 5m rule in the Building Regulations. Service zones are identified along the carriageways with specified crossing points. The zones will be contained by an impermeable membrane to reduce risk of damage to services. Subbase blanket for conveyance - Where infiltration rates are low the subbase blanket acts as a storage blanket. Also, even where infiltration rates are high, during critical storms the inflow will exceed infiltration and water will accumulate and the subbase blanket will act as storage. As the volume of water increases it will flow through the subbase following the profile of the road formation and accumulate at low spots. Unless the water is removed the concentration of water at such points will act as a soakaway and the 5m rule should apply. Figure 3 Section through permeable paving showing mini reservoirs and overflow. Figure 4 Infiltration overflow chamber. 6 Infiltration Blankets & CIRIA 697:The Bicester Experience

7 Generally the flow of water through the subbase should not cause too great a problem. At Bicester this feature is being used to convey flows at shallow depth and eliminate the piped system as far as possible, thus achieving cost savings by eliminating gullies and pipework which offset the cost of the thicker pavement construction. Attenuation and flow control within the subbase blanket. - In order to control the depth and flow within the subbase and prevent surface flooding a series of mini dams are created to contain mini reservoirs. Where the subgrade is permeable overflow pipes are set just below the mini dam crest level to collect excess flows (see Figs 3 and 4). Where the subgrade is relatively impermeable collector drains are installed at the base of the mini dams (see Figs 5 and 6).. Figure 5 Section through permeable paving showing mini reservoirs and attenuation. Figure 6 Flow Rate Control chamber for attenuation in impermeable areas. The collector pipes lead to a small inspection chamber which has an orifice plate within it to attenuate flows thus optimising the storage potential. The orifice is set to control the rate of discharge to the greenfield run off rate. The orifice is consequently relatively small, often only a centimetre or so in diameter. Consequently the risk of blockage is high. The top of the orifice plate is set at a similar level to the mini dam crest and acts as an overflow spill to the other side of the chamber. The inspection chambers have a gully grating for ease of inspection to allow maintenance crews to identify if the orifice is blocked. It is then a simple process to lift the grating and remove the blockage or lift out the plate and clear the orifice. OCC welcomed this simple solution to overcome the small orifice risks. Construction Traffic The spine road and main parcel access roads will carry a significant amount of construction traffic and consequently permeable paving is likely to be damaged by mud and silt. This will reduce the efficiency of the system. Thus permeable paving will not be used on construction access roads but the infiltration blanket will be deployed. Discharge from adjacent properties - OCC agreed that by complying with the layout recommendations in Manual for Streets many properties would abut the highway and it would be difficult to drain the front of buildings to on plot SUDS. Consequently they offered to allow roof drainage to be connected into the subbase blanket. The subbase blanket may need to be increased in capacity and there would be a minor increase in maintenance liability. 7 Infiltration Blankets & CIRIA 697:The Bicester Experience

8 Commuted sum for additional maintenance - Negotiations were entered into with OCC and agreement reached over the additional maintenance costs associated with permeable paving compared with the cost of maintaining conventional impermeable paving comprising gullies and pipes. The increased suction sweeping was costed and the rate per square metre of carriageway was agreed. The same rate is applied to any roof areas draining to the subbase blanket. Design Standards - Development Parcel Design Criteria Soakaways and infiltration blankets shall be designed for a 10 year storm with a factor of safety of 10 Excess flows for events up to a 30 year return period shall be conveyed below ground in pipework Excess flows for events up to a 100 year return period may be routed overland through development parcels to the site wide detention facilities providing such routes are identified and do not present a risk to buildings. 2D overland flow modelling techniques should be employed to determine the approximate depth, velocity and extent of overland flows. Permeable block paving or other permeable surfacing shall be used on drives, mews, parking areas and minor estate roads. Where there is a risk to foundations the 5m rule shall be met by installing an impermeable membrane. Maximum permitted rate of discharge for 10year storm per 1ha. = not more than 7.8l/s/ha Minimum permitted stored volume per 1 ha. of development = not less than 100m 2 The 100m 3 is the typical volume achievable. It is based on identifying the maximum storage required for storms of varying duration less the discharge through the control orifice and through subgrade infiltration for the same period. The critical storm for a one hectare land parcel is calculated to be 2 hours. A typical but conservative infiltration rate of 7E-05m 3 /s/m 2 has been used The impermeable areas used were as follows; Gardens = 25% Roofs& drives = 22% Roads, parking, Mews Courts etc = 53% Net permeable paving area allowing for 5m rule and areas of low permeability = 840m 2 /ha. By applying these design constraints a parcel developer is encouraged to maximise the use of on plot soakaways and permeable paving to reduce the volume of storage above the minimum 100 m 3 requirement. If no infiltration is provided the storage volume required for a two hour storm increases by about 40% With 840m 2 of permeable paving with an average stored depth of 300mm the volume available is approximately 250m 2. Thus it should be relatively easy to meet these parameters even where there is no infiltration into the subgrade and the subbase acts only as storage and conveyance. 8 Infiltration Blankets & CIRIA 697:The Bicester Experience

