Earth fissures are a significant threat to various types of infrastructure in the Southwestern United States and elsewhere in the world. They are caused by differing subsidence rates and magnitudes associated with groundwater pumping in alluvial basins. In this article, Ravi Murthy and his co-author Jon Benoist, both with the Arizona Department of Water Resources, examine the impact that earth fissuring has on embankment dams used for flood control. One of their main dilemmas as regulators is: "Is it possible to safely rehabilitate dams in fissure risk zones to continue to provide economical flood protection to the public, or does the risk of dam failure due to an undetected fissure present too great a threat to the public?"

EMBANKMENT DAMS IN EARTH FISSURE RISK ZONES - A REGULATOR'S DILEMMA

Jon M. Benoist, P.E., Arizona Department of Water Resources, Phoenix, AZ
Ravi Murthy, P.E., Arizona Department of Water Resources, Phoenix, AZ

Abstract

Excessive groundwater withdrawal from alluvium-filled basins in the southwest has triggered large-scale ground subsidence in the Southwestern United States. Differential compaction of the alluvium has induced tensile stresses in the ground. Earth cracks or fissures have developed at locations where the induced tensile stresses exceed the tensile strength of the soil. Earth fissures associated with groundwater withdrawal and ground subsidence have been identified in Arizona, California, and Nevada. Historically, agencies such as the U.S. Army Corps of Engineers (USCOE) and the Soil Conservation Service (SCS) have avoided constructing, or have suggested abandoning embankment fill dams across known fissures and in areas of known high potential fissure risk. Presumably, this approach was taken to avoid potential catastrophic dam failure from erosion along a fissure in the dam foundation, or an associated embankment crack. Despite this cautious and conservative stance, single purpose flood control dams remain operational in developing fissure risk zones in Arizona.

In 2005, the Arizona State dam safety regulator is faced with thousands of people living in housing located immediately downstream of existing flood control embankment dams located in recently developed or developing fissure risk zones. The primary dilemma facing the state dam safety regulator: Is it possible to safely rehabilitate dams in fissure risk zones to continue to provide economical flood protection to the public, or does the risk of dam failure due to an undetected fissure present too great a threat to the public?

Typically, failure of a dam in an urban environment presents a significant risk of losing a large number of lives and extensive property damage. Clearly, a very low level of risk for a dam failure must be the primary objective for the state dam safety regulator, given his responsibility to protect the lives and property of the public. However, if dam rehabilitation is deemed to be unsafe due to fissure risks, the dam safety regulator must require the community to remove the existing dam. The only remaining alternative for the community is to construct significantly more expensive flood control alternatives, such as large floodways or flood basins. The problem is this path also results in risks to the public as it is likely that it will be years before funding is available to provide safe flood protection. Given the increased costs and immediate loss of flood protection, the removal of an existing dam will put the dam safety regulator at odds with the public as well as other government entities sensitive to the immediate needs of the public for adequate and economical flood protection. The authors believe that today's dam safety regulator has a dilemma in that they cannot simply categorize the presence or potential presence of a fissure risk zone as a fatal flaw for rehabilitation of an existing flood control dam.

The variables that the dam safety regulator must consider with regard to fissure risk zones are complex and include such issues as understanding fissure development, erodibility of the foundation soils, adequacy of state-of-the-art computer models and laboratory testing to assess magnitudes of erosion, adequacy of state-of-the-art monitoring to detect existing and developing fissures, and the ability to implement safe repairs to the foundation fissures after they are detected. Where all the factors indicate a safe dam rehabilitation, the dam safety regulator must still appreciate the major unknowns in the present state of the art for fissure evaluation. Advances will occur in the science and engineering related to building and maintaining safe dams in fissure zones. Thus, an essential part of the regulatory approval of a dam rehabilitation in a known or potential fissure risk zone is a periodic full reassessment of all technical factors to confirm the safety of the dam.

This paper discusses these issues from a regulator's point of view in light of two proposed embankment rehabilitation projects at single-purpose flood control dams within fissure risk zones in Maricopa County, Arizona.

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