How extreme was Storm Angus?

On 21st November 2016 heavy rainfall battered the UK. This was storm Angus, which brought severe flooding in different parts of the UK, but in particular in the South West of England (SWE). According to the newspapers, nearly 70 flood warnings were in place, a river burst in Devon, heavy rain flooded the railway between Exeter and Bristol, several locations were flooded in and around Bristol (Backwell, Whitchurch, Fishponds), cars were reported under water in South Bristol, and flooding was reported in the Chew Valley. So, how extreme was this rainfall event and why did it cause so much damage in the SWE?
Figure 1a shows the average November rainfall for the last 25 years [1]. Figure 1b shows the daily rainfall amounts recorded by the Met Office raingauge network on 21st Nov 2016 [2]; the data have been interpolated to show the spatial distribution of precipitation across the UK. The actual rainfall depths recorded by the raingauges are also shown in the same figure. It is interesting to see that the rainfall depths recorded around Bristol (42.4 mm) and other places in the SWE on 21st Nov 2016 were equivalent to more than 50% the expected average rainfall in November. A rainfall sensor installed at the University of Bristol recorded 47.8 mm of rain during the same period. This rain sensor is a disdrometer, which is able to measure the raindrop size distribution that can be used to derive rainfall intensities with 1-minute temporal resolution. The largest rainfall depth was recorded in Devon with 63.2 mm of rain on the same day. Figure 1c shows the daily rainfall depths recorded by the Met Office weather radar network at 1km resolution on the same day [3]. There are similar patterns in the distribution of precipitation by radar rainfall and raingauge measurements, but radar clearly shows regions with very heavy localised rainfall (not observed by the raingauge network) that caused flash flooding in different places across the SWE.

figure1_161208
Figure 1. Average November rainfall (a); Raingauge rainfall (b) and Radar rainfall (c)

Figure 2 shows the depth-duration-frequency (DDF) rainfall curves (colour solid lines) for storms with different return periods (T) for south Bristol using the DDF model proposed in the Flood Estimation Handbook [4].  The figure also shows an estimation of the storm return period for south Bristol using raingauge, disdrometer and radar rainfall for different storm durations. Note that the storm started in the evening of 20th Nov and finished in the afternoon of 22nd Nov and this is why some durations are longer than 24 hr. There are two important points to mention about Figure 2. The first one is related to the fact that the return period of the storm is sensitive to the rainfall measurement used in the analysis, with maximum return periods of 3, 5 and 5 years when using raingauge, disdrometer and radar observations respectively. It is fair to say that all the measurements were not taken exactly at the same location and that all the rainfall sensors have different error characteristics, but the results highlight the uncertainty in the estimation of the storm return period when there is such a high spatial variability of precipitation.  The second point is related to the fact that the storm return period changes depending upon the selected duration of the storm. For instance, with the disdrometer measurements, a duration of about 10h gives a storm with a return period of less than 2 years, but a duration of about 20h gives a 5-year return period storm.  This result highlights the uncertainty in the estimation of the storm return period when using a given storm duration.  Sewer urban systems are often designed to cope with storms with return periods of 1-2 years, and in many cases return periods of 5 years are adopted in areas vulnerable to flooding; more recent guidelines suggest the design of systems for storms with return periods of up to 30 years in order to prevent surface flooding [5]. The results from radar and disdrometer measurements indicate that the storm had a return period of 5 years for South Bristol. However, given the large spatial variability of the storm and the severe localised flooding that occurred in some locations in Bristol, it is very likely that the return period of this storm was actually higher. It is also fair to say that the catchment initial conditions might have increased the risk of surface flooding, with factors such as catchment wetness and drainage blockages (e.g. due to autumn dead leaves) having important effects.

figure2_161208
Figure 2. Rainfall Depth-Duration-Frequency curves

We live in a world with environmental uncertainty and the EU QUICS project will tackle some of the important issues related to the quantification of uncertainty in integrated catchment studies.

figure3_161208

References
[1] Keller, et al. (2015) CEH-GEAR: 1 km resolution daily and monthly areal rainfall estimates for the UK for hydrological and other applications, Earth Syst. Sci. Data, 7, 143-155, http://dx.doi.org/10.5194/essd-7-143-2015
[2] Met Office (2006): MIDAS UK Hourly Rainfall Data. NCAS British Atmospheric Data Centre, 2016. http://catalogue.ceda.ac.uk/uuid/bbd6916225e7475514e17fdbf11141c1
[3] Met Office (2003): 1 km Resolution UK Composite Rainfall Data from the Met Office Nimrod System. NCAS BADC, 2016. http://catalogue.ceda.ac.uk/uuid/27dd6ffba67f667a18c62de5c3456350
[4] Faulkner D (1999) Flood Estimation Handbook, Vol 2, Institute of Hydrology, ISBN 0948540907.
[5] Butler D and Davies JW (2011) Urban Drainage, Spon Press.

Miguel Angel Rico-Ramirez, University of Bristol.

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How extreme was Storm Angus?

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