Addis Ababa University Faculty of Technology Department of Civil Engineering HYDRAULLIC STRUCTURES I LECTURE NOTE Bayou Chane Shimelis B. Dessu Addis Ababa May 2006 Course Syllabus Course Objective This course provides a broad understanding of the basic principles of hydraulic structures. The emphasis is on design and analysis of different types of dams and spillways. Computer applications included. Course Content 2. Foundation of dams 1. Introduction Definition Types of dams Forces acting on Characteristics of dams Combination of loads on Reservoir planning 3. Gravity Dam Types and Causes of 6. Spillways, Hydrology and geology failure Energy Dissipators, 7. Stable channel Investigation Design Principle Intake and Outlet Design Topography, Geology, Principal and Shear works and Sociology stresses Types Stream diversion Resource 4. Arch and Elementary profile of Design Budget Inventory Buttress Dams Stress intensities Feasibility Stability analysis Merits and Demerits of 5. Embankment Dams Learner Assessment 100% Instructor Reference Books: Assignment 10 % Shimelis B. Dessu. 1. P. Novak, Hydraulic Structures E-mail [email protected] Project 10% 2. S. R. Sahasrabudhe, Irrigation Engineeering and Hydraulic Structures Tel 3. S. K. Garg Irrigation Engineering and Hydraulic Structures Mid Exam 30 % 4. V. T. Chow, Open Channel Hydraulics Final Exam 50 % 5. USBR, Design of Small Dams Addis Ababa University Hydraulic Structures I Table of Contents Course Syllabus ........................................................................................................................... i Course Objective .......................................................................................................................... i Course Content............................................................................................................................. i Table of Contents ......................................................................................................................... i 1 Introduction ................................................................................................................ 1 1.1 General ............................................................................................................................ 1 1.2 Types of dams ................................................................................................................. 1 1.3 Characteristics of dams ................................................................................................... 2 1.4 Reservoir Planning .......................................................................................................... 6 1.5 Hydrology and geology................................................................................................... 7 1.6 Environmental, Social, Economical and Political investigation ................................... 11 1.7 Location criteria for dam and spillway site ................................................................... 11 1.8 Dam site investigation................................................................................................... 12 1.9 Data collection .............................................................................................................. 12 1.10 Site Requirement ........................................................................................................... 14 1.11 Stages in Project Planning and Implementation ........................................................... 15 2 Foundations of Dams and their Treatment ........................................................... 16 2.1 General .......................................................................................................................... 16 2.2 Rock foundation ............................................................................................................ 16 2.2.1 General .................................................................................................................. 16 2.2.2 Foundation treatment ............................................................................................ 16 2.2.3 Measures against leakage ...................................................................................... 17 2.2.4 Drainage ................................................................................................................ 17 2.2.5 Stability of Dams and Strength of Rock Foundation ............................................ 18 2.3 Earth foundation............................................................................................................ 18 2.3.1 General .................................................................................................................. 18 2.3.2 Gravel and sand foundation .................................................................................. 19 2.3.3 Estimation of seepage amount .............................................................................. 19 2.3.4 Piping .................................................................................................................... 20 2.3.5 Uplift pressure and control of seepage.................................................................. 21 2.3.6 Theories of Seepage Flow ..................................................................................... 24 2.3.7 Uplift pressure and seepage under masonry structures on pervious foundations . 25 2.3.8 Silt and clay foundation ........................................................................................ 27 3 Concrete Gravity Dam ............................................................................................. 28 3.1 General .......................................................................................................................... 28 3.2 Forces acting on gravity dams ...................................................................................... 28 3.2.1 Water pressure ...................................................................................................... 28 3.2.2 Wight of Structure................................................................................................. 