FREEBOARD 197
Equation (4.7) applies to a free-flowing spillway without gates (for a
water level in the reservoir above the crest). In the event of the flow being
controlled by one or more gates it will apply to one position of the gate(s)
only. A series of similar relationships is therefore required for each succes-
sive position of the gate(s), making flood routing with a gated spillway
much more complicated than for a free (ungated) spillway. Therefore, it is
often assumed in this case that, for a major flood, the gates have been
raised and the reservoir emptied to crest level prior to the flood, the
routing of which is then treated in the conventional manner applicable for
a free-flowing spillway.
In design we thus first estimate the maximum outflow from the
reservoir (for a given inflow hydrograph), and then choose the spillway
size and type and test our assumption by routing the inflow with the
chosen spillway. In this procedure it is assumed that the crest and
maximum permissible water levels are fixed by other considerations
(reservoir use, flooding upstream, economic height of the dam, etc.). In
flood routing we assume the initial reservoir level to be as high as can be
expected at the start of a major flood (usually at spillway crest level).
In general, narrow gated spillways require higher dams and can,
therefore, be highly effective in flood routing. Wide free or gated spillways
save on dam height, but are usually not very effective for regulating floods.
Thus the spillway type influences the benefits to be gained from flood
control. The required size of the spillway – and hence its cost – decreases
with increase of the dam height, which in turn increases the dam cost;
combining the two costs (dam and spillway), the level of the crest for a
minimum total cost can be ascertained, although usually the crest level is
determined by other considerations such as reservoir operation.
4.4 Freeboard
Freeboard is the vertical distance between the top of the dam and the full
supply level in the reservoir; the top of the dam is the highest watertight
level of the structure and could thus be the top of a watertight parapet.
The freeboard has several components:
- rise in reservoir level due to flood routing (flood surcharge; Section
4.3); - seiche effects;
- wind set-up of the water surface;
- wave action and run-up of waves on the dam.
The last three components are often considered as the freeboard proper, or
wave freeboard. Sometimes (in gated spillways) an additional component is