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HomeMy WebLinkAbout08_SURFACE_DRAINAGE8.1Surface Drainage surface drainage section 8 8.2 Bar Slash X Ranch LLC - Annexation and Stage Road PUD/Subdivision: Final Submission This page intentionally left blank 8.3Surface Drainage John Lifton Bar/X Ranch LLC Planning Office, P.O. Box 997, Telluride CO 81435 January29, 2004 Stage Road PUD?Subdivision Surface Drainage Study For the purposes of this study the Stage Road Subdivision drainage basins were analyzed based on the initial phase of development, see sketch DS-1 for basin designation. The initial phase of development consists of the South Road, serving Lots 1-5 and the North Road, which is an extension of the existing Old Stage Road serving Lots 9-12. Also a ranch access road and road to the proposed Cultural Use Area are included in this phase of the development. In some cases these proposed roads modify the existing undeveloped drainage characteristics. In these instances, culverts and channels have been designed to convey the volume of water developed by a 5-year storm event using the City of Aspen’s time-intensity frequency curves. As the specific lots are developed, each lot and its corresponding development will be required to detain storm water within the area of the lot. The following is a summary of the various drainage basins: Basin #1 (22 acres) Using Maps from the Soil Conservation Service and site soils investigations the soil in this area is identified predominantly as Morval Loam. The basin has an average slope of 3% with hydrologic soil classification of “B” resulting in a runoff coefficient of 0.15 for agricultural surfaces. Using software to calculate the time of concentration for a flow distance of approximately 1750 feet and a Manning’s value of 0.03 resulting in a time of concentration of 17.8 minutes. The rainfall intensity (I) for this time of con- centration for a 5-year storm is 1.7 inches per hour resulting in a peak discharge Q = CIA = 0.15*1.7*22 = 5.5 cfs. The smooth culvert diameter required for this flow is 15 inches while the open ditch should have a cross-sectional area of 2 square feet. Basin #2 (30 acres) The soil in this area is identified predominantly as Uracca. The basin has an average slope of 30% with hydrologic soil classification of “B”. The historic flow of this area is not restricted or modified by the proposed development and therefore does not require analysis. Basin #3 (15 acres) The soil in this area is identified predominantly as Uracca. The basin has an average slope of 5% over the pasture area with a hydrologic soil classification of “B”. The runoff coefficient is 0.2 with a flow dis- tance of 1750 feet. The time of concentration is calculated as 14 minutes. The rainfall intensity (I) for this time of concentration for a 5-year storm is 1.8 inches per hour resulting in a peak discharge of 5.4 cfs. The smooth culvert diameter required for this flow is 15 inches while the open ditch should have a cross-sectional area of 2 square feet. 8.4 Bar Slash X Ranch LLC - Annexation and Stage Road PUD/Subdivision: Final Submission Basin #4 (5 acres) The soil in this area is identified predominantly as Uracca. The basin has an average slope of 20% over the bare ground area with a hydrologic soil classification of “B”. The runoff coefficient is 0.35 with a flow distance of 300 feet. The time of concentration is calculated as 4 minutes. The rainfall intensity (I) for this time of concentration for a 5-year storm is 2.8 inches per hour resulting in a peak discharge of 4.9 cfs. The smooth culvert diameter required for this flow is 15 inches while the open ditch should have a cross-sectional area of 2 square feet for diverting water in this basin. Basin #5 (5.5 acres) The soil in this area is identified predominantly as Uracca. The basin has an average slope of 10% over the bare ground area with a hydrologic soil classification of “B”. The runoff coefficient is 0.35 with a flow distance of 300 feet. The time of concentration is calculated as 5.5 minutes. The rainfall intensity (I) for this time of concentration for a 5-year storm is 2.8 inches per hour resulting in a peak discharge of 5.4 cfs. The smooth culvert diameter required for this flow is 15 inches while the open ditch should have