30 3.2.3 Earth and silt pressure ........................................................................................... 31 3.2.4 Wind pressure ....................................................................................................... 31 3.2.5 Wave pressure and wave height ............................................................................ 31 3.2.6 Earthquake forces.................................................................................................. 32 3.3 Load combination for Design ....................................................................................... 34 3.4 Reaction of the foundation ............................................................................................ 34 3.5 Rules Governing the Design of Gravity Dams ............................................................. 35 June 12 i Addis Ababa University Hydraulic Structures I 3.6 Theoretical versus practical section of a dam ............................................................... 38 3.7 Design procedure of gravity dams ................................................................................ 38 3.7.1 Design methods ..................................................................................................... 38 3.7.2 Multiple step method of determining profile of gravity dam ............................... 39 3.7.3 Single Step Method ............................................................................................... 40 3.8 Gravity dam Construction, Quality control and the Future .......................................... 49 3.8.1 The Construction Process ..................................................................................... 49 3.8.2 Quality Control ..................................................................................................... 51 3.8.3 The Future ............................................................................................................. 51 4 Concrete Arch and Concrete Buttress dams ......................................................... 52 4.1 Concrete Arch Dam ...................................................................................................... 52 4.1.1 Constant radius arch dam ...................................................................................... 53 4.1.2 Constant Angle Arch dam ..................................................................................... 53 4.1.3 Variable radius arch dam ...................................................................................... 53 4.1.4 Loads on arch dam ................................................................................................ 54 4.1.5 Methods of design of massive arch dams. ............................................................ 54 4.1.6 The thin cylinder theory ........................................................................................ 54 4.1.7 Design procedure for variable radius arch dam .................................................... 56 4.1.8 Elastic arch theory (Arch dam analysis) ............................................................... 57 4.2 Buttress dams ................................................................................................................ 59 4.2.1 Classification of buttress dams ............................................................................. 59 4.2.2 Advantages and Disadvantages of Buttress Dams ................................................ 59 4.2.3 Forces on buttress dams ........................................................................................ 60 4.2.4 Design Principle for Buttress Dams ...................................................................... 60 5 Embankment Dam ................................................................................................... 62 5.1 Introduction ................................................................................................................... 62 5.2 Key elements and appurtenances of Embankment dam ............................................... 62 5.2.1 Foundation: ........................................................................................................... 62 5.2.2 Core or membrane................................................................................................. 63 5.2.3 Shell ...................................................................................................................... 64 5.2.4 Height of dam: ...................................................................................................... 65 5.2.5 Top Width: ............................................................................................................ 65 5.2.6 Appurtenances....................................................................................................... 65 5.2. Types of Embankment dam .......................................................................................... 66 5.3. Causes of Failure of Embankment dams ...................................................................... 67 5.4. Design features.............................................................................................................. 68 5.5. Seepage analysis ........................................................................................................... 70 5.6. Determination of Phreatic Lines ................................................................................... 70 5.7. Stability analysis ........................................................................................................... 74 5.7.1. Stability of side slopes of earth dam ......................................................................... 74 5.7.2. Stability of earth dam against horizontal shear developed at the base of the dam ... 78 5.8. Foundation analysis ...................................................................................................... 79 5.9. Internal drainage system ............................................................................................... 84 6 Spillways.................................................................................................................... 