SMALL DAMS

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Chapter II SYNTHESIS OF FLOW WITH A RECURRENCE INTERVAL OF 1000 YEARS CALCULATED WITH THE GRADEX 1 METHOD The GRADEX method has been applied by EDF for numerous French catchments of surface areas varying between a few square kilometres to a few thousand square kilometres. The regression established on 170 catchment areas of the peak discharge with a 1000-year recurrence interval relative to the surface area of the catchment is: Q = .S 0,72 S is the area of the catchment in square kilometres and a parameter given in the table hereafter for the following three zones: !"zone I : the catchment areas of tributaries to the lower Loire river (Vienne, Creuse, etc.) located in the north of Central France, those of the Saône and Moselle rivers, and Brittany; !"zone II : the catchment areas of the eastern and central Pyrenees, of Aude and Ariège, of the Dordogne and the Lot rivers, the catchment areas of the Durance, the Fier and the Arve rivers, the Dranses, and the Isère rivers; !"zone III : the catchment areas of Haute Loire, Cévennes, the Tarn river, the right-bank tributaries of the Rhône river downstream of Lyons (Eyrieux and Ardèche rivrers, etc.), Alpes-Maritimes, and Corsica. ZONE Bracket for 90 % Bracket for 70 % I 4.05 3.07 - 5.36 3.4 - 4.8 II 7.4 5.2 - 10.4 5.9 - 9.2 31 III 16.4 9.1 - 29.7 11.3 - 23.9 This formula does not apply to catchment areas smaller than a few square kilometres. It only gives an order of magnitude which must always be rendered more precise with a local study. This order of magnitude is only to give a first opinion on the spillway capacity. It should never replace a more complete study. RAINFALL RUNOFF ANALYSIS IN A WELL DOCUMENTED CATCHMENT It is very rare to find a small catchment where we have a good understanding of the hydrology. The subject of this study is therefore not determination of a dam's design flood. It presents data from an experimental catchment which support the recommended methods and justify the conclusion that the peak discharge of the design flood depends very little on how long the considered rainfall lasts. 1. See Bibliography, reference 5, p. 36.
P reliminary determination of design flood PRESENTATION OF THE CATCHMENT AREA Located in the Massif des Maures, a Mediterranean mountain zone, this catchment has been monitored by Cemagref since 1967, in the frame of the Réal Collobrier Research Project. The lands adjacent are mostly compact gneiss. There is little soil cover. The vegetation consists of scrubby briar and evergreen arbutus. The catchment area measures 1.47 square kilometres. Mean annual rainfall (1967-1990) is 1164 mm and the corresponding runoff is 626 mm. The largest river floods are caused by intense storms occurring mostly in September and October. The following two events, with contrasting time histories, are an excellent illustration of the different types of response of the catchment (table 2). Date Maximum rainfall Maximum runoff Peak Ratio (in mm) (in mm) discharge Peak discharge/ 24 h 1 h 24 h 1 h (m 3 /s) Runoff in 24 h 13.09.68 159 88 57 21 15.5 15.5 29.10.83 165 25 25 3,4 1.61 3.6 Table 2 - Floods of 13/09/68 and 29/10/83 32 APPLICATION OF THE GRADEX METHOD The GRADEX method is applied with time steps of 1, 2, 4, 8, 12 and 24 hours, although these last times are much longer than the catchment's characteristic time. Figures 1 and 2 (p. 35) are examples of observed 4-hour rainfall and runoff. The plots show all events during which rainfall in excess of a given value was recorded. CATCHMENT RETENTION Figure 1 (p. 35) shows that the retention loss was between 70 and 100 mm in the three heaviest rainfall events, and between 55 mm and 100 mm in the three largest flood events. Statistical analysis as described later indicates that the "pivot point" for the GRADEX method should be taken as the 0.95 frequency (20-year recurrence interval, reduced GUMBEL variable, u = 2.97). Table 3 shows retention versus duration of rainfall. Duration 1 2 4 8 12 24 (hours) Retention 30 43 57 76 86 98 (mm) Table 3 - Retention for a 20-year recurrence interval for different durations of rainfall.
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
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Page 23 and 24: P reliminary determination of desig
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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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