ASDSO, 2005). As a result of these inspections, many dams have undergone safety modifications for hydrologic, seismic, and other deficiencies. But efforts to improve dam safety are not complete; about half of the dams that should have EAPs do not (Altinakar et al., 2008; see Box 3.2), and there is a backlog of safety repairs to be addressed. Moreover, improving safety needs to be a continuing and adaptive process that is responsive to changing structural and societal conditions. The concept of safety among dam and levee professionals has not evolved beyond reducing the likelihood of failure.

BOX 3.1

Dam and Levee Hazard Classification

The National Inventory of Dams (NID) hazard classification system (see Table 1) is broad, qualitative, and based on the potential threat to life and property in the event of dam failure. The criteria for inclusion in the inventory are provided in Table 2. A dam is given a "high" hazard rating if its failure can result in fatalities, whether the dam is small or large and has the potential for a single or thousands of fatalities. The rating is also regardless of its condition (e.g., its likelihood of failure). Current emphasis is appropriately on high-hazard dams, but there can be a wide disparity in the consequences of failures of these structures. Other consequences of dam failure, such as economic and environmental losses, are qualitatively evaluated and defined in equally broad terms. The hazard classification process does not include an assessment of the sociological or other effects on a community, nor does it consider the broader local and regional effects (economic and other) of the loss of a critical infrastructure (power, water supply, flood protection). Hazard classification is assigned primarily by state or federal regulatory agencies. The Federal Emergency Management Agency guidance states that classifications "should be based on the worst-case, probable scenario of failure or mis-operation of the dam, i.e., the assigned classification should be based on failure consequences that will result in the assignment of the highest hazard potential classification of all probable failure and mis-operation scenarios" (FEMA, 2004b, p. 7).

TABLE 1 Hazard Classification for Dams


LifeLikelihood of Economic, Environmental, or

Hazard Classification

Likelihood of Loss of Human Life

line Loss


Low

None expected

Low and generally limited to owner

Significant

None expected

Yes

High

Probable; one or more expected

Yes (but unnecessary for this classification)


SOURCE: FEMA (2004b).

TABLE 2 National Inventory of Dams, Dam or Reservoir Size Criteria


Categorya

Criteria

Excluded


Dam height

Over 25 ft

6 ft or lower, regardless of reservoir capacity

Reservoir size

At least 50 acre-ft

Maximum, 15 acre-ft or less, regardless of dam height

Hazard

Any dam that poses a man life or property in “significant threat to hu- the event of its failure”


_____________

aHeight is measured from the dam crest to the downstream toe; size is reservoir impoundment capacity. SOURCE: USACE (2011a).

DAM AND LEVEE INFRASTRUCTURE

Before a community can address risks associated with dam or levee failure, it must know that a dam or levee is present and poses risk. Information on dam and levee location, physical properties (e.g., size and type), design requirements, ownership, maintenance responsibility, and regulatory framework is critical for understanding the hazards and risks



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