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WWRC 89-10
Selecting Appropriate Flood Design Frequencies for Drainage Basins in Wyoming


1.1 Purpose and Tasks
The main thrust of this study is to develop a scientifically and technically sound procedure for determining the appropriate flood design frequency for drainage basins in Wyoming. The results of the study would provide a mechanism for selecting a design frequency for various roadway crossing structures such as culverts and bridges in Wyoming which can serve to complement the current Wyoming Highway Department (WHD) Design and Operating Policy.

Tasks performed to this point in time to accomplish the purpose of the study primarily involved (1) identification, definition, and collection of social, economical, and physical parameters and variables relevant to the design of highway drainage structures; (2) determination of a flood frequency associated with the least total expected cost (LTEC) design; (3) determination of an extended LTEC frequency by considering, in addition to the economic aspect, other intangible factors that affect the selection of design frequency; (4) development of a mechanism to relate LTEC design frequency and/or extended LTEC design frequency to economic, social, hydrologic/hydraulic characteristics of drainage basins which are typical to Wyoming. Types of highway drainage structures considered are box and pipe culverts and bridges.

It is important to note the fact that there have been several modifications and additions to the original proposal as the study evolved. The goal of the original proposed study was to develop some type(s) of empirical regression equations that relate LTEC to relevant economic, social, and drainage basin characteristics. Currently, it is felt that the scope can be extended to bring a new technology to bear on the development of an expert system or decision-support system to aid in the selection of a flood design frequency through LTEC. This would be the approach followed when this research project is continued.

The reasoning behind this change in scope was the fact that determination of an appropriate design frequency for highway drainage structures is an important element in the overall decision-making process and both tangible and intangible factors are important to the selection of the final design frequency in order to make a balanced decision between technical quantities (tangibles) and such items as environmental impacts, public convenience and legal liability of the state highway department (intangibles) in the final determination of the flood frequency to be used.

1.2 Scope of Report
This interim final report summarizes the work performed thus far for the cooperative research project entitled, "Selecting Appropriate Flood Design Frequencies for Drainage Basins in Wyoming" between the Wyoming Highway Department (WHD) and the Wyoming Water Research Center (WWRC). The project was terminated by the WHD on February 27, 1988. Therefore, this report contains the research work performed up to the termination date.

This report includes the following:

  1. Identification and definition of relevant variables and parameters in the least total expected cost (LTEC) design of highway drainage structures.
  2. Development of cost functions for pipe culverts, box culverts, and bridges.
  3. Collection of relevant information on basin and channel characteristics representative of Wyoming drainages.
  4. Collection of relevant information on economic variables in assessing flood damages.
  5. Factors and mechanisms for determining the extended LTEC design frequency.
  6. Identifying design and operational performance standards of Policy 18-6.

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