Hysteresis Loop in OSD Outflow Rating Curve

On-site detention (OSD) is imposed on many new developments in urban areas as a means of reducing site discharge. An OSD facility may either be an underground storage tank or an above ground ponding area. In both cases the discharge is often released to the gutter at the kerb line. In designing these type of facilities 'inlet control' or 'orifice control' is assumed. Experiments were conducted at the University of Technology, Sydney to study the head-discharge relationship of outflow from OSD through an orifice. The head versus outflow rate follow either one of two rating curves. The first is the 'inlet or orifice' control rating curve and occurs when the discharge pipe is flowing part-full. The other is governed by 'outlet control' conditions and generally occurs when the discharge pipe is flowing full. The latter rating curve is lower, i.e for a given flow rate the head is smaller. The two rating curves are commonly thought to be independent of each other and the applicable rating curve depends primarily on the tailwater conditions. However, he experimental results shows that the head-flowrate relationship can revert between the two rating curves and a hysteresis loop exist between the two curves. The mechanism of its formation is explained. The flow rates over which the hysteresis loop occurs virtually covers the range of outflow from OSD facilities. Hysteresis loops means that OSD facilities designed using 'inlet control' assumption nil1 not perform as intended. The PSD may actually be exceeded and the OSD storage will not be fully utilised since smaller amount of runoff is detained.