of channel drainage is greater than point drainage and the excavation required is usually much less deep. The surface opening of channel drainage usually comes in the form of gratings (polymer, plastic, steel or iron) or a single slot (slot drain) that runs along the ground surface (typically manufactured from steel or iron). Review process Drainage in retaining walls Earth retaining structures such as retaining walls also need to consider groundwater drainage. Typical retaining walls are constructed of impermeable material which can block the path of groundwater. When groundwater flow is obstructed, hydrostatic water pressure buildups against the wall and may cause significant damage. If the water pressure is not drained appropriately, retaining walls can bow, move, fracture and seams separate. The water pressure can also erode soil particles leading to voids behind the wall and sinkholes in the above soil. Traditional retaining wall drainage systems can include, French Drains, drain pipes or weep holes. To prevent soil erosion, Geotextile filter fabrics are installed with the drainage system. Reasons for artificial drainage Wetland soils may need drainage to be used for agriculture. In the northern United States and Europe, glaciation created numerous small lakes which gradually filled with humus to make marshes. Some of these were drained using open ditches and trenches to make mucklands, which are primarily used for high value crops such as vegetables. The largest project of this type in the world has been in process for centuries in the Netherlands. The area between Amsterdam, Haarlem and Leiden was, in prehistoric times swampland and small lakes. Turf cutting (Peat mining), subsidence and shoreline erosion gradually caused the formation of one large lake, the Haarlemmermeer, or lake of Haarlem. The invention of wind-powered pumping engines in the 15th century permitted drainage of some of the marginal land, but the final drainage of the lake had to await the design of large, steam powered pumps and agreements between regional authorities. The elimination of the lake occurred between 1849 and 1852, creating thousands of km2 of new land. Coastal plains and river deltas may have seasonally or permanently high water tables and must have drainage improvements if they are to be used for agriculture. An example is the flatwoods citrus-growing region of Florida. After periods of high rainfall, drainage pumps are employed to prevent damage to the citrus groves from overly wet soils. Rice production requires complete control of water,