33 Alma Rd, Windsor
The Brief
RIDA Reports was appointed to undertake a detailed flood risk assessment for a proposed extension to an existing residential property located at 33 Alma Road in Windsor. The scheme involves expanding the current dwelling, which includes an existing basement, while ensuring the new additions are safe and sustainable without increasing flood risk elsewhere.
Why the Client Chose RIDA
The client required specialist flood risk expertise to navigate the planning process, as the property is located in an area with a documented history of varying flood risks. They chose RIDA for our proven ability to deliver robust assessments and practical mitigation strategies, ensuring the development would strictly comply with national planning policies and satisfy local authority requirements.
The Challenge
The site presented a unique combination of environmental and structural constraints. It is situated within River Flood Zone 2, which carries a medium probability of fluvial flooding, and it faces a high risk of groundwater flooding due to seasonally varying groundwater levels. While the risk of surface water flooding is very low, the presence of an existing basement significantly complicated the groundwater risk profile. Additionally, architectural constraints dictated that the finished floor level of the new extension had to match the existing building, limiting the ability to simply raise the entire development above extreme flood levels.
The RIDA Difference
We conducted a detailed flood risk assessment to evaluate all sources of flooding in accordance with the National Planning Policy Framework (NPPF). To mitigate the fluvial risks, we proposed a comprehensive water exclusion strategy.
This included specifying flood-resilient materials with low permeability, suspended concrete floor slabs, and ensuring all new sensitive services and fittings were placed at least 600mm above the flood level.
To directly address the high groundwater flood risk affecting the existing basement, we recommended moving all accommodation to the upper floors and breaking up the basement to install a robust 3-barrier tanking system.
This solution incorporated Type A (Barrier Protection), Type B (Structurally Integral Protection), and Type C (Drained Protection) alongside a gravity drain membrane and sump/pump system.
Furthermore, we demonstrated that surface water run-off would be safely managed using Sustainable Drainage Systems (SuDS) to replicate the existing drainage regime, and identified a safe, dry evacuation route via Alma Road.
The Outcome
The comprehensive assessment successfully demonstrated that the proposed extension will be safe for its design life and fully compliant with the NPPF and Environment Agency guidelines. By embedding tailored resilience measures and a highly specific groundwater mitigation strategy, the report provided the essential technical evidence required to support the planning application while effectively managing the flood risk for the property’s future occupants.
