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:
- Anchor: #LOMHJJJI
- Previous hydrology/hydraulic studies and reports Anchor: #GKKMGGIL
- Hydrological data (rainfall, gage data, flood history, etc.) Anchor: #NSLINKGH
- Site visit and reconnaissance Anchor: #NQLKMFLL
- Aerial/site photography Anchor: #GMKKHKNH
- Survey and mapping Anchor: #LMMJIFEM
- Land use, ground cover, soils information Anchor: #JGJLLLKG
- Fluvial geomorphic data (plan forms, bed and bank sediment characteristics, etc.) Anchor: #NJINIEEH
- As-built plans Anchor: #UINFMHFK
- Bridge inspection reports Anchor: #EGPEGJNL
- Maintenance reports
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.
Anchor: #i1020892Hydraulic 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:
- Anchor: #MHFJMFML
- A culvert that is to be extended to accommodate roadway widening Anchor: #EQIFFMII
- A culvert needing repair due to heavy corrosion Anchor: #NHUJELMK
- A bridge deck to be reconstructed or widened Anchor: #JMHIJLLM
- A cross drainage structure beneath a road that is to be reconstructed Anchor: #SGINLJHF
- A structure being retrofitted for fish passage Anchor: #ENMIKGLG
- An existing storm drain receiving additional or improved curb inlets Anchor: #LIHNIGGN
- A storm drain outfall requiring mitigation of scour problems
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.
Anchor: #i1020942Hydraulic 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.