An Innovative Way to Detect and Quantify Combined Sewer Overflows in Tidal or Free Discharge Conditions.

Reporting requirements for Combined Sewer Overflows (CSO) are becoming more stringent and often the ‘estimation’ of CSO volume is being replaced with a required measurement of CSO volume. Measuring overflow in either tidally-influenced water bodies or free discharge conditions is considered by many as the simple task of installing a flow meter in the outfall pipe and perhaps a meter in the incoming pipe for model calibration. In reality the challenges of measuring flow in a normally-wet pipe (tidal influence) or a normally-dry pipe (free discharge) introduce great uncertainty into the measurement process.In pipes that are normally dry, a flow meter (designed to operate in water) generates erratic depth and velocity data as the meter strives to take a reading. Often it is difficult to discern valid overflow data in the midst of erratic data. In pipes that are submerged due to flow and ebb tides, the meter is always measuring a velocity (flow) due to incoming tides, outgoing tides or minor changes from waves. In both cases it is difficult to determine when the overflow event started or ended, let alone to determine the overflow volume.Experience of measuring combined sewer overflow in both tidal and non-tidal conditions over several years has revealed a more reliable and less expensive technique for detecting and quantifying overflows. The method relies on a scattergraph of depth and velocity data of a flow meter installed in the sewer entering the CSO regulator structure. In regulators discharging to tidal areas the technique may include an open-close switch on the tide gate as a supplemental ‘trigger’. A meter in the overflow pipe can be avoided.Although all CSO structures are hydraulically complex, they can be divided into categories:Category 1 structures are those in which the Underflow Pipe is full prior to start of Overflow.Category 2 structures are those in which the Overflow can commence prior to the Underflow Pipe being full.This method is applicable to Category 1 structures.