FLOOD RISK ASSESSMENT
3.0 EXISTING DRAINAGE
3.1 The nearest Public sewers are the existing combined sewers that run in Hepworth Lane. The depth of the combined sewers appear to be sufficient to service the site for foul flows but the water authority have stated that there is no capacity for surface water flows from the site. They will require the surface water to discharge to a watercourse if at all possible or utilise infiltration methods. A small amount of surface water from the site may already discharge to this point by overland flows.
5.0 PROPOSED SURFACE WATER DRAINAGE
5.1 A preliminary site investigation has shown that the geology in this area is a shallow clay layer overlying a gravel of weathered sandstones from 400mm down. Sandstone rock head was found around 2m deep. Initial soak-away tests suggest that the use of soakaways and infiltration systems would appear to be suitable for a surface water drainage system on this site.
5.2 The discharge of un-attenuated surface water run off from this site would significantly affect the downstream catchment in either combined sewers or the land drainage catchment. However the surface discharges would be minimised by the use of infiltration systems for the houses and associated paved areas and the highway could use a similar system.
5.3 Due to the size of the site and its development proposals it will be possible to provide storage for surface water flows from highways, or areas that are not suitable for Infiltration systems. There was a small area on the north of the site that has predominantly clays down to sandstone rock head at 2m which may not be able to utilise infiltration systems efficiently in this area. These would have to connect to soak aways, outside of this area where infiltration systems will be feasible. This should allowed for in the design of the storage and/or infiltration basins to cater for storms up to and including the 100year storm with due allowances for climate change. In accordance with NPPF this would mean an extra 20% based on the site usage and possible design life of the development.
5.4 The sizes of the storm water storage facilities would need to be determined accurately in the final designs once allowable discharge rates to infiltration systems are agreed. This is all in accordance with the National Planning policy Framework Technical guidance issued in March 2012 and previously in PPS2
Foul WaterYorkshire Water has indicated that foul water should be discharged to the 225mm diameter public combined sewer in Hepworth Lane. Yorkshire Water have not raised any capacity issues
. Surface WaterThere is a preferred hierarchy for the disposal of surface water. This preference indicates that first priority should be given to soakaways, then infiltration, followed by watercourse and finally to the sewers. Site investigations have been carried out which indicate that the geology beneath the surface of the land is suitable for soakaway drainage. The surface water from the development will discharge to the ground via soakaways.