SMALL DAMS

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Chapter II DESIGN FLOOD AND SAFETY FLOOD The design flood is the flood with the longest recurrence interval considered in the reservoir. It is taken into account to determine the Maximum Water Level (MWL) and dimension the spillway, incorporating possibilities of flood routing. Often, the considered design flood is the flood with the highest peak flow. It is not always certain that this flood is the worst case in calculation of the spillway. A flood with a lower peak flow but lasting a longer period could have worse consequences. The minimum recurrence interval recommended for such a flood is between 100 and 10 000 years (10 -2 to 10 -4 ). The choice of a recurrence interval depends on the risk involved in dam failure. The dam's intrinsic risk can be quantified by means of the parameter H 2 V . Table 1 sets out recommendations for the choice of a design flood relative to this criterion. However, global risk is also related to the vulnerability of the downstream area (population density in the zone likely to be flooded in the event of a failure). The recommendations in table 1 must be beefed up in case of serious vulnerability (for example by going from a 500-year flood to a 1000-year flood). When the dam is of public safety interest, the recurrence interval should never be less than 1000 years, whatever the value of H 2 V . Once MWL has been calculated, the dam crest is set at a higher level. The difference between these two levels is called freeboard. Freeboard is essentially intended to avoid overtopping due to wave action but also plays an essential role in safety from flooding. A method for calculating it is given in Chapter IV, p. 73. Thanks to freeboard, a dam should be able to withstand a flood (which is known as safety flood) higher than the design flood. This is by definition the worst case flood that could occur at the dam without any risk of failure. In the case of an ungated spillway at an embankment dam, the safety flood would be any flood that would cause overspillage, provided that it did not cause overtopping at any point in the chute that would jeopardise the fill itself. For a gravity dam, the safety flood would also correspond to the crest of the non-overflow part. For a dam with an impermeable core, the safety flood would be reached when the reservoir water level reaches not the dam crest but the crest of the core. 25 H 2 V < 5 5 to 30 30 to 100 100 to 700 > 700 Recurrence interval 100 500 1 000 5 000 10 000 in years Table 1 - Minimum recurrence interval of the design flood for an earthfill dam without consideration of vulnerability downstream (H: dam height in metres; V: reservoir volume in hm 3 )
P reliminary determination of design flood THE GRADEX 1 METHOD This statistical method, developed by Electricité de France (EDF), is the standard used in France. Its success is in particular due to its (relative) simplicity of use, which results from an extreme simplification of the process of transforming rainfall into flow. ASSUMPTIONS OF THE GRADEX METHOD 26 Rainfall is considered globally over a certain period of time, equal to the average duration of the hydrographs. The probability of precipitation events lasting various durations is a simple exponential decay function. The main parameter is proportional to the standard deviation of maximum precipitation values. It is called the exponential gradient, GRADEX. GUMBEL's law is often applied. Its distribution function is as follows: F(P) = EXP (-EXP (-(P-Po)/a) The GRADEX (a) may be obtained using the event method. In this case, it is equal to 0.78 times the standard deviation. a is of course a function of the duration of the precipitation considered. Remarks : !"When P ➔ ∞, F (P) ➔ 1 - EXP (-(P-Po)/a) and the Napierian logarithm at recurrence interval T = (1/(1 - F (P)) is equal to (P - Po) /a. The rainfall depth varies linearly with the logarithm of the recurrence interval, the slope (a) of this straight line being equal to the GRADEX.. !"If P 1 000 and P 100 indicate respectively the rainfall depth at recurrence intervals of 1 000 and 100 years, then: P 1 000 - P 100 = a (ln1 000 - ln 100) = 2,3 a (ln designating the Napierian (or natural) logarithm). When the catchment area reaches a certain saturation level, any increase in rainfall generates an equal volume of runoff for the same lapse of time. As a first approximation, this state is reached for recurrence intervals of ten years (impermeable catchments, with low retention), to 50 years (permeable catchments, with high retention). The runoff law is obtained quite simply by translating the rainfall depth law to the point of the 10 or 50 year recurrence interval. A physical interpretation of this process can result from observation of the graph of runoff variations relative to the amount of rainfall (see fig. 1, p. 35). The retention capacity of the catchment area is schematically represented by the difference between the bisector (rainfall = runoff) and its parallel, plotted in the middle of a cloud of points. 1. See Bibliography, references 5, 7 and 11, p. 36.
Page 1 and 2: FRENCH COMMITTEE ON LARGE DAMS COMI
Page 3 and 4: PHOTOGRAPH REFERENCES Cover: CACG,
Page 5 and 6: M E M B E R S O F T H E R E A D I N
Page 7 and 8: TABLE OF CONTENTS Foreword - What n
Page 9 and 10: Table of contents FILL DAM DESIGN..
Page 11 and 12: Table of contents Chap. VII - Life
Page 13 and 14: W hat need for guidelines for small
Page 15 and 16: W hat need for guidelines for small
Page 17 and 18: C hoice of site and type of dam TOP
Page 19 and 20: C hoice of site and type of dam AVA
Page 21 and 22: C hoice of site and type of dam CON
Page 23: P reliminary determination of desig
Page 27 and 28: P reliminary determination of desig
Page 37 and 38: G eological and geotechnical studie
Page 67 and 68: F ill dams GEOTECHNICAL STUDIES 68
Page 69 and 70: F ill dams For fill, the wetter the
Page 71 and 72: F ill dams 72 If the latter is unal
Page 73 and 74: 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
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Chapter V Finally, the slope of the
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Chapter V FOUNDATION TREATMENT Hydr
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Chapter V and account will be taken
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Chapter V Thrust of ice This force
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Chapter V The commonly accepted cri
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Chapter V MONITORING SYSTEMS The ge
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Chapter V The typical cross-section
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Chapter V LOUBERRIA DAM Louberria d
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Chapter V Some of these barrages, a
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Chapter V Fig. 8 - Dam with a tilti
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Chapter V BIBLIOGRAPHY 1 - Ministè
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M anagement of water quality EUTROP
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M anagement of water quality !"the
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M anagement of water quality Proces
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M anagement of water quality 148 Al
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M anagement of water quality While
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M anagement of water quality It con
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M anagement of water quality It sho
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M anagement of water quality These
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4. Grégoire (A.), 1987- Caractéri
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L ife of the dam SPECIAL FEATURES O
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L ife of the dam GENERAL SURVEILLAN
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L ife of the dam ORGANISATION OF SU
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L ife of the dam During such period
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L ife of the dam MAINTENANCE 168 Th
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L ife of the dam BIBLIOGRAPHY 1 - C
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C onclusion FLOOD DATA It is fairly
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SMALL DAMS

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