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Doing their level best

Analysis - Engineers had to work fast last week to prevent the collapse of Ulley dam near Rotherham after torrential rain.

By the evening of Monday 25 June, it had been raining heavily all day at the Ulley reservoir.

Council workers at the dam had been told to monitor it, but it was clear all was not right. The embankment had a large scour hole at its base, compromising the structure's integrity.

According to Peter Kelham, an associate director at Arup who was on site that evening, the water had exploited a weak-spot in the dam's main spillway wall, which runs down and along the front of the embankment.

'There was extreme rainfall, and the water came out of its channel, went behind the walls and it all unravelled. This attacked the dam and made it unstable, ' explained Kelham.

The earth l dam has three spillways. Two are Victorian, one running from the north end of the embankment and one from the south. Both feed water down to a channel along its base. The third is a concrete spillway that was installed in the 1960s to increase capacity.

The southernmost Victorian spillway takes the initial overspill. If this reaches capacity, water is supposed to feed into the 1960s spillway. If this is overwhelmed, the northern spillway takes the rest of the water.

According to Kelham the southern spillway should have been able to cope with this amount of water. But because of the volume of water and the weakness in the spillway wall, it eventually collapsed. Water continued to flow, scouring the embankment and causing a partial collapse of the face in the middle of the dam close to the base. Water also began to pass through the dam at this point, accelerating the loss of earth.

The bank became eroded signicantly, displacing 3,000m 3 of material and exposing a 15m x 5m x 40m area of the base.

Rotherham Metropolitan Borough Council site teams had been keeping a watchful eye on developments. Ivan Machin, Rotherham's forward liaison officer, was on site at 8pm when video and photographs were taken of the initial damage.

Machin contacted Dave Crook from Arup's Leeds office to supervise emergency work. He in turn brought in Kelham, and Jim Claydon, an All Reservoirs Panel engineer, to help bring the situation under control.

While the danger to the embankment was becoming clear, Rotherham's civils department, Streetpride, had problems of its own. Director Tom Knight had been forced out of his offices due to the rising waters and operations had to be relocated to the local police station.

'It was managed chaos, ' said Knight later. 'Prospects for the dam to collapse were estimated by Jim Claydon to be 50:50 that night. Now the chances are about one in 100.' Engineers had to divert water from the damaged spillway onto the concrete spillway to get the water levels in the reservoir down, to reduce pressure on the weakened structure.

They then needed to plug the collapsed face as quickly as possible. Water weeping through the dam was the major concern - if this continued, there was a risk that the dam's puddle clay core would become exposed and begin to lose material. This had to be stopped or the dam would indeed burst.

'We had to get the water level down - one to reduce pressure on the embankment, which will reduce pressure through the embankment, then also provide more storage in the reservoir, storage which could help everybody, ' said Claydon.

The consequences of the dam bursting were especially alarming as a high-pressure gas main and electricity substation were in the ng line. 'The high-pressure gas main was a genuine worry, ' said Claydon. 'Transco reduced the pressure from 37 bar to 12 bar. This was the minimum they could operate at and still provide a service. Damage to that main could have been catastrophic.' Had the substation been destroyed it would have cut power to large areas of Shefield and Rotherham.

Rescue services evacuated 1,000 people from the surrounding area and closed junctions 32 to 34 of the M1, 2km from the reservoir. Junction 35 was also later closed as ood waters rose - much of the River Rother had already ooded between Shefield and Rotherham. Were the dam to fail, the M1 and all other low-lying areas would have been doubly swamped.

The fire service brought 11 portable pumps onsite, each capable of removing 5,000l/ minute. These were later supplemented with fi ve larger 300mm diameter pumps, which could pump 25,000l/minute. These took the water directly onto the 5m concrete spillway and away to the floodplain.

Ironically, this area - Canklow - was already fl ooded. 'It is the lesser of two evils, ' said Crook.

'We simply had to get the levels in the reservoir down as the number one priority.' Most of the pumps worked at the southernmost corner of the embankment, with four more pumps at the northern end. Pumps were also installed at the reservoir head to divert water from entering. 'We had to get the level down by 2m from the top water level in the reservoir, ' Claydon said. At its highest point, the water level was 500mm above the top water level - the level at which the first spillway begins to work.

Meanwhile, a geotextile membrane was pinned to the embankment face. Some 500t of rock fill was imported to stabilise the dam.'We used a Terram sheet between the original earth and the rock fill and used rocks with a diameter of up to 150mm, ' said Claydon on the Tuesday night, when the fate of the dam was still in the balance.

The Terram acted as a sieve, keeping mud fines on the embankment face. 'The geotextile acts like a filter - there is some seepage coming through the embankment - some weeping. The Terram will stop the fines from getting through, ' said Kelham.

Work was overseen by the Environment Agency's technical specialist Mark Chapman. In the hours leading up to midnight on Tuesday, Chapman's colleagues in Canklow telephoned him at 15-minute intervals.

'They know that if there is no answer they have to get to dry land - and quickly, ' said Chapman. But he was full of praise for Rotherham Metropolitan Borough Council.

'Some councils may not have taken the danger seriously and we would have had to step in and manage the operation.

Fortunately, they took this extremely seriously, ' he said.

Pumping throughout Tuesday night brought the level in the reservoir down by 650mm. And the 500t of fill was in place on the dam face by Wednesday morning. Another 1,500t arrived throughout the day.

By Wednesday mid-morning, the danger of collapse was signifi antly less. 'The risk was then down to about 1 in 25, ' said Knight. With the morning's light and dry weather, Crook had the opportunity to check the embankment. 'The core was not exposed, ' he said.

But the long-term fix for the embankment was not yet considered a priority. 'This is a reconstruction and stabilisation - long term we may have to do some remodelling.' The council will most likely have to drain the remainder of the reservoir and then drill into the clay core to check its integrity before making any final decisions on its future.

By Friday, the reservoir's water level was down by 2m below the top water level and with more rain expected, the reservoir was now ready to take more water.

The damaged spillway was plugged with concrete to disable it. The consequence of this was to raise the top water level by 1.1m, to the level of the 1960s concrete spillway.

alculations by Claydon showed that the dam would hold the extra water. 'Water is not normally expected at this level.

We have done some hydraulic calculations, and the 2m drawdown has proved about right. A 25mm of rainfall event, which would not overflow, is consistent for the weather expected this weekend, and we have the pumps on standby' he said.

Finally, the fire service removed their portable pumps, leaving the high-capacity pumps, ready for more water. 'We are very grateful to the fire service.

At one point, there were 18 such pumps in the north of England, and 17 were committed to Ulley, six on standby' said Claydon.

With the immediate danger over, the local authority is taking no risks. 'We are still monitoring the embankment every hour, ' says Knight. 'But I think it is okay - I feel it is okay.'

Who's who

Site owner: Rotherham Metropolitan Borough Council Consultants: Rotherham MBC Streetpride, Arup Contractor: JN Bentley Craning: Emsley Pumps: Selwood Pumps, JLD Electrical, DV Howells

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