Float the pumps, boys
May Gurney turned to ITT Water & Wastewater to devise a plan to remove water from a Scottish reservoir without introducing suspended solids into the water discharge. It proved a challenging but successful project for the company.
Birkenburn reservoir lies high in the Kilsyth Hills in North Lanarkshire, near Glasgow. The reservoir has a water capacity of 780,592m3 and supplies water to the Forth and Clyde Canal.
Like most reservoirs, Birkenburn requires almost constant upkeep and maintenance, but its location – two and a half miles up a dirt track – makes even routine maintenance a difficult undertaking.
Following an inspection by the All Reservoirs Panel Engineer (ARPE) in 2006 several remedial works were identified for the reservoir, including refurbishing existing discharge pipe and an upgrade to existing leakage monitoring, along with improvements to dam and spillway wave protection.
May Gurney, the infrastructure support services company, was contracted to complete the work on behalf of British Waterways, but it was clear that in order to complete the works, the reservoir level would have to be reduced by 8.5m from a spillway weir crest and have that level maintained for the duration of the works, a three and a half month period.
Also, the flow rate had to be monitored and was not to exceed 600l/s.
May Gurney turned to ITT Water & Wastewater to devise a plan to remove water from the reservoir without introducing suspended solids into the water discharge.
Colin Rae, regional rental manager at ITT Water & Wastewater, takes up the story: “A previous attempt at water removal, utilising the existing discharge pipe resulted in suspended solids pollution of the waterways downstream of the reservoir. It was of paramount importance therefore that silt or suspended solids were not introduced into the discharge.”
ITT Water and Wastewater’s solution was to install a floating pontoon at the deepest point in the reservoir. The pontoon would contain three off-electric submersible pumps powered by a temporary generator on the bank.
The plan required the pumps to be located in a basket, which would ensure that the suction side remained at a constant depth of 500mm below water level. This would then allow only clean water being pumped and a minimum disturbance of any silt layer.
However, a plan for de-watering without silting was only part of the problem.
The key logistical issue was how to get the equipment to
“The task of getting the equipment up a dirt track in appalling weather conditions, despite the fact it was July, was a logistical headache to put it mildly,” says Rae.
“The solution was to have a base camp and assembly point to break up the journey, and create an area where the equipment could be put together before installation.”
The plan was for the discharge hoses from the pumps to be directed into the spillway and, as the water level fell, the pontoon would drop to the required level. The pumps would operate at a capacity of 575l/s, enough to overcome incoming flow and reduce the water level while keeping within the maximum output permissible.
“With this pumping solution we had to take into account the fact that the maximum discharge was only required at the start of pumping operations while the level was being dropped,” says Rae.
“Once the minimum level was achieved only maximum incoming flow needed to be pumped. Because we were locating the pumps 500mm below water level at all
times, we could monitor the silt level below the pumps and
stop the pumps if the silt was disturbed.”
Once the pontoon and pumps were in position, separate retrievable mooring blocks were lowered to the reservoir bed on lifting chains and a 2.5m length of steel cable was fitted to the mooring block with control devices attached. This ensured that once the pumps reached a level of 2.5m from the silt, telemetry signals would be sent to ensure that the pumps would stop automatically to allow manual monitoring and control to commence, and ensure no disturbance of the silt layer.
One of the key issues with the plan was water quality testing. “Prior to the activation of the pumps, the water quality was tested within the reservoir and spillway channel,” says Rae.
“Monitoring stations were placed in the reservoir and granite spillway channel, in the form of hand held units to measure turbidity and silt levels.
“What’s more a telemetry unit was fitted to the equipment to monitor the status of the pumps to ensure that an automatic dialup service was activated to a series of predetermined telephone numbers 24 hours a day in the event of pump malfunction.”
ITT Water & Wastewater completed the de-watering of the reservoir allowing May Gurney to complete the refurbishment works on schedule. “We did a job that at first sight seemed impossible to do, namely lowering the water level of a remote reservoir in arduous conditions, without allowing any solids to contaminate downstream,” says Rae.
“This method will become the standard for de-watering a reservoir whilst maintaining clean water downstream.”
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