SMALL DAMS PETITS BARRAGES

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ρw = Density of water in kg/m 3 Q = Discharge in m 3 /s b = Width of channel in m s = Slope of channel (m/m) 7.2.4 Spillway Chute or Spillway Return Channel Alignment Change of direction in the spillway chute or spillway return channel during supercritical flow conditions provides problems regarding erosion of materials. Water in supercritical flow does not change direction and tends to carry on straight except if the floor is elevated correctly. Straight chutes for supercritical flow conditions must be provided. 7.2.5 Deformation of Spillway Structures Provided Over Embankments It is possible to construct the spillway over the embankment, with two conditions: Expected settlement of the foundation must be very low or must occur mostly during construction of the original embankment (short-term settlement). Expected settlement of the original embankment must be very low, that means good compaction of the material. A concrete spillway constructed over an earthfill embankment on soft foundations will therefore settle with movement of the concrete panels. Failure may be caused due to ingress of overflowing water and uplift of concrete panels. Although often expensive, (or costly) minimum risks and best solutions for the improvement of spillways include the “tried and tested” approach of location of spillways on solid foundations. Nevertheless, construction of an additional spillway over the embankment of an existing dam may be a solution to consider, as settlement has often ceased after some decades. 7.2.6 Erosion of the Abutment Wall Between the Embankment and the Spillway The solution recommended for the transition between the embankment and the spillway is a concrete wall i.e. mass gravity or cantilever (depending on foundation, height and economics). If the protection of an earthfill abutment (or the training wall of one side of a spillway return channel) comprises a layer (or layers) of rock (or rip-rap), it may be damaged (or washed away) after cracking of soil and erosion by storm water or high powered flowing water. Materials used for this type of erosion protection must be carefully designed to accommodate the unit power of the flowing water. The protection must also be designed to accommodate the safety evaluation check discharge. 7.2.7 Gabions Used as Sill in by-pass Channels Gabion baskets must be considered with care as they have shortcomings: Very often the galvanized steel wire that makes up the rectangular (or square) panels of the baskets corrodes and results in individual stones in the baskets being exposed to 116
flowing water. Flowing water with high power (kW/m 2 ) can remove stones if too small. Where there has been poor maintenance, total breakdown of a Gabion will take place if most of a panel has corroded and “broken down” before repair (or rehabilitation to its original functional condition) Stones and the “upper panels” of Gabions may be removed by vandalism. A cut-off structure connected to rock should be considered. 7.3 TECHNIQUES FOR IMPROVING EMBANKMENT SAFETY 7.3.1 Techniques to Overcome Flood Handling Problem 7.3.1.1 Parapet Wall A parapet wall can be used to raise the crest of a consolidated embankment crest. The concrete parapet must be designed to accommodate water loads including uplift but the safety factors against overtopping and shear failure if exposed to the PHC 3 floods can be selected close to 1 in this case. An impermeable membrane must be designed between the impervious core of the embankment and the base (or underside) of the parapet wall. The footing normally have to be imbedded into the embankment crest. 7.3.1.2 Raise Embankment Crest Generally (but not always) the lowest cost option to overcome inadequate flood handling problem is to increase the total freeboard of the dam by raising the crest of the embankment. This is normally achieved by adding earthfill to the downstream face of the embankment starting from the toe of the embankment (see Figure 7.2). In addition to the fact that the spillway must be designed to accommodate higher “recommended design and safety evaluation check floods”, respectively, the three (3) topics that are listed below must be addressed in the context of the structural integrity of the embankment: The stability of the downstream slope (and “in-situ” foundation) must be analyzed taking into account that the existing embankment (or portions thereof) are structural zones (or components) of the raised embankment. The engineering properties of the earthfill materials of the existing embankment and the “new” earthfill section should be determined with reasonable confidence (i.e. a representative (or appropriate) sampling and laboratory testing program) and used in any analyses. A “sand chimney drain” must be considered at the interface between the downstream face of the existing embankment and the “new” earthfill section (or zone). This will draw down the phreatic surface and intercept any possible seepage through the embankment. A new toe drain possibly connected to the existing toe drain must be considered. An example is shown in Figure 7.2. 117
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SMALL DAMS Design, Surveillance and
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FOREWORD The ICOLD - International
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ACKNOWLEDGEMENT The Ad Hoc Committe
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FOREWORD ACKNOWLEDGEMENT SUMMARY TA
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5.5.5 Compaction in confined areas
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7.2.1 Introduction 7.2.2 Spillway c
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1. SMALL DAMS DEFINITION AND CLASSI
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2 Fig. 1.2 - Relationship H . V wit
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1.3 REFERENCES [1] French Committe
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2.3 ZONED EARTHFILL DAMS In arid pa
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sloping core guarantees water impou
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Fig. 2.11 - Hlinky Concrete Gravity
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Fig. 2.13 - Photograph of Neusberg
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The gabion dam as the gabion retain
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Some inflatable dams are operated b
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3.1 INTRODUCTION 3. SAFETY OF SMALL
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Condition Effect on safety 13. Dama
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Fig. 3.2 - Massive dam break overto
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and suffusion or internal instabili
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downstream part of the embankment m
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small dam impounding a lot of water
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Whenever the Supervising Authority
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emergency management organizations
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c) The decommissioning plan should
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5.1 INTRODUCTION 5. FEATURES OF THE
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5.2 DESIGN FLOODS Inadequate flood
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downstream slope will show seepage
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Fig. 5.2 - Design Freeboard 5.4.3 F
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2 U f Fe S 1400 D Where Uf is the d
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Maximum compaction is obtained at c
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characteristic must be determined.
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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
Page 107 and 108: the others weir gauges need to be i
Page 109 and 110: � Earthquakes sensible at the dam
Page 111 and 112: APPENDIX INSPECTION CHECKLIST FOR S
Page 113 and 114: Exposed reinforcement � The condu
Page 115: These spillways can be lined or unl
Page 119 and 120: Fig. 7.3 - Installation of the arti
Page 121 and 122: Fig. 7.7 - Dam overtopping protecti
Page 123 and 124: complete construction was in 2001,
Page 125 and 126: In some cases, the downstream face
Page 127 and 128: Tube-a-manchettes injection system
Page 129 and 130: 7.3.5 Slope Protection Measures 7.3
Page 131 and 132: Pipe Rehabilitation SLIPLINING MODI
Page 133 and 134: Conditions Pros Cons Comments types
Page 135 and 136: Embankment dams that have properly
Page 137 and 138: and minimize erosion. Design of sui
Page 139 and 140: [20] SMEC- “Draft Guidelines for
Page 141 and 142: present visual information. It must
Page 143 and 144: Fig. 8.1 - Three small dams in casc
Page 145 and 146: Social Disruption low (rural area)
Page 147 and 148: Fig. 8.4 - Flood water level after
Page 149: 8.9 REFERENCES [1] BURREC (1994) -
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SMALL DAMS PETITS BARRAGES

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