A numerical assessment of land reclamation as a strategy for nearshore diamond mining

Abstract

On the south-west coast of Namibia, diamonds were trapped during the Quaternary on bedrock platforms and in bedrock gullies. These were formed by differential erosion due to wave action. The diamonds originated from Kimberlite diamond pipes within the catchment area of the Orange River in South-Africa and Botswana. This river eroded the diamonds from the pipes and transported them over a distance up to 1000 km to the sea. Subsequent deposition resulted in the formation of placer deposits on the south-west coast of Namibia. These diamond deposits form the mineral reserve for the Southern Coastal Mine, operated by NAMDEB, which is the area of interest for this study. The currently used extraction strategy in the mine is based on the philosophy that the mining method must be selective and land-based. Conventional loading and hauling of overburden and bulk material is applied after which a final cleaning of the bedrock is done with vacuum cleaners. A seawall made out of sand protects the mine along the entire coastline against flooding. Overburden sand and tailings are constantly transported and dumped onto the beach to achieve land reclamation. This is referred to as the Sand-2-Sea program, developed to expand mining activity in a seaward direction. When enough land is accrued the sandy seawall, protecting the mining operation against flooding, is shifted seawards and new mining area is claimed. The coastal area is characterized by a highly energetic wave climate. This causes a high erosion rate of the sand nourishment along the coast. The strategic business plan of NAMDEB states that diamond extraction, using the current mining- and land reclamation method, must continue until 2031 reaching a maximum seaward distance of 545 m from the average high water line (Mean Sea Level (MSL) +2m) of 2014. It was hypothesized in this thesis that the feasibility limit of the current mining method is reached before this maximum distance is obtained. In addition, severe seawall erosion can occur during storm events. A storm in May 2015 was an example of this, causing seawall failure and flooding of an area with ongoing mining activity. This event is used as a case study in this thesis. The aim of this thesis was to indicate how far seawards the land reclamation strategy for diamond mining can be applied. In addition, research was conducted to define standards for a seawall to effectively protect a mining area against flooding.