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 th ...

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 energy that it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acce ...

of the descending water. This eliminates or 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 and channels, have been used for over 3,500 years since the first structures were built in

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and

Crete Crete ( el, Κρήτη, translit=, Modern: , Ancient: ) is the largest and most populous of the Greek islands, the 88th largest island in the world and the fifth largest island in the Mediterranean Sea, after Sicily, Sardinia, Cyprus, ...

. During

Antiquity Antiquity or Antiquities may refer to: Historical objects or periods Artifacts *Antiquities, objects or artifacts surviving from ancient cultures Eras Any period before the European Middle Ages (5th to 15th centuries) but still within the histo ...

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

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, 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 The Gold Creek Dam is an earth-fill embankment dam with an un-gated spillway across the Gold Creek that is located in the South East region of Queensland, Australia. The main purpose of the dam is for potable water supply of the Brisbane regi ...

(1890) in Brisbane, Australia. During the first half of the 20th century, the stepped cascade design became out of fashion, partly because of the maintenance costs but also because of the development in 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 upon 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 o ...

, 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 are maintained by the transmission of shear stress 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 stepped weirs are commonly used for embankment protection, river training and flood control; 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

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External links

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

Historical developments

Basic flow characteristics

Discussion

See also

References

External links