22. Hydraulic Hysteresis

This scattergraph displays data from a flow monitor installed in a 21-in trunk sewer.  During wet weather, backwater and surcharge conditions are observed in a distinct manner referred to as hydraulic hysteresis.  During the intial response to this rain event, downstream storage is available within the system, allowing it to accommodate flows up to 4.25 MGD.  However, once this storage is used, the throughput of the system is reduced to about 2.50 MGD.  A similar condition is often seen in river flooding when the channel velocity is higher during the ascending part of the flood as flood waters fill the flood plain.