DAM ENGINEERING

Amjad Agha*

INTRODUCTION

Most rivers in the World have seasonal flow pattern. There is more water in these rivers during rainy or snow melting season. In other parts of the year the flow in the rivers reduces and sometime becomes very small. The crop water requirement is however also important in low river flow season. Therefore the need to store water in the water surplus months to be used in the water deficient period has led to construction of dams and reservoirs. Another important use of the dams is the production of hydroelectric energy. The head of water created by the dam is used for creating water falls which run the turbines and generate electric power.

The importance of creating reservoirs is becoming more critical, because the availability of water per capita is constantly decreasing due to increasing population and it is extremely important that the seasonal surplus water is stored, instead of letting it go to the sea. With global warming in the horizon, the river flows are likely to become more erratic and unpredictable, therefore the need for storages would become even more important in the future.

Majority of the dams built in the World are earth dams, for instance out of 5000 important dams registered in U.S.A, 73% are the earth dams. In the recent times there is increasing trend to construct Concrete Faced Rockfill (CFRD) and Roller Compacted Concrete (RCC) dams. Such dams are relatively more economical and take less time to build.

Soil Mechanics and Geotechnical Engineering play a very important role in the design of all types of dams. The development of these subjects has improved the confidence in building large dams under more difficult site conditions.

The construction of large dams received a setback in the 1990s when the World Bank and other International donor and lending agencies significantly decreased and even ceased the provision of funds to developing countries for dam construction, because of their negative effects on environmental and forced human resettlement. This trend is changing again, primarily due to better planning and awareness of mitigating these issues.

HISTORY OF DAMS

The river basins are renowned as cradles of civilization and cultural heritage. The earliest evidence of river engineering is ruins of irrigation canals over eight thousand years old in Mesopotamia. Remains of water storage dams found in Jordan, Egypt, Yemen and other parts of Middle East date back to 3000 BC. The primary objective of

*1.President Associated Consulting Engineers – ACE (Pvt) Ltd. 2.President Pakistan Geotechnical Engineering Society

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early dams was serving as water storages, but also for controlling floods and allowing or improving navigation. With the advent of industrial revolution and technology for generating power from water movement, humans started to construct large dams to obtain energy. The first dams to incorporate an electric power station were built at the end of nineteenth century in Western Europe and the United States. During the middle of twentieth century there was great impetus in dam building which continued till 1980. The boom in dam construction came with start of 21st century, particularly in China. The construction of large dams by decades and world wide regional distribution of large dams is shown in chart below:

CONSTRUCTION OF WORLD DAMS BY DECADE (1900-20000)

Regional distribution of world large dams at the end of the 20th century

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0

1000

2000

3000

4000

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Before1900

1900s 1910s 1920s 1930s 1940s 1950s 1960s 1970s 1980s after 1990

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5,000

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China Asia North andCentralAmerica

WesternEurope

Africa EasternEurope

SouthAmerica

Austral Asia

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ams

In all about 800,000 dams have been built worldwide out of which about 50,000 are large (higher than 15 meter). These dams generate altogether about 19% of the world’s electricity and supplying water for 30 – 40% of the irrigated crop lands. The dams and diversion system (barrages and canals) are thus meeting 35 percent of world cereal requirements.

Until recently large dams were considered a milestone on the development plans of nation and they were often viewed as a symbol of modernity and economic progress. However effects derived from their building on the environment and the society did somewhat changed this vision. As a consequence some developed countries such as France and United States have interrupted dam construction and even started some demolition. Since the primary effect of dams is modifying river flow, this can result in several changes on natural habitats. These together with water pollutant’s concentration may have a large impact on aquatic and fish culture. Social consequences are linked with forced human resettlement. It is estimated that construction of large dams have displaced 40 to 80 million people worldwide (WCD-2000), with the associated changes in their livelihood.

These effects were significant, but were highly publicized and a great anti-large-dam lobby was created. The result was that funding agencies almost totally stopped providing funds for large dam construction to the developing countries. The countries needed the dams for their agriculture sustainability and for cheaper electric power. Therefore the environment effect mitigation and resettlement issues have now become a part of the planning of such projects. Better ways of resettlement and land acquisition are being adopted. Out of area resettlement is avoided as far as possible, land and dwelling compensation is becoming generous. Similarly mitigation of environment effects on the infrastructure and aquatic life are given due attention. With this change of attitude the World Bank etc. are coming back to funding the large dams. Recently completed (2004) Ghazi Barotha Hydroelectric Project in Pakistan (1450 MW) is being sited as a “good illustration of the usefulness of dams and water resource projects in the 21st century” (HRW Dec. 2005) primarily because particular attention was paid to minimize environmental and social effects.

