Data processing script that derives FLOWRATE from DEPTH in a pipe under gradually varied flow conditions
The purpose of this document is to describe the post-processing method for the monitoring data from California Department of Transportation (Caltrans) 1194R Austin Sand Filter (ASF) in Newport Beach, CA. Caltrans, a member agency of the Southern California Stormwater Monitoring Coalition (SMC) is conducting water quality monitoring at a suitable BMP location as part of a Regional Monitoring Network.
In an optimal monitoring setup, an autosampler translates real-time water depths into estimates of flowrate. The functional relationship between water depth and flowrate is called a rating curve, and is commonly established with a combination weir + pressure transducer. The flowrate data can then be used by the autosampler to pace the physical collection of water quality aliquots into a composite sample. By collecting each aliquot after a fixed volume of water has passed, the concentration of the composite sample is the flow-weighted event mean concentration (EMC). The emergent problem with conventionally monitoring the Caltrans ASF is the shallow slope of the inflow conduit to the sand filter from an upstream detention basin. Due to the shallow slope and the ponding depth of the ASF, significant backwater effects have been observed. Rating curves developed using a weir + pressure transducer combination have the assumption of a free outfall from the mouth of the weir, which is not present during backwater conditions. Without a valid rating curve, there is no way to communicate in real time to the autosampler.
In lieu of the conventional, integrated autosampler approach to monitoring at Caltrans 1194R ASF, a manual approach was implemented involving collaboration between SCCWRP and the Caltrans consultant, WSP. Discrete grab samples (1 sample per bottle) were collected by the autosampler, but at a time frequency set by the WSP operator, rather than based on flow volumes estimated from the rating curve. Flowrate data was obtained through the use of two pressure transducers at either end of the inflow conduit. Grab-sample times were combined with the flowrate data to generate a flow-weighted composite sample. The flow-weighted composite sample is created by mixing aliquots of each grab sample in volumes proportional to the volume of water that passed through the system between each grab sample.
The remaining sections of this document describe the monitoring configuration at the ASF 1194R interbasin conduit (i.e., the ASF inflow), as well as the programmatic flow-calculation approach. Backflow conditions are illustrated, and the derivation of an equation to derive flow from the difference between two depths is presented. A summary of the 2022-2023 monitored storm events is included.