SMALL DAMS

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Chapter VII C H A P T E R V I I I Conclusion 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. 171 GEOLOGICAL AND GEOTECHNICAL STUDIES 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.
C onclusion FLOOD DATA 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 <strong>DAMS</strong> 172 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 <strong>DAMS</strong> 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. SPILLWAY DESIGN 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.
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FRENCH COMMITTEE ON LARGE DAMS COMI
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PHOTOGRAPH REFERENCES Cover: CACG,
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M E M B E R S O F T H E R E A D I N
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TABLE OF CONTENTS Foreword - What n
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Table of contents FILL DAM DESIGN..
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Table of contents Chap. VII - Life
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G eological and geotechnical studie
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F ill dams GEOTECHNICAL STUDIES 68
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F ill dams For fill, the wetter the
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F ill dams 72 If the latter is unal
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F ill dams As concerns freeboard R,
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F ill dams Wave height h (m) Thickn
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F ill dams In the case of a widely
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F ill dams Fig. 3 - Sloped granular
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F ill dams It is therefore necessar
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F ill dams The selected profile sho
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F ill dams 86 Case of compressible
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F ill dams In present practice, the
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F ill dams MONITORING SYSTEM 1 The
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F ill dams The most common measurem
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F ill dams 94 When H 2 V > 30, the
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F ill dams DESIGN OF THE FREE OVERF
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F ill dams 98 In the case of very w
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F ill dams TENDERING AND DAM CONSTR
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F ill dams In the case of a rock fo
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F ill dams However, when the materi
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F ill dams All of the inspections m
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F ill dams SPECIFIC FEATURES OF VER
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F ill dams !"engineering costs. The
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▲ 1 - Compacting clay in the cut-
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▲ 7 - Scarification after passage
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▲ 12 - Rip-rap on a dam built for
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▼ 20 - Overspill part of a side i
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C H A P T E R V Small concrete dams
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Chapter V Arch dams Arch dams trans
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Chapter V Masonry dams are no longe
Page 125 and 126: Chapter V Finally, the slope of the
Page 127 and 128: Chapter V FOUNDATION TREATMENT Hydr
Page 129 and 130: Chapter V and account will be taken
Page 131 and 132: Chapter V Thrust of ice This force
Page 133 and 134: Chapter V The commonly accepted cri
Page 135 and 136: Chapter V MONITORING SYSTEMS The ge
Page 137 and 138: Chapter V The typical cross-section
Page 139 and 140: Chapter V LOUBERRIA DAM Louberria d
Page 141 and 142: Chapter V Some of these barrages, a
Page 143 and 144: Chapter V Fig. 8 - Dam with a tilti
Page 145 and 146: Chapter V BIBLIOGRAPHY 1 - Ministè
Page 147 and 148: M anagement of water quality EUTROP
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Page 163 and 164: 4. Grégoire (A.), 1987- Caractéri
Page 165 and 166: L ife of the dam SPECIAL FEATURES O
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Page 175: L ife of the dam BIBLIOGRAPHY 1 - C
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SMALL DAMS

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