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Evaluation of methods of measuring flowrates for bioretention planters
Citation:
Nissen, K., M. Borst, AND E. Fassman-Beck. Evaluation of methods of measuring flowrates for bioretention planters. Presented at Urban Drainage Modeling Conference, Costa Mesa, California, January 10 - 12, 2022.
Impact/Purpose:
Research and monitoring initiatives have increased with more frequent installation of green infrastructure (GI), including bioretention systems. Monitoring often measures components of a water balance and determines system performance based on the assumption of a closed balance. Bioretention flow-through planters provide a unique opportunity to monitor a nearly¿closed system and attempt to close a water balance to determine measurement method accuracy. The efforts to quantify the performance of bioretention systems vary with monitoring ability and design challenges. Direct flow measurement is feasible using a flume, weir, or area-velocity meter where concentrated inflow or outflow occurs, such as in a pipe or channel. However, many bioretention systems receive diffuse inflow or outflow via exfiltration into the surrounding soils, limiting or eliminating the opportunity for direct measurement of water balance components. Many studies use a weir or flume with a depth measurement to calculate inflow or outflow, but the accuracy of these measurements is rarely characterized. Because performance is often determined by the water balance (e.g., measuring inflow and outflow volume and declaring difference as infiltration and ET), knowing the limitations and accuracy of the setup is necessary to provide complete performance assessments.
Description:
Monitoring methods vary throughout green infrastructure monitoring, but many use a coupled weir and pressure transducer. This presentation investigates this method for green infrastructure monitoring using the nearly-closed system of a bioretention planter. The methods were evaluated by determining how often the water balanced closed. The water balance consistently did not close during the 2019 season, due to the variability in total depth of the pressure transducers differed for each storm. The inconsistent measurements are thought to be partially due to measuring at the end of the instrument’s operating range; there was significant data drift and noise. Topping off the water level in a weir box or sump to the level of the weir crest or V-notch before the storm improved the mass balance in the 2021 monitoring season. This change eliminates uncharacterized intra-event losses and provides weirs with volumes to measure storm flows rather than first capturing storm flow to attain weir height, however filling the weir boxes and sumps is labor intensive and may not always be practical.