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15-438: First envisaged in 1897, Hardap Dam has a capacity of 320 million cubic metres (420,000,000 cu yd) and a surface area of 25 km (2,500 ha). Construction began in 1960 and was completed in 1963. The Hardap Dam supplies Mariental and the surrounding settlements with potable water. Its location close to the city, however, also poses a danger of flooding when sluices have to be opened fully due to good rains in

30-697: A carpet or rubber matting, which trap the heavy minerals, gemstones, and other valuable minerals. Since the early 2000s more miners and prospectors are relying on more modern and effective matting systems. The result is a concentrate which requires additional processing. In the Somerset Levels , sluice gates are known as clyse or clyce. Most of the inhabitants of Guyana refer to sluices as kokers. The Sinhala people in Sri Lanka, who had an ancient civilization based on harvested rain water, refer to sluices as Horovuwa. Black sand Black sand

45-430: A common mineral-suite that becomes black sands after weathering and erosion. Several gemstones , such as garnet , topaz , ruby , sapphire , and diamond are found in placers and in the course of placer mining, and sands of these gems are found in black sands and concentrates. Purple or ruby-colored garnet sand often forms a showy surface dressing on ocean beach placers. An example of a non-volcanic black sand beach

60-552: A simple, hand-operated, chain pulled/lowered, worm drive or rack-and-pinion drive , or it may be electrically or hydraulically powered. A flap sluice, however, operates automatically, without external intervention or inputs. Sluice gates are one of the most common hydraulic structures used to control or measure the flow in open channels. Vertical rising sluice gates are the most common in open channels and can operate under two flow regimes: free flow and submerged flow. The most important depths in designing of sluice gates are: In

75-561: Is sand that is black in color. One type of black sand is a heavy, glossy, partly magnetic mixture of usually fine sands containing minerals such as magnetite , found as part of a placer deposit . Another type of black sand, found on beaches near a volcano , consists of tiny fragments of basalt . While some beaches are predominantly made of black sand, even other color beaches (e.g. gold and white) can often have deposits of black sand, particularly after storms. Larger waves can sort out sand grains leaving deposits of heavy minerals visible on

90-429: Is a sluice channeling water toward a water mill . The terms sluice , sluice gate , knife gate , and slide gate are used interchangeably in the water and wastewater control industry. "Sluice gate" refers to a movable gate allowing water to flow under it. When a sluice is lowered, water may spill over the top, in which case the gate operates as a weir . Usually, a mechanism drives the sluice up or down. This may be

105-415: Is also a large variety of bird species to be observed in and around the dam. Great White Pelican , cormorant , darter and spoonbill can be seen on the dam itself, as well as fish eagle and a small number of osprey . The vegetation in the area is classified as dwarf shrub savanna . The following trees occur: camel thorn, wild green-hair tree and buffalo thorn. After a four-year period of renovation

120-506: Is at Langkawi in Malaysia. When lava contacts water, it cools rapidly and shatters into sand and fragmented debris of various size. Much of the debris is small enough to be considered sand. A large lava flow entering an ocean may produce enough basalt fragments to build a new black sand beach almost overnight. The famous "black sand" beaches of Hawaii, such as Punaluʻu Beach and Kehena Beach , were created virtually instantaneously by

135-449: The surface of erosion escarpments . Black sands are used by miners and prospectors to indicate the presence of a placer formation. Placer mining activities produce a concentrate that is composed mostly of black sand. Black sand concentrates often contain additional valuables, other than precious metals: rare earth elements , thorium , titanium , tungsten , zirconium and others are often fractionated during igneous processes into

150-576: The Fish River's catchment area . Reed grasses growing in the riverbed of the Fish River, slow down the flow of water and further aggravate the danger of flooding. Before the dam was built, Mariental was flooded in 1923 and 1934. Floods after the commissioning of the dam occurred in 1972, 1974, 1976, 2000, and 2006. Since then, the dam's water level is kept at a maximum of 70% of its capacity to prevent both an overflow and an uncontrolled outflow through fully opened sluices. The name Hardap derives from

165-568: The Nama word meaning "nipple" or "wart", which is how the surrounding area of low conical-shaped hills appeared to the early inhabitants. There are fishing spots at various points along the northern shore of the lake. Permits, and a map of permitted fishing areas, are available from the resort office, or from the magistrate's office in Mariental. The area is home to the black rhino, gemsbok, Hartmann's zebra, kudu, ostrich, springbok and steenbok. There

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180-510: The dam's recreational sites have been reopened in 2016. Activities at the dam include swimming, fishing, and bird watching. There is a restaurant and a shop. Sluice A sluice ( / s l u s / SLOOS ) is a water channel containing a sluice gate , a type of lock to manage the water flow and water level. It can also be an open channel which processes material, such as a river sluice used in gold prospecting or fossicking . A mill race , leet , flume , penstock or lade

195-464: The mountains of the United States, sluices transported logs from steep hillsides to downslope sawmill ponds or yarding areas. Nineteenth-century logging was traditionally a winter activity for men who spent summers working on farms. Where there were freezing nights, water might be applied to logging sluices every night so a fresh coating of slippery ice would reduce friction of logs placed in

210-416: The sluice the following morning. Sluice boxes are often used in the recovery of black sands , gold , and other minerals from placer deposits during placer mining operations. They may be small-scale, as used in prospecting , or much larger, as in commercial operations, where the material is sometimes screened using a trommel , screening plant or sieve. Traditional sluices have transverse riffles over

225-428: The violent interaction between hot lava and sea water. Since a black sand beach is made by a lava flow in a one time event, they tend to be rather short lived since sands do not get replenished if currents or storms wash sand into deeper water. For this reason, the state of Hawaii has made it illegal to remove black sand from its beaches. Further, a black sand beach is vulnerable to being inundated by future lava flows, as

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