Drop structure

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.

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.

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.

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 1 foot (0.30 m) to 10 feet (3.0 m). 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 form the drop structure. Another less common type of drop structure, the sculpted sloping boulder drop, is derived from this. The sculpted sloping boulder drop is used to create a more natural appearance to the drop structure. Both of these structures also tend to suffer from downstream erosion.

The baffle chute drop is built entirely of concrete and is effective with low maintenance needs. They typically consist of a concrete chute lined with "baffle" teeth to slow velocity of water as it passes over the structure. Despite these appeals, however, they have very "limited structural and aesthetic flexibility, which can cause them to be undesirable in most urban settings."

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Wikipedia