Drop structure

From Wikipedia, the free encyclopedia

Please help improve this article or section by expanding it.
Further information might be found on the talk page or at requests for expansion. (December 2007)

A drop structure is a grade control structure built in an artificial or natural riverbed, primarily to stabilize steep grades and to aid the water's descent. Most drop structures are built of concrete or large boulders.

1 Types and function
2 Parts
3 Riprap
4 Effect on stream environment
5 Miscellaneous
6 Sources

[edit] Types and function

The main types of drop structures are basic vertical drop structures ("vertical hard basin"), riprap drop structures, "riffle" drop structures, and weirs. Other types include "baffle chutes" and "Grouted Sloping Boulder". Drop structures are sometimes built as retaining walls to stabilize a descent in the riverbed that is in danger of collapsing (see St. Anthony Falls). In times of flood, drop structures slow the velocity of the water by turning otherwise a fast-flowing rapids into a vertical descent. Artificial riverbeds and flood control channels especially need drop structures because there are no natural drops.

A weir in Warkworth, New Zealand. A weir is a type of drop structure.

[edit] Parts

Most drop structures are built with a plunge pool at the base. The plunge pool acts like a plunge pool below a waterfall, but its basic function is to dissipate the flow of the water and reduce erosion. Some plunge pools contain energy dissipating blocks that slow down the velocity even more. Many drop structures cause pooling behind when the water erodes away the material behind the structure, especially in natural riverbeds.

[edit] Riprap

A riprap drop structure is made by piling large amounts of broken rocks or concrete on a steep slope in the riverbed. Many examples of this can be seen on the Arroyo Trabuco in Southern California.

[edit] Effect on stream environment

Drop structures segment the stream and prevent natural movement up or downstream by fish and other wildlife. An exception is riffle drop structures. Many vertical drop structures segment the stream in the same way as waterfalls, however, riffle" drop structures allow the water to drop not vertically, but instead is segmented into a series of pools with water spilling from one drop to the next, allowing fish to jump through, like a fish ladder. Weirs can be like vertical drop structures, but some are built across waterways with holes cut into the weir. The stream passes through the holes at low flow, but is allowed to spill over the top in a flood.

[edit] Miscellaneous

A sloped drop structure, also can be curved, is built when the upper streambed is very unstable. Sometimes, drop structures are built with a slot in the center to handle normal stream flow and prevent stagnation, but in a flood, the water is allowed to overflow the slot.