| Conditions or Reasons: | 19)
No development shall take place until the detailed design and associated management and maintenance plan for the proposed surface water drainage for the site, based on sustainable drainage principles and a detailed and site specific assessment of the hydrological and hydrogeological context of the development, has been submitted to and approved in writing by the Local Planning Authority. The scheme shall include provision of attenuation and a restriction in run-off rates as outlined in the Indicative Drainage Layout (3787-301-300-G) and Drainage Note (3787 301). The approved drainage system shall then be implemented in accordance with the approved detailed design and shall be managed and maintained thereafter. The scheme to be provided shall include, at a minimum:
Full calculations detailing the existing surface water runoff rates for the 1 year, 30 year, 100 year and 100 year plus climate change;
Full results of the proposed drainage system modelling in the above-referenced storm events (as well as 1% AEP plus climate change) , inclusive of all collection, conveyance, storage, flow control and disposal elements and including an allowance for urban creep, together with an assessment of system performance;
Detailed plans and drawings showing the proposed drainage system in its entirety, including location, pipe run reference numbers, dimensions, gradients and levels (in metres above Ordinance Datum). This shall include all elements of the system proposed, including source control, storage, flow and pollution control etc.;
Details of flow control measures to be used, demonstrating that runoff rate and volume will not the agreed post-development discharge rate. For a pump based solution, the residual risk of flooding due to the failure of the pumps will be fully considered and the flood level determined under the following criteria: the pumps were to fail, the attenuation storage was 50% full, and a design storm occurred;
Full calculations of the attenuation storage volume required including allowances for climate change, based on the simulated rainfall runoff and the agreed post-development discharge rates;
Flooded areas for the 1 in 100 year storm when system is at capacity, demonstrating flow paths for design for exceedance and any additional measures required to protect people/property;
Measures taken to prevent pollution of the receiving network and evidence the downstream system is of sufficient condition/capacity to receive flow;
Integration of sustainable drainage components with wider site objectives, including water quality treatment, amenity, biodiversity and amenity;
Details of the structural integrity, proposed construction of the system, and any phasing of works;
Full details of the maintenance and/or adoption proposals for the entire drainage system, including all elements listed above, and any proposed split of the surface water management system and/or maintenance responsibilities between private (i.e. within curtilage) and public (i.e. in public open space and/or highway).
Reason: To ensure the approved system will function to a satisfactory minimum standard of operation and maintenance and prevent the increased risk of flooding both on and off site, in accordance with para 167 of the NPPF and its supporting technical guidance. This is required to be pre-commencement as the drainage strategy is intrinsic in the early design strategy. |
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