Contents preface (VII) introduction 1—37

17.6.4. Drum Gates
Drum gates are hollow (and, therefore, buoyant), triangular in section, and made of steel plates. The drum gate is hinged at the upstream lip of a hydraulic chamber in the weir structure in which the gate floats. Water introduced into or drawn from the hydraulic chamber causes the gate to swing upwards or downwards. The inflow or outflow of water to the chamber is governed by controls located in the piers adjacent to the chambers.
17.7. CAVITATION EROSION ON SPILLWAY SURFACE
Surface roughnesses of a spillway causes separation of the boundary layer from the surface, thereby forming a lower pressure zone immediately downstream of the roughness. If the pressure in the wake region of the roughness falls to the vapour pressure of water, vapour bubbles are formed and carried downstream. When these bubbles reach a high pressure region, they suddenly collapse giving rise to extremely high pressures. These extremely high localised pressure cause damage to any surface adjacent to the collapsing bubbles (or cavities). The continued process of this nature, named as cavitation pitting or cavitation erosion, may cause serious damage to the structure in due course of time.
With the increasing demand for water, high dams are being proposed, planned, and constructed. The flow velocities over the spillways of such projects often exceed 30-40 m/s which are enough to cause cavitation pitting of the normal concrete surface of the spillway. The methods to prevent cavitation, used till recently, consisted of either using cavitation-resistant materials (such as steel lining, epoxy concretes, epoxy mortars, and fibrous concrete) for the construction of the surface or the adoption of specification criteria for limits of construction finish. The first method, being costly, is reserved for small areas such as near the outlet gates, or for repairing damaged surfaces. The second method defines objectionable irregularities. However, the standards are difficult to obtain especially when the flow velocities exceed 20 m/s. Besides, the method is relatively uncertain because of the defects which may develop on the surface as a result of climatic, atmospheric or chemical conditions.
More recently, a new method has been developed to protect spillway surfaces by aeration devices. This method is based on the known fact that the presence of air bubbles hinders cavitation. Hence, present-day design of spillways with high velocities envisages provision of aeration devices, known as spillway aerators, across the spillway face. Three basic types of aerators, viz., groove, deflector, and offset (Fig. 17.26) have been used. An aerator of the groove-type has been constructed for the Karjan dam in the Narmada basin.

598 IRRIGATION AND WATER RESOURCES ENGINEERING

A
Deflector

B

Deflector and offset

Offset

C	F

Deflector, offset and groove

D

Offset and groove

E Groove