Effective flood risk planning is a complex and challenging task given the climate crisis and the increase in the frequency and intensity of rainfall events. With that in mind, Causeway Technologies recently hosted a webinar that delved into the issue with expert insights from UKCEH and Wallingford HydroSolutions to help those responsible for flood planning, design and management.
In the first in a series of blogs, we summarise the knowledge shared. Before we dive into the sections of the webinar, let’s quickly discuss the history of flood risk planning and FEH.
The Flood Estimation Handbook (FEH) is a guide for estimating flood frequency in the UK and it is widely used for flood risk management and planning. However, put simply, due to the ongoing climate change threat, there needs to be a step change in how we design flood defences in the UK, and this should be undertaken using the latest advancements in rainfall estimation and hydrological modelling. Here, we offer an overview of some of the key points from our two main speakers:
Understanding rainfall DDF models with Dr Gianni Vesuviano, Hydrological Modeller at UK Centre for Ecology & Hydrology (UKCEH)
The new FEH22 rainfall depth duration frequency model
With the need to evolve our approach to flood risk planning and designing flood defences, the current FEH22 rainfall depth-duration-frequency model is the UK Centre for Ecology and Hydrology’s (UKCEH) latest rainfall frequency estimation model.
Figure 1: FEH22 rainfall graph (Credit: UKCEH)
There is a long history of national scale rainfall depth, duration and frequency (DDF) models in the UK, starting with the FSR in 1975, which is still the most recent model on a national scale for Probable Maximum Precipitation (PMP) and as such, it is still used for the design of Category A dams. However, a limited amount of data went into the FSR.
It was eventually replaced by the FEH99 in 1999. Major advances were made with the FORGEX pooling method, which relates growth curves to observed events rather than fixed factors, and a large increase in calibration data. Building on this, came FEH13 in 2015, which improved the standardisation of events even further with more spatial consistency and a larger increase in hourly calibration data.
The motivations behind FEH22
The FEH22 rainfall depth and duration frequency model was released in December 2022. This came to fruition due to two record breaking events, both in Cumbria, in 2009 and 2015 that were not captured in the FEH13 calibration dataset, as well as other significant events. Therefore, multiple requests were made for the UKCEH to update the FEH13 calibration with new data. The need to release this as a national model became clear when better access to sub daily rainfall data was also achieved.
Figure 2: Recalibration of FEH13 rainfall model for Cumbria (Credit: UKCEH)
Finally, FEH22 has been designed so that if you know just two of the factors – then you can calculate and model the third.
Considering the Environment Agency’s position that FEH22 should be used in preference to FEH13 for all new projects, alongside recalibrated ReFH 2, it’s wise to use a drainage design software like Causeway Flow which has the latest rainfall data available. The solution allows you to design complaint drainage schemes and streamline the approval process.
Figure 3: Causeway Flow software
Calibration of ReFH 2 using FEH22 rainfall with Jude Jeans, Director of Wallingford HydroSolutions
Design flood estimation in the UK
There are two FEH methods for design flood estimation in the UK - both of which are recommended as best practice by all four of the UK regulators (EA, SEPA, NRW and DFI) and the SuDS Manual for run off rates and rainfall.
The first is the FEH statistical method, which uses catchment descriptors to identify similar gauged catchments and applies statistical techniques to the annual maxima data to estimate peak flows. This has been implemented in the latest WINFAP 5 software.
The second is the FEH rainfall-runoff method, which uses both catchment descriptors and design rainfall to estimate peak flows and hydrographs. This is the method which has been implemented within ReFH 2.
Figure 4: WINFAP 5 and ReFH 2 (Credit: Wallingford HydroSolutions)
What is ReFH 2?
ReFH 2 is the latest version of the Revitalised Flood Hydrograph (ReFH) model, which was released in June 2023 in the ReFH 2 software, and crucially, draws on the FEH22 and FEH13 rainfall model data available. ReFH 2 enables you to estimate design and observed flood hydrographs for rural and urbanised ungauged catchments across the UK. By using ReFH 2 in conjunction with the UKCEH FEH Web Service you can estimate runoff rates and volumes for specific development sites.
Within the latest calibration and assessment overview for RFH2.3, there were no methodological changes in the ReFH 2.3 rainfall run off model, so any differences were related to changes in the rainfall dataset itself.
ReFH 2.3 has now been calibrated for the FEH22 rainfall where the two-year event peak flow is calibrated against the gauged QMED (the median of the gauged AMAX data). Performance has also been assessed through comparison of the ReFH 2.3-FEH22 peak flows with observed data and those estimated using the FEH statistical method and ReFH 2.3-FEH13.
Figure 5: The 2 year, 100 year and 100 year peak flow estimates (Credit: Wallingford HydroSolutions)
Causeway’s drainage design software – adopting FEH22
Given our aim to support in the efficient design of compliant drainage schemes; the inclusion of FEH22 in Causeway Flow was absolutely crucial. With our software, users can remain up to date with the UK requirements and all the latest rainfall methodologies.
Causeway's end-to-end drainage design solution can design any type of drainage system including traditional, foul, and existing systems. Whether it is preliminary or detailed, the optimised design will be based on the current UK regulatory requirements, assuring that your drainage schemes will be automatically compliant.
Furthermore, with the government’s announcement that it intends to implement Schedule 3 of the Flood and Water Management Act of 2012 in England as of 2024, this will really impact the future of SuDS design. Developers will not have an automatic right to connect drainage systems to sewers. Instead, they will need to show that SuDS are incorporated into schemes and show how the system can be maintained. Therefore, the FEH22 rainfall model as well as SuDS design are going to be critical moving forward.
To watch the full webinar on FEH22 and ReFH 2: Bridging the gap between theory and application – click here
Alternatively, if you are ready to apply FEH22 to your designs, request pricing information or book a demo to see how Causeway Flow can work for you.