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Proposed Practice for Alternative Bidding of Highway Drainage Systems (2015)

Chapter: Chapter 8 - Hydraulic Evaluation

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Page 51
Suggested Citation:"Chapter 8 - Hydraulic Evaluation." National Academies of Sciences, Engineering, and Medicine. 2015. Proposed Practice for Alternative Bidding of Highway Drainage Systems. Washington, DC: The National Academies Press. doi: 10.17226/22157.
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Page 51
Page 52
Suggested Citation:"Chapter 8 - Hydraulic Evaluation." National Academies of Sciences, Engineering, and Medicine. 2015. Proposed Practice for Alternative Bidding of Highway Drainage Systems. Washington, DC: The National Academies Press. doi: 10.17226/22157.
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Page 52

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51 Hydraulic Evaluation During Phase 2A the user compares each evaluated pipe system’s hydraulic capacity to the hydraulic requirements of the drainage application. The Recommended Practice recommends conducting these evaluations not through detailed hydraulic design of each pipe system, but rather through comparison of pipe size (equivalent circular diameter) and pipe roughness. While independent rigorous hydraulic capacity evaluation for each pipe system is not considered necessary for most applications, verification of equivalency or the adequacy of the defined pipe roughness categories to adequately achieve all hydraulic requirements can be conducted in Phase 3A for critical drainage applications if desired. Minimum pipe diameters for standard roughness categories were established in Phase 1B. If the range of pipe roughness in the pipe system inventory can be adequately represented through grouping into one of the Manning’s n categories defined in Phase 1B then no further hydraulic evaluation is required and pipe systems are considered hydraulically acceptable if they meet the minimum and maximum pipe size requirements defined for the representative roughness category. If it is desired to hydraulically evaluate each pipe system’s specific Manning’s n value or to define the pipe size require- ments for additional pipe roughness categories not previously set in Phase 1B, these evaluations are recommended to be performed in Phase 2A using hydraulic equivalency charts. Starting from the baseline hydraulic designs completed in Phase 1B, the Manning’s equation may be used to determine the equivalent hydraulic capacity of different pipe materials under the drainage system flow conditions. An example hydrau- lic equivalency equation and chart are shown in Figure 25. The completion of the hydraulic evaluation step results in each pipe system option being rated as either hydraulically acceptable or unacceptable. This rating is then carried forward to the reporting and presentation of results stage. The systematic matrix approach of the Recommended Practice is used for tracking the results of each evaluation phase. While many of these aspects are accounted for in the evalu- ation of the baseline hydraulic design, the following design aspects are not incorporated directly into the Recommended Practice. These design considerations should either be set in Phase 1 or accounted for in the final design and policy checks completed in Phase 3: • Flow control and measurement • Low head installations • Siphons • Aquatic organism passage • Scour at inlets/outlets • Sedimentation and debris control • Multiple barrels • Perforated pipes As with the other technical evaluation steps, the results matrix presents the evaluation results from the hydraulic evaluation stage. The hydraulic matrix is denoted with “H” identifiers within the box for each pipe system type and high- lighted green in cases calculated to be suitable, or highlighted red and crossed out for pipe system options that do not meet the design criteria. C H A P T E R 8

52 Note 1. Chart based on Manning equation assuming pipe, full flow conditions, constant slope, and constant total flow 2. Actual flow conditions may vary and warrant a more detailed analysis Figure 25. Hydraulic equivalency chart.

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 801: Proposed Practice for Alternative Bidding of Highway Drainage Systems explores the application of a performance-based process for selection of drainage pipe systems. The selection process is based on satisfying performance criteria for the drainage system while considering the full range of suitable pipe materials.

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