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Section 2: Scope of Hydrologic and Hydraulic Activities

Scoping and reconnaissance are the investigative processes aimed at determining which issues are to be addressed by the project. Scoping initially identifies the major needs, issues, constraints, and feasibility of proposed improvements from which the more comprehensive, interdisciplinary preliminary engineering activities, surveys, investigations, environmental studies, and analysis can be effectively planned and budgeted. This includes the major elements of hydrologic and hydraulic work necessary to develop the project.

Reconnaissance is the collection of information that would generally be sought, collected, and used, as standard practices for the design whenever available and applicable. The following list includes broad categories for the H&H portion of the work:

The nature and extent of work proposed for drainage structures will affect the level of H&H analysis and the applicability of the standards and criteria presented in this manual. The scoping and reconnaissance effort should always include an appropriate assessment of the existing physical condition and the hydraulic performance of all drainage structures. A site visit is usually required for proper reconnaissance. The findings of the assessment will lead to recommendations as to whether existing structures should be replaced, rehabilitated, modified, abandoned, or left undisturbed.

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Hydraulic Considerations for Rehabilitated Structures

This chapter defines rehabilitated structures as existing structures that are not to be replaced, but may be substantially repaired, modified, or extended as part of the project. Common examples of rehabilitated structures include, but are not limited to:

If a structure is to be rehabilitated, the standards and criteria presented in this manual may not be feasible, applicable or appropriate because of constraints imposed by project budget, right-of-way, or schedule. However, the impacts of a rehabilitation project, whether safety or maintenance projects, must be considered and evaluated. Such projects, for instance may cause changes to the flood surface profile, stream stability, or increase flood risk to neighboring properties. In determining whether a variance from standards and criteria is appropriate, an assessment of the risk involved should be undertaken, as discussed in the next section. The complete replacement of an existing structure that has exhibited no history of past problems must be justified by a compelling reason; simply that it does not meet current hydraulic criteria for new design is not sufficient.

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Hydraulic Considerations for New Structures

The standards and criteria presented in this manual should be regarded as the minimum acceptable for projects involving new drainage structures or replacements of existing structures. Exceptions or variances may be justified by a risk assessment or detailed risk analysis. New and replacement structures should be, to the extent feasible, located, oriented, and sized so as to minimize the potential for hydraulic problems such as excessive scour or adverse impact on flood profiles.

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