The Cumbrian Floods In Perspective
By Paul Homewood
Main Street, Cockermouth
As the floods in Cumbria begin to recede, it’s time for a round up of the facts, and a bit of historical perspective.
We have heard of claims that 24-hour rainfall totals at Honister Pass have broken the UK record, with 341mm. This is what DEFRA have to say:
Over the past 48 hours, Environment Agency rain gauges have recorded widespread totals of more than 200mm across Cumbria. Provisional figures from a rain gauge in Honister in Cumbria show that 341mm rain fell in 24 hours (18:30 Friday- 18.30 Saturday). If verified, this will be a UK record for 24 hours. This is more than a month’s worth of rainfall in a day.
The gauge belongs to the Environment Agency, and I have asked them to confirm how long it has been operational. As I reported earlier, it only appears to have been operational since 1970, and records since then are incomplete.
I am still not aware of any confirmation from the Met Office.
It is worth noting however that this record only applies to any 24-hour period, and not for a rainfall day, which runs from 9.00am. The record for the latter still remains at Martinstown, Dorset, in 1955, when 279mm fell.
Many more stations are now automatic and give hourly recordings than was the case in the past. Consequently we are statistically much more likely to find 24-hour records nowadays.
If found to be valid, the Honister record will replace the one of 316mm set at Seathwaite, just a mile down the road, in 2009. As Honister lies at an altitude about 700 feet higher, this extra rainfall would not be unexpected.
Recent Weather in Cumbria
One of the big contributory factors to this week’s flooding in Cumbria has been the large amount of rainfall that accumulated over the region during November. Consequently, the rivers and flood storage reservoirs were already pretty much full even before Storm Desmond hit.
The cause of this weather has been the track of the jet stream, which brought depressions one by one over the North West, rather than elsewhere.
According to the Lake District National Park, Seathwaite is the wettest inhabited place in England, experiencing around 3500mm of precipitation each year. (The average for the UK as a whole is 1154mm).
The reason is simple. The mountains of the lake district rise up sharply from the sea to the west, only 20 miles away. Any depressions coming from that direction are immediately impacted.
As the map below indicates, Cockermouth is in a particularly vulnerable position, lying at the confluence of the Rivers Derwent and Cocker.
The Environment Agency highlights the problems faced by Cockermouth, and lists some of the flooding events that have affected the town in the past.
Note well the clustering, particularly in the 1930’s.
The Lake District Flood of 1898
There is an interesting paper, published in 2003, “Estimating areal rainfall during the 1898 flood in the English Lake District and the implications for probable maximum precipitation”, by Colin Clark.
The paper gives some of the contemporary accounts of what was a significant flooding event, but there are some interesting snippets of information, of relevance to the current flood.
These are actual rain gauge data.
The figures for the lowland sites of Ambleside and Grasmere in 1898 are remarkably similar to Keswick, supposed by the Met Office to be the worst of the lowland ones this time. (Shap is about 800 feet up).
This gives good reason to suggest that the 1898 storm was every bit as bad.
As the paper makes clear, there were no rain gauges up in the mountains in those days, but, using data from lake levels etc, the paper estimates:
Estimation of areal rainfall, especially in mountainous areas, is difficult. The use of lake levels and soil hydraulic conductivity data has enabled an estimate of 260- 280mm over a 24- hour period to be made for the 1898 storm in the Lake Windermere catchment area. This result is more robust and reliable than any estimate obtained from raingauges which are often, as in this case, located in the valleys. The results presented in this paper are based on a historical flood record
This of course is an average for the whole catchment area, so it is inevitable that some places would be above the 280mm figure. The paper reports on other studies which arrive at higher figures for the rainfall:
The Flood Studies Report (FSR) (Natural Environment Research Council 1975) and Flood Estimation Handbook (FEH) (Institute of Hydrology 1999) both suggest a 24-hour PMP of 300-350mm for the Lake District, with the higher value occurring on the Borrowdale Fells to the north of the Windermere catchment. The FSR/FEH PMP value for an area of 187km2, as distinct from the point values given above, is 280-327mm.
Borrowdale, incidentally, is the valley leading up to Honister Pass.
If any of these figures are anywhere near right, it is clear that there was nothing unprecedented about Storm Desmond, as far as the lake District is concerned.
Watkins & Whyte
There is one more study I want to have a quick look at, by Watkins & White, which was published in 2008.
Note again the reference to “concentrations”. They include the following chart:
This study has demonstrated the value of historical sources in supplementing data derived from scientific techniques such as lichenometry and radiocarbon dating. For Cumbria it was possible to identify 34 extreme floods which affected upland catchments since AD1600. The evidence supports the information derived from geomorphological approaches indicating that there have been marked changes in the frequency of such floods in the last 400 years, but that the frequency of such floods in the last three decades is not unusual and has fallen markedly from the mid-twentieth century. The occurrence of extreme flood events in upland areas appears to be linked closely with negative NAO values.
There was no rain gauge up Honister Pass in the 1960’s or earlier. If there had been