1.0 Introduction

Nagarjuna Sagar Dam across the Krishna River in India

Dams have been part of human civilization for thousands of years and have one basic function – to impound water – though the reasons for this vary greatly. Dams are used for flood control, water supply, and hydropower in a variety of forms. The total number of actual water impoundments is essentially uncountable since they range in size from small ponds to huge reservoirs.

Usoi Dam, a Natural Landslide Dam Along the Murghab River in Tajikistan

In addition to human-engineered dams, dams also form through natural processes such as landslides, glacial moraines, ice, and animal activity.

Dam failures and the resulting floods have been around as long as dams have. Although small dams fail more frequently, the greatest concern is with large dams above densely populated regions.

Kariba Dam, a Hydroelectric Dam in the Kariba Gorge of the Zambezi River Basin Between Zambia and Zimbabwe

This lesson describes the characteristics of large dams and modes of failure including the roles of dam type and age. In the process we will discuss terminology associated with dams and dam breaches. We present simulations of dam failure impacts using the U.S. Army Corps of Engineers’ Hydrologic Engineering Center, River Analysis System (HEC-RAS) model, which is used throughout the world.

Homepage of the Hydrologic Engineering Center,  River Analysis System (HEC-RAS) website

Four simulations will provide examples of how the flows out of a reservoir are sensitive to the characteristics of a dam breach. The characteristics include breach size and the time it takes a breach to form. The simulations are based on the HEC-RAS model run for a dam in the U.S. state of Pennsylvania. Although they are based on the characteristics of that dam, the results illustrate important considerations common to dams around the world.

After completing this lesson, you should be able to:

  • Identify the characteristics of large dams and modes of failure associated with them.
  • Match reservoir and watershed characteristics with the type of model most likely needed to produce a forecast hydrograph during a major dam failure.
  • Explain the changes to headwater, tailwater, and outflow at a dam during a dam failure and how breach size and formation time affect these.
  • Describe how HEC-RAS model simulations can be used to explore the impact of breach size and breach formation time on the resulting downstream flows.
  • Describe the basic characteristics of downstream inundation and hydrographs associated with dam failures.