SMALL DAMS PETITS BARRAGES

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core is flanked by zones of materials considered more pervious, called shells. These pervious zones or shells enclose, support and protect the impervious core, the upstream pervious zone affords stability against rapid drawdown; and the downstream pervious zone acts as a drain to control seepage and lower the phreatic surface. In many cases, a filter between the impervious zone and downstream shell and a drainage layer beneath the downstream are necessary. In Table 5.4 are presented some examples of zoned and homogeneous small dams in Japan, illustrating their typical sections and also some rehabilitation measures under way in order to improve their safety conditions. 5.4 FREEBOARD 5.4.1 Definitions of Freeboard Freeboard is the vertical distance between the crest of the dam and a specified stillwater reservoir water surface elevation. Common definitions of freeboard are as follows: � Normal freeboard – vertical distance from dam crest elevation to normal operating reservoir still-water surface elevation. � Minimum freeboard – vertical distance from dam crest elevation to maximum reservoir still-water surface elevation during inflow design flood with spillway operating 5.4.2 Freeboard Design Considerations Freeboard must be sufficient to prevent overtopping of the dam that could result from reasonable combinations of a number of factors, as illustrated on Figure 5.2: and � Wind-generated wave action, wind setup, and wave runup; � Earthquake and/or landslide-generated waves and runup; � Post-construction settlement of embankment dams and foundations; � Provision for malfunction of spillways (especially gated structures) and outlet works, � Site-specific uncertainties including flood hydrology. 54
Fig. 5.2 – Design Freeboard 5.4.3 Freeboard Design for Wave Run-up and Wind Setup In general, freeboard design should consider reasonable combinations of appropriate components defined on Figure 5.2. However for most small dams, wind setup and wave runup are the principal design criteria that are most often used to determine freeboard requirements. Wave run-up and wind setup are highly sensitive to site-specific conditions. In the U.S., for example, hurricane wind conditions may control freeboard requirements for reservoirs located along the Atlantic and Gulf coasts, whereas controlling wind conditions in mountainous regions are more strongly influenced by “orographic” effects of topography and seasonal meteorological conditions that can produce sustained high winds (USACE, 1997). Widely used design procedures in the U.S. for calculating freeboard based on wave runup and wind setup are outlined in USBR (1992) and EM 1110-2-1100, Part II (USACE, 2003). These procedures generally involve the following steps: Evaluate Effective Fetch (Fe) Fetch is the straight-line distance across a body of water subject to wind forces, and is limited for inland reservoirs by the surrounding topography. Effective fetch (Fe) is defined as the average horizontal distance in the general direction of the wind over water, corrected for reservoir plan geometry, over which a wind acts to generate waves. Evaluate Design Wind Use of the actual wind records from a site is the preferred method for establishing wind speed-duration curves (USACE, 2003). Alternatively, generalized maps have been developed for the continental U.S. that show contours of overland wind velocities for summer, fall, winter, and spring (USBR, 1992). Maps for the fastest mile (approximately 1-minute duration) and sustained 1-hour winds are published. USBR (1992) suggests developing 2-hour wind velocity 55
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SMALL DAMS Design, Surveillance and
Page 3 and 4: FOREWORD The ICOLD - International
Page 5 and 6: ACKNOWLEDGEMENT The Ad Hoc Committe
Page 7 and 8: FOREWORD ACKNOWLEDGEMENT SUMMARY TA
Page 9 and 10: 5.5.5 Compaction in confined areas
Page 11 and 12: 7.2.1 Introduction 7.2.2 Spillway c
Page 13 and 14: 1. SMALL DAMS DEFINITION AND CLASSI
Page 15 and 16: 2 Fig. 1.2 - Relationship H . V wit
Page 17 and 18: 1.3 REFERENCES [1] French Committe
Page 19 and 20: 2.3 ZONED EARTHFILL DAMS In arid pa
Page 21 and 22: sloping core guarantees water impou
Page 23 and 24: Fig. 2.11 - Hlinky Concrete Gravity
Page 25 and 26: Fig. 2.13 - Photograph of Neusberg
Page 27 and 28: The gabion dam as the gabion retain
Page 29 and 30: Some inflatable dams are operated b
Page 31 and 32: 3.1 INTRODUCTION 3. SAFETY OF SMALL
Page 33 and 34: Condition Effect on safety 13. Dama
Page 35 and 36: Fig. 3.2 - Massive dam break overto
Page 37 and 38: and suffusion or internal instabili
Page 39 and 40: downstream part of the embankment m
Page 41 and 42: small dam impounding a lot of water
Page 43 and 44: Whenever the Supervising Authority
Page 45 and 46: emergency management organizations
Page 47 and 48: c) The decommissioning plan should
Page 49 and 50: 5.1 INTRODUCTION 5. FEATURES OF THE
Page 51 and 52: 5.2 DESIGN FLOODS Inadequate flood
Page 53: downstream slope will show seepage
Page 57 and 58: 2 U f Fe S 1400 D Where Uf is the d
Page 59 and 60: Maximum compaction is obtained at c
Page 61 and 62: characteristic must be determined.
Page 63 and 64: 5.6.1 Foundation Watertightness The
Page 65 and 66: � A continuous vertical chimney d
Page 67 and 68: Thickness of pervious layer Thick D
Page 69 and 70: t 5.7.1.4 Practical Considerations
Page 71 and 72: It is generally accepted that cohes
Page 73 and 74: � Simple Bishop (1955) � Spence
Page 75 and 76: � The elements for surface draina
Page 77 and 78: The supporting layer for the rip-ra
Page 79 and 80: For very small reservoirs (fetch of
Page 81 and 82: The junction of the downstream slop
Page 83 and 84: Fig. 5.14 - Hongshiyan homogenous d
Page 85 and 86: material has been left in place or
Page 87 and 88: The homogeneous dam is recommended
Page 89 and 90: [3] OL and OH soils are not recomme
Page 91 and 92: part (core) lies in the zone (Fig.
Page 93 and 94: 6. GUIDELINES ON SURVEILLANCE OF SM
Page 95 and 96: cases the owner may retain the serv
Page 97 and 98: Surface displacement on an embankme
Page 99 and 100: Fig. 6.1 - Leakage measurement usin
Page 101 and 102: Water Level Sounder - Details of th
Page 103 and 104: pressure) Left abutment 2 5 Weir ga
Page 105 and 106:
Fig. 6.9 - Recommended standpipe pi
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the others weir gauges need to be i
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� Earthquakes sensible at the dam
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APPENDIX INSPECTION CHECKLIST FOR S
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Exposed reinforcement � The condu
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These spillways can be lined or unl
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flowing water. Flowing water with h
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Fig. 7.3 - Installation of the arti
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Fig. 7.7 - Dam overtopping protecti
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complete construction was in 2001,
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In some cases, the downstream face
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Tube-a-manchettes injection system
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7.3.5 Slope Protection Measures 7.3
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Pipe Rehabilitation SLIPLINING MODI
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Conditions Pros Cons Comments types
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Embankment dams that have properly
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and minimize erosion. Design of sui
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[20] SMEC- “Draft Guidelines for
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present visual information. It must
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Fig. 8.1 - Three small dams in casc
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Social Disruption low (rural area)
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Fig. 8.4 - Flood water level after
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8.9 REFERENCES [1] BURREC (1994) -
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SMALL DAMS PETITS BARRAGES

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