The Flood of June 2007
To start we need to put June 2007 into some kind of context in order to examine the potentially calamitous events. Climatic records in South Yorkshire are collected at Weston Park in Sheffield. This has been the case since 1883 when records were first collected. Since records began June 2007 is the wettest month ever recorded with 285.6mm of rain falling. (Only two other months have recorded over 200mm in that time.) Since 1883 on only 20 times had the month of June had rainfall exceeding 90mm, 9 of those have occurred since 1980 which may suggest that our climate is becoming more extreme. June 2009 was the 3rd wettest June on record and caused more flooding in the South Yorkshire region.
The statistics show that a ‘normal’ June should produce 66.7mm of rain and annual rainfall should be 824.7mm. 2007 was not a ‘normal’ year. 8 months received less than the average rainfall which included the last 5 months of the year. The year, as a whole received 948.8mm of rain which is above average. June 2007 contributed 30% of the rain to fall in 2007. The Meteorological Office recorded June 2007 in South Yorkshire as a greater than 250 year event.
The dam at Ulley Country Park was constructed in the 1870’s of earth with a clay core. The specifications for the dam show that it was constructed from ‘the best materials on site.’ Had the dam been constructed this century there is little doubt that the original design would not have been accepted. Since its construction mining subsidence has occurred behind the dam. (Pillars of coal were left under the dam to support its weight, but not behind the dam to support the weight of water.) This rendered the by wash channels, constructed round the dam to take excess water straight into Ulley Brook without filling the dam, useless as the water would need to flow uphill. Therefore the two streams filling the dam did just that. The 1945 spillway was at a slightly higher level than the by wash outflows which meant that the bulk of the water flowing out of the dam did so down the Victorian spillways rather than the later construction. These spillways were not designed to take this amount of water and it was one of these which collapsed and led to the earth dam being eroded.
Water level – June 26th
The two photographs show the extent to which the reservoir had risen overnight on June 25th/26th. The signpost in the foreground of the picture to the right is the same signpost. (Note the levels of the reservoir were lowered to protect the dam in the days following June 25th.)
Victorian by wash channel – 25th June
The same by wash channel on June 25th – The dam is eroding.
If we examine the events leading up to the flood of the 25th and 26th of June we need to analyse the rainfall statistics. I have stated that June 2007 was the wettest June on record. That 285.6 mm of rain fell. This rain fell in two very significant ‘events.’ Between June 13th and 15th 135.4 mm (Over 5 inches) of rain was recorded at Weston Park with 88.2 mm falling on June 15th. This rain effectively filled all the ‘stores’ in the earth. The soil store, which allows infiltration through the surface layers, was completely filled. If the soil store has capacity it will allow water to infiltrate and then release the water slowly to the nearest river channel. If the soil store is full any further rain falling flows over the surface and straight into the river channel.
The second event occurred less than ten days later. Water was still being held in the soil store when a further 87.1 mm (Measured at Weston Park) fell in the late afternoon and overnight on the 25th and 26th of June. It was as if it was falling on a concrete surface. There was no infiltration into the soil. The water ran as what is called surface flow into the stream channels and into the reservoir.
Path around the reservoir – 25th June
Standing water ready to flow directly into the reservoir, as was all the rain falling within the catchment area of the reservoir. This pattern was repeated across South Yorkshire. Rivers rose to new historical levels. The Wicker in Sheffield became part of the River Don’s channel. Ulley Brook and Morthen Brook, feeding the reservoir were swollen and caused local flooding. Ulley Reservoir was filled beyond capacity and the dam wall was eroding.
Rotherham Advertiser – 27th June
The photograph of the front page of the Rotherham Advertiser shows the collapse of the Victorian Spillway to the left. The water flowing down the spillway overtopped the stone facing of the spillway and started to erode the earth dam, had the Rangers and the emergency services not responded so rapidly then it is possible that the dam would have collapsed. Part of the response may be seen in the top right of the photograph as pressure is taken off the dam wall by pumping water out of the reservoir and down the 1945 spillway.
On a more positive note the dam did act as flood storage. Water which would otherwise have flowed down Ulley Brook in the immediate aftermath of the rain was held behind the dam for a further 24/48 hours and did not, therefore, contribute to the immediate flood risk downstream.
Given the nature of the potential disaster and the possibility of another rainfall event causing similar problems the Rotherham Borough Council were left with the conundrum of what to do with a severely damaged dam in a well loved Country Park. Could, or indeed should a rehabilitation process be put into place or should the dam be dismantled totally?
As the Friends of Ulley Country Park we knew what we wanted the Borough to do. Could they be persuaded to rehabilitate the dam and the Park?