than other types of gates; they require protection against freezing
and are particularly sensitive to aeration demand and vibration, which
may be prevented, however, by the use of splitters at the edge of the flap
(Fig. 6.3).
6.2.6 Roller gates
A roller gate consists of a hollow steel cylinder, usually of a diameter
somewhat smaller than the damming height; the difference is covered by a
steel attachment, most frequently located at the bottom of the cylinder
(in the closed position). The gate is operated by rolling it on an inclined
track (Fig. 6.5). Because of the great stiffness of the gate, large spans (of
up to 50 m) may be used, but roller gates require substantial piers with
272 GATES AND VALVES
Fig. 6.3 Drum gate (after Thomas, 1976)
