Cyngor Bro Morgannwg
Vale of Glamorgan Council

The site should therefore not be allocated within the RLDP.

Objection re: HG1 KS1: Flood Risk and Sustainable Drainage
I object to the proposed development at Weycock Cross, Barry because the developer’s
own Preliminary Drainage Strategy (May 2025, Persimmon) shows that the site has
major flood-risks and future drainage problems that have not been resolved.
Existing surface-water flood risk remains unresolved due to missing data: Although
the site lies within Flood Zone 1, the drainage report confirms that:
o certain areas of the development land are already at risk of surface water
f
looding
o NRW flood mapping shows local watercourse flooding within parts of the
developable area
o critically, 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
f
lood 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.
Significant inaccuracies exist between NRW flood maps and surveyed ground
levels: the drainage report acknowledges diAerences of up to 2.5 metres between the
site’s topographical survey and NRW flood model outputs. Such a discrepancy is
extreme - flood models typically operate within tolerances of 0.10–0.25 m, not 2.50 m.
This means:
o the modelled flood outlines may be wholly incorrect
o flooding could be far greater or extend into additional areas of the site
o the identified development platform may not be safe.
A site with such substantial mapping errors should not be advanced to allocation.
The central watercourse must accommodate up to 70.4 litres per second of
discharge, yet its capacity is unknown. The strategy proposes discharging all surface
water from the development into the on-site watercourse at a controlled rate of 70.4
litres per second (l/s). However:
o the downstream culvert has not been surveyed
o no hydraulic modelling has been completed
o no evidence exists to show that either the open channel or culvert(s) can take an
additional 70.4 l/s, let alone the exceedance flows during extreme rainfall.
If this watercourse lacks capacity, the result would be flooding of new homes, existing
homes, or both. Allocating a site before verifying its outfall capacity is unsafe and
contrary to PPW drainage principles.
Soil conditions make the eAectiveness of infiltration SuDS highly unlikely - forcing all
runoA into a constrained ditch. Soilscapes and strategy data show the site is comprised
of slowly permeable, seasonally wet loamy and clay soils, with impeded drainage.
As a result, infiltration rates are expected to be too low to meet BRE365 standards:
o 100% of the developable area’s runoA must be attenuated and discharged to the
watercourse. This places huge pressure on the ditch and culvert, especially
during high-intensity storms.
Reliance on a single outfall in these conditions is high-risk and non-compliant with
SuDS hierarchy best practice.
The development requires extremely large attenuation volumes: over 9,580 cubic
metres; the drainage strategy calculates the required storage for the 100-year + 40%
climate-change storm event as:
o Attenuation volume: 9,580.7 m³
o Storage basin footprint: 8,926 m² (approximately the size of a football pitch)
o Storage base dimensions: 163.6 m × 54.5 m
o Design depth: up to 1.14 m for the 200-year event
The need for such enormous attenuation demonstrates that the land naturally produces
high runoA, that the site is hydrologically sensitive and also that the developable area
must be dramatically reshaped. Such significant engineering intervention is a strong
indicator that the land is not suitable for housing development.
Overland flow from neighbouring farmland already enters the site - the report
confirms that runoA from land outside of the proposed site boundary flows onto the
development area. This means the drainage design must intercept and reroute external
water as well as onsite runoA. Any mis-calculation in flow direction, levels, or
attenuation capacity could:
o push this water towards existing housing
o increase flood risk on site
o cause contaminated exceedance flows to migrate toward sensitive receptors,
including the nearby Ancient Semi-Natural Woodland (ASNW).
This represents a major and unmanaged risk.
Foul drainage is entirely unconfirmed - with all options requiring major oA-site works.
The strategy presents multiple foul disposal options, all of which are speculative and
depend upon:
o pumping stations and rising mains as long as 650 metres
o the need for third-party land agreements
o possible network upgrades
o pending confirmation of capacity from Welsh Water.
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 further delays,
uncertainty and risk.
In conclusion, the flood risk management strategy is incomplete, with many
critical elements remaining “to be confirmed”.
Key required data and assessments are missing, including:
o BRE365 infiltration testing
o Downstream culvert surveys
o Watercourse capacity assessment
o Full SAB pre-application review
o Confirmation of foul capacity from Welsh Water
o Flood hazard data (NRW Product 6).
A site cannot be allocated when its fundamental drainage viability is still unproven.
The numerical evidence within the developer’s own drainage report confirms that:
o flood mapping is unreliable
o flood-risk data is missing
o outfall capacity is untested
o infiltration is unlikely
o over 9,580 m³ of attenuation is required
o a discharge rate of 70.4 l/s must be imposed on an unmodelled watercourse
o overland flows enter the site from surrounding farmland
o foul drainage has no confirmed solution.
These issues are significant, unresolved, and represent fundamental constraints
that cannot be conditioned away or resolved later. The site should therefore not be
allocated within the RLDP.