This report presents the results of Phase 1 of Project SC090031 ‘Estimating flood peaks and hydrographs for small catchments’. This work was jointly funded by the Environment Agency, the Centre for Ecology & Hydrology (CEH) and JBA Consulting. The objectives of the scoping study were: • to review existing datasets and techniques available for flood estimation in small rural and urban catchments and to suggest a preferred technique; • to assess a range of possible methods that could be applied to small catchment hydrology in the future; • to recommend interim procedures if appropriate. The study has focused on flood estimation in small rural and urban catchments in the UK. For the purposes of the study, a limit of 25 km2 was selected as the maximum catchment area considered, although it is recognised that many flood risk assessments are required for much smaller areas, some of which may form only part of a catchment and may not contain a watercourse. The scoping study has included a review of the sources of flow data in the UK for small catchments. The Environment Agency’s HiFlows-UK dataset provides flood peak data for over 950 gauging stations, but less than 10% are from catchments smaller than 25 km2 in area. A particular problem is the current lack of adequate flow data for small urban catchments. The prospects for augmenting the dataset have been explored and a number of sources of potential new data have been identified. It is proposed that the expansion of the flow data available for further analysis should form one of the early tasks of Phase 2 of this project. Consideration of the types of process that give rise to floods in small catchments and plots highlights the fact that practical flood models rely on conceptual simplifications and empirical data on a range of scales. Every year, many flood risk assessments are carried out on small catchments in the UK, often to meet the requirements of planning guidance such as Planning Policy Statement 25 (PPS25), and for other reasons such as the design of storm sewers and road drainage. A review of the guidance available to practitioners has demonstrated that a number of different methods, some of which were developed more than 35 years ago, are recommended for different applications. In some cases, the guidance to use particular methods seems to be related more to the ease with which they can be applied, rather than to how appropriate individual methodologies are. The most up-to-date methods available are those from the Flood Estimation Handbook (FEH) and its subsequent updates, which are based on long, reliable gauging station records and were specifically developed to be applicable to a range of catchment sizes and types. The report provides details of an analysis comparing the performance of a number of widely used flood estimation methods in small catchments. The results suggest that the FEH statistical method and the Revitalised Flood Hydrograph (ReFH) event-based method both outperform the older methods, although the former may be more uncertain on small catchments than on larger ones. While there is little evidence to suggest that the accuracy of the FEH methods when applied to ungauged catchments is particularly scale dependent, it is recommended that further flood peak data from small catchments, both rural and urban, should be analysed. The conclusions of the scoping study are as follows: • The FEH methods (both statistical and ReFH methods) are applicable across the range of catchment sizes used in their development and thus the continued recommendation of outdated methods such as IH 124 and ADAS 345 is inappropriate. • Since small catchments are not well represented in HiFlows-UK, further flood peak data for small rural and urban catchments should be sought and analysed. • Despite lack of bias, uncertainty in flood estimation remains high and there is a need to develop and test improved catchment descriptors, especially of soils and watercourse extent. • There is a requirement for improved hydrological methods to support fluvial flood risk assessment in very small catchments and also to support drainage design. • Estimates are required of both flood peaks and hydrographs to give flood volumes. The following interim recommendations are made to practitioners: Based on the results of the analysis in Section 6, it is recommended that flood estimates on small catchments should be derived from FEH methods in preference to other existing methods. The current versions of the FEH statistical approach or the ReFH rainfall-runoff model should be used except on highly permeable catchments (BFIHOST>0.65), where ReFH should be avoided, and possibly on urban catchments (URBEXT2000>0.15), where the results of the ReFH model can be less reliable. Checks should be carried out to ensure that the flood estimates are within expected ranges based on what is known about the history of flooding and the capacity of the channel (including evidence from previous flood marks). For catchments smaller than 0.5 km2 and small plots of land, runoff estimates should be derived from FEH methods applied to the nearest suitable catchment above 0.5 km2 for which descriptors can be derived from the FEH CD-ROM and scaled down by the ratio of catchment areas. The decision to translate FEH estimates from catchment scale to plot scale should be accompanied by an assessment of whether the study site is representative of the surrounding catchment area. One of the outputs of the study is a proposal for further research within a second phase of the project, which is presented in a separate document. Following on from Phase 2, it is expected that a new software tool would be developed, which would form part of the FEH suite.
|Place of Publication||Bristol, U. K.|
|Publisher||European Environment Agency (EEA)|
|Commissioning body||The Environment Agency|
|Number of pages||65|
|Publication status||Published - 2011|