Tailings pond freeboard before cyclone season
An upstream-raised tailings storage facility has reached 85 m height. The beach is 60 m wide, the pond is 40 m from the crest, and freeboard is 1.5 m. The underdrainage system is working, and a piezometer in the outer slope reads 3 m below the crest. The decant pump can remove 0.8 m³/s. The catchment above the pond is 1.2 km², and the 1-in-100, 72-hour storm for the site is 280 mm. The impoundment surface area at current level is 350,000 m². The operation has been told that the cyclone season starts in six weeks, and the tailings delivery spigots are currently at the pond edge, not on the perimeter. The supernatant pond has risen 0.4 m in the last month, and the beach is narrowing. The operator must decide whether to continue deposition, lower the pond, or start a downstream raise.
1. Estimate the pond rise from the design storm using a runoff coefficient of 0.7 for the catchment, allowing for direct rainfall on the pond and pumping at 0.8 m³/s for 72 hours. Assess the remaining freeboard.
Read the worked answer
Catchment runoff = 0.7 × 0.280 × 1,200,000 = 235,200 m³. Direct rainfall on the pond = 0.280 × 350,000 = 98,000 m³. Total inflow = 333,200 m³. Pump removal over 72 h = 0.8 × 72 × 3,600 = 207,360 m³. Net volume = 125,840 m³. Pond rise = 125,840 / 350,000 = 0.36 m. Remaining freeboard = 1.5 − 0.36 = 1.14 m. If the runoff coefficient is 1.0, total inflow is 434,000 m³, net is 226,640 m³, rise is 0.65 m, and freeboard falls to 0.85 m. Neither result includes wave run-up or the risk that the pump is unavailable. A freeboard of 1.14 m or less is not adequate for a 1-in-100 storm at a facility with a narrow beach and a pond already close to the crest. The storm assessment therefore shows the facility cannot be left as it is through the cyclone season.
2. Recommend immediate actions and changes to deposition practice. Justify the priorities.
Read the worked answer
Before the season, lower the pond by at least 1.0 m using the decant system and, if necessary, temporary pumping. That creates a freeboard of about 2.5 m and restores an operating margin. Move the spigots to the perimeter and rotate deposition around the facility so that a wide beach is rebuilt and the pond is kept away from the crest. Verify the decant pump, standby power and emergency spillway capacity, because the calculated storm removal assumes continuous pumping for 72 hours. Desilt the decant intake and survey the freeboard and beach daily. Do not start a downstream raise as the immediate response; the first priority is to restore safe operating freeboard and beach width with the existing facility. If deposition must continue, reduce the volumetric rate and deposit only on the perimeter, away from the pond. The pond rise of 0.4 m in one month shows that current operation is already losing freeboard faster than the design storm allowance.
3. Explain why an upstream-raised facility with the pond close to the crest is a geotechnical concern, and list the monitoring and trigger levels you would require.
Read the worked answer
An upstream raise relies on the deposited tailings and the beach to form the retaining structure. If the pond is close to the crest, the phreatic surface in the outer slope rises, reducing effective stress and shear strength and increasing the risk of slumping or flow liquefaction. A narrow beach means the supernatant cannot be kept away from the crest, and the 1.5 m freeboard is insufficient for the design storm. Monitor piezometers in the outer slope, seepage weirs, survey monuments, beach width, pond level and freeboard. Set trigger levels in advance, for example: stop deposition and lower the pond if the piezometric level rises above a defined line, if the beach narrows below 40 m, if freeboard falls below 2.0 m before the season, or if seepage flow or turbidity increases. Any trigger should lead to a documented stop, inspection and remedial action, not simply more monitoring.