Abstract
AbstractIn November 2009 and December 2015, two record‐breaking 24‐hr rainfalls occurred in Cumbria, UK, significantly changing the perception of flood risk for local communities. FEH13, the current UK rainfall depth‐duration‐frequency (DDF) model, estimated return periods of around 1,000 years for both events. The previous model, FEH99, received criticism from panel engineers responsible for making technical safety decisions relating to reservoirs for appearing to estimate relatively short return periods for extreme events. Although FEH13 is more consistent with current probable maximum precipitation (PMP) estimates, there is high uncertainty in both models due to the limited number of extremes captured by UK rain gauges. Furthermore, neither model included the 2009 or 2015 event in its calibration. Here, we re‐calibrate FEH13 using additional gauged rainfall data collected in Cumbria during 2006–2016, including the record‐breaking 2009 and 2015 storms. Using the updated calibration data set reduces the estimated return periods of the 2009 and 2015 events to approximately 140 years each. This case study illustrates the considerable uncertainty in short‐sample records, demonstrates the importance of maximising the quantity of relevant calibration data, shows that perception of risk depends upon the method and data used, and illustrates the difficulty of separating trends and natural variability.
Highlights
From 0000 UTC on November 19, 2009, 316.4 mm of rainfall was measured in one 24-hr period at Seathwaite Farm in Cumbria, setting a new UK record and surpassing the previous record, from July 1955, by 37 mm
We recalibrated the UK's standard rainfall depth-duration-frequency (DDF) model, FEH13, in Cumbria, using more up-to-date rainfall data that included the extreme rainfall events that led to severe flooding during 2005–2015
Inclusion of the more recent data resulted in instances where the relationship between rainfall depth and return period ( Gumbel reduced variate) followed an S-shape, whereas typical extreme value plots in hydrology tend to show points following either an arc or a relatively straight line
Summary
From 0000 UTC on November 19, 2009, 316.4 mm of rainfall was measured in one 24-hr period at Seathwaite Farm in Cumbria, setting a new UK record and surpassing the previous record, from July 1955, by 37 mm ( that record was accumulated over a standard rainfall day, from 0900 UTC to 0900 UTC). The Revised FORGEX (FOcused Rainfall Growth EXtension) method is used to estimate the rainfall depthfrequency relationship for a given duration at a given focal point, using annual maxima pooled from increasingly wider circles to define the relationship for longer return periods It was initially described in Faulkner (1999) and extended by Stewart et al (2013). The number(s) of records used for each stage is shown, where ranges indicate that not all records were suitable for all accumulation durations Most of these gauges are located outside Cumbria, the method requires that RMED be mapped simultaneously for the whole of the UK and that gauges up to 200 km away from the edge of the study area be included in the frequency analysis
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