Spillway Success
This article was contributed by Sofien GABSI, Technical Director and Siwar KAROUI, Hydraulic Engineer, MECATER Ingénierie.
Making tailings safe: Tunisia-based engineering consultancy MECATER Ingénierie turns to FLOW-3D HYDRO to solve a critical environmental concern in New Caledonia.
One thousand miles off the coast of Australia, deep in the South Pacific, sits an island that many of us have never heard of, and those who have would struggle to find it on a map. Welcome to New Caledonia, a land of lush tropical rainforests, expansive coral reefs, and one of the largest nickel and cobalt reserves in the world.
Remote or not, the abundance of these last two natural resources explains why this small French territory plays such a prominent role in the global energy market. It currently produces roughly 40,000 tons annually of the nickel hydroxide cake needed to make lithium-ion batteries for electric vehicles, a volume that is sure to increase as drivers everywhere look for more sustainable alternatives to internal combustion engines.
Mining of any mineral has its environmental downsides, however, and nickel is no different. Originally operated by Vale Nouvelle-Calédonie, the Goro mine in New Caledonia’s southern region first began processing this vital mineral in 2010.
Cleaning up
The hydrometallurgical plant extracts nickel and cobalt from the “red earth,” or laterite, using a process of leaching. The resultant waste slurry—the wet tailings—are then pumped to a massive storage pond that was built during the industrial complex’s construction nearly two decades ago. “They’ve been storing the waste onsite since the start of mining operations,” says MECATER’s hydraulic engineer.
Prony Resources New Caledonia, the owner of the Goro mine, launched Lucy, a solid waste storage project that aims to avoid potential flooding by constructing an overflow spillway at the tailing storage facility’s (TSF) southern end. That’s when they reached out to MECATER Ingénierie, a company with a long history in mine infrastructure, water management, and geotechnical engineering.
KO2 Spillway design criteria
“The spillway is a stepped structure measuring 63 meters high by 70 meters wide and is capable of handling probable maximum flood (PMF) flows of roughly 290 cubic meters per second,” says the project director, who played a lead role on the MECATER project team responsible for the spillway’s optimization.
Building Lucy
Note the term “optimization” just now. An equally appropriate term is redesign. That’s because the original spillway—first proposed in 2009—was only partially completed, and Prony Resources charged MECATER Ingénierie with improving the design while simultaneously reducing construction costs. It was a big ask. “We had to make certain it would withstand a PMF event without failing, and do so in the most efficient manner possible,” says the engineer.
The stakes were quite high, both for the facility and the surrounding community. MECATER’s hydraulic engineer began by developing elaborate mathematical models, but soon found the project needed tools better suited to Lucy’s enormous complexity. The team contacted Dr. Amr Refay Abdelghany, the director of technical services at El Refay for Engineering Services and the official distributor of FLOW-3D computational fluid dynamics (CFD) software products in the Arab countries.
"CFD was new to us, so we began with two-dimensional modeling tools from another provider. These helped us to improve the design and deliver a preliminary proposal to the client, but we needed a better way to visualize the water flow and identify additional areas for optimization. After speaking with El Refay about our requirements, we switched to FLOW-3D HYDRO."
At the request of Prony Resources, MECATER also partnered with the Moroccan hydraulic laboratory LPEE to conduct experiments on a 1:30 scale physical model of the stepped spillway, under the technical assistance of the independent expert Professor Hubert Chanson from the University of Queensland, Australia. The testing results from the physical model and its simulated FLOW-3D HYDRO counterpart were “nearly identical” to one another.
“We found that CFD modeling was a powerful tool for evaluating the design of our spillway structure. In fact, with its capability to simulate hydraulic jump and real conditions such as air entertainment model, fluid characteristics and also its accurate FAVOR™ method to generate meshing, we were able to confirm the results we’d obtained earlier with 2D software tools as well as those of our scale model.”
Getting physical
Based on the data provided by numerical, physical, and CFD-generated models, the team proposed numerous modifications to the original design. These include eliminating two of the steps within the main channel, overcutting of the dissipation basin by a further two meters, limiting the return channel’s slope to a 3% grade, and creation of two waterfalls, which were protected from erosion by free riprap and equipped with two large plunge pools at the staircase bottom.
Prony Resources accepted these and several additional proposals, and construction began soon after. Today, the company and those who live nearby can rest assured that the hazardous tailings within the TSF structure will remain safely contained, even if a PMF event were to occur.
"As it did in New Caledonia, FLOW-3D HYDRO also allows us to show videos of our designs to clients. This helps them to visualize water flow under realistic conditions and gives them greater confidence that the solution will work as promised."
None of this surprises Amr Abdelghany of El Refay for Engineering Services, who has been a FLOW-3D distributor and technical sales partner since 2019 and has seen similar successes with many of his other clients. “Even though MECATER’s team had no extensive prior CFD experience, they were up and running with the software within a few days. That’s a testament to their skill, of course, but it also speaks highly of the software, which is both intuitive and easy to navigate. It gives us great pleasure to see MECATER Ingénierie in Tunisia enjoying significant success with FLOW-3D HYDRO, and we look forward to working with them again soon.”
MECATER’s team also looks forward to the next project. “Using simulation software easily saved us a month or more of effort while helping us to deliver an optimized design that met our client’s criteria. We recently visited the New Caledonia facility and we were very proud to see the completed spillway. Based on these results, we now see FLOW-3D HYDRO as an invaluable tool in any large-scale hydro engineering project.”
Results
The results of the numerical simulations and the experimental tests carried out have showed that:
- The flow in the spillway is mainly concentrated in the central channel with complete development of hydraulic rise on each step for the different flows studied, particularly for the probable maximum flood (PMF);
- The flow passes from the nappe flow regime for the ten-year flood to the transition flow regime for the probable maximum flood;
- Controllable overflows without damage are observed in the lateral sections of the course during the probable maximum flood.
