by Gérard DEGOUTTE
Small dams have many specific features, at every stage in their life: preliminary studies, design, construction, and later surveillance. If we are to be perfectly strict, two types of specific features should be distinguished:
* technical: for example, the lack of any inspection adit for small earthfill or concrete dams, or the technique of drilling a small vertical drain after construction of a small earthfill dam;
* statistical: for example, the fact that small structures may belong to an unspecialised owner or that they generally control small catchment areas.
Without indulging in this distinction, we shall deal with the main points specific to small dams, listed according to the stage at which they are considered.
The cost of preliminary studies is not entirely proportional to the size of the dam, and therefore it is not always possible to undertake all the geological studies that might be desired for a small dam. In this case it may be preferable to abandon sites that appear doubtful, whereas in the case of a large dam the means would be provided to settle any doubt.
In simple cases, on the other hand, it may be interesting and economical to group several types of site investigation in a single stage: for example, it is routine to move a mechanical shovel only once in the case of a small dam that presents no special geological or geotechnical difficulties.
It is fairly common to discover that a small stream is completely lacking in gauging stations, thus increasing the uncertainty about flood flows. This must then be taken into account in selecting the design flood. In addition, such uncertainty will lead the designer to prefer certain technical options:
* an ungated spillway, as the increased discharge capacity is greater than that of a sluice spillway when the reservoir water level rises above maximum;
* a very long spillway crest, which decreases the impact of uncertainty on the stability of a small gravity dam.
DESIGN OF EARTHFILL DAMS
The technique of the vertical drain drilled after placement of the fill with a mechanical shovel is advantageous for most small dams, whereas for larger dams, the drain is placed in layers like the rest of the material.
Outlet structures generally take the form of prefabricated piping assembled on site, instead of reinforced concrete structures built with formwork.
Given how easy it is to build, the homogeneous earthfill design is conventionally used for the smallest dams addressed in this volume, even when it results in placing slightly greater quantities. It is rarely selected for dams over 30 m high, as project optimisation often results in a decision to use materials of different characteristics in the core and the shoulders. The simplicity of the profile is therefore favoured for small dams, while the performance of different materials is mobilised for medium-sized to large ones, with clay to provide watertightness and coarse materials to stabilise the shoulders.
DESIGN OF CONCRETE DAMS
Small concrete dams generally have no internal galleries; they are limited to about 15 metres high for conventional concrete and 20 to 25 metres for RCC.
Small dams are rarely provided with drainage in their mass.
It may be acceptable to build a small concrete dam on a foundation of loose material, provided certain precautions are taken.
A very long ungated spillway is preferred to facilitate passage of floating debris.
If possible, all moving parts will be eliminated from the spillway in order to decrease the risk of breakdown and the consequences of insufficient maintenance: small dams are always unattended.
Robust monitoring equipment that is simple to read is preferred. The frequency at which the devices are read, as well as that of in-depth inspections, must be suited to the size of the dam.
All of these decisions combine to fulfill two objectives that can never be dissociated: economics and safety.
In conclusion, a small dam must always be simple in design, simple in construction, and simple in surveillance. But this must not be to the detriment of safety: for example, the filter of coarse material around a drain can never be sacrificed for reasons of economy!
Unfortunately, for the designer of a small dam, simple never means easy. On the contrary, it can be harder to design a small dam than a large one, as the investigations – among other things – are not necessarily adequate.
We hope that this manual will facilitate the dialogue between dam owners, the administration, contractors, and engineers, and will help in reconciling simplicity and safety.