A drop structure, also known as a grade control,

sill Sill may refer to: * Sill (dock), a weir at the low water mark retaining water within a dock * Sill (geology), a subhorizontal sheet intrusion of molten or solidified magma * Sill (geostatistics) * Sill (river), a river in Austria * Sill plate, ...

, or

weir A weir or low head dam is a barrier across the width of a river that alters the flow characteristics of water and usually results in a change in the height of the river level. Weirs are also used to control the flow of water for outlets of l ...

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

spillway A spillway is a structure used to provide the controlled release of water downstream from a dam or levee, typically into the riverbed of the dammed river itself. In the United Kingdom, they may be known as overflow channels. Spillways ensure th ...

, 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 Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is d ...

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 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 below average. Formation A stream pool may be bedded with sediment or armoured with gravel, and in some cases t ...

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

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 ...

(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 is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand ...

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 Riprap (in North American English), also known as rip rap, rip-rap, shot rock, rock armour (in British English) or rubble, is human-placed rock or other material used to protect shoreline structures against scour and water, wave, or ice erosion. ...

, 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

Wildlife

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,

eddies In fluid dynamics, an eddy is the swirling of a fluid and the reverse current created when the fluid is in a turbulent flow regime. The moving fluid creates a space devoid of downstream-flowing fluid on the downstream side of the object. Fluid ...

, 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 () is the common name for several commercially important species of euryhaline ray-finned fish from the family Salmonidae, which are native to tributaries of the North Atlantic (genus '' Salmo'') and North Pacific (genus '' Onco ...

, 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 control

Erosion is usually reduced by drop structures, and natural river channel processes, such as channel migration,

meander A meander is one of a series of regular sinuous curves in the channel of a river or other watercourse. It is produced as a watercourse erodes the sediments of an outer, concave bank ( cut bank) and deposits sediments on an inner, convex ban ...

ing, and creation of stream pools and riffles, are also reduced. Drop structures can be used for flow control and to stabilize waterways and prevent the formation of gullies. They also have the potential to operate as inlets and outlets for other conservation structures, such as

culvert A culvert is a structure that channels water past an obstacle or to a subterranean waterway. Typically embedded so as to be surrounded by soil, a culvert may be made from a pipe, reinforced concrete or other material. In the United Kingdo ...

References

{{Rivers, streams and springs Dams by type Stormwater management Soil erosion