SMALL DAMS

F ill dams
DESIGN OF THE FREE OVERFLOW SILL
For a given flow, there are infinite solutions between:
!"a very long sill, resulting in very low hydraulic head;
!"a very short sill with high hydraulic head.
The general principle consists in assuming a given overspill length, calculating the
head on the sill with account taken of attenuation in the reservoir, and possibly
increasing or decreasing the length of the sill.
A maximum head on the sill of 0.50 to 3 metres, and more generally 1 to 2 metres,
is selected.
96
Front or side inlet for a overflow spillway
An overflow spillway is called straight drop spillway if the direction of flow is upstreamdownstream,
and side channel spillway if the flow changes direction by 90° at the
spillway (see photos 19 and 20 p. VII).
When the reservoir has a large surface area, it can provide good attenuation. It is
then advantageous to store as much of the volume as possible temporarily and therefore
have higher head and the smallest overspill length possible. In this case, the inflow is
generally frontal (i.e upstream - downstream).
In contrast, a long overspill length will decrease the surface area that must be
purchased or expropriated since the highest water level will be low. The cost of the fill
is then reduced because the crest is lower, but of course the cost of the spillway will
be increased. Freeboard gives greater safety for a flood greater than the design
flood. Inflow is generally lateral, which often helps to reduce earthworks.
Discharge of floating debris
It is vital to have a safety margin for discharge of floating debris, especially for shaft
or sluice spillways. If possible, this means avoiding having a walkway above a
spillway with frontal inflow. If such a structure is selected, care must be taken to keep
sufficient air gap during the design flood, which may mean moving the walkway
towards the downstream side of the sill (or the contrary). Finally, as a last recourse,
the walkway could be designed to be washed away.
When the catchment area is wooded, trees may be torn away from the banks during
strong floods. This possibility becomes a certainty during exceptional floods. But
experience has shown that a lot of other floating debris can arrive in front of the
spillway, in particular trailers!
The minimum dimensions that can be recommended to discharge such debris are as
follows:
!"sill length of 10 - 15 metres between piers;
!"air gap 1.5 to 2 metres under a walkway or a bridge;
!"shaft 6 to 8 metres in diameter.