The East Bay MUD was experiencing venting problems in its Main Treatment Plant outfall. Flow Science and Carollo Engineers conducted a line fill slam test to determine the conditions under which the venting occurred. The field test and hydraulic modeling revealed a range of flows and tidal conditions where the outfall is flowing partially full. An increase in outfall flow which causes the pipeline to transition from a partially full condition to a full flow pipe condition was predicted to create a line fill slam in the outfall as the air was rapidly vented.