IMPACT OF WIND: PLANNING FOR SUSTAINABLE PIT LAKES

Pit lakes are usually one of the lasting features in the footprint of an open pit mine site. Morphometry, orientation, and watershed topography of the pit lake, are mainly dictated by maximization of safe resource extraction, physical reclamation and cost minimization, and not by final pit lake sustainability. However, relatively simple measures in the early phases of site planning and feasibility studies can help to maximize the future stability of these manmade lakes. Since wind is usually the dominant force for lake mixing, taking into account such things as pit lake morphometry, orientation of the future lake to prevailing winds, fetch length, and watershed topography could decrease the impact of wind forced mixing in pit lakes.

We applied the modified Schmidt stability, Wedderburn Number, and Lake Number calculations to a filling pit lake near Ridgeway, South Carolina, USA from 2001 to 2002. Modified Schmidt stability calculations showed that the lake was most stable during the summer and prone to mixing in winter. The lake was highly unstable during reclamation and during silt laden density current influents.

Model results showed that average daily wind speeds of up to 20 mph in summer would not force surface water to the metalimnion or to the bottom of the lake. However, calculations for winter lake configurations showed that average daily wind speeds ³3 mph would force surface water to the metalimnion and average daily wind speeds ³13 mph would force surface water to the bottom of the lake. Assessment of the wind field showed that most wind impacted the lake from the east-west axis, which coincided with the lake’s longest axis. Average daily wind speeds indicated that the lake had little chance of mixing to the thermocline or lake bottom in summer but was prone to surface-to-thermocline mixing in the winter because average daily wind speeds in winter were 3.9 mph. The lake was less likely to undergo surface to bottom mixing because maximum average daily wind speeds were 12.1 mph. However, consistent daily wind speeds, which exceeded the surface-to-thermocline threshold, deepened the epilimnion. Slightly less than half (46%) of the days during February02 had wind speeds which exceeded 3.0 mph.