Now once again large scale dam construction is taking place. According to ICOLD information at present some 1700 large dams are under construction. out of which about 900 are in India and many in China and other countries in the region.

The distribution of water runoff (river and stream flow) is very uneven from the point of view of continents. Asia which supports 60 percent of global population has only 36 percent of runoff, while South America with 6 percent population receives 26 percent of world runoff. Higher temperature prevailing in major parts of Asia and Africa and highly skewed rainfall, places Asia and Africa at a tremendous disadvantage due to greater evapotranspiration losses. In Asia with the high population growth, it is vital that water resource is harnessed and preserved and building of new dams and reservoirs is absolutely essential to sustain the cereal needed for its growing population.

Storage Capacity of Dams and Reservoirs of some selected countries is illustrated in the following table.

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TYPES OF DAMS

The two principal types of dam are the concrete or masonry dams and embankment dams. Embankment dams can be divided into two categories i.e. earthfill and rockfill. Recently some variation of these types are being frequently adopted. These are RCC (Roller Compacted Concrete) and CFRD (Concrete Faced Rockfill Dams). The selection of the type of a dam depends on site conditions and economy in construction. Earthfill dams are selected for sites with wide stream valley, considerable depth of soil overlying rock in the river bed and availability of sufficiently quantities earth and core materials. Rockfill dams are selected for relatively narrow valleys, and where construction requires large rock excavation. Earlier rockfill dams had impervious core, similar of earthen dams, but now wherever rockfill dam is selected, mostly it has a concrete facing, instead of soil impervious core. This type of construction is faster and safer. Such dams are also more economical as the side slope of the CFRD can be steeper than those with impervious core.

Presently a number of CFRD's are under construction throughout the world, the largest being Malaysia's Bakun Dam which is 200m high.

Similarly, the conventional concrete gravity dams are now being replaced by RCC dams on many sites. By the end of year 2005, China had completed 90 RCC dams and Spain has completed 22 RCC dams in the definition of ICOLD large dams. Very high RCC dams are under construction and planning. For instance Longtan RCC dam in China which is in advance stage of construction is 216.5 meter high. Basha RCC dam presently being designed in Pakistan will be 270 m high. It appears in the future majority of dams will be of this type because of their ease of construction and much lower costs. Techniques for RCC construction are improving. One of the problems for RCC dams was the bonding of the layers, where seepage developed. In the modern RCC dams the problem has been greatly overcome.

Some sections of the various types of dams are illustrated here.

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Typical Cross-section of CFRD (Beris Dam Malaysia)

Roller Compacted Concrete (RCC)

Use of RCC in dam construction is becoming increasingly popular in the recent times. The RCC mixture includes low heat cement, pozzolan (mainly fly ash), sand, aggregates, additive etc. Using pozzolan as much as possible and decreasing content of cement can reduce hydration heat of RCC, also simplifying measure of thermal control. In China two other materials are also used, they are stone powder and MgO . It is considered by Chinese builders that use of stone powder improves workability of RCC, by filling voids in the aggregate, which results in easy compaction and anti segregation. In China 15 – 20 percent of stone powder has been used in the RCC mixture. Another additive used is MgO,

which is helpful in reducing shrinkage by drop of temperature in mass concrete. Maximum of 5% of MgO . is recommended by the Chinese builders.

The cementations material in the RCC varies from 160 to 220 Kg/m3 , out of this 50% or more can be the pozzolan or fly ash. The required strength of the RCC varies from 30 to 50 MPa, depending on the quantity of cementations material.

Since RCC is placed in discrete, thin lifts or layer of about 300 mm, there has been concern expressed as to achieving adequate bond between these layers. It is well known that bond of the RCC across the layer joints depend on the age of the joint at the time next layer is placed over it. This is no different to CVC, which is also placed in lifts. The difference is the CVC is placed in 1.5 m lifts as compared to RCC lift of 0.3m, and thus the CVC has only 20% number of joints compared to RCC. The bond across lifts joints governs the shear resistance of the dam to failure by sliding or cracking in tension due to overturning forces. There is little or no residual cementations capacity left on the aged surface to develop a bond, this must come from the adhesion of the covering lift or the bedding mortar. Earlier RCC dam built had bonding problem between the layers and seepage developed along these joints. In modern RCC dam the retarders and chemical admixtures are used to extend the initial setting time from 2 to 6 hours. Further improvement in RCC construction has come through a “Slope Layer Method” being used in China; which reduces the number of joints and also increases productivity and speed of RCC.