a cross-sectional area of 2 square feet for diverting water in this basin. Generally this water will discharge into the existing pond in this basin with controlled outflow. Basin #6 (17.8 acres) The soil in this area is identified predominantly as Morval Loam. The basin has an average slope of 6% over the agricultural area with a hydrologic soil classification of “B”. The runoff coefficient is 0.15 with a flow distance of 1000 feet. The time of concentration is calculated as 5.5 minutes. The rainfall intensity (I) for this time of concentration for a 5-year storm is 2.2 inches per hour resulting in a peak discharge of 5.8 cfs. Generally, this water will discharge into the proposed pond with controlled outflow. The remaining 121.7 acres to the north are not disturbed except for the proposed Burlingame Afford- able Housing, which will require a separate drainage study when the proposed development is better defined. In conclusion, because the overall added impervious surface of the roadways (139,000 sq.ft.) is less than 5% of the overall area of the subdivision, there is not a significant impact to the existing runoff peaks. Therefore detention areas are not required for this phase of the development. All storm water runoff eventually discharges to Maroon Creek or the Roaring Fork River. 8.5Surface Drainage Appendix A: Culvert and Ditch Calculations Manning Pipe Calculator for 15” smooth culvert Given Input Data: Shape Circular Solving for Diameter Full Diameter 1.2146 ft Depth 1.2146 ft Flowrate 5.5000 cfs Slope 0.0200 ft/ft Manning's n 0.02 Computed Results: Area 1.1586 ft2 Wetted Area 1.1586 ft2 Wetted Perimeter 3.8157 ft Perimeter 3.8157 ft Velocity 4.7470 fps Hydraulic Radius 0.3036 ft Percent Full 100.00% Full flow Flowrate 5.5000 cfs Full flow velocity 4.7470 fps Critical Information Critical depth 0.9909 ft Critical slope 0.0176 ft/ft Critical velocity 5.2619 fps Critical area 1.0452 ft2 Critical perimeter 2.6751 ft Critical hydraulic radius 0.3907 ft Critical top width 1.2146 ft Specific energy 1.4856 ft Minimum energy 1.4863 ft Froude number 0.7071 Flow condition Subcritical Channel Calculator for typical 24” deep x 12” wide ditch Given Input Data: Shape Rectangular Solving for Depth of Flow Flowrate 5.5000 cfs Slope 0.0300 ft/ft Manning's n 0.025 Height 2.0000 ft Bottom width 1.0000 ft Computed Results: Depth 1.0906 ft Velocity 5.0430 fps Full Flowrate 11.1783 cfs Flow area 1.0906 ft2 Flow perimeter 3.1812 ft Hydraulic radius 0.3428 ft Top width 1.0000 ft Area 2.0000 ft2 Perimeter 5.0000 ft Percent full 54.53% 8.6 Bar Slash X Ranch LLC - Annexation and Stage Road PUD/Subdivision: Final Submission Tc CALCULATION FOR BASIN #1 Sheet Flow Description Manning's n 0.03 Flow Length 300.0000 ft Two Yr, 24 hr Rainfall 1.8000 in Land Slope 0.0300 ft/ft Computed Sheet flow time > 7.3817 min > 7.3817 min Shallow Concentrated Flow Description Surface Unpaved Flow Length 1750.0000 ft Watercourse Slope 0.0300 ft/ft Velocity 2.7946 fps Computed Shallow flow time > 10.4369 min Channel Flow Description Flow Area 0.0100 ft2 Wetted Perimeter 0.0100 ft Flow Length 1.0000 ft Channel Slope 0.0100 ft/ft Manning's n 0.02 Hydraulic radius 1.0000 ft Velocity 7.4300 fps Computed Channel flow time > 0.0022 min Total Time of Concentration > 17.8208 min Tc CALCULATION FOR BASIN #3 Sheet Flow Description Manning's n 0.03 Flow Length 300.0000 ft Two Yr, 24 hr Rainfall 1.8000 in Land Slope 0.0500 ft/ft Computed Sheet flow time > 6.0175 min Shallow Concentrated Flow Description Surface Unpaved Flow Length 1750.0000 ft Watercourse Slope 0.0500 ft/ft Velocity 3.6078 fps Computed Shallow flow time > 8.0844 min Channel Flow Description Flow Area 0.0100 ft2 Wetted Perimeter 0.0100 ft Flow Length 1.0000 ft Channel Slope 0.0100 ft/ft Manning's n 0.02 Hydraulic radius 1.0000 ft Velocity 7.4300 fps Computed Channel flow time > 0.0022 min Total Time of Concentration > 14.1041 min 8.7Surface Drainage Tc CALCULATION FOR BASIN #4 Sheet Flow Description Manning's n 0.03 Flow Length 300.0000 ft Two Yr, 24 hr Rainfall 1.8000 in Land Slope 0.2 Computed Sheet flow time > 3.4562 min Shallow Concentrated Flow Description Surface Unpaved Flow Length 300.0000 ft Watercourse Slope 0.2000 ft/ft Velocity 7.2156 fps Computed Shallow flow time > 0.6929 min Channel Flow Description