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Construction of dams and reservoirs in karst is historically known as a very risky task. Inspite of very detailed geophysical investigations and repeated sealing treatments, the possibility for dam failure cannot be eliminated. In the karst environment, with its highly random distribution of dissolution features, some uncertainties always remain. The final determination of the adequacy of sealing measures comes after the first reservoir impoundment or even later. In many worldwide examples, watertightness treatment during dam construction was only partially successful, with some remedial work after impoundment being quite common. However, in some cases, the problem is simply too complicated and cannot be overcome. Special approaches have to be undertaken in order to prevent seepage from reservoirs. The key elements are a good geological map and proper geophysical investigations. These investigations are key prerequisites of dam construction in karst and cutting costs through restricting them usually results in increasing the chance of project failure. To deal with karst successfully, innovation, engineering practice, execution feasibility, and commercial understanding have to be undertaken. Grouting alone is definitely not adequate in the case of large karst conduits. Special treatment of large caverns and flexibility during grout curtain execution, including modifications and adaptations on the basis of the geological findings, should be the standard procedure for dam construction in karst to minimize risk. Such an approach is the basic worldwide rule in the fight against leakage from dam sites and reservoir abutments.

Weather systems of the tropics and extra tropics generate heavy to very heavy rainfall for periods of days in various parts of the world, as discussed in Chapter 5. The heavy rainfall events may last 3 to 4 days and are responsible for causing floods, landslides, levee breaches, dams overtopping, sedimentation, erosion and other such occurrences. Flooding from some of the abnormally heavy rainfall events have destroyed dams, causing enormous loss of life and property damage. This happened in India in 1979 when a heavy rainfall event over the Machhu River catchments totally destroyed the Machhu-2 dam on 11 August. The disaster killed as many as 2000 people and totally devastated urban and rural property downstream of the dam (Purohit et al., 1993). Another instance is the catastrophic flood generated from a heavy rainfall event caused by typhoon Nina of August 1975 in the Hongru River basin in China which destroyed the Banqiao and Shimantan dams (Tan and Lu, 1994). There are many dam failures (see Table 8.11) Table 8.11 Major dam failures S. No. Dam Height (m) Country Year of Failure Lives Lost 1. Southfork 22 USA 1889 2209 2. Briseis 24 Australia 1929 14 3. Sellazerbino 47 Italy 1935 - 4. Kadam 23 India 1958 - 5. Malpasset - France 1959 421 6. Panshet 53 India 1961 - 7. Khadakwasla 20 India 1961 - 8. Valnot 265 Italy 1963 2600 9. Gibson 61 USA 1964 - 10. Chikkhole 37 India 1972 - 11. Shimantan - China 1975 - 12. Banqiao - China 1975 - 13. Teton - USA 1976 11 14. Euclides Da Cunha 40 Brazil 1977 - 15. Machhu-2 25 India 1979 2000 16. Noppikoski 19 Sweden 1985 19 17. Tailings - Italy 1985 200 18. Kantle - Srilanka 1986 100 19. Spitskop 13 South Africa 1988 13 caused by flooding due to heavy rains in various parts of the world (Lemperiere, 1993). In fact, the first recorded dam failure in the USA occurred on 16 May, 1874 in Williamsburg, Massachusetts. It is evident from the above examples that tropical and extra tropical storms and associated heavy rainfalls cause floods which are undoubtedly one of the mightiest and most devastating forces of nature. However, there are ways to prevent or minimize the impacts of flood disasters from heavy rainfall on people and property by adopting the following measures: Constructing flood walls and levees to constrict the overflow of rivers Increasing capacity of existing channels by river improvement and cutoff Constructing flood control dams on rivers to store runoff and reduce flooding downstream Constructing diversion channels for flood waters Monitoring spillway gates in times of heavy rainfall Creating and enforcing effective building codes to prevent property from tropical storms.

There exists a considerable body of literature accumulated over nearly a century and half on movement of flood waves resulting from sudden or instantaneous dam failure. In the last two decades or so, public interest in dam safety has heightened significantly which, in turn, has stimulated and augmented research on various facets of dam failure. As a result, a number of mathematical models have been developed and reported in the literature (Seemanapalli and Singh, 1990, 1992). The mathematical models which are concerned with earth dam breaching are briefly discussed here.

A comprehensive investigation of the hydraulics of dam failure involves flood hydrology, mechanics of erosion and sediment transport, hydraulics of flow over the crest and through the breach, geomechanics of slope instability, hydraulics of channel flow and floodplains, dam regulation and management, disaster alleviation, and damage assessment. Some of these elements are interconnected as shown in Figure 3.1. A complete mathematical model incorporating all of these elements does not appear to have been reported yet. Most models are based on two or more elements that hover around breach formation and dam-breach flood routing.

Dam-break flows or floods (D.B.F.) have become a very important topic for engineers. As a part of a risk and safety analysis it is now current to produce inundation maps consistent with different dam failure scenarios and conditions. These maps are based on numerical simulations and on computational models of dam-break floods. The dam-break problem is now as a real problem in engineering pratice that must be solved by computational models according to real experience and knowledge, based on historical events, physical models and sensitivity analysis. A “state-of-the-art” of D.B.F. modeling is presented, including recent contributions to numerical techniques advances on shock simulation and 2-D modeling.

The computation of steady and unsteady flows with shocks and bores is discussed in this paper. Two different formulations are presented to model steady flows. In the first, which is valid only for super-critical flows, a steady form of the shallow water equations is numerically integrated. In the second formulation, the unsteady gradually varied flow equations are solved and time is used as an iteration parameter. A k-∈ model is used to include turbulent stresses for the simulation of a radial hydraulic jump. To demonstrate application to unsteady flows, two-dimensional flows produced by a dam failure are discussed.

The problem of risk reduction due to dam failure of existing dams with a limited, reasonable expenditure of funds is modeled and solved through the use of quantitative risk-based decision analysis procedures. This approach aids in the identification of which dams are candidates for modification and given that modification is to occur the study can be used to aid in selecting the appropriate type and level of modification to be made. The elements of the procedure as required for practical application to the complex engineering, social, and environmental problem encountered in water resources reliability and risk investigations are illustrated.