URBAN DRAINAGE


Delving deeper into flooding issues


Most people think that flooding occurs when rivers burst their banks, but backflow flooding from sewers is an even more serious problem. Chris Ricketts, a specialist in drainage at BSS Industrial, outlines the scale of the problem and offers a guide to the prevention products available


T


he once-in-a-generation floods we have seen in recent years in areas such as Cumbria, Herefordshire and Cornwall have


been life-changing events for the individuals caught up in the devastation. At the heart of the issue lies our drainage network, which is struggling to cope with the effects of climate change and its altering of our rainfall patterns. Television coverage of burst river banks only


tells half the story. The often untold story of many of these floods is the flow of greywater and even blackwater sewage back into commercial properties and residential homes. This phenomenon, known as backflow flooding, is resulting in a significant increase in orders for prevention devices at our specialist BSS Drainage centres across the UK. The Environment Agency estimates that 80%


of UK buildings are at risk from backflow flooding at times of heavy rainfall. And the number of properties that are vulnerable to damaging sewer backflows is increasing. This is due to factors including the greater use of hard landscaping, the increased use of flood plains for development and the popularity of installing equipment which requires connection to the drainage system, such as WCs and showers, in basement areas. In cases of severe flash flooding, a storm


and need protecting with prevention measures. One of the most serious implications of backflow flooding is on an individual’s health with, on average, more than 10,000 reported cases of backflow contamination each year. Pathogens present in water can cause infection with micro-organisms entering through the oral route via ingestion, or by eyes, ears, nose or through an open wound. Pathogens associated with sewage contaminated water include bacillary dysentery, pneumonia, botulism, hepatitis A, meningitis and septicaemia. Backflow flooding is best controlled via


prevention with the fitting of a non-return valve within the private sewer of a property upstream of the public sewerage system. There are a number of specialist backflow prevention products available in the UK market from BSS Drainage and manufactured by companies like ACO Building Drainage, Kessel and OSMA. These products have


Quatrix- The ACO Quatrix backflow valve is an in-line Type 3 automated anti- flooding device


different specifications and uses, including motorised versions which include acoustic alarms and pneumatic sensors to alert when the potential for backflow is signalled. The key to specifying this type of


sewer has the potential to surcharge. In this situation, the water level in the manhole rises above the top of the pipe and hydraulic pressure causes the drains below the rising water level to back-up. When this happens, contaminated floodwater can flow back though foul sewers causing flooding inside buildings. The backflow level is the highest level to which rainwater can rise in a drainage installation and is usually defined by local authorities. It is usually taken as the level of the road above the sewer connections. All drainage installations below this level are at risk of backflow flooding


product is for building contractors, groundworkers, and drainage contractors to ascertain what the area of backflow risk is to be used for. If a backflow risk area, such as a basement only contains a washing machine or hand basin, then greywater compatible non- return backflow valves could be fitted. If the area of potential risk contains a toilet then blackwater compatible non-return valves must be fitted. It is important that any product specified


conforms to BS EN 13564-1:2002 Anti-flooding devices for buildings. Anti-flooding devices require careful installation and must, of course, be regularly maintained. ■■■


www.bssindustrial.co.uk 18 Water & Wastewater Treatment September 2010


Stormwater project gets under way


Five sewer overflow shafts of varying depths and diameters will be constructed


U


nited Utilities has started work on a £114M stormwater improvement contract in Preston. KMI Water – a joint


venture between Kier, J Murphy and Interserve Project Services – has been appointed preferred bidder with GHA Livigunn acting as their lead designer.


The scheme involves the construction of


3.5km of 2.85m internal diameter interceptor and storage tunnels 30m below ground to the west of Preston complete with five drive and reception shafts. The tunnels will hold up to 40Ml of stormwater to prevent overflows discharging to the River Ribble. In addition, five sewer overflow shafts of varying depths and diameters will be constructed in the area to spill flows into the new interceptor / storage tunnel system via connecting micro-tunnels. Tunnelling operations are due to commence in December. A new terminal pumping station 300m


diameter and 34m deep will be constructed at Penwortham to lift flows from the tunnel and transport them to the wastewater treatment works at Clifton Marsh via twin 900mm diameter rising mains. This will require a further tunnel crossing of the River Ribble near the Clifton Marsh Treatment Works. During severe storm events the terminal pumping station overflows to a new screened overflow system which is pumped via five 1,200mm diameter outfall rising mains to the River Ribble. The screening chamber is 50m long 20m wide and 13m deep, it incorporates four Longwood escalator screens and 55 outfall pumps. Work is predicted to complete in March 2012. Diaphragm walling operations have commenced on the terminal pumping station, shaft SIA and the screening chamber. ■■■


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