The experience of improved quality and bond across RCC layer joints can be checked through the unbroken length of cores extracted from RCC dams. In some Chinese RCC dams continuous length of 7 to 13 metre of core have been extracted.

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The confidence in the facing layers of RC dams are also improving. Previously on RCC dams PVC geo-membranes have been used on the upstream face to make it impervious, however now the better quality RCC is used on the upstream face with good results. On very high dams, the upstream face of reinforced concrete is also used. RCC constructed is faster and about 15 – 20% cheaper as compared to CVC.

Concrete Faced Rockfill Dams (CFRD)

Earlier rockfill dams (mostly constructed prior to 1965) comprised of dumped rockfill with a compacted clay core and filters. Subsequently with the advent of heavy vibratory rollers, the practice changed and rockfill was compacted instead of dumping. The compaction helped minimize post-construction deformations and settlement of rockfill. This achievement have allowed introduction of concrete facing in the rockfill dam, and eliminated the clay core.

Barry Cooke from California, U.S.A, contributed in introducing and improving the design of CFRD. Now wherever rockfill dam is to be built it is mostly CFRD, this type of dam has three elements, the plinth, the zoned rockfill section of the dam and the concrete face.

The plinth is the non-erodible cutoff element. On non-erodible rock the foundation treatment is straight forward. On rock with possible erodible features thorough geotechnical attention is given to grouting and to sealing and filters downstream of plinth. (see sketch)

The rockfill section is the structural element. It is zoned to accept the water loading with minimum settlement and to provide an embankment safe against through flow in the event of leakage from concrete face. The rockfill is compacted in 300 to 600 mm layers (and downstream zones even in 1000mm layers), using a 10 ton vibratory roller.

The concrete face is the impervious feature. It is watertight, but incidents of perimeter joint leakage and of face crack leakage have occurred. The recent practice of three tier protection system of perimeter joint and two tier sealing of construction joints have successfully controlled leakages through face slab. As a second line of defence the leakage through the face slab is controlled by face zone semi-pervious rockfill. Also leakage can be substantially sealed by underwater placement of impervious silty fine sand. The formula for face thickness of t = 0.30 +002 H has been and is being satisfactorily used. The reinforcement percentage of 0.3 Horizontal and 0.35 to 0.4 vertical is generally provided.

Although the design of CFRD is empirical and is based on experience and precedent, but there are inherent safety features in CFRD dam that make empirical design acceptable. Very large number of CFRDs are now being planned and built; about 100 such dams have been built in China alone. There are most than 25 CFRD built around the World which are over 130 meter high and several dams are presently under construction or under design which will be over 200 meter high.

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MODERN TRENDS IN DAM ENGINEERING

As stated earlier knowledge of Geotechnical Engineering plays a very important role in the Dam Engineering. The advancement in this speciality particularly with respect to computer-aided analysis tools and the quantum jump in Seismo-tectonic approach have brought important changes in the State of the Art of Dam Engineering in the modern times and the confidence level on design is substantially improved. However in the practice of geotechnical engineering one always works with incomplete data, since the full knowledge of subsoil conditions can never be obtained. Therefore certain “calculated risks” are selected by the geotechnical engineer in the formulation of his recommendations. With each risk selected for a project, there is an associated construction cost. Obviously the greater the risk owner selects, the less cost of the project and vice versa. Successful application of risk-cost approach requires that the geotechnical engineer makes all practical efforts to obtain reliable and adequate data and he be involved in all phases of the project to ensure himself and the owner that soil-structure interaction is compatible with that predicted and within the risk selected. It is well known that three necessary attributes for the successful practice of geotechnical engineering are

i) knowledge of precedentsii) familiarity with soil mechanics andiii) working knowledge of engineering geologyiv) availability of sophisticated analysis tools made possible by help of

computersv) Automated performance monitoring instrumentation.

With proper and better use of above mentioned essentials, it has been possible to improve the design of the modern dam to produce better safe and more economical structure.

