Drainage Assessment Form hounslow gov uk

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Please see the CIRIA SuDS Manual C753 for further details about above ground attenuation techniques Examples could include but are not limited to swales detention basins rain gardens planters etc Page 4 of 9 Attenuation of rain water through tanks or sealed water features Underground storage features which gradually release water Please note that these are less sustainable than


Is the site currently known to be at risk Please attach surface water and fluvial flood risk maps as. of flooding from any sources If so shown on the Environment Agency s website and any. please state and provide evidence records of known historic flooding at the site. The Greenfield runoff rate from the development which is to be used for assessing the requirements for limiting discharge flow rates and attenuation storage from a site should be calculated for the. area that forms the drainage network for the site whatever size of site and type of drainage technique Please refer to the Rainfall Runoff Management document or CIRIA SuDS Manual for details. 2 Impermeable area,DIFFERENCE, EXISTING PROPOSED NOTES FOR APPLICANTS LOCAL AUTHORITIES. PROPOSED EXISTING,Impermeable area Ha, If proposed existing then runoff rates and volumes will be. Surfaces which do not permit infiltration of water. increasing,into the ground, Drainage Method See the London Plan Policy 5 13 Drainage Hierarchy If the. Rainwater harvesting infiltration SuDS existing drainage was via infiltration and the proposed is. watercourse sewer not section 3 should provide evidence as to why. 3 Is infiltration on site suitable Storage is required for the additional volume from site but also for holding back water to slow down the rate of discharge from the site This is. known as attenuation storage and long term storage The idea is that the additional volume is not permitted to flow rapidly overland into watercourses or into the sewer system. and hence potentially increase flood risk on site and or downstream of the site You can either infiltrate the stored water back into the ground or if this is not possible hold it back. with on site storage allowing gradual discharge at a controlled rate Please fill in the table to show the extent of your investigations as to whether infiltration is a possible route for. runoff to be discharged to,NOTES FOR APPLICANTS LOCAL AUTHORITIES. State the site s geology including Infiltration rates are highly variable and infiltrating into made i e unnatural. superficial deposits where known ground should be avoided. State the site s known Source Please refer to the Environment Agency website to identify any source protection. Protection Zones SPZ zones SPZ, Infiltration What is the development site s Infiltration rates should be worked out in accordance with BRE 365 If infiltration is.
infiltration rate the preferred method of drainage then rates should be no lower than 1x10 6 m s. If it is not feasible to access the site to carry out infiltration tests before the. Were infiltration rates obtained via, application is submitted a desktop study could be undertaken looking at the. a desktop study or from infiltration, underlying geology of the area and assuming a worst case infiltration rate If a. desktop study demonstrated that infiltration could be feasible then a ground. Page 2 of 9, investigation will need to be completed to evidence that infiltration can be used as. part of the scheme without increasing risk, Where known please use borehole test results and state the time of year these. At what depth below ground is the, were carried out If high groundwater levels are considered to be an issue then.
water table groundwater level, borehole testing should be completed as part of a ground investigation. If the ground conditions are found to be suitable from a desktop survey or ground. State the distance between the investigation a minimum of 1m depth between the base of the infiltration device. proposed infiltration device base and the water table is required to protect groundwater quality and ensure. and the water table groundwater does not enter infiltration devices Avoid infiltration where this is not. Is the site contaminated If yes A ground investigation will need to consider potential contamination to. consider advice from others on groundwater Water should not be infiltrated through land that is contaminated. whether infiltration is a safe The Environment Agency may provide bespoke advice in planning consultations for. solution contaminated sites that should be considered. In light of the above information is infiltration If infiltration is not feasible the applicant should consider the options in section 4 If. feasible infiltration is feasible then it can be combined with the methods in section 4. 4 Method proposed for discharge of surface water in line with the London Plan Policy 5 13 Please select multiple options where necessary Where an. option is not deemed possible sufficient justification must be submitted with supporting evidence where necessary. YES NO EVIDENCE THAT THIS IS OR IS NOT POSSIBLE NOTES FOR APPLICANTS LOCAL AUTHORITIES. Rainwater harvesting is where rainwater is stored on site. for reuse For example water for gardening domestic use. Rainwater harvesting, etc Harvesting features could include but are not limited. to rainwater harvesting tanks and water butts, Allowing space for rainwater to soak into the ground as. Infiltration per natural methods If proposed these must satisfy the. requirements shown in section 3 of this DAF,Please see the CIRIA SuDS Manual C753 for further. Attenuation of rain,details about above ground attenuation techniques.
