Dam Systems
During Hurricane Katrina a number elements of the Hurricane Protection System (HPS) that was intended to protect the city failed, resulting in extensive and loss of life. A significant lesson learned from these events and the follow-up IPET1 study was recognition the HPS was a system in name only. It was not designed, operated or maintained as a system and as a consequence did not perform as one.
The safe and reliable performance of a dam system involves a broad system of activities that includes, the philosophy and standards used to design all the structures and components, the methods of construction, the owner’s approach to project and risk management, the dam operating and emergency procedures, maintenance practices, methods of monitoring and condition assessment, and regulatory oversight. A dam system should be considered in terms of the physical, operational, and management infrastructure that is responsible for safe retention of the reservoir and reliable system performance.
The systems aspect of dams is recognized during dam incidents which typically involve a combination of events related to the performance of different structures and components, operator actions, post-event emergency operations, etc. The performance of the Lower San Fernando Dam during the 1971 earthquake is a good illustration of this. While the embankment failed due to liquefaction, there was no uncontrolled release of the reservoir.
Figure 1 View of the Lower San Fernando Dam prior to the earthquake. Note the presence of two intake towers. Also notice the population-at-risk just below the dam.
Figure 2 View of the failed upstream embankment of the Lower San Fernando Dam after the earthquake and the reservoir had been lowered. Note the presence of only one of the two intake towers.
Figure 3 Cross-section view of the Lower San Fernando Dam at the time of the earthquake.
The scenario that played out prior to, during and following the earthquake resulted in the reservoir being safely lowered, preventing a breach and uncontrolled release of the reservoir involved a combination of random (unrelated) events as well as events that were a direct effect of the earthquake. The sequence of events that played out are:
- A downstream berm is added to the embankment (reasons unknown), and
- California Division of Safety of Dams restricts the reservoir level due to seismic safety concerns, and
- The restrictions are lifted just prior to the earthquake in January 1971, and
- The winter is dry in terms of the rain that occurs in southern California, so the reservoir level remains low, and
- Since it is winter, available water that will fill the reservoir is stored as snow in the mountains, and
- An earthquake of magnitude 6.6 occurs close to the dam, and
- Liquefaction occurs in the embankment and a significant slide occurs that results in a loss of most of the available freeboard, and
- One of the intake towers fails, likely a result of the embankment failure, and
- The other intake survives, and
- Seepage/piping in the damaged embankment does not occur, and
- Aftershocks do not lead to additional damage and breaching, and
- The Upper San Fernando Dam survives the earthquake, thus not releasing the contents of its reservoir into the Lower San Fernando Dam reservoir, and
- The reservoir is lowered in a timely manner, averting an uncontrolled release of the reservoir.
Aspects of the dam system that were involved in this
incident include the design and method of construction of the dam, the design
and seismic integrity of the intake towers allowing one of the towers to
survive, the seismic integrity of the intake valves, timely response of the Los
Angeles Department of Water and Power to arrive at the dam after the earthquake
and to immediately lower the reservoir to prevent further damage and possible
uncontrolled release of the reservoir.
1Interagency Performance Evaluation Team, Executive Summary Report, June 2006.