Bed-mounted Electro Magnetic meters: Implications for robust velocity measurement in Urban Drainage systems

Flow monitoring in Urban Drainage Systems (UDS) is required for a successful system control and operational assessment. Commonly used methods can lead to erroneous results in partially filled pipes and hostile environmental conditions, normally encountered in UDS. Recent studies focused on the flow rate measurements in UDS revealed that the capability of acoustic Doppler velocimeters to estimate mean flow velocity is impeded by several factors. Most prominent issues are the operation under low flow depths and velocities, as well as in the case of the sedimentation at low flow velocities. This study is focused on an alternative method for the velocity measurements in the UDS, based on Electro-Magnetic Velocity (EMV) meters. The study also determines the sensor's capacity to operate when covered by a porous sediment layer, using a newly developed procedure. A brief theoretical background is given to support the idea behind the usage of EMV in UDS. Measurement uncertainties were firstly benchmarked in the laboratory flume without sediment. After local, site-specific (re) calibration, EMV operated with combined uncertainty of only few cm/s. Furthermore, the EMV measured the flow rates with depths low as 4 cm and velocities bellow 5 cm/s. Additionally, a series of tests were performed with sediment layers above the EMV meter, varying in height from 0 to 80 mm. Observational uncertainty analysis showed that EMV meter can be used even in these conditions. Since the bias uncertainty increased with the rise of the sediment depth, a correction function model was derived for the transformation of the output signal, reducing the observational uncertainties below 5 cm/s. Subsequently, practical implications of the EMV usage in the UDS are considered.