Recalibration of FEH13 rainfall model for Cumbria. Final report

Vesuviano, Gianni ORCID:; Stewart, Elizabeth ORCID: 2021 Recalibration of FEH13 rainfall model for Cumbria. Final report. Wallingford, UK Centre for Ecology & Hydrology, 82pp. (UKCEH ref. NEC06121/NEC06813/Issue 1.2) (Unpublished)

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This report presents the results of a study to re-calibrate the UK’s current rainfall depth-duration-frequency model (FEH13) over Cumbria with more recent data that were not included in calibration of the FEH13 model currently available through the FEH Web Service. These new data include the record-breaking November 2009 and December 2015 storms. The study includes a re-mapping of the median annual rainfall for the whole UK, for different durations from 1 hour to 8 days. It then focuses on the depth-duration-frequency (DDF) relationships estimated at ten case study sites. The steps undertaken to generate the final DDF estimates from the raw annual maximum rainfall data are described in detail. As the FEH13 method was almost unchanged during this study, this report describes in detail the steps undertaken to generate the data available on the FEH Web Service. The new model is used to estimate spatial return periods for the maximum 36-hour total of the November 2009 event, and point return periods for the November 2009 and December 2015 events for selected locations and durations. The rarest return period within the November 2009 storm’s spatial field is found to decrease from almost 8000 to just over 500 years, while return periods at the periphery of the storm are practically unaffected. Point rainfall return periods for the November 2009 event are found to decrease considerably relative to FEH13 but to be similar to those estimated by the original Flood Estimation Handbook model (FEH99). However, the return period of the 24-hour record breaking rainfall at Honister Pass in December 2015 is found to be reduced approximately sevenfold compared to both the FEH13 and FEH99 models. Finally, model outputs for the whole of Cumbria, consisting of mapped rainfall depth for specified durations and return periods, are presented and compared to estimates from the current FEH13 model, indicating that the vast majority of duration-frequency combinations result in greater modelled depths, or that specified depth-duration combinations are increasing in frequency. However, peak 1-hour rainfalls of specified return periods (here 100 and 1000 years) may be getting smaller in the north of Cumbria. Although the new estimates for rainfall DDF relationships presented in this report are theoretically less uncertain than those from previous UK-wide rainfall models, including the FEH13, FEH99 and FSR, it is important to note that uncertainty in extreme events is still very high, and can only be lowered through the observation and incorporation of more extreme events from a longer monitoring period.

Item Type: Publication - Report (Project Report)
UKCEH and CEH Sections/Science Areas: Hydro-climate Risks (Science Area 2017-)
Funders/Sponsors: Environment Agency
NORA Subject Terms: Hydrology
Date made live: 03 Mar 2021 11:46 +0 (UTC)

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