Review of methodology for estimating flood peaks and hydrographs for small catchments
This project explored ways of reducing uncertainty in flood estimation in small catchments and development-sized plots of land
Summary
The aim of this research project was to review and improve flood estimation in small catchments and plot-scale areas of land in England and Wales. The project contained 2 phases.
Phase 1 reviewed the existing techniques available for flood estimation in ungauged small rural and urban catchments. We found Flood Estimation Handbook (FEH) techniques to be the most suitable, but there was scope for improving the methods.
Phase 2 of the project developed and tested FEH methods to reduce the uncertainty in flood estimates.
The project reports will be of interest to anyone wanting to understand the evidence behind the Environment Agency’s and Natural Resources Wales’ flood estimation guidelines. They will also be useful for hydrologists and scientists wanting to know more about sources of uncertainty in flood estimation and the tools and techniques available to reduce it.
Background
To improve flood risk management in the UK we need reliable flood frequency estimation in small catchments. This is important as most river catchments here are small (less than 25km2) - and most plots of land that require drainage estimates are even smaller (the area is often less than one hectare).
This project aimed to reduce the uncertainty in flood estimates for small catchments. This would lead to improved flood risk management as high degrees of uncertainty can lead to over/under design of drainage systems and flood defences - or inappropriate development in a floodplain.
Currently estimation in small catchments is difficult because of:
- a lack of river flow data
- difficulties with catchment descriptors at small scales
- the susceptibility of flood peaks to be affected by local features
The project contained 2 phases.
Research approach - Phase 1
Phase 1 was a scoping study to assess the different methods used for estimating flood flows within small catchments and plots of land. It concluded that the current generation of Flood Estimation Handbook (FEH) methods were the most appropriate to use. However, given the nature of small-catchment flood hydrology, uncertainties still existed and so Phase 1 recommended a follow-on project to:
- build an expanded data set of small catchments peak flow data
- develop improved methods to model flood flows in small ungauged catchments and plots of land
Research approach - Phase 2
Phase 2 followed up and expanded on the Phase 1 recommendations with a number of actions:
Additional data collection
New data allowed the team to:
- develop a new flood peak dataset for small fluvial catchments
- collate gauged plot-scale runoff data
Literature review and data analysis
The review and analysis was used to:
- suggest how to improve flood estimation in small fluvial catchments and sub-catchment plots
- investigate the applicability of depth-duration-frequency rainfall models for use in small catchments and development sized plots
- explore the potential for improving flood estimates in small catchments and sub-catchment plots of land by taking local data into account, such as vegetation type and land management
Develop and test proposed improvements
The main activities in this stage were to:
- use the existing FEH and version 2 of the Revitalised Flood Hydrograph (ReFH2) frameworks for development and testing
- evaluate plot-scale greenfield peak flow rate and volume estimation
Report and summarise the scientific process and findings
The objective of this stage was to:
- demonstrate how to use the new methods
- compile an overview report linking all separate studies under this project
Findings
Flood and hydrograph estimation in small catchments and small sub-catchment plots of land is still a highly uncertain process. To minimise this uncertainty as much as is feasible, our research recommends that users continue to use FEH methods, but follow the specific recommendations made in our report SC090031/R0. These recommendations are summarised as follows:
- in small ungauged catchments continue to apply the existing catchment descriptor equation for the estimation of the median annual flood (QMED) - but for catchments smaller than 25km2 only use the single closest donor flow gauging station
- in small catchments use our revised pooling procedure to improve flood estimation for return periods greater than QMED - in general, it is also recommended for use in intermediate sized catchments (those up to 40 km2)
- the ReFH2 Hydrograph shape and FEH recommended storm duration are broadly appropriate for most catchment types, but new recommendations are made regarding the application of design storms in urban catchments - these recommendations are scale independent and apply to all urban catchments of any size, not just small catchments
- our evaluation of version 1 of ReFH2 found it is broadly appropriate for simulating plot-scale runoff
The project produced a freely available (providing it is appropriately attributed) 1 km resolution greenfield plot-scale peak flow-rate screening dataset. The dataset spans England and Wales for return periods of 1, 2, 30 and 100 years. In addition, runoff rates and volumes are provided for the 100-year event of 6-hour duration.
We recommend limiting the use of this dataset to initial screening for the pre-planning stage of new developments. Further work is required before we can recommend its application to assess flood risk or development planning purposes. We are also making the data available for general awareness and to allow wider scrutiny.
In line with other flood estimation guidance, the recommendations are generally applicable to the whole of the UK. However, users will need to seek local information and expertise when applying these general methods.
These recommendations are not instructions - they should be considered alongside proper guidance.
The scope for further refinement of these recommendations is limited within the current FEH framework, and future efforts should explore more novel methods.
Documents
There are 12 documents and 3 appendices available to download:
Phase 1
Phase 2
This report details the main findings and recommendations and summarises the conclusions from the other 9 reports. It is the main report intended for practicing hydrologists and general users.
SC090031/R0/Appendix B - Free greenfield plot-scale peak flow rate and volume screening data
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(zip/ASCII Raster)
: describes the process used for collecting and finalising the new small-catchment flood peak dataset, and an analysis of the representativeness of the dataset
(Excel)
(Excel)
: describes the re-analysis of existing FEH methods against the new small-catchment flood peak dataset
: summarises the main findings and recommendations from the reassessment of existing FEH methods against the new small-catchment flood peak dataset
: describes the new QMED model and the research and development around reducing uncertainty in estimating QMED in small catchments
: describes the development of a new pooling group model and the research and development around selecting pooling groups for flood peak estimation in small catchments
: describes the research and development around estimating design hydrographs in small catchments
: describes the research and analysis of the relationship between estimated and observed outflows from small sub-catchment scale plots of land
: describes the results of a literature review into the effects of vegetation and land management on runoff generation
: describes research into the depth–duration–frequency (DDF) characteristics of short-duration rainfall data, to assess the reliability of current national models in estimating rainfall frequency for durations shorter than the basic intervals used in developing them
Project Information
Project Manager: Mark Whitling
This project was commissioned by the Environment Agency’s FCRM Directorate, as part of the joint Flood and Coastal Erosion Risk Management Research and Development Programme.
Last updated 22 March 2024 + show all updates
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Phase 2 findings and reports added
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First published.