BACKGROUND

Highway drainage structures often are located near the confluence of two streams where they may be subject to inundation by high flows from either stream. These structures are designed to meet specified performance objectives for floods of a specified return period (e.g., the 100-year flood). Because the flooding of structures on one stream can be affected by high flows on the other stream, it is important to know the relationship between the coincident exceedence probabilities on the confluent stream pair (i.e., the joint probability of the coincident flows).

Accurate estimates of the joint probability of design flows at stream confluences are a crucial element in the design of efficient and effective highway drainage structures. No accurate generally accepted estimation procedure for determining coincident flows exists for use in the design of highway structures. A practical procedure for the determination of joint probabilities of design flows at stream confluences is needed.

OBJECTIVE

The objective of this research is to develop practical procedures for estimating joint probabilities of design coincident flows at stream confluences and guidelines for applying the procedures. The scope of this research is limited to riverine areas and does not include coastal areas.

Accomplishment of the project objective will require at least the following tasks.

TASKS

Phase I (1.) Critically review the literature pertaining to joint probabilities for flows at stream confluences. Identify and summarize procedures used to estimate joint probabilities of coincidental events (e.g., hydrologic, traffic-flow, seismic, and other stochastic processes). (2.) Critically review the procedures identified in Task 1 and evaluate the suitability of promising procedures for use in hydrologic modeling by highway agencies. Determine the data requirements, data availability, and the level of effort needed to apply each procedure. (3.) Using available stream-gauge and other hydrologic data, identify confluent stream sites with concurrent data of sufficient record to estimate joint probabilities. The sites should be regionally diverse throughout the United States and include a wide range of basin characteristics and hydrologic conditions. (4.) Propose the development of one or more innovative procedures or refinements of existing procedures for estimating joint probabilities of design coincident flows. For each proposed procedure, address the availability of data, ease of use, statistical robustness, and accuracy of the procedure. (5.) Submit an interim report documenting the work completed in Tasks 1 through 4. The interim report should include the sites identified in Task 3 and a detailed description of the Task 4 proposed procedures. Include an updated, detailed work plan for completing Phase II of the research as a separate appendix to the interim report. Meet with the NCHRP panel to discuss the interim report and Phase II work plan. Work on Phase II will not begin without approval of the NCHRP panel.

Phase II (6.) Collect and compile stream-gauge and other hydrologic data from the sites identified during Task 3 and agreed upon during the interim panel meeting. Archive the data in an electronic database. (7.) Using the Task 6 data, develop the practical procedures proposed in Task 4 for estimating joint probabilities of design coincident flows at stream confluences for riverine areas. (8.) Validate the Task 7 procedures. Refine the procedures as necessary. Rank the procedures in order of predictive abilities (e.g., bias, standard error, or mean square error) and develop guidelines for applying the most promising procedures. The guidelines should address the regional applicability of the procedures. (9.) Submit a final report documenting the entire research effort. The final report should include illustrative examples showing the application of the procedures and the ranges of variables.