88 6.1 General ...................................................................................................................... 88 6.2 Types of Spillway ..................................................................................................... 88 6.2.1 Free overfall (straight drop) spillway ................................................................... 88 6.2.2 Ogee (overflow) spillway ..................................................................................... 89 June 12 ii Addis Ababa University Hydraulic Structures I 6.2.3 Side channel spillway ........................................................................................... 90 6.2.4 Siphon spillway ..................................................................................................... 91 6.2.5 Chute spillway ...................................................................................................... 91 6.2.6 Shaft spillway........................................................................................................ 92 6.3 Spillway Crest Gates ................................................................................................. 92 6.4 Spillway design ......................................................................................................... 97 6.4.1 Background ........................................................................................................... 97 6.4.2 Stilling Basin ....................................................................................................... 112 7 Sediment Transport and Stable Channel Design ................................................ 115 7.1 Mechanics of Sediment transport............................................................................ 115 7.2 Lacey’s regime method of stable channel design. .................................................. 115 7.3 Force on sediment particle ...................................................................................... 118 7.4 Stream Diversion .................................................................................................... 118 References: ..................................................................................................................... 122 Sample Questions from Previous Exams ..................................................................... 123 June 12 iii Addis Ababa University Hydraulic Structures I 1 Introduction 1.1 General Hydraulic Structures are engineering constructions designed and mechanically fit for managing and utilizing water resources to the best advantage of the human being and environment. Dam is a barrier across flowing water that obstructs, directs or retards the flow, often creating a Reservoir. Reservoir is an artificial lake created by flooding land behind a dam. Some of the world's largest lakes are reservoirs. Spillway is a section of a dam designed to pass water from the upstream side of a dam to the downstream side. Many spillways have gates designed to control the flow through the spillway. Flood is an overflow or an expanse of water submerging land. Dams differ from all other major civil engineering structures in a number of important regards: Every dam, large or small, is quite unique; foundation geology, material characteristics, catchment flood /hydrology etc. are each site-specific. Dams are required to function at or close to their design loading for extended periods. Dams do not have a structural lifespan; they may, however, have a notional life for accounting purposes, or a functional lifespan dictated by reservoir sedimentation. The overwhelming majority of dams are of earth fill, constructed from a range of natural soils; these are the least consistent of construction materials. Dam engineering draws together a range of disciplines, e.g. Structural and fluid mechanics, geology and geotechnics, flood hydrology and hydraulics, to a quite unique degree. The engineering of dams is critically dependent upon the application of informed engineering judgment. Hence the dam engineer is required to synthesize design solutions which, without compromise on safety, represent the optimal balance between technical, economic and environmental considerations. 1.2 Types of dams Dike is a stone or earthen wall constructed as a defense or as a boundary. The best known form of dyke is a construction built along the edge of a body of water to prevent it from flooding onto adjacent lowland. Levee is a natural or artificial structure, usually earthen, which parallels the course of a river. It functions to prevent flooding of the adjoining countryside. However it also confines the flow of the river resulting in higher and faster water flow. Weir is a small overflow type (designed to be overtopped) dam commonly used to raise the level of a small river or stream. Water flows over the top of a weir, although some weirs have sluice gates which release water at a level below the top of the weir. Check dam is a small dam designed to reduce flow velocity and control soil erosion Diversion dam is a type of dam that diverts all or a portion of the flow of a river from its natural course Masonry dam is a type of dam constructed with masonry. It is made watertight by pointing the joints with cement. A plaster of cement is also applied. The interior could be either in coursed masonry or rubble masonry. June 12 1 Addis Ababa University Hydraulic Structures I Dams may be classified according to Material of construction, structure, intended purpose or height; Hence, no classification is exclusive. Classification according to material of construction Timber dams Steel dams Concrete dams Earth dams Rockfill dams Combined dams Classification according to design criteria Hydraulic design Stability consideration Non-overflow dams Gravity dams Overflow dams Non-gravity dams Composite dams Classification according to Purpose Storage dams Stage control dams Barrier dams Flood control Diversion Levees and dykes Water supply Navigation Coffer dams Detention storage Classification according to height (H) H ≤ 30m low dam 30 ≤ H ≤ 100m medium H ≥ 30m high dam When the size of the dam has