RAINWATER SYSTEMS which is a big storm. Although many underground Sustainable Drainage Systems (SuDS) are now designed to cope with 100-year storms, the pipe systems which take the water to them are commonly only designed for a maximum of 30 years. That may not seem to make any sense; why design storage to cope with a 100-year event and the pipes for only 30 years? It is all to do with how long the rainstorm lasts. The longer the time period that the rainstorm lasts, the lower the intensity is. Think about a summer thunderstorm, it rains very heavily for a couple of minutes; the intensity is very high, but the overall amount of rain is not that great. The usual design durations are: Roof drainage: 2 mins Underground drainage: 5-15 mins SuDs storage: 1-12 hours This matters because a 100-year storm running off a 1000m² area in London will be: Roof drainage: 2 mins – 68 l/s Underground drainage: 5-15 mins – 50 l/s to 29 l/s SuDs storage: 1-12 hours – 12 l/s to 2 l/s Thus, the SuDS storage may be designed for 30 times less flow than the gutters. There is obviously a huge difference between what flows may be allowed in design, and thus the crunch point is where the underground meets the below ground. So, what can be done? 072 DECEMBER 2017 RCIMAG.COM Vented manhole lid for siphonic system Those designing underground systems need to be aware of just how large the flows may be from the roof and make provision for them, certainly in the first legs from the downpipes. Vented manhole covers For siphonic roof drainage systems it is particularly important that vented manhole covers are fitted on the first manhole, where the siphonic system from the building meets the gravity underground drainage. This achieves two things; firstly, it ensures that air can be vented from the siphonic systems as they fill, which helps them fill quickly and effectively. Secondly, it ensures that if there is a lack of capacity in the underground system, flow can be harmlessly released onto external surfacing, rather than flooding at high level in the building. It is vital that any vented lids meet the requirements of BS 8490:2007, with an open area of at least 2 x the incoming pipe area. To achieve this, ‘vented lids’ with little holes drilled in them are not adequate; full open grate covers, usually used as large highway gully tops, must be used. However, it is not just siphonic systems which need careful detailing; less severe problems can occur on gravity systems too. External downpipes should be discharged into gullies, and back inlet gullies with open grates should be fitted to pipes serving internal downpipes. Both these ensure that if there is a capacity problem in the underground drainage, water can harmlessly escape outside the building instead of backing up the downpipes and causing issues. Further advice and guidance is available from any MGMA member company whose details can be found on the MGMA website: www.mgma.co.uk “It is vital that any vented lids meet the requirements of BS 8490:2007, with an open area of at least 2 x the incoming pipe area” Imitation is always fl attery... UNIFOLD® Unrivalled excellence in gutter lining technology. www.gutterliners.com But none can match the excellence of a system that is truly complete THE UNIFOLD GUTTER LINING SYSTEM. Having faced and solved every problem experienced in its long life with consummate ease UNIFOLD remains THE GOLD STANDARD to which others aspire! UNIFOLD – Unrivalled excellence in Gutter Lining Technology.
RCI December 2017
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