Flow Area 0.0100 ft2 Wetted Perimeter 0.0100 ft Flow Length 1.0000 ft Channel Slope 0.0100 ft/ft Manning's n 0.02 Hydraulic radius 1.0000 ft Velocity 7.4300 fps Computed Channel flow time > 0.0022 min Total Time of Concentration > 4.1513 min Tc CALCULATION FOR BASIN #5 Sheet Flow Description Manning's n 0.03 Flow Length 300.0000 ft Two Yr, 24 hr Rainfall 1.8000 in Land Slope 0.1000 ft/ft Computed Sheet flow time > 4.5604 min Shallow Concentrated Flow Description Surface Unpaved Flow Length 300.0000 ft Watercourse Slope 0.1000 ft/ft Velocity 5.1022 fps Computed Shallow flow time > 0.9800 min Channel Flow Description Flow Area 0.0100 ft2 Wetted Perimeter 0.0100 ft Flow Length 1.0000 ft Channel Slope 0.0100 ft/ft Manning's n 0.02 Hydraulic radius 1.0000 ft Velocity 7.4300 fps Computed Channel flow time > 0.0022 min Total Time of Concentration > 5.5426 min 8.8 Bar Slash X Ranch LLC - Annexation and Stage Road PUD/Subdivision: Final Submission Tc CALCULATION FOR BASIN #6 Sheet Flow Description Manning's n 0.03 Flow Length 300.0000 ft Two Yr, 24 hr Rainfall 1.8000 in Land Slope 0.06 ft/ft Computed Sheet flow time > 5.5943 min Shallow Concentrated Flow Description Surface Unpaved Flow Length 1000.0000 ft Watercourse Slope 0.0600 ft/ft Velocity 3.9521 fps Computed Shallow flow time > 4.2171 min Channel Flow Description Flow Area 0.0100 ft2 Wetted Perimeter 0.0100 ft Flow Length 1.0000 ft Channel Slope 0.0100 ft/ft Manning's n 0.02 Hydraulic radius 1.0000 ft Velocity 7.4300 fps Computed Channel flow time > 0.0022 min Total Time of Concentration > 9.8136 min 8.9Surface Drainage Additional Information - SUDS Using on-site storage in ponds and wetlands, and slowing water movement by percolation through soils, are techniques of Sustainable Drainage Systems or SUDS. SUDS technology started in the United Kingdom but is rapidly spreading in international applications. Most land is naturally porous, allowing water to infiltrate soils and move into surface water fea- tures such as ponds, lakes, rivers and streams and underlying water storing rocks (aquifers). The movement of water through soils slows the transla- tion of rainfall to river flow while filtering and break- ing down pollutants. In non-urban areas rainfall fills up (recharges) the aquifers in winter. In summer much of the rainfall is lost through transpiration and evaporation, so recharge of aquifers is limited. In urban catchments, water flow is altered by the materials we use to build our towns, cities, shopping centres and developments. Tarmac and concrete are impermeable and water collects on the surface. Throughout most of the 20th century the water that fell on these impermeable materials was seen as a problem and moved as quickly as possible into our rivers and watercourses through dedicated surface water sewers or combined foul and surface sewers. The presence of impermeable cover prevents water from seeping into the ground while the drainage systems transmit rainfall rapidly to the nearest watercourse. This leads to a more “flashy” flow regime in urban catchments with lower flows in summer and higher peak flows during storms. Sustainable Drainage Systems (SUDS) aim to reduce the impact of developments by mimicking natural processes of infiltration of water into the soil and underground rocks. As an added bonus SUDS methods can “filter” and degrade pollutants gathered by water as it moves across urban areas, reducing pollution in watercourses. A range of techniques can be used to minimise water run-off including: • soakaways to handle uncontaminated roof drainage • permeable paving which allows water to soak into the ground • filter strips which encourage infiltration and sediment removal • water butts which collect water for later reuse Where reducing run-off is not possible, techniques can be used to store and/or filter runoff includ- ing: • vegetated channels (swales) • french (filter) drains • infiltration basins • storage ponds or wetlands These methods not only help to balance flows but can also offer efficient treatment for a wide range of contaminants and provide wetland habitats in an urban environment. Such habitats can give communities an educational resource and provide opportunities for recreation as well as contributing to the delivery of local biodiversity