Backwater and Surcharge Orifice - ADS

11. Backwater and Surcharge Orifice

The scattergraph show here reveals the presence of a dead dog and a downstream restriction that cause backwater and surcharge conditions.  However, note that flow rate increases from 6.9 MGD to nearly 9.2 MGD as the flow depth increases during surcharge conditions.  This phenomenon is described as orifice flow and indicates that free flow conditions exist downstream from the restriction.  A review of as-built records indicated both the dead dog and the downstream restriction are attributed to a pipe segment that was constructed at a negative slope.