Design of OSD with Hysteresis Loop in Outflow Rating Curve

When designing on-site detention (OSD) facilities the 'inlet control' condition is normally assumed. Experiments conducted on OSD show a hysteresis loop exists in the head-discharge relationship of outflow through an orifice. This occurs because flow can self-prime or be induced to flow full in the outflow pipe. When this happens a hysteresis loop forms and outflow is larger than designed for. Permissible site discharge (PSD) can be exceeded by about 20 %. A solution is suggested.