In this context the approach to seismic-resistant design of dams have been completely modified and requires proper understanding. The availability of fast running finite element computer modules and various non-linear models for concrete and particulate materials have allowed simulation of deformation behaviour of dam body and foundation under seismic loadings. Consequently, confident decisions are now made with respect to aseismic design of dam structures. The recent development of Roller Compacted Concrete (RCC) and its use in the construction of dams have had significant effects on the cost and time saving, similarly the modern Concrete Faced Rockfill Dams (CFRD ) have brought about a safer and economical advancement in the dam construction.

Development of automated and remote performance monitoring instrumentation systems have enabled to understand the behaviour of dam in terms of pore water pressures and deformations during events of short-lived incidents of seismic shaking and passing of flash floods.

The use of satellite images and GIS maps have enhanced the hydrological analysis and evaluation of reasonable and accurate assessment of design floods and risks associated with them.

With respect to dam engineering it may not be irrelevant to mention the positive role of internet. It has added efficiency in the sharing of knowledge in the field of dam

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engineering. A particular example is the availability of seismic records and time history of seismic events on the net.

DAM SAFETY AND DAM FAILURES

The history of dam building since the dawn of civilization is a long series of failures. Man learns little from success but a lot from his mistakes. The failure of a dam is however the result of complex causes and mechanism, which should be interpreted with extreme care. Learning from our errors is vital for improving our knowledge and promoting safer design, as designing and building a successful earth dam is still nowadays more an art than a science. Soil Mechanics is a powerful tool, which has all the characteristic of an experimental science. Its proper use in design, however, implies the use of simplified models and selection of parameters which may be far from representing the actual complexity of nature. In addition, the floods in the river are ill-known and remain a threat over the period of construction and afterwards during operation of the dam, although hydrology also in an experimental science. The best proof that designers are not yet able to carry out their works in a scientific manner is given by the recent spectacular failure of earthdams, where conventional good engineering practice was followed. The frequency of such incident has however substantially reduced, a statistical study shows that during the period 1900 – 1940 cases of dam failures were 0.7%, these were reduced 0.2% between 1940 – 1960 , and further reduced in the new dams by a ratio of 10, between 1900 and the present time.

The major causes of failure have been –

a) Failure by Overtopping

Overtopping due to inadequate spillway capacity. Such failures are primarily in earth dams, and constitute over 30% of all failures. The recent improvements in hydrology particularly the use of Probable Maximum Flood Concept, use of fuse plugs have resulted in substantial increase in the safety of earth dams as far overtopping is concerned. However many dams designed and built in the past are not safe, and their spillway capacity should be enlarged. The failure of Oros Dam, (1960), S Chunha and Olivera Dams (1977) in Brazil and Bolan Dam in Baluchistan Province of Pakistan are relatively recent examples of failures due to overtopping North East Brazil and Baluchistan are semi-arcid regions, with irregular rain fall, varying greatly from one year to another. Since these dams were operating without any problem for the last 15 years or more, the arrival of sudden large flood was not anticipated.

b) Failure by Sliding

About 15 percent of dam failures have been attributed to sliding. The usual concept to check the stability against sliding is to carry out stability analysis and determine the factor of safety. If the factor of safety was considered in the safe limits the dam design was considered safe. It should however be realized that 85 percent of failures have taken place for causes other than an inadequate factor of safety. Such causes which are not covered by

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conventional stability analysis, like piping, cracking, uplift pressure are difficult to predict shows that engineers have still to learn to improve their practice.

One cause of sliding failure, which was not fully appreciated, is the existence of shear zone in the overconsolidated clays. These zones are not easily detected in geotechnical investigation, because the shear zones are only few millimeter thick and are almost always obscured by weathering in material exposure. These zones are particularly missed in core drilling, therefore wherever over consolidated are found, extensive trenching and exposure are necessary to study them. The existence and significant of such layers was first discovered during the construction of Mangla Dam in Pakistan (1960s). Discontinuities were noticed in the Siwalik Clays, which turned out to be shear zones. Professor Skempton who was engaged as a special consultant on this project, carried out pioneering research on these zones and concluded that wherever over consolidated clays are existing in the foundation, the normal shear strength determined through tri-axial tests etc. is not applicable. For such materials only residual strength of such clays must be used in the stability analyses against sliding for failure surfaces parallel to the bedding planes. Special multi-reverse shear tests were devised at the Imperial College London to get a value of residual strength. The value of residual strength could be over half or one third of the angle of internal friction measured in the laboratory in normal tri-axial test. At Mangla Dam original design had to be extensively modified. The design modification resulted in an increase of about 22 percent in the estimated cost of civil works. Similar design problems were faced during the construction of Gardiner Dam on the south Saskatchewan River in Canada, where a 17 percent increase in cost took place due to design modifications. Subsequently Waco Dam in Taxas, USA failed due to sliding on a shear zone in the foundation shale. It was later recognized that the failures of Seven Sisters dam and North-Ridge dam in USA were due to the presence of over consolidated plastic clay of low residual strength.

