A drop structure, also known as a grade control, sill, or weir

Weir

A weir is a small overflow dam used to alter the flow characteristics of a river or stream. In most cases weirs take the form of a barrier across the river that causes water to pool behind the structure , but allows water to flow over the top...

, is a manmade structure, typically small and built on minor streams, or as part of a dam's spillway

Spillway

A spillway is a structure used to provide the controlled release of flows from a dam or levee into a downstream area, typically being the river that was dammed. In the UK they may be known as overflow channels. Spillways release floods so that the water does not overtop and damage or even destroy...

, to pass water to a lower elevation while controlling the energy and velocity of the water as it passes over. Unlike most dam

Dam

A dam is a barrier that impounds water or underground streams. Dams generally serve the primary purpose of retaining water, while other structures such as floodgates or levees are used to manage or prevent water flow into specific land regions. Hydropower and pumped-storage hydroelectricity are...

s, 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

Erosion

Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...

 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

Stream pool

A stream pool, in hydrology, is a stretch of a river or stream in which the water depth is above average and the water velocity is quite below average.-Formation:...

, to catch the discharged water. The purpose of the vertical hard basin drop is to force the water into a hydraulic jump

Hydraulic jump

A hydraulic jump is a phenomenon in the science of hydraulics which is frequently observed in open channel flow such as rivers and spillways. When liquid at high velocity discharges into a zone of lower velocity, a rather abrupt rise occurs in the liquid surface...

 (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

Sediment

Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....

 is often deposited in the basin, requiring frequent removal, and erosion downstream of the base of the structure.

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 1 foot (0.3048 m) to 10 feet (3 m). These structures are built by creating a slope of riprap

Riprap

Riprap — also known as rip rap, rubble, shot rock or rock armour or "Rip-rap" — is rock or other material used to armor shorelines, streambeds, bridge abutments, pilings and other shoreline structures against scour, water or ice erosion.It is made from a variety of rock types, commonly granite or...

, 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.

Baffle chute

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."

Environmental effects

Drop structures have been shown to either be beneficial or detrimental to habitat in the stream. They create complexity of habitat by breaking up a stretch of stream into a series of pools. Surface turbulence, eddie

Eddy (fluid dynamics)

In fluid dynamics, an eddy is the swirling of a fluid and the reverse current created when the fluid flows past an obstacle. The moving fluid creates a space devoid of downstream-flowing fluid on the downstream side of the object...

s and bubbles are generated by drop structures that provide hiding and cover for fish and other aquatic organisms. Water is aerated as it passes over drop structures. Sediment is collected and sorted in scour pools, which provide energy dissipation.

On the other hand, drop structures may also become barriers to fish. The downstream channel may erode and slowly and unexpectedly increase the height of the structure, to a point where migratory fish, such as salmon

Salmon

Salmon is the common name for several species of fish in the family Salmonidae. Several other fish in the same family are called trout; the difference is often said to be that salmon migrate and trout are resident, but this distinction does not strictly hold true...

, cannot pass over the structure. Other causes may be that the plunge pool is obstructed or the water flow is too shallow. However, many properly functioning drop structures themselves may impede the upstream and downstream migration of fish.

Unless the structure is designed to maintain them, existing fish spawning pools will be impacted or lost. Erosion is usually reduced by drop structures, and natural river channel processes, such as channel migration, meandering

Meander

A meander in general is a bend in a sinuous watercourse. A meander is formed when the moving water in a stream erodes the outer banks and widens its valley. A stream of any volume may assume a meandering course, alternately eroding sediments from the outside of a bend and depositing them on the...

, and creation of stream pools and riffles, are also reduced.