In this article, we review the UK’s current flood estimation methods and how Flood Modeller’s Hydrology+ module allows hydrologists and hydraulic modellers to accelerate project delivery.
Overview of flood estimation methods:
Flood estimation methods can broadly be divided into two independent approaches. Firstly, the statistical method, which uses observed river flow records going back decades to estimate peak flows for extreme events. The second method is conceptual catchment modelling (rainfall-runoff) which derives design flood hydrographs from estimates of extreme rainfall.
These methods are often combined, drawing on each other's strengths.
Following the publication of the Flood Studies Report (NERC, 1975), the FSR techniques became gradually popular among the hydrology community to derive flood hydrology for UK and Irish catchments. With more hydrometric data and computing tools available, the FSR methods were supplemented by various new research in the post-FSR era through the publication of FSSR (Flood Studies Supplementary Reports), including the IH124 method.
Following the publication of the Flood Estimation Handbook (Institute of Hydrology, 1999), the FEH method replaced the FSR method in flood hydrology. The FEH rainfall-runoff method was essentially an extension and improvement of the original FSR rainfall-runoff method, which was replaced by the Revitalised Flood Hydrograph (ReFH1 and currently ReFH2) for deriving T-year event peak flows and hydrographs. Many older methods continue to be used for niche applications such as reservoir safety.
Industry standard for deriving flood hydrology in the UK
The current UK industry practice for deriving flood hydrology involves applying the FEH methodology using the WINFAP and ReFH2 hydrology software packages developed by Wallingford HydroSolutions. Both solutions have been approved by the Environment Agency, the Scottish Environment Protection Agency, Natural Resources Wales, and the Department for Infrastructure Northern Ireland.
According to the Environment Agency Flood Estimation Guidelines (2022), the improved set of procedures incorporated in the latest version of WINFAP has replaced most of the previous peak flow estimation methods for ungauged sites. Similarly, the latest version of the Revitalised Flood Hydrograph (ReFH2) method has replaced the original 1999 FEH rainfall-runoff method for most applications.
WINFAP is capable of undertaking statistical analysis for both gauged catchments (single site and enhanced single site analysis using AMAX data) and ungauged catchments (pooling group analysis using peak flow dataset at UK gauges and FEH catchment descriptors). ReFH2 is capable of deriving both peak flow estimates and flood hydrographs at catchments using FEH catchment descriptors and the latest rainfall data, both available within the FEH web service.
ReFH2 is a recommended method within the CIRIA SuDS Manual for undertaking the estimation of greenfield runoff rates and volume using the rural model, replacing many older methods that have been used. It is also a recommended method for undertaking the estimation of post-development runoff rates and volumes for simple developments using the urban modelling capabilities. Brownfield runoff rates for developments are also modelled using ReFH2.
Accelerating project delivery using Hydrology+
The latest release of Flood Modeller, version 7.1, directly integrates WINFAP and ReFH2 into Hydrology+, bringing advanced workflows, tools and automation to both hydrologists and hydraulic modellers.
Hydrology+ digitises the hydrological analysis workflow, bringing consistency, repeatability and easy adoption of the latest UK hydrological methods. Hydrology+ covers all stages of hydrological analysis, including data collection and visualisation, catchment schematisation, WINFAP and ReFH2 analysis, reconciliation of methods, application of hydrology and reporting.
Organisations already undertaking hydrology using WINFAP and ReFH2 can use their existing licences alongside Hydrology+. Those looking to undertake flood risk assessments or looking to transition from drainage design software can derive greenfield/brownfield discharge rates and volume using the latest tools, and can also benefit from the WINFAP and ReFH2 integration with Hydrology+, enabling the rapid update of methods within a structured workflow.
Key Hydrology+ functionality includes
Access to the latest datasets, including FEH Online, Environment Agency gauge data and UKCEH National River Flow Archive (NRFA), all from within Flood Modeller, making sure you are using the most up-to-date data whilst simplifying access.
Fully integrated WINFAP and ReFH2 workflows, providing access to the most current methods and tools, including FEH22 rainfall and the latest climate change uplifts.
Direct application of hydrology to both 1D and 2D models, allowing rapid assessment of flood risk and removing the need for reprocessing of hydrological analysis.
Automated reporting, aligned to Environment Agency reporting requirements, allows for the automatic generation of hydrology deliverables, saving time and reducing human error (coming soon).
For further information on Hydrology+, visit our website or watch our recent webinar.
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