discharge coefficient will depend on the gate opening and upstream and
downstream water depths – for guidance see Fig. 6.10 (e.g. Henderson,
1966, see also Worked example 6.1). For other types of gates and further
particulars, see Hager (1988) and Naudascher (1987, 1991).
For gates with planar and cylindrically shaped skin plates (Tainter
gates) with free flow and a downstream horizontal apron and the outflow
edge of the gate inclined at to the flow (90° and varies according to
the gate position) Toch (1955) suggested for Ccthe equationCc 1 0.75
90
0.36
90
2. (6.5)
6.5.2 Forces on high-head gatesIn closed positions hydrostatic forces determined by standard procedures
will again apply. For vertical lift gates, the hoist lifting force has to be
dimensioned to overcome the gate weight, frictional resistance and, most
importantly, the downpull forcesresulting from the fact that during the
gate operation the pressure along the bottom edge of the gate is reduced
(to atmospheric or even smaller pressures), whereas the pressure acting on
the top of the gate is practically the same as under static conditions (i.e.
full reservoir pressure). This condition applies both to gates located within
a conduit or at the upstream face of the dam or an intake (it should be
noted that the seals for the gates at the intakes have to be on the down-
stream side, and for gates in conduits are usually in the same position).
The downpull (or uplift) forces acting on a gate with a given housing and
seal geometry and the gate vibrations have to be analysed at various oper-
ating conditions using theoretical considerations and experience from field
trials, as well as model experiments, if necessary.
The lifting force of a gate will also be influenced by the geometry of
the bottom edge and the seal. In order to avoid negative pressures and
downpull there should be no separation of the flow until the downstream
edge of the gate is reached. By investigating the shape of a jet flowing
under a sharp-edged plate opening by an amount aunder a head h,
Smetana (1953) determined the shapes of outlet jet surface curves for
varioush/aratios in the same manner as shapes for overfall spillway crests
were determined (Section 4.7). For h/a3.3 this curve had a constant
shape, while for ratios h/a3.3 the curves were flatter and of varying
shapes. By forming the bottom edge of the gate according to these curves
it is possible to avoid separation of flow, increased downpull, vibration,
and resulting instabilities (for further details, see Novak and Cˇábelka
(1981)).
Forces acting on an emergency gate closing into the flow (with the
downstream gate/valve open and failing to close) can be particularly com-