targets. SUDS have not been widely accepted to date because of a lack of detailed technical and legal guidance, reluctance to take on “novel” techniques and uncertainty over the long-term implications for both management and maintenance. Bar/X Ranch is a perfect candidate for SUDS because of the high percolation of the soils, and its historical immunity from flooding. We have therefore designed the subdivision improvements to minimize the additional amount of non-porous surface constructed, and for roads, roofs and other non-porous areas to drain into constructed wetlands and percolation sumps. Construction of new ponds also helps buffer the movement of water through the system. 8.10 Bar Slash X Ranch LLC - Annexation and Stage Road PUD/Subdivision: Final Submission The following brochure from the United Kingdom provides additional background information. 8.11Surface Drainage Sustainable Drainage Systems South Gloucestershire design guide This is one of a series of leaflets aimed at encouraging best practice in the planning & design of new development. It was adopted in September 2002 to supplement the policies of the Development Plan. August 2002A guide for developers Towards Sustainable Development South Gloucestershire Council and the Environment Agency share the ultimate aim of seeking to achieve sustainable development, and within this the conservation and enhancement of the water resource and biodiversity. Surface Water Runoff - The Problem Development can cause a range of adverse impacts on our water resources: By diverting rainfall to piped systems the amount of water infiltrating the ground is reduced. This contributes to depletion of ground water and low flows in water courses. Surface water runoff can contain a wide range of contaminants such as oil, organic matter and toxic metals. Although often at low levels, cumulatively these can result in poor water quality in rivers and streams which affects biodiversity and amenity. After rainfall, the first flush can often be highly polluting. Increased runoff as a result of more extensive hard paving and roofing can increase the risk of flooding downstream, as well as give sudden rises in water levels and flow rates as the water is discharged into watercourses. Although some pollution arising from urban runoff may be unavoidable, and water treatment at every outfall may be impractical, by moderating flows and filtering runoff, Sustainable Drainage Systems (SuDS) can deliver significant reductions in impact on the water resource. What are Sustainable Drainage Systems? SuDS are physical structures and techniques designed to receive surface water runoff. They can be incorporated into planted and paved areas of the site. The aim is to: a) Prevent or remove pollutants by means of ground infiltration, sub base storage and filtration or bio-filtration. b) Recharge ground water supplies and aquifers. c) Attenuate (storage) to reduce flow and delay discharge to collector watercourses that may be subject to flooding. 8.12 Bar Slash X Ranch LLC - Annexation and Stage Road PUD/Subdivision: Final Submission Development which would have an unacceptable effect on the water environment, including surface water and groundwater quality and quantity, river corridors and associated wetlands, will not be permitted. Development proposals will be required to incorporate sustainable drainage systems (SuDS) for the disposal of surface waters. Where this is not practicable it must be demonstrated that an acceptable alternative means of surface water disposal is incorporated. What are the benefits of SuDS? The implementation of SuDS may lead to cost savings, by for example avoiding or reducing the need for constructing or requisitioning surface water sewers or for providing piped connections to distant outfalls. SuDS can be cost effectively designed to work with retained natural features such as ditches or ponds and to form an integral part of hard and soft landscaped areas. In this way they can help to produce an attractive scheme which enhances the nature conservation and amenity value of the development while also recycling the valuable water resource. For example, the introduction of water features into new development can improve the quality of the built environment, the quality of life and