Stepped spillway
Encyclopedia
Recent advances have permitted the construction of large dams and reservoirs and channels. These have required the development of new spillway
designs. A key feature of a spillway system is the safe dissipation of a major component of the kinetic energy of the waters to avoid damage and failure. It can be achieved by a high velocity water jet taking off from a ski jump and impinging into a downstream pool of a standard stilling basin downstream of the chute where a hydraulic jump dissipates the flow energy, or the construction of steps on the spillway chute to assist in energy dissipation: i.e., the stepped spillway. On a stepped chute, the steps increase drastically the rate of kinetic energy dissipation taking place in the spillway, thus eliminating or reducing greatly the need for an energy dissipator at the spillway downstream end.
and reservoirs. This progress has required the developments of new design methods and construction methods, particularly to provide the adequate flood release facilities to pass the flood flows and the assoсiated safe dissipation of the energy of the flow. The latter may be achieved by the design of a staircase spillway
, called stepped spillway. A stepped spillway design increases considerably the rate of kinetic energy dissipation taking place down the spillway channel. In turn, the design eliminates or reduces greatly the needs for a sizable energy dissipator at the toe of the chute in the form of a hydraulic jump stilling basin or flip bucket and plunge pool. Although the stepped spillway design was common up to the beginning of the 20th century, a lot of expertise was 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.
and Crete
. During the Antiquity
, 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 Mediterranean sea
, 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 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.
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.
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...
designs. A key feature of a spillway system is the safe dissipation of a major component of the kinetic energy of the waters to avoid damage and failure. It can be achieved by a high velocity water jet taking off from a ski jump and impinging into a downstream pool of a standard stilling basin downstream of the chute where a hydraulic jump dissipates the flow energy, or the construction of steps on the spillway chute to assist in energy dissipation: i.e., the stepped spillway. On a stepped chute, the steps increase drastically the rate of kinetic energy dissipation taking place in the spillway, thus eliminating or reducing greatly the need for an energy dissipator at the spillway downstream end.
Presentation
The recent advances in construction technology have permitted the construction of large damsDAMS
Driot-Arnoux Motorsport is a racing team from France, involved in many areas of motorsports. DAMS was founded in 1988 by Jean-Paul Driot and former Formula One driver René Arnoux. It is headquartered near Le Mans, only 2 km from the Bugatti Circuit.- History :The year after its foundation,...
and reservoirs. This progress has required the developments of new design methods and construction methods, particularly to provide the adequate flood release facilities to pass the flood flows and the assoсiated safe dissipation of the energy of the flow. The latter may be achieved by the design of a staircase 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...
, called stepped spillway. A stepped spillway design increases considerably the rate of kinetic energy dissipation taking place down the spillway channel. In turn, the design eliminates or reduces greatly the needs for a sizable energy dissipator at the toe of the chute in the form of a hydraulic jump stilling basin or flip bucket and plunge pool. Although the stepped spillway design was common up to the beginning of the 20th century, a lot of expertise was 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.
Historical developments
The stepped spillway, weirs and channels have been used for over 3,500 years since the first structures built in GreeceGreece
Greece , officially the Hellenic Republic , and historically Hellas or the Republic of Greece in English, is a country in southeastern Europe....
and Crete
Crete
Crete is the largest and most populous of the Greek islands, the fifth largest island in the Mediterranean Sea, and one of the thirteen administrative regions of Greece. It forms a significant part of the economy and cultural heritage of Greece while retaining its own local cultural traits...
. During the Antiquity
Classical antiquity
Classical antiquity is a broad term for a long period of cultural history centered on the Mediterranean Sea, comprising the interlocking civilizations of ancient Greece and ancient Rome, collectively known as the Greco-Roman world...
, 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 Mediterranean sea
Mediterranean Sea
The Mediterranean Sea is a sea connected to the Atlantic Ocean surrounded by the Mediterranean region and almost completely enclosed by land: on the north by Anatolia and Europe, on the south by North Africa, and on the east by the Levant...
, 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
Gold Creek (Queensland)
Gold Creek rises in Brisbane Forest Park in the suburb of Upper Brookfield, Brisbane, Queensland, Australia; is dammed by Gold Creek Dam and on leaving the Forest Park runs alongside Gold Creek Road until it joins Gap Creek at Brookfield....
(1890) in 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.
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, 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.