Cyngor Bro Morgannwg
Vale of Glamorgan Council

The drainage report confirms that certain areas of the development land are already at high risk of surface water flooding. NRW flood mapping shows strong possibilities of local watercourse flooding. NRW’s detailed Product 6 flood data is not available, meaning there is no numerical assessment of depth, velocity, or hazard rating. This omission makes it impossible to determine if there is safe access/egress during a flood event, whether proposed levels will displace water, or whether the attenuation system is correctly sized. Without this essential dataset, no defensible flood risk assessment can be completed. The drainage report acknowledges differences of up to 2.5 metres between the site’s topographical survey and NRW flood model outputs. Such a discrepancy is extreme. This means the modelled flood outlines may be wholly incorrect. A site with such substantial mapping errors should not be advanced to allocation. The capacity of the central watercourse is unknown and no evidence exists on either the open channel or culvert(s). If this watercourse lacks capacity, the result would be flooding of new homes and existing homes. KS1 site comprises slowly permeable, seasonally wet loamy and clay soils, with impeded drainage. Infiltration rates are expected to be too low to meet BRE365 standards. KS1 runoff must be discharged to the watercourse. This places huge pressure on the ditch and culvert. Reliance on a single outfall in these conditions is high risk. The land naturally produces high runoff and is hydrologically very sensitive. Overland flow from neighbouring farmland already enters the site. This means the drainage design must reroute external water as well as onsite runoff. Foul drainage all remains entirely unconfirmed - with all options requiring major off site works. The strategy presents multiple foul disposal options, all of which are speculative. This means that even if surface water solutions were viable (which they are not), the foul network may be unable to support the development, creating uncertainty and risk. The strategy is incomplete, with many critical elements remaining “to be confirmed”. Key required data and assessments are missing, including: BRE365 infiltration testing, downstream culvert surveys, Watercourse capacity assessment, confirmation of foul capacity from Welsh Water. A site cannot be allocated when its fundamental drainage viability is still unproven: flood mapping is unreliable; flood risk data is missing; outfall capacity is untested; foul drainage has no confirmed solution. These issues represent fundamental constraints. The site should therefore not be allocated within the RLDP.