water in ponds and, Examples could include but are not limited to swales. open water features,detention basins rain gardens planters etc. Page 3 of 9, Underground storage features which gradually release. Attenuation of rain, water Please note that these are less sustainable than. water through tanks or, above ground methods and are usually more complex to.
sealed water features, Is there a watercourse nearby If so please name stating. approximate distance from site Evidence of discharge. To watercourse agreement with the Environment Agency for main. rivers or the Lead Local Flood Authority for ordinary. watercourses i e all non main rivers will be required. The confirmation from sewer provider that sufficient. To surface water sewer, capacity exists for this connection will be required. This would only be acceptable where other options are. not reasonably practical and will not be accepted where. To combined sewer, separate sewer systems currently exist on or close to the. 5 Supporting calculations in order to check that the proposed development is designed to conform to standards please complete the following three tables and. submit your supporting calculations, A Peak discharge rates this is the maximum flow rate at which storm water runoff leaves the site during a particular storm event. IH124 method FEH method Other please, Please circle which method was used to calculate the Greenfield Runoff Estimation.
London Plan policy 5 13 Developers should aim for a Greenfield runoff rate from their developments. London Plan Sustainable Design and Construction SPG section 3 4 10 All developments on Greenfield sites must maintain Greenfield runoff rates On previously developed sites. runoff rates should not be more than three times the calculated Greenfield rate. EXISTING RATES IF, GREENFIELD RATES L S PREVIOUSLY DEVELOPED PROPOSED RATES L S NOTES FOR APPLICANTS LOCAL AUTHORITIES. QBAR QBAR is approximately the 1 in 2 year storm event. Page 4 of 9, Proposed discharge rates with mitigation should be no greater than the. 1 in 1 year Greenfield rates for all corresponding storm events If restrictions to Greenfield. rates cannot be achieved then sufficient justification with supporting evidence. 1 in 30 year must be submitted Please note that discharging all flow regardless of the. corresponding storm event intensity from site at the existing 1 in 100 year event. rate would increase flood risk during smaller events and therefore would not be. 1 in 100 year permitted Applicants must also check and adhere to existing Local planning. policy where further runoff restrictions may apply. To mitigate for climate change the proposed 1 in 100 year CC runoff rate must. be no greater than the Greenfield 1 in 100 year event runoff rate The peak rainfall. 1 in 100 year plus, intensity allowance used must be in line with the latest Environment Agency s. climate change, climate change allowance guidance Currently this requires allowances of 20. and 40 to be added and the most appropriate one used and justified Typically. for heavily urban sites we would ask for 40 to be used. B Discharge volumes post development, The Non Statutory Technical Guidance for SuDS Where reasonably practicable for Greenfield development the runoff volume from the development to any highway drain.
sewer or surface water body in the 1 in 100 year 6 hour rainfall event should never exceed the Greenfield runoff volume for the same event Where reasonably practicable for. developments which have been previously developed the runoff volume from the development to any highway drain sewer or surface water body in the 1 in 100 year 6 hour. rainfall event must be constrained to a value as close as is reasonably practicable to the Greenfield runoff volume for the same event but should never exceed the runoff volume. from the development site prior to redevelopment for that event. IF THE PROPOSED STORAGE VOLUME ON SITE POST DEVELOPMENT IS LESS THAN THE STORAGE VOLUME. STORAGE VOLUME REQUIRED TO, PROPOSED STORAGE VOLUME ON REQUIRED TO ACHIEVE THE GREENFIELD RUNOFF RATE PLEASE PROVIDE A JUSTIFICATION AS TO WHY. ACHIEVE THE GREENFIELD RUNOFF, SITE POST DEVELOPMENT M3 APPLICANTS MUST ALSO CHECK AND ADHERE TO EXISTING LOCAL PLANNING POLICY WHERE FURTHER. RUNOFF RESTRICTIONS MAY APPLY,1 in 100 year,6 hour event. Page 5 of 9, C Storage methods Attenuation storage is provided to enable the rate of runoff from the site into the receiving watercourse or sewer to be limited to an. acceptable rate to protect against erosion and flooding downstream The volume needing storage is a function of the amount of development relative to the. Greenfield discharge rate, TYPE OF SUDS FEATURE VOLUME M3 NOTES FOR APPLICANTS LOCAL AUTHORITIES.