economic value of the development. Other benefits include: Reducing the flood risk from development within a river catchment. Minimising dispersed pollution arising from surface water runoff. Minimising the risk of pollution to groundwater. Minimising environmental damage e.g. bank erosion and damage to habitats. Maintaining or restoring the natural flow regime of the receiving watercourse. Maintaining recharge to groundwater. The implementation of appropriate SuDS techniques at the start of a development which can prevent the pollution of watercourses during construction. Eliminating the need for petrol/oil interceptors The Policy Context In line with the guidance issued by government in PPG25 South Gloucestershire Council has included a SuDS policy in the South Gloucestershire Local Plan (Revised Deposit Draft). Policies L17 and L17A state that: SuDS also constitute suitable measures for the mitigation of adverse impacts of development, and support the waste conservation objectives set out in Policy 13 of the Joint Replacement Structure Plan. South Gloucestershire design guide Sustainable Drainage Systems L8 8.13Surface Drainage SuDS and the Planning Process SuDS include tried and tested techniques which are already being implemented on a range of projects in South Gloucestershire and elsewhere. They incorporate cost effective techniques which are applicable to a wide range of schemes, from small developments through to major residential, leisure and commercial or industrial operations with large areas of hardstanding and roof. They can also be successfully retro-fitted to existing developments. As with other key considerations in the planning process, the incorporation of SuDS needs to be considered early on in the site evaluation and planning process, as well as at the detailed design stage. All key existing landscape features should be incorporated into the site layout and it is essential that SuDS be considered at this initial stage. This Council will expect planning applications, whether outline or detailed, to demonstrate how SuDS will be incorporated into development proposals, and for detailed design information to be submitted at the appropriate stage. Technical officers will be happy to discuss these matters with applicants at any stage before or during the planning application process. The Council will make use of conditions to secure the implementation of SuDS, where appropriate. Adoption and Maintenance Adoption and maintenance of SuDS are associated structures and grills must be considered at the design stage. It is important to incorporate permanent utility vehicle and maintenance access into the landscaping or works. The developer must consider his responsibility, both during and after construction, and submit a management strategy plan to the Council for approval as part of the full planning application. The Council may be prepared to adopt such areas and associated structures under a Section 106 Agreement of the Town & Country Planning Act. However, this would be subject to a commuted sum since the degree of maintenance will be higher than open land. The Environment Agency and other relevant organisations are working together to ensure that the issue of adoption is resolved. Currently a draft adoption plan is being disussed to ensure that a clear and consistent responsibilty procedure is reached leading to an agreed code of practice. Choosing the right SuDS Mechanism The choice of SuDS mechanism will depend on a number of factors: the pollutants present in runoff the size of and drainage strategy for the catchment area the hydrology of the area and infiltration rate of the soil the presence of Groundwater Source Protection Zones Large scale ponds and wetlands are generally more appropriate for larger (> 5ha) sites. Infiltration trenches, swales, filter strips and porous pavements are suitable for both large and small sites. Many large sites will incorporate a mix of different mechanisms. SuDS can be incorporated into areas where there is clay subsoil or there is a fairly steep gradient. South Gloucestershire design guide Sustainable Drainage Systems L8 8.14 Bar Slash X Ranch LLC - Annexation and Stage Road PUD/Subdivision: Final Submission Sustainable Drainage Techniques Filters runoff Some losses through