been determined, the type of dam envisaged requires certain geological and topographical conditions which, for the main types of dams, may be stated as follows. Concrete Dams Embankment Dams Gravity dams Rock fill dams Buttress dams Hydraulic fill dams Multiple ach dams Earthen embankments Thick arch dams Composite dams Thin arch dams 1.3 Characteristics of dams Coffer dam is a temporary structure constructed of any material like timber, steel, concrete, rock or earth. It is built to enclose certain work site or to divert the flow to enable construction activity in the main river channel. After the main structure is built (bridge, barrage or dam) either the coffer dam is dismantled or it becomes part6 of the structure if the design so provides. Gravity Dams Stability is secured by making it of such a size and shape that it will resist overturning, sliding and crushing at the toe. June 12 2 Addis Ababa University Hydraulic Structures I The dam will not overturn provided the resultant force falls within the base. to prevent tension at the upstream face and excessive compression at the downstream face, the dam cross section is usually designed so that the resultant falls within the middle third at all elevations of the cross section good impervious foundations are essential inspires more confidence in the layman than any other type; it has mass that lends an atmosphere of permanence, stability, and safety Shasta Dam impounds the Sacramento River in northern California. Like all concrete gravity dams, Shasta Dam holds back the water in its reservoir, Shasta Lake, by the sheer force of its weight. Built of solid concrete, the massive structure rises 183 m (602 ft). It measures 165 m (542 ft) at the base and just 9 m (30 ft) at the crest. This shape, typical of concrete gravity dams, counteracts the force of the water pressing against the dam at the bottom of the reservoir, where the pressure is most intense. Courtesy of U.S. Bureau of Reclamation/Microsoft Corporation Illustration Microsoft ® Encarta ® Reference Library 2005. © 1993-2004 Microsoft Corporation. All rights reserved. Figure 1-1 Example of concrete Gravity dam Gravity dams are classified as "solid" or "hollow." The solid form is the more widely used of the two, though the hollow dam is frequently more economical to construct. Gravity dams can also be classified as "overflow" (spillway) and "non-overflow." Buttress Dams The buttress dam is suitable where the rock is capable of bearing pressures of 2 - 3 MPa. Buttress dams require between one thirds and half of the concrete required for a gravity section, hence making it more economical for dams over 14m. Additional skilled labor is required to create the formwork. Threat of deterioration of concrete from the impounded water is more likely than from a thick gravity section. There is also an elimination of a good deal of uplift pressure, the pressure resulting from the water in the reservoir and possibly of water from the hillside rocks gaining access through or under any grout curtain and exerting upwards underneath the mass concrete dam. June 12 3 Addis Ababa University Hydraulic Structures I Lake Tahoe Dam impounds the Truckee River in northern California. Like all flat slab buttress dams, it has a flat slab upstream face supported by a series of buttresses on the downstream side. Lake Tahoe Dam measures 5.5 m tall and 33 m long. It was completed in 1913 to raise the water level in Lake Tahoe, a natural lake, to provide additional water for crop irrigation. Figure 1-2 Profile of buttress dam Arch dam Stability is obtained by a combination of arch and gravity action Utilizes the strength of an arch to resist loads placed upon it by 'arch action' The foundations and abutments must be competent not only to support the dead weight of the dam on the foundation but also the forces that are directed into the abutments because of arch action in response to the forces acting on the dam. The strength of the rock mass at the abutments and immediately down valley of the dam must be unquestionable and its modulus of elasticity must be high enough to ensure deformation under thrust from the arch is not so great as to induce excessive stresses in the arch. Monticello Dam impounds Putah Creek west of Sacramento, California. The solid concrete structure stands 93 m tall. The dam’s arched upstream face transfers some of the pressure from its reservoir, Lake Berryessa, onto the walls of the canyon. This design enables an arch dam to be much less massive than an equivalent gravity dam, which relies solely on the force of its weight to hold back the water in a reservoir. While Monticello Dam measures 30 m at its base, an equivalent gravity dam might be more than five times as thick at the base. Figure 1-3 Sample of Arch dams June 12 4 Addis Ababa University Hydraulic Structures I Rockfill dams Can be built where the following conditions exist - Uncertain or variable foundation which is unreliable for sustaining the pressure necessary for any form of concrete dam. Suitable rock in the vicinity which is hard and will stand up to variations of weather. An adequate amount of clay in the region which may be inserted in the dam either as a vertical core or as a sloping core. Accessibility of the site and the width of the valley is suitable for the manipulation of heavy earth-moving machinery, caterpillar scrapers, sheepfoot rollers and large bulldozers. Hydraulic Fill Dams Hydraulic fill dams are suitable in valleys of soft material and are constructed by pumping soft material duly consolidated up to moderated heights up to 30m. A hydraulic fill is an embankment or other fill in which the materials are deposited in place by a flowing stream of water, with the deposition being selective. Gravity, coupled with velocity control, is used to effect the selected deposition of the material. Earthen Embankment dams Near the site there must be clay to fill the trench and embanking material capable of standing safely, without slipping, to hold up a clay core. An advantage of earthen embankments is that troubles due to the deterioration of the structure by peaty waters of low pH do not arise. Figure 1-4 Typical profiles of Earth dam June 12 5