Using Flow Monitoring to Understand CSO Structure Performance

This paper discusses the challenges encountered during the monitoring program and subsequent model analysis phase associated with National Pollutant Discharge Elimination System (NPDES) Combined Sewer Overflow (CSO) outfall 44 in Seattle Public Utilities (SPU) combined sewer system. The CSO structure at outfall 44 is located in the northern portion of the Henderson Area and contains two separate diversion weirs –44A and 44B – as well as an off-line storage structure and a HydroBrake – a flow control device that limits downstream discharge. The structure is at the downstream end of a basin roughly 170 acres in size and has an annual CSO frequency of about 13 events per year. The purpose of the flow monitoring program and modeling analysis was to determine alternatives viable to reduce the frequency of overflows to one event per year for this outfall.Three temporary and two permanent monitors were located in the vicinity of this structure to help illustrate the hydraulics through the structure. Specifically, HydroBrake performance needed to be characterized in order to simulate the device in the hydraulic model. CSO events needed to be detected and quantified for reporting purposes, as well as for use in calibrating and validating the hydraulic model response.During the monitoring data review and model analysis challenges encountered included:Defining HydroBrake performanceVerifying the quality of the monitoring data being collectedQuantifying CSO volume over a long, uneven weirThe monitoring data underwent continuous review so that modifications needed to improve the flow monitoring could be quickly identified and implemented. With a good understanding of the current performance of this structure, SPU has been able to 1) develop confidence in the measured CSO volumes and modeling results and 2) evaluate and identify the appropriate improvements to reduce the number of CSO events at this outfall from 13 events per year to one event per year.