Chapter 1      


INTRODUCTION

Dams provide many benefits to our society, but floods resulting from the failure of constructed dams have also produced some of the most devastating disasters of the last two centuries. Major causes of failures identified by Costa are overtopping due to inadequate spillway capacity (34 percent), foundation defects (30 percent), and piping and seepage (28 percent). Simulation of dam breach events and the resulting floods are crucial to characterizing and reducing threats due to potential dam failures. A dam break may result in a flood wave up to tens of meters deep traveling along a valley at quite high speeds. The impact of such a wave on developed areas can be very devastating. Such destructive force comes as an inevitable loss of life, if advance warning and evacuation was not possible.

The two primary tasks in the analysis of a dam breach are the prediction of the reservoir outflow hydrograph and the routing of that hydrograph through the downstream valley. Predicting the outflow hydrograph can be further subdivided into predicting the breach characteristics (e.g., shape, depth, width, rate of breach formation) and routing the reservoir storage and inflow through the breach. The routing tasks through the breach and through the downstream valley are handled in most of the widely used computer models with various one-dimensional routing methods. However, the programs differ widely in their treatment of the breach simulation process. Many models do not directly simulate the breach; rather, the user determines the breach characteristics independently and provides that information as input to the routing model.

In spite of great advancements in design methodologies, failures of dams and water retaining structures continue to occur. The numerous dam failures that occurred in the mid-1970's, including Buffalo Creek coal waste dam (West Virginia, 1972), Teton Dam (Idaho, 1976), Laurel Run Dam and Sandy Run Dam (Pennsylvania, 1977), and Kelly Barnes Dam (Georgia, 1977) led to comprehensive reviews of Reclamation's dam safety program. Failure of the Malpasset Concrete Dam in France in 1959 caused 433 casualties and eventually prompted the introduction of dam safety legislation in France. In October 1963, 2000 people died in Italy when a landslide fell into the Vaiont reservoir creating a flood wave more than 100 metres high that overtopped the dam and flooded the downstream valley. In July 1985, about 90% of the 300 people living in Stava near the Stave Dam in Italy also died when this mine tailing dam failed. More recently, in May 1999, a dam failed in Southern Germany causing four deaths and over 1 billion Euro of damage. In Spain 1997, failure of a dam on the Guadalquivir River, not far from Sevilla, caused immense ecological damage due to the release of polluted sediments in the river valley. Similarly, in Romania, failure of a mine tailings dam released lethal quantities of cyanide in the river system, polluting the environment and a major source of drinking water for both Romania and Hungary.

In India's worst dam failure in 1979, the Machu II Dam in Gujarat unleashed floodwaters in which about 2000 people died and the flood wave of order of 10 metres caused a heavy devastation in Morvi town and nearby villages. The breaching of Kodaganar Dam (Tamil Nadu) in year 1977 caused a huge loss of property in downstream area. Among the recent dam failures, the breaching of Pratapura Dam, Badodara Dam in Gujrat, and Nandgaon Dam in Nagpur, Maharashtra both in year 2005, caused heavy flooding in the downstream area. These instances of dam breaks establish that hazard posed by dams, large and small alike, is very real. As public awareness of these potential hazards grows and tolerance of catastrophic environmental impact and loss of life diminishes, managing and minimizing the risk from individual structures is becoming an essential requirement.

Dam break failures are often caused by overtopping of the dam due to inadequate spillway capacity during large inflows into the reservoir from heavy rainfall-generated runoff. Dam failure may also be caused by seepage or piping through the dam or along internal conduits, earthquake and landslide generated waves in the reservoir. For a cascade of dams, the breaking of one dam may cause subsequent damage to other dams located downstream due to their overtopping. Partial or catastrophic failure of a dam leading to uncontrolled release of water causes severe damages to lives and properties of people situated downstream. The effect of such a flood disaster can be mitigated to a great extent, if the resultant magnitude of flood peak and its time of arrival at different locations downstream of the dam can be estimated, facilitating planning of the emergency action measures. The most suitable instruments for analysis and prediction of a dam break flood are mathematical hydrodynamic simulation models. These models can be used for prediction of dam breach flood hydrograph and its routing through downstream valley to obtain the time series of discharge and water level at different locations of the valley.


1.1 NEED FOR DAM BREAK MODELLING

The first European Law on dam break was introduced in France in 1968 following the earlier Malpasset Dam failure that was responsible for more than 400 causalities. Since then, many countries have also established requirements and in others, dam owners have established guidelines for assessment. In India, risk assessment and disaster management plan has been made a mandatory requirement while submitting application for environmental clearance in respect of river valley projects. Preparation of Emergency Action Plan after detailed dam break study has become a major component of dam safety programme in India.

The extreme nature of dam break floods means that flow conditions will far exceed the magnitude of most natural flood events. Under these conditions flow will behave differently vis-a-vis conditions assumed for normal river flow modelling and areas will be inundated that are not normally considered. This makes dam break modelling a separate study for the risk management and disaster management plan. The objective of dam break modelling or flood routing is to simulate the movement of a dam break flood wave along a valley or indeed any area downstream that would flood as a result of dam failure. The key information required at any point of interest within this flood zone generally includes.

»      Time of first arrival of flood water

»      Peak water level- extent of inundation

»      Time of peak water level

»      Depth and velocity of flood water (allowing estimation of damage potential)

»      Duration of flood

The nature, accuracy and format of information produced from a dam break analysis will be influenced by the end application of the data.


1.2 SCOPE OF WORK

As envisaged in the agreement, the title of the project is "Dam Break Analysis of Mullaperiyar and ldukki Dams (ldukki Arch Dam, Cheruthoni & Kulamavu Darns), Lower Periyar Dam & Boothathankettu Barrage up to Arabian Sea". The objective of the work is to carry out state-of-the-art Dam Break Analysis using appropriate Darn Break Model, which is Hydrologic Engineering Centre - River Analysis System (HEC-RAS) 4.1.0 version.

The scope of work consists of the following.

1.2.1 Part I: Dam Break Analysis of Different Scenarios of Failure

The state-of-the-art dam break analysis shall be carried out using appropriate dam break model. Different combinations of dam failures are possible. Seven dam break scenarios postulated as given in table 1.1, shall be carried out.



Table 1.1: Dam break scenarios

Dam Failure Combinations

 

 

 

Scenario

 

 

 

Idukki

 

Lower Periyar

Boothathankettu Barrage

 

Mullaperiyar

 

 

 

 

 

 

 

 

 

Arch

Cheruthoni

Kulamavu

 

 

I

Y

 

 

 

 

 

 

 

II

Y

 

Y

 

 

 

 


III

Y

 

 

Y

 

 

 

 

IV

Y

 

 

 

Y

 

 

 

V

Y

 

Y

Y

Y

 

 

 

VI

Y

 

Y

Y

Y

Y

 

 

VII

Y

-

Y

Y

Y

Y

 

Y

 

 

 

 

 

 

 

 

 

Note: Y= Dam / Barrage fails

River survey data consisting of river cross sections at 100-500 metres interval from Mullaperiyar Dam up to the downstream of Boothathankettu Barrage are supplied by the Irrigation Department for Dam Break Study. For topographical details, Survey of India maps in the scale of 1:25,000 are also supplied by the Department Flooding conditions for the above scenarios using an appropriate dam break model (such as National Weather Service DAMBRK Model or MIKE II or HEC-RAS model) shall be generated.

1.2.2 Part II: Inundation Mapping for Various Failure Scenarios

The present study includes the dam break flood analysis of the Mullaperiyar Dam up to idukki Reservoir.

Executive Summary

Analysis and discussion of results

Conclusions


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