The recommended guidelines is to make residual strength measured on all over consolidated clayey soils, and to base the stability analysis on the low strength parameter, with a factor of safety not much above unity.

Another possible cause of sliding failure is the liquefaction of the loose saturated sand. Liquefaction could result from a sudden pore pressure increase which might be induced by an earthquake or from a strain building up induced by a slide. Although no large dam has actually failed due to liquefaction, but several near failure are known. Upper and lower San – Fernando dams (USA) developed large downward displacements due to liquefaction during Sen – Fernando earthquake of 1971.

(c) Failure by Seeping Water

Piping is probably the most detrimental factor of failure in earth dams nowadays. Discovery of the dispersivity of some clays has emphasized the concern regarding danger of piping through dam cores. Piping along the contact with concrete works, such as culverts or retaining wall is also a threat.

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Uplift pressure is another aspect to be considered for safe design. Almost 25 – 30 percent of failure have taken place due to piping and uplift. The various failures due to piping have amply emphasized that whatever the details of penetration of water through the core, it is certain that the silty material requires protection by filters. Some important failures have taken place where the designers were negligent of this sensitivity. For instance the failure of Teton Dam ((Idaho USA) 1976 is a glaring such example. Teton Dam is he highest dam which ever failed with 93 m above river bed. Piping occurred through the core and foundation rock during the first filling of the reservoir. No instruments whatsoever were installed and there was no inspection gallery under the core. On 5 June 1976 at 8.30 a.m two large leaks appeared at the contact between the dam and its right abutment. Within few hours the piping hole increased and embankment was breached to the crest. The erosion washed away 3 million cubic meters of earthfill, releasing a peakflow of 70,000 M3. Alarm was given to all living downstream in a very effective way, so that only 11 people were drowned, although the flood reached over 200 Km of river plain. Several inquiries were conducted and they were definite in condemning “faulty design”. It appears that silty material from the core was not properly protected by filters and open cracks in the cut off trench were not properly plugged, and erosion took place where the plugging of cracks by concrete was not done, because it was not specified and there was no coordination between USBR design and field staff. Dr. .L. Sherard declared in a public lecture the Teton did not offer any technical lessons to the designs, as it was a “bureaucratic failure”. Piling along the contact with concrete works such as culverts or retaining walls is also a thread. Fortunately most designers are well aware of these dangers and they can remedy them in complete safety by proper use of filters and drains.

(d) Gap Graded Gravels

Gap graded gravel layers found in some river deposits have also to be treated with care. Such deposits were given a name of open-work gravels at the Tarbela Dam in Pakistan where these caused a serious problem of formation of sink holes in the upstream clay blanket on first fitting of the reservoir. The sink holes were most probably formed due to internal erosion of blanket material and migration of this material and in the voids of open work gravel. Various theories were propagated for the sink hole formation including differential settlement, washing away of sand etc. In 1992, piping tests conducted at Imperial College London under the supervision of Professor Skempton (Ref) showed that for gap graded internally unstable sand gravel mixtures, movement of sand can take place at hydraulic gradients far lower than critical gradients given by classical theory. Subsequently a series of piping tests were carried out on samples of representative gap graded material collected from the foundation strata of Ghazi Barotha Barrage, this material was similar to the foundation material of Tarbela Dam upstream Blanket. These piping tests were carried out in the Central Material Testing Laboratory (CMTL) of WAPDA at Lahore in a large 600 mm diameter parameter. The result of a total of nine piping tests showed that a safe value of critical gradient for such material is 0-.3, which was adopted for the design of underdrainage system at Ghazi-Barotha Barrage.

