Embankment Dams in Fissure Risk Zones

The Regulator’s Dilemma

Massive urbanization is presently occurring in the Southwest and is consuming agricultural lands and low density rural lands previously protected by flood control dams constructed under programs administered by the SCS and USCOE. Most of these dams extend for miles, crossing numerous, typically dry, alluvial fan drainages. Historically, these desert agricultural lands have been irrigated by over-drafting groundwater. Present groundwater levels have declined hundreds of feet resulting in surface subsidence and associated ground fissures in close proximity to the existing dams. These older dams have become at-risk for failure due to recently developed or developing fissure risk zones. These dams are even more problematic to the state regulator because they are now being reclassified from low or significant hazard, when protecting agricultural lands, to high hazard due to the downstream urbanization.

The Arizona State dam safety regulator is faced with thousands of people living in or moving into new housing developments located immediately downstream of existing, singlepurpose, flood control embankment dams located in known and developing fissure risk zones. These homeowners are often unaware of the need for flood protection or even the existence of the upstream flood control dam. Although owners typically conduct public outreach programs in advance of major dam and flood control projects, it is likely that many individuals living downstream are not aware of the threat to their lives and property from a potential dam failure where these older dams are located in existing or potential fissure risk zones. These homeowners are also likely to be extremely unhappy and vocal if the dam safety regulator requires the removal of an unsafe dam and their homes are suddenly declared to be without flood protection.

The regulator’s dilemma is complex and difficult to quantify: Can flood control dams in fissure risk zones be safely rehabilitated to the same level of risk associated with embankment dams located outside of fissure risk zones? If this is not practical, is a higher level of risk still acceptable? Should this risk be commensurate with the risk a community faces when it is without flood protection for a period of years, until funding is available for other more expensive types of flood control protection? Is it safe to rehabilitate these dams or must they be breached? The dam safety regulator obviously understands the need for community flood protection and will be at odds with the immediate community needs if it is determined that an existing flood control dam cannot be safely rehabilitated, especially if the community does not have readily available funding for other flood protection alternatives.

Historically, the construction of a new dam or the rehabilitation of an existing dam has not been allowed by regulating agencies in existing or developing fissure risk zones. Fissure occurrence in the foundation of an earth dam and or the potential extension of the fissure feature into the embankment has been considered a fatal flaw, since the dam will likely fail catastrophically due to erosion upon filling. Historically, there has not been a practical method of detecting fissures developing in the foundation of a dam and thus, there has been no practical means of monitoring the safety of a dam as impacted by the occurrence of a fissure. Since an unexposed fissure could not be detected, it was also not possible to implement the rehabilitation of a fissure in the foundation of a dam. The discovery of a fissure has typically been the sudden emergence of a large, linear surface depression associated with the collapse and erosion of the surface soils into the fissure from surface runoff. The dam safety regulator cannot accept a dam failure related to the sudden collapse and the erosion of an undetected fissure beneath a dam when a major flood suddenly fills the reservoir.

If dam rehabilitation cannot be safely accomplished, the dam safety regulator is forcing the community to pursue significantly more expensive alternatives such as a large floodway or flood basin project. Limited flood control budgets would stall development of control structures and any development would have to proceed such that the public would be in defined flood plains and incur millions of dollars in flood insurance costs. Given limited available funding and multiple projects requiring rehabilitation – is the greater public safety served by rehabilitating a dam to a failure risk that still may be of significance but is much less than that posed by the existing dam, or by waiting until public funding is available to construct a clearly low risk flood protection alternative, leaving the public exposed to the risk of flooding for a number of years before funding becomes available? Clearly, the dam safety regulator’s primary responsibility and objective is to be confident that the risk of a dam failure is very low and there is not an unacceptable risk to loss of life and property. If the dam safety regulator perceives that the risk is unacceptable and the dam must be breached, it is highly likely that this decision will be at odds with the lay public and potentially other government entities that are sensitive to the dissatisfaction of the public.

Recognizing the immediate need that the ongoing massive urbanization process has for adequate and economical flood control protection, the dam safety regulator must search for new and creative means to provide safe dams for flood protection. To provide economical levels of adequate flood protection the regulator is forced to seriously consider the construction or rehabilitation of dams in fissure zones. It is no longer possible to simply consider the occurrence of a fissure as a fatal flaw. The dam safety regulator in Arizona has had to ask the question with respect to rehabilitation of dams in fissure risk zones: Are elements present today within the context of investigations, structural designs, monitoring, rehabilitation, and future developments in all technical areas related to fissures that, when incorporated together, comprise a safe flood control dam?

The variables that the regulator must consider today in locating a dam in a fissure risk zone include many related elements. These elements are delineated below and are investigated and evaluated in close coordination with the owner and his design engineer.

  1. Are fissure development and occurrence understood? Is there a slow development or a sudden development of a fissure of a given width?
  2. Can potential fissure risk zones be identified based on surface settlement and subsurface geology?
  3. Is it is possible to remotely detect fissure development below the ground surface?
  4. What width of fissure can be remotely detected below the ground surface?
  5. At what depth below the ground surface can fissures be remotely detected?
  6. Which types of earth materials will express a fissure as a result of differential settlement?
  7. Which types and sizes of the components of earth materials are susceptible to large levels of erosion?
  8. Are the mechanics of fissure erosion understood? Do the same mechanics apply for a small amount of runoff versus the volume of water contained in a reservoir?
  9. Is modeling computer code developed to simulate erosion on a fissure? Are laboratory tests and field tests of soil erosion developed where the erosion parameters are accurate enough for input to the model computer code? Is the seepage flow gradient occurring initially and during erosion of a fissure understood and incorporated into the model computer code? Is cementation and micro-structure occurring in soils incorporated into the erosion parameters input to the model computer code?
  10. Can structural elements such as hardened dam sections or cut off walls and upstream aprons be adequately modeled and evaluated for increasing the safety of the dam against erosion failure on a fissure?
  11. Can state-of-the-art monitoring, such as GPS surveys, low sun-angle photography, time-domain reflectometry cables, etc. detect existing and developing fissures?
  12. Is it practical to safely remediate the adverse impacts of a fissure after it is detected in the foundation or reservoir of a dam?