SMALL DAMS

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Chapter II Figure 2 (p. 35) illustrates this principle: the adjustment of the rainfall values has the GRADEX as slope. In this application, the recurrence interval retained for the hypothesis concerning saturation of the catchment area is 20 years (this corresponds to a reduced GUMBEL variable equal to 2.97). For volumes exceeding the runoff corresponding to this recurrence interval, adjustment is made by plotting a straight line with a slope equal to the GRADEX. In this example the catchment area is instrumented and it is therefore possible to make a statistical adjustment of runoff, up to the 20 year recurrence interval. In the case of small catchment areas without flow records, this is not possible. A regional approach based on nearby, if possible similar, catchment areas is necessary. It is however possible to consult national analyses such as SOCOSE or CRUPEDIX 1 . These methods essentially require rainfall data and give an order of magnitude for peak flow at recurrence intervals of 10 years (10 and 20 years for SOCOSE). It appears that even a sizeable error on a 10-year (or 20-year) flood has a relatively weak influence on the 1000-year flood or the 10 000-year flood calculated with the GRADEX method. It is noteworthy that for small catchment areas with no gauging stations, the evaluation of rare flood flows is almost exclusively based on rainfall information. Luckily this information is generally available for most of France. A simple ratio of affinity is used to go from runoff in the considered time-frame to peak flow. This ratio is estimated from hydrographs; its average value is used (laws governing the probabilities of ratios and average runoff may also be combined, resulting in ratios that increase with the recurrence interval). For catchment areas with no water level gauging stations, we may use a ratio determined for similar catchment areas. An example of this application is given farther along in this chapter (p. 31). 27 DIFFICULTIES IN APPLYING THE GRADEX METHOD Strictly exponential decrease in precipitation with recurrence intervals leads to assigning extremely high recurrence intervals to certain observed events at a random point in France. It is true that total rainfall over 500 mm in 24 hours is not really exceptional in certain areas of France, but it is limited to certain regions: around 1000 mm in the Canigou region of the eastern Pyrenees, in October 1940; 800 mm in the Solenzara region of Corsica, in October 1993, etc. The standard design flood used in dimensioning a dam is therefore not the maximum flood that could occur. There is no well-defined rule to calculate the time during which the hypothesis of equal increase in rainfall and in runoff is applied. Only a detailed coupled analysis of rainfall and floods will result in an estimate that is not too risky. In the absence of 1. See Bibliography, reference 1, p. 36. The SOCOSE method is derived from the work of the American Soil Conservation Service.
P reliminary determination of design flood precise data, the formulation of the characteristic time of the catchment area, developed in the SOCOSE 1 method, may be used. In this method, the characteristic time is defined as the period during which the runoff is more than half of peak runoff. If no data is available on runoff on the site, the following regionalised formula may be used: Log D = - 0,69 + 0,32 Log S + 2,2 (Pa/Pta) 0,5 D : characteristic time (hours) S : surface area of the catchment area (sq. km) Pa : average annual rainfall (mm) P : daily rainfall with a 10-year recurrence interval (mm) ta : average yearly temperature (°C) Note: This method is frequently used at a daily time-step when the catchment area is of a certain size, by virtue of a greater availability of daily information on rainfall and runoff. The sudden rupture that affects the runoff equation at the pivot point (start of the rainfall equation) leads to an over-estimation of flow with intermediate recurrence intervals (50 to 500 years). 28 The affinity ratio to obtain the peak runoff is extremely variable. The method recommends keeping its average value. If we have properly chosen the duration in which the increase in runoff is equal to the increase in rainfall, it should be of the order of 1.5 to 2.0. This method does not give a design hydrograph in a form suitable for simulation of flood attenuation. A bi-triangular shape which respects the duration, the peak runoff, and the volume of runoff can be used. In general, these design hydrographs result in over-estimations of attenuation capacity, as they represent only a part of the flood. One must often take into account the base flow in the river before the flood, when it represents a non-negligible proportion of the flood flow. THE AGREGEE MODEL 2 A recent development by Cemagref 3 , this model is an extension to the GRADEX method. Re-using its statistical concepts and the hypothesis that when the catchment area is saturated, any increase in rainfall generates an equal increase in runoff. The modifications are based on: !"combining the laws of probability of rainfall and runoff to progressively go from the runoff equation to the rainfall equation; 1. See Bibliography, references 1 and 12, p. 36. 2. See Bibliography, reference 9, p. 36. 3. Cemagref : The french agricultural and environmental research institute – http://www.cemagref.fr
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 and 24: P reliminary determination of desig
Page 25: 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,
Page 75 and 76: 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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