Siphonic roof drainage (SRD) systems are an efficient method of removing rainwater rapidly from roofs. Such systems are designed to run full-bore, resulting in sub-atmospheric system pressures, higher driving heads and higher system flow velocities. Hence, SRD systems normally require far fewer downpipes, and the depressurised conditions also mean that much of the collection pipework can be routed at high level, thus reducing the extent of any underground pipework. But, they work properly at only one roof run-off rate and therefore suffer from sizing and operational problems which limit their performance. Climate change is creating situations where normal ranges of rainfall intensity are being frequently exceeded, and this may have an impact on the performance of SRD systems.A multiple parallel pipe SRD system appears to offer benefits and avoids sizing problems associated with current SRD systems. A movable cap covering the inlet to a small bank of parallel pipes has the potential to avoid noise associated with making and breaking siphonic action through flow modulation. Laboratory scale tests demonstrate the basic feasibility of the multiple parallel pipe system and indicate that handover of flow between pipes occurs smoothly, and that the flow modulation cap functions reliably.
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