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

Abstract

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, sitespecific (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 2 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.

Metrology in Urban Drainage and Stormwater Management: Plug and pray

“…Similarly, if the water depth h is less than the sum of the sensor height Z surf and reach of the control volume t max , the upper integration limit is shifted downwards. Direct laboratory comparisons between EMVs and acoustic Doppler flowmeters (Aguilar et al, 2016;Ivetić et al, 2018) revealed that, although the control volume of the EMV is close to the probe, due to the operating principle, it is more precise and robust in the assessment of the average cross-sectional velocity U. Also, the EMV can operate in full-bidirectional flows with velocities lower than a few centimetres per second, so they are convenient to use in flows influenced by backwater effects (Figure 3.30).…”

“…Potentially, the most interesting benefit of EMV use in urban drainage, experimentally examined in Ivetić et al (2018), is the ability for velocity measurements in the case of sedimentation over the sensor housing (Figure 3.31). This is a common situation in sewers and even a small layer of sediment (or plastic bags, rags, toilet paper, etc.)…”

Bed-mounted electro magnetic meters: Assessment of the (missing) technical parameters

“…Bed-mounted EMV meters are not commonly used for flow measurements in UDS or in open channel flows. The operating principle of these devices along with the overview of the used EMV characteristics are presented in Ivetić et al (2018a), while the basics are recapitulated here. Afterward, the simplified mathematical model of an EMV sensor is proposed and the importance of the missing technical parameters is highlighted in the scope of the accurate mean flow velocity assessment.…”

“…Due to the nature of the operating principle, the EMV meters are potentially more robust and reliable when compared to the ADV. It was shown that EMV meters can provide measurements of the flow velocity even covered by few centimeters of porous sediment (Ivetić et al, 2018a). Additionally, EMV meters have good performance for flows with low depths (smaller than 5 cm) and low, or even reverse velocities (below few cm/s), found in pipes under the back-water effect.…”

“…Furthermore, an experimental methodology is proposed for the assessment of these technical parameters, based on two lab flume experiments. The first experiment involves investigation of the EMV output in the conditions where the sensor is covered with porous sediment (Ivetić et al, 2018a) of varying depth. The second experiment is focused on describing the distribution of the longitudinal velocity within the lab flume, or more accurately within the CV of the EMV device.…”