Comrie Flood Protection Scheme
The village of Comrie is located at the confluence of the rivers Earn, Lednock and the Water of Ruchill. Due to the village’s history of flooding, Sweco was appointed to design a scheme to protect 420 properties from a 1:200 year fluvial event, taking on the role of Lead Consultant and Principal Designer. Sweco’s multidisciplinary project team include hydrologists, structural engineers, geotechnical engineers, ecologists, landscape architects and environmental consultants.
The design will deliver infrastructure and environmental improvements whilst also enabling the regeneration of areas previously at risk of flooding. The design makes use of novel bank protection such as root wad revetments and included extensive geomorphological assessments to ensure a robust solution is provided in an area of rapid change.
Multiple climate change and asset degradation scenarios were assessed to identify trigger points enabling a managed adaptive approach to future flood risk. A detailed economic appraisal was undertaken to confirm that the scheme provided a positive return to the economy. The scheme will be entirely passive ensuring that lifetime carbon is minimised.
Sweco’s multi-disciplinary team has led this project from the feasibility study through initial investigations outline and detailed design to produce the Flood Order documentation; this has completed on time and to budget. Sweco is currently developing the detailed design and will be providing design support services during construction.
The Result and Benefit to the Client
Consultation with upstream landowners regarding natural flood management was recommended by Sweco leading to a separate project that is seeking to mitigate the effects of climate change on the flood defence performance by developing natural flood management measures and land management changes in the catchment.
Sweco recommended the development of a sediment transport/riverbed evolution model to estimate the impact of new bank protection measures. This has allowed the client to manage landowner concerns with regards to changing erosion and deposition patterns. The model provided a detailed understanding of shear force on the bank allowing for the efficient design of bank protection works.
The use of grey bank protection was likely to result in a downgrading of the rivers WFD rating. The scheme would need a derogation to proceed in this instance. By placing defences as far back as possible and by using alternatives to grey bank protection Sweco was able to reduce the impact of the scheme on river morphology and avoid the derogation. One added value measure put forward by Sweco is the use of root wad revetment (over 300m of bank), this makes use of local materials, and is cheaper than the traditional alternative whilst also reducing the embodied carbon of the project.
Sweco’s design removed the need to jack-up a historic bridge that was included in the feasibility design, undertaken by a different consultant. The jacking of the bridge was considered necessary as a result of coarse assumptions made by the previous consultant, identified by Sweco during a due diligence review. Sweco made recommendations to revise the hydrology, particularly the method in which joint probability had been calculated. Once revised, the hydrology removed the over prediction at the bridge location, and it did not need to be unnecessarily disturbed. SEPA accepted the revised hydrology.
Passive operation – Sweco (NCE100 Low Carbon Leader Award winners 2018 & 2019) continually strives to improve sustainability focussing on opportunities to create environmental, economic, and societal benefits such as reducing whole life carbon.
On the Comrie Flood Protection Scheme we applied carbon reduction interventions that allowed us to exclude pumping stations (testing secondary flooding in our integrated hydraulic model), liaising with statutory bodies to gain agreement. This, together with the elimination of flood gates and grey bank protection, resulted in one of the first intentionally passive schemes in Scotland – reducing the lifetime project cost by £1M and carbon footprint by 1000tCO2e.