Floods On The Somerset Levels: A Sad Tale Of Ignorance And Neglect
Research and consultant hydrologist, Dr Colin Clark quantifies a possible solution to the current flooding on the Somerset Levels.
When King Alfred hid in the Somerset marshes from the Danes in 878, he could scarcely have imagined that the marshes of Athelney and the lands of Wessex to the North East would be drained and occupied during both winter and summer, since beforehand, these lands could only be used during the summer: thus the name of my home county – the land of the summer pastures. The Saete of Somersaete, comes from the summer pastures of Norway which are called Saeters or Seters.
In showing our respect for the freedom of others from serious floods, I would like to describe some of the ignorance and neglect that has led to the recent flooding of the Somerset levels, and to then shine a beam of hope for the future, which quantifies a solution to the problem.
No one disputes that the past three months have been very wet. But how does it compare with the past? Using the monthly series of rainfall for England and Wales we find the following results for November and December only.
There was serious flooding in 1854, 1872-3 and 1929-30. During the latter flood at Taunton, from November 1929 to January 1930 537mm were recorded, which is over 70% of the annual average. The floods lasted from December to February. For the same time period in 2013-14 the rainfall in the upper Brue, which drains into the Levels, was 434mm.
An analysis of the highest consecutive three monthly rainfall since 1766 shows that this is the fifth highest, giving it a return period of about 1 in 50 years. This result excludes 1960, when Bedlamgreen in the upper Brue recorded 556mm. Before 1766 possibly the worst flood in historical times was that of 1607: I will mention this event at the end of this paper, since it was not caused by prolonged rainfall.
It would therefore not be correct to state that the present floods are in anyway linked to changes in air temperature, both local, or global.
Several Acts of Parliament in the early 19th century led to improvements in the drainage of the Levels. In the uplands, field boundaries often consisted of a hedge and ditch. Both were maintained, often on an annual basis. Where they did not lead directly or not at all into a watercourse, ditches retained a considerable volume of water.
During the late 1970′s I calculated the likely volume of potential storage of field ditches in the upper Brue catchment. The Brue is one of three main rivers that drains into the Levels. Figure 1 shows the extent of field ditches, excluding those by highways. Assuming a cross section area of 0.7 m^2 gives a storage volume of about 77000m^3. Many ditches have a bigger cross section. This is big enough to have cancelled out nearly all of the overflow volume during the flood at Bruton in May 1979, when 36 houses were flooded. Applying the same density of ditches over the upland catchments of the Tone, Parrett, and Brue and assuming a cross section area of 5m^2 gives a storage volume of about 1.6Mm^3. I will describe the way in which this could improve the situation in the penultimate section of this paper.
Rate of pumping
From the 17th century pumps were used to remove excess water from the land. The earliest pumps were driven by windmills, but steam power, followed by diesel and electricity enabled much greater volumes of water to be carried away. In the scholarly account of the draining of the Somerset levels by Dr Michael Williams, published in 1970, Williams never gave a single figure for the capacity of the pumping stations. In 2014 the situation is more complicated as Table 2 shows.
|Environment Agency spokesman||72,000 m^3 per day||January 2014|
|David Rook EA.||3,000,000 m^3 per day||February 2014|
|BBC News Website||86400m^3 per day||February 2014|
|Water Briefing Website||1.5 million tonnes over 65 km^2||February 2014|
|Environment Agency website||1,500,000m^3 per day||February 2014|
Not all of these volumes can represent the truth. None of these sources give an estimate of the effect of the removal of these volumes of water. However, all sources are united in giving the area of flooded land as 65km^2. Figure 2 shows the extent of the present flooded area.
Figure 2: Extent of the flooded area
Taking a value of 1.5Mm^3 from the Environment Agency website, over an area of 65km^2, gives a reduction in water level of 2.3cm per day. This means that a house with 0.6m of floodwater can expect, all else being equal, to have to wait 26 days for the pumps to remove all the flood water from indoors. This rather sorry situation must be amended because as with some of the estimates of pumping rates, it too is in error. From the evidence in Figure 2, the likely area that is now flooded in February 2014 is about 125km^2. Clearly the Environment Agency needs to check its basic data via a peer review process before it is published. Given a flooded area in excess of 100km^2, yields a decrease in water level of about 1.5cm per day, and an evacuation time of 40 days.
Dredging of the river channels
During the mid 1980′s considerable sums of money were spent on dredging the main rivers (Basil Tinkler, personal communication). The effects of an increased channel width and depth, possibly with a compound channel to reduce the effects of silting, can be calculated. If the capacity of the rivers Tone, Parrett, and Brue in their lower reaches were to be increased by 10m^3 s-1 then allowing for the effects of the daily tidal regime, would be expected to evacuate an additional 1.29Mm^3 per day. This is probably more effective because the pumping in February 2014 is to remove water from the flood plain, whereas an increase of channel capacity helps to prevent floodwater from overflowing into people’s houses and farmland in the first place.