SuDS can be adapted for most situations even where infiltration isn t feasible e g. 5 impermeable liners beneath some SuDS devices allows treatment but not infiltration. See the CIRIA SuDS Manual C753, If no storage features have been proposed please explain why this is the case and. 7 provide evidence to support this reasoning in the box below. This value should be equal to or greater than the Proposed storage volume value in. section 5B,Page 6 of 9, IF NO STORAGE FEATURES HAVE BEEN PROPOSED IN THE SECTION ABOVE PLEASE EXPLAIN WHY THIS IS THE CASE AND PROVIDE EVIDENCE TO BACK UP THIS REASONING IN THIS BOX. EVIDENCE MUST BE SUFFICIENTLY DETAILED TO DEMONSTRATE THAT A SUDS BASED SYSTEM IS IMPRACTICAL FOR THIS SITE. 6 Please confirm,NOTES FOR APPLICANTS LOCAL AUTHORITIES. PLEASE NAME RELEVANT EVIDENCE DOCUMENT S, The Non Statutory Technical Standards for SuDS states that no part of. That the drainage system can contain the 1 the site should flood during a 1 in 30 year event unless that area is. in 30 storm event without flooding designated to hold and or convey water as part of the design This is. also a requirement for Sewers for Adoption and is good practice. That any flooding between the 1 in 30 1 in Safely not causing property flooding or posing a hazard to site users. 100 plus climate change storm events will be i e no deeper than 300mm on roads footpaths Flood waters must. safely contained on site drain away at section 5A rates. As per the Non Statutory Technical Standards for SuDS proposed. How runoff flows from storm events in methods for managing excess flows should be demonstrated so as to. excess of 1 in 100 years will be managed on minimise the risks to people and property e g through evidence of. site exceedance routes These exceedance proposals should be for runoff. in storm events greater than 1 in 100 year plus climate change. Page 7 of 9, Recent development in some flow control devices now allow flow.
How are rates being restricted hydrobrake, rates to be controlled as low as 0 7l s without pipe blockages see. Hydrobrake, 7 Adoption and maintenance please provide the following information Note This space should only be used to reference a standalone document that. covers details of both maintenance and adoption of the scheme. ADOPTION AND MAINTENANCE INFORMATION NOTES FOR APPLICANTS LOCAL AUTHORITIES. Please confirm the proposed owners adopters, If there are multiple owners a drawing illustrating. of the entire drainage systems throughout the, exactly which features will be within each owner s. life of the development Please list all the,remit must be submitted with this DAF.
owners and contact details,Clear details of the maintenance proposals of all. elements of the proposed drainage system over the,lifetime of the development must be provided. Poorly maintained drainage can lead to increased, How is the entire drainage system to be flooding problems in the future. maintained,Please note that a maintenance plan should be. provided as a standalone document so that it can,then be implemented easily by whoever adopts the.
drainage scheme going forward,Page 8 of 9, 8 Evidence Please identify where the details quoted in the sections above have been taken from i e supporting plans calculations reports etc Please. also provide relevant drawings that need to accompany your submitted DAF in particular exceedance routes and ownership and location of SuDS. maintenance access strips etc, FORM SECTION DOCUMENT REFERENCE WHERE DETAILS STATED ABOVE ARE TAKEN FROM PAGE NUMBER S. Section 5A,Section 5B,Section 5C, This DAF should be completed using evidence from the documents submitted with this application This should include site plans and if necessary for the site a Flood Risk. Assessment FRA see the Local Planning Authority s Strategic Flood Risk Assessment for details of when a FRA is required The DAF serves as a summary sheet of the drainage. proposals and should clearly show that the proposed runoff rate and volume as a result of development will not be increased If without the use of SuDS there would be an. increase in runoff rate and or volume the rate and volume sections should be completed to set out how the additional rate volume will be managed. Form completed by, Qualification of person responsible for signing off this Drainage Assessment Form.

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