Drainage

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Foul sewerage

The development can be drained by gravity to the existing Anglian Water sewerage system, however the existing sewers adjacent/near to the site do not have sufficient capacity to accommodate all foul flows in a single connection. It is proposed to utilise several foul sewer connections to the existing sewer system, subject to confirmation of capacities from Anglian Water.

However, the majority of foul flows shall connect to the existing sewer which leads to Histon Road from Thornton Way which runs across the north of the site. This existing sewer is 225mm diameter and shall most likely require upsizing up to a foul pumping station located in Hazelwood Close near to Histon Road. The pumping station shall also require upgrading to ensure it can accommodate the additional foul flows.

Surface water sewerage

The ground conditions of the site have been assessed for the potential to dispose of surface water runoff in to the subsoil by soakage. Infiltration tests have been carried out in trial holes across the whole site. Only one location close to the eastern boundary adjacent to Tavistock Road provided a positive result. All other tests failed due to the presence of clayey materials in the subsoil. It is therefore considered that soakage is not a feasible form of disposal for surface water runoff from the proposed development.

The site topography falls from the Huntingdon Road end of the site towards Histon Road. Surface water drainage should therefore follow this general slope of the land. The existing buildings at the Huntingdon Road end of the site already have existing surface water drainage which connects to an existing ditch adjacent to Whitehouse Lane. It is proposed to retain the existing outfalls to this ditch for the land which is to be retained by NIAB. This allows for this part of the redevelopment to be drained independently from the remainder of the site.

Surface water flows from the NIAB retained land shall need to be balanced to no more than the existing flows from the existing buildings etc. The remainder of the development shall be drained by a positive surface water sewer system to be adopted by Anglian Water. The sewers shall lead to the north-eastern boundary of the site where the new access road is to be constructed, and leading to an existing ditch. The ditch is a tributory of Histon Brook. Flow to the ditch shall strictly be restricted to greenfield runoff rates, thus creating no impact on flows in the watercourse.

To restrict flows to the ditch to greenfield runoff rates it is necessary to balance flows from the site. The primary storage facility for this balancing shall be from localised swales which are to be incorporated into the landscaping design for the site. A swale is a flat, wide form of ditch which in this scenario would be utilised mainly for temporary storage of surface water runoff from the site. Along perimeters of the site, the swale will be close to the existing boundary and shall be incorporated into the planting scheme.

In other areas within the site the swale can be incorporated in to the street scene and recreational areas. Some of the swales will be formed by the widening of existing ditches within the site which shall be undertaken in a sympathetic manner to protect existing habitats for wildlife. Typically swales shall be up to 10 metres wide and shall incorporate landscaping/ planting to enhance the appearance of the street scene and open areas.

Additional surface water balancing shall be provided by an off-line balancing pond close to the proposed new access off Histon Road. The pond shall provide a shallow depth of water during a storm event, however it would be incorporated in to a landscaping feature, possibly with a permanently wet pond and recreational/ landscaping features.

The swales and balancing pond shall be owned and maintained in conjunction with the open spaces from the development. All sewer pipes shall be adopted by Anglian Water, which shall provide continuity of drainage to the outfall point to the existing ditch. Connections to the swales and balancing pond shall be achieved by diversion of storm flows by throttle pipes or similar flow controls.