infiltration Provides temporary storage Slows flood response during heavy rainfall A swale is a shallow depression used to convey runoff slowly through grassed/ planted areas Outlet to swale Grassed swale Nealon Tanner Architects, Bristol Permeable Pavements The need for surface water drains and off site sewers can be reduced where runoff is encouraged to permeate through pavements such as concrete blocks, crushed stone, permeable asphalt based or other surfacing. Depending on the ground conditions, the water may infiltrate directly into the subsoil, or be stored in an underground reservoir (e. g. crushed stone layer) before slowly soaking into the ground. If necessary an overflow can keep the pavement free of water in all conditions. Pollutant removal occurs either within the surfacing material itself, or by the filtering action of the reservoir or subsoil. Swales and Basins These can be created as features within the landscaped areas of the site, or they can be incorporated into ornamental, amenity and screen planted areas where they would be maintained as a part of the normal maintenance contract. They provide temporary storage for storm water, reduce peak flows to receiving waters, facilitate the filtration of pollutants (deposited and incorporated into the substrate) and microbial decomposition as well as facilitating water infiltration directly into the ground. Swales and basins are often installed as part of a drainage network connecting to a pond or wetland prior to discharge to a natural watercourse. They may be installed alongside roads to replace conventional kerbs, therefore saving construction and maintenance costs. Overflowpipe Gravel filter course Permeable paving Stores water and attentuates runoff Filters runoff water Allows water to slowly infiltrate to groundwater Only occassionally overflows to watercourse Paved Area Geotextilemembrane Porous paving A South Gloucestershire School South Gloucestershire design guide Sustainable Drainage Systems L8 8.15Surface Drainage Ponds and Wetlands Although these can be designed as wet or dry ponds, or wetlands, they are most likely to contribute to visual amenity and biodiversity where they include a permanent water body. Ponds or wetlands can be designed to accommodate considerable variations in water level during storms, thereby enhancing flood storage capacity. By allowing adequate detention time, the level of solids removal can be significant. The algae and plants of wetlands can provide a particularly good level of filtering and nutrient removal as well as having the potential to recycle grey water. Ponds and wetlands can be fed by swales, filter drains or piped systems, and the use of inlet/ outlet sumps will help reduce sedimentation. Planting of reeds at inlets and outlets will cleanse water as it enters and leaves the pond. InletStructure ShallowMarshWedges Outlet Sump Stores water and attentuates runoff Filters runoff Removes nutrients Can recycle grey water SedimentForebay Runoff filters through5m wide buffer zone Trench1.2m deepfilled with30-60mmdia cleanstone Geotextilelining to preventclogging Protectivelayer of filterfabric Emergencyoverflow berm Schematic wetland An infiltration trench Planted Areas Well vegetated areas play an important role in attenuation by increasing soil organic matter and its ability to retain moisture. The incorporation of planting within a scheme will aid SuDS, whilst visually enhancing the development, promoting its integration within the wider landscape. Cisterns, Dry Wells & Retention Grading Runoff can also be stored in cisterns and dry wells for recycling purposes. Cistern collection systems (plumbed into roof drainage), retention grading and driveway dry wells can significantly intercept rainfall. Such solutions are practical and affordable if installed at the initial building stage rather than as an addition to an existing scheme. University of the West of England ©Avon Wildlife Trust Infiltration Trenches, Basins and Filter Drains Infiltration trenches comprise stone filled reservoirs to which stormwater runoff is diverted, and from which the water gradually infiltrates the ground. Their longevity is enhanced through the incorporation of a filter strip, gully or sump pit to remove excessive solids at the inflow. Widely