Concrete fuse plugs are simple massive blocks placed side by side on a spillway sill. They are free standing and stable until the water level in the reservoir reaches a certain elevation and they start tilting when this elevation is exceeded, for instance for a flood of yearly probability 2%.
If associated with a gated spillway, they may be designed to tilt before being overtopped and, in such a case, will have a significant height compared to their width.
But the present paper only refers to concrete fuse plugs used upon a single free flow spillway; they should tilt only for exceptional floods and thus require a significant overtopping by ordinary floods before tilting. Such fuse plugs are designed for overtopping by an upstream depth “h” which can be up to several times the block height “P”. This type of fuse plug has a significant length and width compared to its height. Its upstream upper corners may also be trimmed to ease overflow discharge. Model tests have shown that floating debris have no effects on the water levels at time of tilting.
Blocks placed on the same sill may have the same height but different width, so that they tilt at different water elevations according to increase of floods discharge. The water elevations for tilting can be calculated based on water loading combined with predictable uplift pressures. To ensure that the magnitude of uplift pressure under each block develops as required, a hollow area is provided under each block which is wide open at the upstream side and completely closed and watertight at the downstream side, as shown on Fig.1. To ensure that the blocks are correctly bedded in normal use and before tilting, it is necessary to use few unsealed supports under their upstream parts.
At least 4 blocks of different width should be used along a spillway to give a progressive tilting as the water level rises for extreme floods.Model tests suggest that intermediate walls should preferably be placed between the fuse plugs. These walls, which can be designed as a part of the spillway sill, will help maintain the water nappe over those plugs adjacent to the tipped one. This will ensure that the water elevations triggering the tipping of each element can be predicted accurately. These walls do not need to be very large or thick to be efficient. General arrangements are shown on Figs. 2 to 4.
To ensure correct tilting and avoid sliding, each block should preferably abut two small support ridges, or abutments, fixed downstream on the spillway sill, for instance as shown in Fig.1 and 2. Alternatively inclined steel bars can be used, anchored into the sill instead of concrete abutments.
Block extremities must be designed to avoid friction against adjacent blocks or the separating walls when tilting. This can be achieved by chamfering the extremities so that one face is slightly longer than the other, see Fig. 3.
It is advisable to ensure correct aeration of the water nappe to avoid possible vibration. This is usually simple to achieve, for instance by means of pipes embedded into the intermediate walls.
Model tests have helped establish the ratio between the width of a block and the water level at time of tilting. For preliminary purposes, the simplified formula:
E = h + 0,4 P may be used, where: h = the upstream water depth over the fuse plug for tilting
E = the upstream to downstream base width of the fuse plug
P = the height of the fuse plug
This relationship may be used for blocks having the same general shape as described above, with a hollow section under the block of 10% of the block height P and with a concrete density in the range of 2.3 t/m3. Concrete density variations of 5% will lead to variations of about 10% in the nappe depth at the time of tilting.
Scope of utilization
The main advantages of concrete fuse plugs are a simple technology and a very low cost.
Concrete Fuse Plugs may be used for new dams. In such case, it is possible, with about the same quantity of concrete and cost as for creager weirs, to double the flow of the extreme flood discharged through the spillway or to increase the storage for a same safety..
They may also be used to improve existing free overflow spillways either by increasing maximum spillage, after lowering the sill, or by increasing reservoir storage or by combining both, as shown on Figs. 7 and 8.