Velocity field in a Best Overflows Monitoring Structure.

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Gislain Lipeme Kouyi, Ahmed Amin Nasri, Adrien Momplot

Tuesday 30 june 2015

9:30 - 9:45h at Africa (level 0)

Themes: (T) Water engineering, (ST) Experimental facilities and instrumentation

Parallel session: 4F. Engineering - Instrumentation

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The flow control of sewer and storm water systems is essential to preserve receiving water bodies and environment from harmful impacts. This implies to set up a reliable, efficient and accurate metrological chain. In order to face this challenge, a hydraulic facility - the BOMs (best overflows monitoring structure) - that is placed in the overflow pipe of sewer overflow structures has been developed. This system is an innovative solution which aims at providing reliable data (quantity and quality) whatever the geometry of the upstream overflow structure is. This new structure is designed as an integrated solution to obtain reliable measurements of overflow discharge and is appropriate for all CSOs (combined sewer overflow structures) monitoring. It is inserted in the overflow pipe which commonly connects CSOs and receiving water bodies and it exhibits an appropriate hydraulic profile in order to perform reliable quantitative and qualitative overflow measurements. Indeed, measurements in overflow pipe are not new and have been already performed. However, there are often outliers due to the fact that the overflow pipe is often dry and disturbances always occur during storm events. Hence, flow characteristics (air entrainment, complex velocity profiles, etc.) in overflow pipes are not often suitable for flow rate and pollutant loads measurements. The BOMs is a compact solution and is able to provide reliable measurements whatever the inlet flow conditions are. In order to better understand the behaviour of the BOMs, velocity measurements were performed using the ADV device. The results show a symmetrical behaviour after energy losses device. Due to supercritical flow conditions, the acceleration occurs in the entry of the BOMS. Then the hydraulic head decreases across energy losses devices. A subcritical flow is obtained downstream to a hydraulic jump. After a small stabilisation zone, the flow enters in a spilling zone which is appropriate for flow measurements thanks to water depth and overflow rate relationship. In addition, the vertical velocity profile is uniform after energy losses devices and before the spilling zone, highlighting an appropriate area for sampling and continuous monitoring of the overflows quality. Thanks to those results, a full-scaled BOMs has been built by the Greater Lyon (France).