Below normal rainfall levels during flow ansd rainfall monitoring conducted this winter have impacted the need for future flow monitoring. Flow monitoring will need to be conducted next fall and winter in order to accurately define infiltration and inflow levels in local collection systems and their impact on the King County regional sewer system. King County is considering its options and costs for continuing the flow monitoring program this next fall and in the future. Monitors operate all transducer combinations, and then derive a single depth reading based on the composite results of all firings. The sensor is mounted at the crown of the pipe where ultrasonic waves are sounded at the surface of the flow.
Echoes bounce back and are received by the appropriate crystal pairs.
The time required for the signal to return to the sensor is directly proportional to the distance traveled. Pressure depth sensors are not used as the primary means of depth measurement because of their propensity for drift, for hysteresis, and for damage caused by over-pressurization due to high storm flows. This depth measurement methodology is typically employed as a back-up to the ultrasonic depth sensor in lines where surcharge conditions are expected. This signal is reflected off particles moving through the flow and intercepted back at the sensor. Flow information will be stored in data loggers installed in the manholes.