Reverse flow in a sewer system is rare, but in this situation a 12-in sewer is overpowered by a much larger downstream interceptor. During a rain event, this sewer experiences backwater and surcharge conditions. However, note the sequence of events that occurs during surcharge conditions. The flow rate begins to slow down at 42 inches and eventually comes to a momentary stop at 72 inches. Reverse flow is observed above this depth. The reverse flow volume is estimated to be 46,000 gallons and most likely caused an SSO somewhere upstream from the monitoring location.
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