Posted by: waterworks | November 12, 2007

5. When sheep cause floods

many-sheep.jpgThere are many more sheep in mid-Wales than there used to be.  From 1970 to 1990, numbers increased six-fold. The animals’ individual weight also doubled, as a result of farmers shifting to heavier breeds.  Changes were, in part, driven by Common Agricultural Policy (CAP) farming subsidies.  One very unexpected consequence of CAP-driven growth in sheep numbers has been increased flooding reported in the upper reaches of the Severn catchment. According to Howard Wheater of Imperial College,  a sheep hoof exerts similar levels of pressure to a tractor.  The net result  of a greatly increased sheep population has thus been widespread soil compaction.  

                                                                                                                                                                                    

Under normal conditions, soil contains millions of tiny air spaces that can soak up and slowly transmit rainwater to river channels during storms.  A vital valve in the rainfall-river system, these air spaces help maintain a steady rate of drainage that rarely results in river levels over-topping banks.  Following soil compaction by sheep, rainwater finds it far more difficult to infiltrate the soil. Instead, it runs overland, arriving at the river channel more directly and in much less time.  All of this results in a “flashy” river response.  Water levels rise rapidly almost as soon as rainfall commences and bank-full discharge is quickly reached.  Too many sheep therefore equates with increased frequency of river flooding.

sheep1.jpg

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Responses

  1. An interesting assertion and I’m sure there’s something in it.

    However, I suspect the vast forestry operations in the area, which includes leaving large areas of land deforested after being harvested, is also leading to flooding issues in the area and placing pressure further down the Severn.

  2. […] 12th, 2007 by Montgomeryshire Witness Here’s an interesting blog which has highlighted the impact sheep in Montgomeryshire might be having on […]

  3. […] The pluvial brouhaha is a direct result of unprecedented insurance losses brought by the June and July 2007 floods to England.  Total losses of £3bn have meant that many of the UK’s major insurers will struggle to post any profit for the financial year.  Fingers are pointing at pluvial flooding, which is not currently included in any systematic risk assessment and warning policy.  The existing UK flood protection paradigm (in non-coastal areas) has evolved out of concerns with fluvial flooding  – those instances when rivers exceed bankfull discharge and inundation of the floodplain follows.  This monolithic approach has now been found wanting.  It is widely being assumed that the incidence and severity of pluvial flooding is likely to increase with climate change.  Additional factors exacerbating pluvial problems in a local context may include the “urban heat island” effect and the presence of too many sheep (see the previous mid-Wales post https://waterworlds.wordpress.com/2007/11/12/5-when-sheep-cause-floods/).  […]

  4. […] extensive floodplain building, poor flood defence maintenance (not to mention increased sheep numbers in Wales) exacerbated the situation further?  Or has a relatively recent rise of 24-hour news […]


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