The Bureau of Reclamation’s New Downstream Hazard Classification Guidelines

Abstract Only - A downstream hazard is defined as the potential loss of life or property damage downstream from a dam and/or associated facility (e.g., dike) due to floodwaters released at the Structure or waters released by partial or complete failure of the structure. Within the Bureau of Reclamation (Reclamation), dams are given a hazard classification for two main reasons: 1. The Department of Interior Departmental Manual establishes that a hazard classification is to be assigned to every DOI dam. 2. Hazard classification serves as a management tool for determining which dams are to undergo the full SEED (Safety Evaluation of Existing Dams) process. Dams having a low downstream hazard classification are excluded, whereas those having a significant or high downstream hazard classification are included. Downstream hazard classification is important as a management tool because it could be the deciding factor that determines whether a formal safety evaluation and possible modification are performed on a dam. For any dam, a situation can always be imagined that would result in loss of life regardless how remote the location of a dam, and/or how little the chance of persons being affected by its failure flood. Thus, guidelines can be very useful in these situations to avoid being unduly conservative and to provide consistency to hazard classification as much as possible. While downstream hazard classification may be obvious for many large dams, it often requires detailed analysis combined with good judgment for small dams. However, detailed analysis does not always result in a firm hazard classification. Many unknowns exist with regard to structural damage to buildings, roads, occupancy, behavior of persons threatened by flooding, etc. Due to these unknowns, agency policy and guidance are important to give objectivity and consistency in assigning a downstream hazard classification. New Reclamation guidelines have been developed for use in determining a dam’s downstream hazard classification. Although these guidelines are intended to be used for all dams, they are especially useful for dams whose failure 93 flood would affect only a small population (e.g., small dams ). The purpose of the guidelines is: (1) to define the SEED method for assigning a dams downstream hazard classification; (2) to provide guidance and present methods, for the purpose of downstream hazard classification, for estimating the downstream area susceptible to flooding due to a dam failure; (3) to provide guidance and criteria for identification of downstream hazards; and (4) to bring objectivity and consistency into downstream hazard classification. One major feature of the guidelines is a set of five depth velocity- danger level curves for use in helping the analyst assess lives-in-jeopardy. These curves are plots of flood depth versus velocity and are made up of three zones: low danger, judgment, and high danger. The curves are presented for the following situations: (1) houses on foundations, (2) mobile homes, (3) passenger vehicles, (4) adults, and (5) children. The guidelines recommend use of the National Weather Service Simplified Dam-Break model (SMPDBK) for estimating flood depths and velocities, and offer specific criteria for breach parameters. SMPDBK is recommended because of its ease of use together with reasonable accuracy. However, it is cautioned that the analyst be knowledgeable of SMPDBK’S limitations. Other methods should be used, such as the National Weather SeNice DAMBRK model, when more accurate results are desired and/or the analyst has the background~ time, and resources needed to apply such methods. Additional information regarding dam-break/inundation studies is provided in the appendix Reclamation’s Downstream Hazard Classification Guidelines can be obtained by sending a request to: Bureau of Reclamation, Denver Office, P.O. Box 25007, Denver, Colorado 80225, Attention: D-7923A.