Appeal Allowed for Retrospective Pond Development
Detailed hydraulic modelling demonstrated that an enlarged retrospective pond reduced flood risk, convincing the planning inspector to allow the appeal against an enforcement notice.
Project challenges
The client had excavated a significantly larger pond to replace an existing one and built an associated embankment for water impoundment, all without planning permission. The Local Planning Authority had taken enforcement action against the retrospective development.
The site sat within a surface water flooding risk zone per Environment Agency data. While ponds qualify as "Water Compatible" under planning policy, the extent of landscaping required assessment of whether off-site flood risk had increased. The enforcement notice raised concerns about both the increased pond volume and the structural integrity of the new embankment.
How we solved it
The team developed two hydraulic models — one representing pre-excavation conditions and another reflecting the as-built scenario using detailed topographic survey data. This comparison quantified changes in flood behaviour across the development.
Separate embankment breach modelling assessed both dry-day and wet-day failure scenarios to evaluate the risk to downstream properties from the larger water body. The analysis considered the full range of return period events including climate change allowances.
Results delivered
The modelling demonstrated that the enlarged pond would "result in a reduction in flood depth of up to 0.11m in the 1 in 100 year event with 45% climate change allowance." Downstream flood extent also decreased slightly.
Regarding embankment breach risks, the analysis showed downstream buildings would not be subject to substantially increased risk despite the larger water volume. The inspector concluded the works had decreased flood risk overall, determining that potential breach risks were outweighed by the low probability of occurrence. The appeal was allowed and the enforcement notice was quashed.
Project Overview
This case involved defending an enforcement notice related to an enlarged pond and embankment constructed without planning permission in Sussex. The Local Planning Authority had taken action against the retrospective development, and the client needed technical evidence to demonstrate that the works had not increased flood risk to neighbouring properties.
The Modelling Approach
The key to this case was a robust comparative modelling exercise. By building two separate hydraulic models — one representing the pre-excavation baseline and one representing the as-built conditions — we were able to quantify the exact change in flood behaviour resulting from the works.
The as-built model was constructed using detailed topographic survey data to accurately represent the enlarged pond geometry, embankment profile, and surrounding ground levels. Both models were run for a range of return period events up to and including the 1 in 100 year event with 45% climate change allowance.
Breach Analysis
The embankment breach modelling was a critical component of the assessment. We modelled two scenarios: a dry-day breach (failure during normal conditions) and a wet-day breach (failure coinciding with a design storm event). In both cases, the modelling demonstrated that downstream properties would not experience materially increased flood risk compared to the baseline condition.
Appeal Outcome
The planning inspector accepted our evidence in full. The modelling demonstrated a net reduction in flood depths during design events, and the breach analysis showed that residual risks were acceptable given the low probability of embankment failure. The appeal was allowed, providing the client with certainty over the future of the development.
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