A drop structure, also known as a grade control, sill, or weir, is a manmade structure, typically small and built on minor streams, or as part of a dam's spillway, to pass water to a lower elevation while controlling the energy and velocity of the water as it passes over. Unlike most dams, drop structures are usually not built for water impoundment, diversion or raising the water level. Mostly built on watercourses with steep gradients, they serve other purposes such as water oxygenation and erosion prevention. Typical designs Drop structures can be classified into three different basic types: "vertical hard basin", "grouted sloping boulder", and "baffle chute". Each type is built depending on water flow, steepness of the site, and location. Vertical hard basin The vertical hard basin drop structure, also called a dissipation wall, is the basic type of drop structure. The vertical hard basin drop consists of a vertical "cutoff wall", usually built of concrete, that is usually laid perpendicular to the stream flow; and an impact basin, not unlike a stream pool, to catch the discharged water. The purpose of the vertical hard basin drop is to force the water into a hydraulic jump (a small standing wave). Though the simplest type of drop structure, it is highest in maintenance needs and less safe, with most problems related to the impact basin. Sediment is often deposited in the basin, requiring frequent removal, and erosion downstream of the base of the structure. Understanding the estuarine turbulent flow from dams, channels, and pipes, as well as the river flow are very important due to the potential to cause damage to the bed of the river or channel and cause scouring of structures such as the saddles of bridges, because of the huge amount of the kinetic energy carried by the flow. One of the most efficient yet simple ways to dissipate this energy is to install a stilling basin at the discharge point to calm the flow. Grouted sloping boulder A grouted sloping boulder drop structure is the most versatile of drop structures. Able to accommodate both a broad floodplain or a narrow channel, they can also handle many different drop heights. Heights of these structures usually range from to . These structures are built by creating a slope of riprap, which consists of large boulders or less commonly, blocks of concrete. These are then cemented together ("grouted") to