BE WARY OF WEALLY WOUGH WAVES Identifying and Quantifying the Impact of Standing Waves on Flow Measurement

Abstract

The authors discuss the effect of waves on the accuracy of flow monitoring. Most people are familiar with waves in streams and channels, but likely don’t think about waves in “out-of-sight” sewers. Waves and hydraulic jumps are close cousins and can reduce the carrying capacity of sewers and junction structures if they occur in nearly full-pipe conditions. Even when flow monitors are working correctly, localized variation in depth and velocity from standing waves can cause an incorrect calculation of flow rate. Because waves in sewers are out of sight, users of flow monitors may be unaware of the effect on data quality. Deviation can be on the order of ±25% of calculated flow rate. The authors encountered standing waves during a flow monitor test conducted by EPA’s Environmental Technology Verification (ETV) field test. Subsequent research has lead to the classification of three types of hydraulic jump phenomena and a methodology for identifying the presence of waves by reviewing depth and velocity flow monitor data.