WET News

20 WET NEWS AUGUST 2014 When Wessex Water needed to increase sewer and drainage capacity in Bristol, the excavation and construction work could have run into hundreds of thousands of pounds. The challenge: Increasing sewer capacity without the cost A s part of Wessex Water's AMP5 flood alleviation schemes programme, the drainage in the Clion area of Bristol is being upgraded by Wessex Water. The company is also upgrad- ing foul and surface water sys- tems throughout the region in a multimillion-pound scheme. The Northern Intercept Sewer (NIS) was constructed in the 1960s as part of a programme of works intended to increase capacity to match the spread of the Bristol urban area. In addition, it gave the oppor- tunity to provide some re-organ- isation of the drainage infra- structure that has grown organically since Victorian times. The sewers discharge to the Avonmouth Treatment Works, and once fully treated finally dis- charges into the Severn. There are still a large number of com- bined foul and surface water sewers within the area. Storm- water flow volumes can be sig- nificant in peak rainfall. When work was needed to increase sewer and drainage capacity in the Whiteladies area, INSIGHT PIPES & DRAINAGE The Energy Dissipation Unit is lowered into the shaft. The unit is a flow receiving chamber that was constructed in-situ Wessex Water could have con- structed a new 50m sha through limestone bedrock. Wessex Water project engi- neer Andy Roberts explains: "The existing sewers within the area were struggling during heavy rain, so we added a new sewer along Woodlands Road and needed to make a connec- tion onto the Northern Intercept. Falling through a height of 50m to the Northern Intercept, the water will carry so much energy that severe damage could be caused at the bottom of the drop unless the energy is dissipated. "Excavating a new 50m sha through the limestone bedrock to reach the sewer would have cost hundreds of thousands of pounds," adds Roberts. It would also have entailed major con- struction and neighbourhood disruption. "The forces involved would tear apart a standard sewer con- struction quite quickly," points out Keith Hutchings, Hydro International group product manager. "The erosive wear, vibration and stress could be very high unless the energy is carefully managed and dissipated. "Standard solutions to sud- den vertical differences greater than a few metres include weirs, cascades or flumes. "However they need a great deal of space and need to be carefully designed to slow the water sufficiently at peak flows. "Other pipe work and vortex schemes oen entrain signifi- cant quantities of air at maxi- mum flow. The results include highly damaging cavitation and in extreme events 'glugging' or water hammer pose danger to the pipework and the receiving sewer. Care must be taken to design conventional drop sha solu- tions to ensure that air is always entrained at all states of flow." While conventional vortex drop shas are sometimes con- sidered, their need for an air core down the length of the ver- tical pipe means a large diame- ter pipe, and the design of the stilling basin at the bottom of the sha also takes up a lot of space. n Constructing a Vortex Drop Shaft using a 'ship in the bottle' solution dissipates dropping water. A solution: Hydro performs 'keyhole surgery' T hrough the engineering contractors on the project, the concept of a drop pipe was discussed with Hydro, says Andy Roberts. It was suggested that a Hydro Vortex Drop Sha should be inserted through the access sha with the construc- tion of an Energy Dissipation Unit at the bottom. Hydro came up with a novel 'ship-in-a-bottle' solution to construct the Vortex Drop Sha via the existing access sha, avoiding major construction and minimising neighbourhood disruption. The new Drop Sha is designed to handle up to 680l/s at maximum flow. All the components were flat packed and inserted down the hole to be re-assembled inside at the bottom; then the pipe work lowered down from above. The Hydro Vortex Drop Sha is designed to run either with controlled air entrainment for lower flows and at pipe full without entrained air and with complete stability. The innovative design and wear-resistant coating to the stainless steel of the Energy Dissipating Unit at the base of the sha enable a very com- pact yet highly effective flow receiving chamber to be con- structed at the bottom. The anticipated maintenance is minimal. The Hydro Vortex Drop Sha is a single-sha self-acti- vating system with no moving parts, designed to quietly and safely drop water or sewage from heights in a space-saving design. It is suitable for drops of up to 100m and flows up to 5,000 l/s, whether foul or stormwater. Flows enter the system through the main horizontal inlet, around a carefully- designed top bend and, if appropriate, taper. Aer drop- ping the desired height, flow enters into the Energy Dissipa- tion Unit before continuing onwards downstream. A key element of installing the Hydro Vortex Drop Sha in the Whiteladies project is its compact dimensions. Compared to the main sha diameter of 3.6m, the diameter of the Vortex Drop Sha top chamber is only 450mm, and the diameter of the pipework is only 300mm, so it was rela- tively simple and quick to cut an access hole through the existing ladder platforms at every 15m or so on the way down. Also, Hydro's system could be dismantled like a flat-pack wardrobe and lowered through the sha, to be re-assembled at the bottom like a model sailing ship in a bottle. As the Hydro Vortex Drop Sha is of a small enough diameter to be easily contained within existing shas, it allows safe access to personnel without the need to create additional access either for construction or maintenance. n THE CONCEPT • The Hydro Vortex Drop Shaft is a single-shaft self-activating system with no moving parts • It is a 'ship in a bottle' solution that uses existing shafts • The Energy Dissipation Unit is installed at the bottom • The solution is suitable for drops of up to 100m • Designed to handle up to 680l/s at maximum flow NEED TO KNOW 1 Compact dimensions: the Vortex Drop Shaft was a small enough diameter to be easily contained within existing shaft 2 Avoided excavation of new 50m shaft through limestone bedrock 3 Avoided major construction costs and minimised neighbourhood disruption 4 Enabled safe access for personnel and minimal maintenance 5 Protects against erosive wear, vibration and stress by carefully managing and dissipating energy THE VERDICT "Excavating a new 50m shaft through the limestone bedrock to reach the sewer would have cost hundreds of thousands of pounds. Through the engineer- ing contractors on the project, the concept of a drop pipe was discussed with Hydro. They suggested inserting a Hydro Vortex Drop Shaft through the access shaft and constructing the Energy Dissipation Unit at the bottom." Wessex Water project engineer Andy Roberts Water falling through a height of 50m would have carried so much energy that serious damage could be caused at the bottom of the drop

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