A stepped spillway is a

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 tha ...

with steps on the spillway chute to assist in the dissipation of the

kinetic energy In physics, the kinetic energy of an object is the form of energy that it possesses due to its motion. In classical mechanics, the kinetic energy of a non-rotating object of mass ''m'' traveling at a speed ''v'' is \fracmv^2.Resnick, Rober ...

of the descending water. This reduces the need for an additional energy dissipator, such as a body of water, at the end of the spillway downstream.

Historical developments

Stepped spillways, consisting of

weirs 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 water level. Weirs are also used to control the flow of water for outlets of l ...

and channels, have been used for over 3,500 years since the first structures were built in

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and

Crete Crete ( ; , Modern Greek, Modern: , Ancient Greek, Ancient: ) is the largest and most populous of the Greek islands, the List of islands by area, 88th largest island in the world and the List of islands in the Mediterranean#By area, fifth la ...

. During Antiquity, the stepped-chute design was used for dam spillways, storm waterways, and in town water supply channels. Most of these early structures were built around the

Mediterranean Sea The Mediterranean Sea ( ) is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean basin and almost completely enclosed by land: on the east by the Levant in West Asia, on the north by Anatolia in West Asia and Southern Eur ...

, and the expertise on stepped spillway design was spread successively by the Romans, Muslims, and Spaniards. Although the early stepped spillways were built in cut-stone masonry, unlined rock, and timber, a wider range of construction materials was introduced during the mid-19th century, including the first concrete stepped spillway of the Gold Creek dam (1890) in Brisbane, Australia. During the first half of the 20th century, the stepped cascade design fell out of fashion, partly because of the maintenance costs but also because of the development of hydraulic jump stilling basins. Yet the long-lasting operation of several famous stepped cascades has demonstrated the soundness of the stepped spillway design. Although the stepped spillway design was common up to the beginning of the 20th century, a lot of expertise has been lost since, and the present expertise is limited to very simple geometries, namely some flat, horizontal, stepped prismatic chutes, despite some recent interest in stepped spillway design.

Basic flow characteristics

The flow over a stepped spillway may be divided into three distinct flow regimes depending on the flow rate for a given stepped spillway geometry:

nappe In geology, a nappe or thrust sheet is a large sheetlike body of rock that has been moved more than or above a thrust fault from its original position. Nappes form in compressional tectonic settings like continental collision zones or on the ...

, transition, and skimming flow regimes with increasing flow rates. For a given stepped spillway geometry, the nappe flows are observed for small discharges. They are characterized by a succession of free-falling nappes at each step edge, followed by nappe impact on the following step. The transition flows are observed for a range of intermediate discharges. Some strong hydrodynamic fluctuations, splashing, and spray near the free surface constitute the main features of this flow regime. To date, the transition flow is avoided for design flow conditions because of past failures. The skimming flow regime is observed for the largest discharges. The waters skim over the pseudo-bottom formed by the step edges as a coherent turbulent flow. Beneath the pseudo-bottom, some intense recirculation and vertical structures fill the cavities. These recirculation

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 ...

are maintained by the transmission of

shear stress Shear stress (often denoted by , Greek alphabet, Greek: tau) is the component of stress (physics), stress coplanar with a material cross section. It arises from the shear force, the component of force vector parallel to the material cross secti ...

from the mainstream, and they contribute significantly to the energy dissipation down the stepped spillway. For a small dam, the nappe flow is considered most efficient in terms of energy dissipation, while the skimming flow is most efficient for long spillway chutes and large dams.

Discussion

Gabion A gabion (from Italian ''gabbione'' meaning "big cage"; from Italian ''gabbia'' and Latin ''cavea'' meaning "cage") is a cage, cylinder or box filled with rocks, concrete, or sometimes sand and soil for use in civil engineering, road building ...

stepped weirs are commonly used for embankment protection, river training, and

flood control Flood management or flood control are methods used to reduce or prevent the detrimental effects of flood waters. Flooding can be caused by a mix of both natural processes, such as extreme weather upstream, and human changes to waterbodies and ru ...

; the stepped design enhances the rate of energy dissipation in the channel, and it is particularly well-suited to the construction of gabion stepped weirs. For very low flow, a porous seepage flow regime may be observed, when the water seeps through the gabion materials and there is no overflow past the step edges.

References

{{Reflist

External links

''Stepped Spillway Research''
nbsp;– (2017), The University of Queensland + Hydraulic structures

Historical developments

Basic flow characteristics

Discussion

See also

References

External links