Case Study: Flood Risk Assessment, Hydraulic Modelling and Drainage Strategy for a Holiday Development on Scotland’s NC500
Aegaea was commissioned to support a proposed holiday accommodation development in Laide, a village on Scotland’s North Coast 500 (NC500) route overlooking Gruinard Bay. Our team delivered a Flood Risk Assessment (FRA), bespoke fluvial hydraulic modelling and a surface water drainage strategy to demonstrate that the site could be safely developed despite fluvial and coastal flood risk constraints.
The site is located close to the Sand Burn and had been identified by Highland Council planners as potentially at risk of flooding. Aegaea was instructed to define the Flood Risk Area, assess flood risk from all relevant sources and demonstrate that proposed hardstanding areas could be managed through an appropriate Sustainable Drainage Systems (SuDS) strategy.
The Challenge: Proving a Safe Layout on a Watercourse-Constrained Rural Site
The proposed development – a small cluster of glamping pods with their own welfare facilities – falls within the “Most Vulnerable” land-use category under SEPA’s Flood Risk and Land Use Vulnerability Guidance. As a result, Policy 22 of NPF4 and Highland Council’s Flood Risk and Drainage Impact Assessment Supplementary Guidance required all proposed buildings to be located outside the Flood Risk Area, while safe pedestrian and vehicular access and egress had to be demonstrated during the 1 in 200 year plus climate change flood event.
SEPA’s national flood maps placed the site in the vicinity of the Medium (0.5% annual probability) to High (10% annual probability) fluvial flood extents. However, SEPA’s published maps are derived from strategic, national-scale modelling, and are generally not suitable for site specific analysis. They also do not always account for structural blockages, despite these being a requirement when defining the Flood Risk Area under SEPA’s Flood Risk Guidance for Stakeholders for a site specific Flood Risk Assessment to accompany a planning application. Bespoke fluvial hydraulic modelling was therefore required.
Aegaea’s Approach: A Linked 1D–2D Model Tailored to the Site
Aegaea developed a fully linked 1D–2D hydraulic model (flood model) using Flood Modeller -TuFLOW. Due to the site’s rural location, no publicly available LiDAR data existed for the site or the surrounding area. A robust topographical survey specification was therefore produced by Aegaea at the outset of the project to capture all key areas within the modelled reach, including the floodplain and other significant topographical features.
This approach ensured the hydraulic model accurately represented both the watercourse and surrounding floodplain, providing a robust basis for defining the site’s Flood Risk Area and informing the development layout.
The 1D domain was constructed from a channel and structure survey using 18 surveyed cross-sections across the Sand Burn and the unnamed ditch, together with the two key culverted structures – including the six-barrelled 1.3 m diameter culvert carrying the burn beneath the A832, which had to incorporate a 50% blockage in accordance with SEPA’s Technical Flood Risk Guidance for Stakeholders. The 2D floodplain was built on a 2 m computational grid, with the site’s detailed topographic survey triangulated and burned in over OS Terrain 5 data; ground truthing in accordance with SEPA’s aforementioned guidance of the two surfaces showed an average vertical discrepancy of just 72 mm, confirming a good correlation between the two datasets.
Design flows were derived following SEPA’s current Technical Flood Risk Guidance for Stakeholders documentation, delineating a 7.03 km² catchment for the Sand Burn and a 0.11 km² catchment for the unnamed ditch. Both FEH Rainfall-Runoff and ReFH2 flow estimates were assessed. However, because the catchment’s FARL value was below 0.9 (the threshold below which SEPA advises caution when using ReFH2), the more conservative FEH Rainfall-Runoff estimates were adopted.
A climate change uplift of 48% was applied to the 1-in-200-year event in line with SEPA’s latest allowances for the West Highland River Basin, taking the peak Sand Burn flow to 33.17 m³/s.A conservative coastal tidal level was applied at the downstream boundary, assuming coincident peak fluvial and tidal conditions to represent a robust worst-case scenario.
The Outcome: Supporting Planning Approval Through Robust Flood Risk Assessment and Drainage Design
The Flood Risk Assessment and supporting hydraulic modelling (flood model) demonstrated compliance with both Policy 22 of NPF4 and Highland Council’s Flood Risk and Drainage Impact Assessment Supplementary Guidance. The assessment confirmed that all proposed pods could be located outside the 1 in 200 year plus climate change (plus blockage) flood extent – i.e. the Flood Risk Area – with finished floor levels set at least 600 mm above the design flood level. It also demonstrated safe pedestrian and vehicular access and egress via Columbia Court to the north-west, which remains dry throughout the design flood event.
Alongside the flood assessment, we prepared a surface water drainage strategy for the development. As a greenfield rural site with no meaningful public sewerage nearby, the strategy prioritised a Sustainable Drainage Systems (SuDS) approach in line with NPF4 Policy 22, minimising impermeable area, managing runoff naturally on site and avoiding reliance on the combined sewer network. This ensured the completed development would not increase surface water flood risk to the surrounding area.
The planning application was subsequently approved by Highland Council’s North Planning Applications Committee on 11 June 2025.
Our Expertise: Supporting Rural and Tourism Developments Across Scotland
This project is a good example of Aegaea’s expertise in supporting developers on rural and remote sites across Scotland, including holiday accommodation and tourism developments such as glamping sites, lodges and campsites. These schemes bring their own challenges, including limited or unavailable LiDAR coverage, , small ungauged watercourses, coastal and estuarine influences, and land-use classifications that demand a high standard of flood risk assessment.
By providing:
- A detailed and appropriate topographical survey specification
- Appropriate hydrological analysis of multiple watercourses – in accordance with SEPA’s Technical Flood Risk Guidelines
- A site specific 1D-2D fluvial hydraulic model
- Analysis of results and determination of the Flood Risk Area and developable footprint
- Suitable mitigation measures, including finished floor levels
- Application of the SuDS hierarchy and drainage modelling to produce a surface water drainage strategy in line with Highland Council’s Flood Risk and Drainage Impact Assessment Supplementary Guidance
This resulted in a successful planning application.
Our team works on these challenges every day, combining practical planning experience with specialist flood risk and hydraulic modelling expertise to turn complex site constraints into workable, consentable development proposals. If you’re planning a rural or holiday accommodation development anywhere in Scotland and require a Flood Risk Assessment, hydraulic modelling or a drainage strategy to support your planning application, we’d be pleased to help.
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