Testun llawn:
HG1 KS1 North West Barry (Weycock Cross)
Objection-KS1: Flood Risk and Sustainable Drainage
HG1 KS1: Flood Risk and Sustainable Drainage
Ref: NWB_RLDP_JVN_007
I object to the proposed allocation of land at Weycock Cross, Barry (HG1 KS1) on the grounds
that the developer’s own Preliminary Drainage Strategy (Persimmon, May 2025)
demonstrates unresolved, significant, and in some cases unquantified flood-risk and
drainage constraints. These issues fundamentally undermine the site’s suitability for
residential development and render the allocation unsound.
1. Surface-Water Flood Risk Remains Unresolved Due to Missing Critical Data
Although the site lies within Flood Zone 1, the drainage report confirms that:
NRW mapping shows local watercourse flooding within the developable area
parts of the site are already at risk of surface-water flooding
NRW’s detailed Product 6 dataset is unavailable, meaning there is no numerical
assessment of flood depth, velocity, or hazard ra ng
Without Product 6 data, it is impossible to determine:
whether proposed site levels will displace water onto adjacent land
whether attenuation features are correctly sized
whether safe access/egress can be maintained during a flood
In the absence of this essential dataset, no defensible flood-risk assessment can be
completed. Allocation of the site is therefore premature and contrary to PPW requirements
for evidence-based flood-risk management.
2. Major Discrepancies Between NRW Flood Maps and Surveyed Ground Levels
The drainage report acknowledges discrepancies of up to 2.5 metres between the site’s
topographical survey and NRW’s flood-model outputs. This level of variance is extreme;
flood models typically operate within tolerances of 0.10–0.25 m, not 2m or more.
Such discrepancies imply that:
flooding may be more extensive than shown
the proposed development platform may not be safe
modelled flood outlines may be fundamentally inaccurate
A site with such substantial mapping errors should not progress to allocation without full
resolution.
3. Outfall Capacity of the Central Watercourse Is Unknown
The strategy proposes discharging all surface water into the on-site watercourse at a
controlled rate of 70.4 litres per second (l/s). However:
no hydraulic modelling has been undertaken
the downstream culvert has not been surveyed
no evidence demonstrates that the open channel or culvert can accommodate 70.4
l/s, let alone exceedance flows during extreme rainfall
If the watercourse lacks capacity, the consequences include:
increased flood risk to existing homes
uncontrolled exceedance flows affecting sensitive ecological receptors
flooding of new homes
Allocating a site before verifying outfall capacity is unsafe and contrary to PPW drainage
principles.
4. Soil Conditions Make Infiltration SuDS Unviable
Soilscapes data and the drainage strategy confirm that the site is comprised of soil that is:
slowly permeable
seasonally wet
loamy and clay based
exhibits impeded drainage
As a result, infiltration rates are unlikely to meet BRE365 standards. Consequently:
100% of surface-water runoff must be attenuated and discharged to the watercourse
this places substantial pressure on a constrained ditch and culvert system
reliance on a single out all is high-risk and contrary to SuDS hierarchy best practice
5. Extremely Large Attenuation Volumes Required
The drainage strategy calculates that the 100-year + 40% climate-change event requires:
Attenuation volume: 9,580.7 m³ (approximately 4 Olympic-sized swimming pools)
Storage basin footprint: 8,926 m² (approximately the size of a football pitch)
Base dimensions: 163.6 m × 54.5 m
Design depth: up to 1.14 m for the 200-year event
Such extensive attenuation demonstrates:
the land naturally produces high runoff
the site is hydrologically sensitive
the developable area would require substantial re-engineering
Planning for this level of intervention strongly indicates that the land is inherently
unsuitable for residential development.
6. Overland Flow from Adjacent Farmland Already Enters the Site
The drainage report confirms that runoff from neighbouring land flows onto the
development area. This means the drainage design must intercept and reroute:
external overland flows
internal site runoff
Any miscalculation in levels, flow direction, or attenuation capacity could:
redirect water toward existing housing
increase on-site flood risk
cause contaminated exceedance flows to migrate toward sensitive receptors,
including the adjacent Ancient Semi-Natural Woodland (ASNW)
This represents a major unmanaged risk.
7. Foul Drainage Solutions Are Unconfirmed and Require Major Off-Site Works
All foul-drainage options presented are specula ve and dependent on:
pumping stations and rising mains up to 650 metres in length
third-party land agreements
potential network upgrades
confirmation of capacity from Welsh Water
Even if surface-water issues were resolved (which they are not), foul-drainage uncertainty
introduces further risk, delay, and potential undeliverability.
8. Critical Elements of the Flood-Risk Strategy Remain “To Be Confirmed”
Key missing assessments include:
downstream culvert surveys
watercourse capacity modelling
BRE365 infiltration testing
full SAB pre-application review
NRW Product 6 flood-hazard data
confirmation of foul-drainage capacity from Welsh Water
A site cannot be allocated when its fundamental drainage viability remains unproven.
9. The Developer’s Own Evidence Demonstrates Fundamental Unsuitability
The drainage report confirms:
flood mapping is unreliable
flood-risk data is missing
outfall capacity is untested
infiltration is unlikely
over 9,580 m³ of attenuation is required
a discharge rate of 70.4 l/s must be imposed on an unmodelled watercourse
overland flows enter the site from surrounding farmland
foul drainage has no confirmed solution
These issues are significant, unresolved, and cannot be conditioned away or deferred to later
stages.
Conclusion
The site presents fundamental and unresolved flood-risk and drainage constraints. The
evidence demonstrates that the site is not hydrologically suitable for residential
development. It should therefore not be allocated within the RLDP
