Hydrometric Monitoring and Effluent Discharge Mixing in Challenging Natural Conditions
Authors: Michael Barrett, Jaime Villaamil, Violeta Martin, Sitotaw Yirdaw, Travis Pastachak, Garnet Cornell
Conference: Mine Water Solutions 2022
Date: June 14-16, 2022
ABSTRACT
River systems prone to backwatering present difficult conditions for collecting reliable hydrometric monitoring data. Furthermore, passive discharge of treated effluent in low velocity receiving environments may not achieve full mixing without additional measures to promote dilution. Knight Piésold Ltd. (KP) designed a hydrometric monitoring network and a treated effluent discharge system for the Rainy River Mine (RRM) in Northwest Ontario that had both these conditions.
The Pinewood River provides a challenging environment for hydrometric monitoring due to backwatering effects from downstream obstructions, which precluded developing stable stage-discharge relationships to facilitate continuous discharge measurements. Consequently, an alternative to the traditional flow measurement approach was required. To overcome the difficulties associated with stage-discharge relationships, an acoustic Doppler type instrument was selected that measures the velocity profile and the water depth continuously, which are used in combination with a user-specified cross section to record continuous real-time discharge data.
The treated effluent to be discharged into the local receiving environment must meet strict mixing and water quality standards, as dictated by RRM’s Environmental Compliance Approval (ECA). The permitted conditions include constraints on the ratio of instream flow to effluent discharge in addition to achieving full transverse mixing within a short distance from the discharge point. An analytical mixing model was developed to evaluate the performance of various diffuser options under a wide range of effluent and ambient discharge conditions, which was then used to guide the final diffuser selection and design. The selected design includes a diffuser system with two planar jets using duckbill-type neoprene rubber check valves. This design also provides back flow and fish entrance prevention, as the rubber check valves close when the outflow is discontinued. To prevent channel erosion from the discharge jets and prevent utilization by fish for habitat in the vicinity of the diffusers, grouted gabion boxes, grouted riprap, and interlocked concrete blocks were used to create a protected and stable cross section.
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