used by highway authorities for draining roads, filter drains are similar structures through which a perforated pipe runs. This facilitates the storage, filtering and some infiltration of water on route from the source to the discharge point. Pollutant removal is by absorption, filtering and microbial decomposition in the surrounding soil. Systems can be designed which successfully incorporate both infiltration and filter systems. South Gloucestershire design guide Sustainable Drainage Systems L8 8.16 Bar Slash X Ranch LLC - Annexation and Stage Road PUD/Subdivision: Final Submission Further Information Protecting the Quality of Our Environment: Sustainable Urban Drainage - An Introduction - SEPA and the Environment Agency - Rio House, Waterside Drive, Aztec West, Almondsbury, Bristol BS12 4UD. (Tel 01454 624400) ISBN 1- 901322- 12- 8. Environment Agency website: http: www.environment-agency.gov.uk contains initial assessment on current research on SuDS Scope for Control of Urban Runoff: CIRIA Report 124. Construction Industry Research and Information Association, London SW1P 3AU (Tel 02072 228 891) CIRIA’s two SuDS manuals: C522 Sustainable urban drainage sys- tems - design manual for England and Wales and C523 Sustainable urban drainage - best practice handbook. Urban Drainage - the Natural Way: Hydro Research and Development, Clevedon, BS21 7RD (Tel 01275 878371). South Gloucestershire Council, Planning, Transportation & Strategic Environment: Castle Street, Thornbury, Bristol, BS35 1HF (Tel 01454 868686) www.southglos.gov.uk Groundwater Source Protection Zones: For further information contact your local EA office Consultation Process This guidance was published as an interim advice note in 1999. It was subject to advertisement and public consultation from 17th December 2001 to 17th January 2002. As a result of comments received a revised leaflet was considered by the Council and the final version was adopted on the 30th September 2002. Full details of the consultation process are available from the Council. Soil Type Constraints SuD design P o l l u t a n t Solids P N BOD Metals Bacteria Overall Extended detention ?good pond,24hr detention As above with ?high shallow marsh Wet pond, ?high large Infiltration trench, good infiltrates first flush Infiltration basin, good infiltrates first flush Porous pavement, good infiltrates first flush Filter strip, ?low 6m wide (grass) Grassed swale, ?low low gradient Key 0-20% removal 20-40% removal 40-60% removal 60-80% removal 80-100% removal insufficient knowledge P Phosphorous N Nitrogen BOD Biochemical Oxygen Demand ? BMP Soil Inf iltration Rate (mm / hr ) 200-12 12-7 7-2 2- 1.5 1.5- 0.5 (sandy) (loam) (silty) (silt/clay) (clay) Extended detention pond Wet pond Infiltration trench Infiltration basin Porous pavement Swale/filter strip Key Feasible Marginal- needs careful design Not Feasible Source:Protecting the Quality of our Environment: A Guide to Sustainable Urban Drainage:SEPA/EnvironmentAgency ISBN 1-901322-01-7 Examples of Pollutant Removal Capacity Leaflet designed by Graphics & Mapping South Gloucestershire Council www.southglos.gov.uk Printed on recycled paper PT&SE 8686 Soil Permeability and Hydrology South Gloucestershire includes a wide range of ground conditions. The area also includes extensive old mine workings which can intercept drainage. Soil permeability can have a significant effect on the selection of SuDS mechanisms. Infiltration techniques may not be effective if the infiltration rate is below 10mm/hr for the upper soil layers. Swales and ponds, working by a combination of filtration and infiltration, are more tolerant of poor soils. In highly permeable soils wet ponds need to be lined. In the vicinity of old mine workings, SuDS must be designed to avoid linking to such features. It is important therefore that developers establish the soil conditions and hydrology of their site at an early stage in the site planning process. The results of such investigations should be provided to the Council as background to the proposals for a drainage system included with the planning application. This guidance should not be read in isolation, but with all other relevant planning policies of the Council. South Gloucestershire design guide Sustainable Drainage Systems L8