9 SUMMARY The challenges at Kingsmere, Bicester are to develop a SUDS system which can be applied throughout the site regardless of the varying infiltration rates. The varying infiltration rates across the site preclude the widespread use of soakaway SUDS. The infiltration blanket approach has been adopted as it will permit infiltration even in areas where the substrate is relatively impermeable. The pressures on high density development and land values preclude the widespread use of land hungry SUDS. The use of the infiltration blanket does not utilise valuable development land. The use of permeable paving combined with swales and ditches where possible and detention basins provide an element of water quality enhancement and habitat diversity. At the outset it was accepted that some forms of SUDS or flows from SUDS exceeding the design standards would not be accepted into a surface water sewer network adopted by Thames Water as the sewerage undertaker. Consequently, with the encouragement of Oxfordshire County Council Highways, the surface water drainage system will be designed to their requirements and adopted under the powers of Section 38 of the Highways Act 1980 The SUDS hierarchy and chain were agreed with the EA, OCC and Cherwell District Council at the outset and included in the Design Code Maximum use of water butts and on-plot soakaways form part of the strategy. Soakaways and permeable paving are designed for the 10 year storm. Soakaways are designed for full discharge of events up to the 10 year event but with overflows into the permeable paving or local drains and swales. Permeable block paving with the voided subbase is being used for a combined infiltration, storage and conveyance system. The permeable paving system is designed to accommodate rainfall from a 10 year storm and where infiltration rates in the subgrade are poor then overflows are attenuated to greenfield rates. Flows in excess of the 10 year storm overflow into the highway drain network. At low points within the permeable paving excess flows are taken into small inspection chambers with orifice plates to attenuate flows. From the chambers a shallow surface water drain is used to convey flows to swales or primary infrastructure drains. The swales and primary infrastructure drains lead to dry detention basins which provide additional flood protection for the rainfall runoff for the 100 year event plus climate change. The storage volumes provided by the upstream SUDS have enabled the detention basins to be reduced in size. The flows from the paving are controlled using small inspection chambers, which have a simple orifice plate placed across the diagonal of a square chamber or widest part of a circular chamber. Grating covers are used to facilitate frequent inspection for blockages or standing water in the system. 9 Infiltration Blankets & CIRIA 697:The Bicester Experience

10 The design guidance implies that permeable paving should not be used within 5m of a building. However as the infiltration is dispersed and generally no greater than an unpaved area it is recommended that the 5m advice in the Building Regulations and CIRIA guidance is modified. This paper was written at the time when the details were being signed off for approvals and construction should be underway before the conference begins. Barry West of Oxfordshire County Council should be applauded for the forward thinking approach to SUDS. They have overcome the artificial constraints imposed by most water undertakers and highway authorities which normally preclude widespread use of SUDS. ACKNOWLEDGEMENTS Vicky Boorman Environment Agency Barry West Oxfordshire County Council John Coates, Nigel Borrell, Stephen Price Countryside Properties Thomas Kinnear, Jonathan Merrick - WSP Development and Transportation John Howe - Interpave REFERENCES Woods-Ballard, Kellagher, Martin, Jefferies, Bray, Shaffer - CIRIA (2007) Report 697 The SUDS manual Building Regulations 2000 Part H Drainage and Waste disposal Department for Communities and Local Government & Department for Transport - (2007) - Manual for Streets Interpave (Dec 2006a) - Permeable Pavements Guide to the design, construction and maintenance of concrete block permeable pavements Edition 4 Interpave (Dec 2006b) - Permeable paving and the law Implementing planning policy, building regulations, government guidelines and adoption Department for Communities and Local Government (Dec 2006) - Planning Policy Statement 25 - PPS25 Development and Flood Risk Department for Communities and Local Government (Nov 2006) - Planning Policy Statement 3 PPS3 Housing National SUDS Working Group (July 2004) - Interim Code of Practice for Sustainable Drainage Systems Water Industry Act Infiltration Blankets & CIRIA 697:The Bicester Experience