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Gap Graded Gravel (Indus River at Tarbela)

SEDIMENTATION IN DAM RESERVOIR

Many rivers in our region carry heavy load of sediments, and these sediments are deposited in the reservoir created by dams. Since almost all dams built uptodate do not have a sediment sluicing arrangement with the result almost 90 – 95 percent of the sediment get trapped. At Tarbela Dam, the Indus River carries a sediment load of about 200 million tons/year. This sediment is not only eating up the live storage capacity of the reservoir, but also a threat that the sediment may liquefy and block the intake of the power tunnel if a heavy earthquake occurs at site. The sediment depth in the reservoir is over 200 feet (see diagram). Many schemes have been considered for evacuation of sediments from the reservoir, the only viable method is lower the reservoir and flush the sediment through outlets in the dam, but the danger of sediment blocking the outlets and secondly the closing down of power generation pose great issues. The dredging of sediments from the large reservoir is highly uneconomical and not worth considering.

It is therefore recommended to consider that sediment sluicing from the reservoir is part of the reservoir operation plans and whenever feasible sluicing should be carried from the early stage of impoundment.

TEN COMMANDMENTS

In the light of the lessons from old and more recent failure of earth dams, Pierre Londe (former President of ICOLD) had put forward following recommendations for improving their overall safety. These ten recommendations are still considered as commandments to the dam designers:

1. During geological and geotechnical investigations look out for clayey soils, sometimes in the shape of very thin seams, the residual strength parameters, particularly in formations where soft layers are interbedded with stiffer layers. Overconsolidated clays are particularly relevant for use of residual strength parameters.

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2. During geological and geotechnical investigations look out for loose silty or sandy soils, and study their liquefaction potential.

3. For design of sequence of construction stages and of diversion of river, make a thorough analysis of the floods in terms of probability of occurrence and corresponding probability of damage downstream.

4. For design of spillway and outlets make the best use of most recent hydrological methods, with a clear appraisal of the catastrophic consequences of overtopping the main embankment. Fuse plug spillway generally is a good safety measure, provided it is correctly designed.

5. For safety gated spillway, very detailed and strict operating rules are required. Any human or mechanical failure has to be envisaged and corrected by emergency arrangements.

6. Ample and well graded filters and drains are vital for preventing piping through dam and foundation.

7. All fine clayey soils should be tested for dispersion potential of clay particles.

8. Instrumentation is the only means of monitoring safety during operation of the dam. If is a vital part of the design of new dam and must be incorporated in old dams where it is missing.

9. Inspection galleries, either at the base of the core or in the foundation rock below, are invaluable for placing instrumentation for direct observation and for quick remedial action if required.

10. Thorough and careful surveillance of dams safeguards safety of people living downstream. It should be done on a 24 hours basis, together with automatic recording instrumentation.

THE LAWS OF NATURE HIT AT RANDOM

I would like to end this Paper by quoting from a letter which Karl Terzaghi wrote to Andre Coyne immediately after the failure of Malpasset Dam in France.

“ When I read in the papers about the failure of the Malpasset Dam, my thoughts turned immediately to you and to the terrible shock you must have experienced when the sad news reached you. In situations of this kind it is at the outset impossible to divorce the technical aspects of the event from the human tragedies involved. Yet every fair-minded engineer will remember that failures of this kind are, unfortunately, essential and inevitable links in the chain of progress in the realm of engineering, because there are no other means of detecting the limits of the validity of our concepts and procedures.

Having known you well for many years, I feel confident that the failure was not a consequence of an error in your design. Therefore, it will serve the vital purpose of disclosing a factor which in the past has not received the attention which it

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requires. The fact that its implications became manifest on one of your jobs is not your fault, because the occurrence of failure at the borderline of our knowledge is governed by the laws of statistics, and these laws hit at random. None of us is immune. You as an individual, and the equally innocent victims of the failure have paid one of the many fees which nature has stipulated for the advancement in the realim of dam construction. Therefore, the torments which you experienced should at least be tempered by the knowledge that the sympathies of your colleagues in the engineering profession will be coupled with their gratitude for the benefits which they have derived from your bold pioneering.”

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DAM ENGINEERING

INTRODUCTION

HISTORY OF DAMS

TYPE OF DAMS

MODERN TRENDS IN DAM ENGINEERING

DAM SAFETY AND DAM FAILURES

SEDIMENTATION IN DAM RESERVOIR

TEN COMMANDMENTS

THE LAWS OF NATURE HIT AT RANDOM

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