Section 2: Planning and Location ConsiderationsAnchor: #i1006508
Generally, a stream crossing location is selected during the planning and location phase of a highway project. The final location should be selected only after obtaining detailed survey information and completing preliminary hydraulic studies. Although they are not the sole consideration in bridge location and design, hydraulic aspects should receive major attention in the initial planning of the highway. The location and alignment of the highway can either magnify or eliminate hydraulic problems at the crossing. Adverse conditions should be identified in the early stages of new location selection so that potential problems receive adequate consideration. If the cost of the required structures is prohibitive, consider rerouting the highway.Anchor: #i1006563
Location Selection and Orientation Guidelines
Bridge location and orientation requirements are covered in general in the Bridge Project Development Manual. The specific hydraulic requirements are covered below:
- The bridge should be centered on the main channel portion of the entire floodplain. This may mean an eccentricity in the location with respect to the entire stream cross section, but allows for better accommodation of the usual and low flows of the stream.
- The bridge waterway opening should be designed to provide a flow area sufficient to maintain the through-bridge velocity for the design discharge no greater than the allowable through-bridge velocity.
- The headers and interior bents should be oriented to conform to the streamlines at flood stage. Standard skew values of 15°, 30°, and 45° should be used where feasible. The piers and the toe of slope of the header must be located away from deep channels, cuts, and high velocity areas to avoid scour problems or interference with stream low flows.
- Consider including either relief openings or guide banks if the intrusion of either or both roadway headers into the stream floodplains is more than about 800-feet.
- Existing vegetation should be incorporated into the overall bridge plan. Where practicable, trees and shrubs should be left intact even within the right-of-way. Minimizing vegetation removal also tends to control turbulence of the flow into, through, and out of the bridge.
- For some configurations, roadway approaches may need to accommodate overflow. Such overflow approaches allow floods that exceed the design flow to overtop the roadway, thereby reducing the threat to the bridge structure itself. Protection of the approaches from overflow damage should be considered.
Environmental impacts must be considered along with the hydraulic issues as one may directly affect the other. (See the Environmental Manual for details.)
Biological considerations in site selection include the effects on habitat and ecosystems in the floodplain, stream, and associated wetlands. Biologists should assess this aspect of site selection, but provide much of the information necessary for a valid assessment of the biological effects and the available alternatives for mitigation, including the following:
- economic viability of using a bridge rather than filling in wetland areas
- cost to replace lost marsh or wetland areas
- circulation of fresh or brackish water in marshes and estuaries
- feasibility of providing mitigating measures for the loss of invertebrate population
- shade and resting areas for fish.
Water Resource Development Projects
Water resources development projects such as reservoirs or stream channel modifications, whether existing or only potentially planned, must be considered when selecting a stream crossing location. Planned resources development projects often require the relocation or reconstruction of existing highways and can interfere with the location or design of proposed highway-stream crossings. Many water resources development projects are planned or authorized for periods of years or even decades before construction begins. Others never come to fruition or may be permanently stopped by court decisions or regulatory agency actions.The roadway designer must carefully plan and construct a highway facility near a water resources project location, designing the highway facility for both existing and future site conditions. The excess cost of building the facility due to the water resources project must be considered in selecting the stream-crossing site. See Chapter 12, Reservoirs.Anchor: #i1006692
FEMA Designated Floodplains
The majority of highway crossings involve floodplains that are in FEMA-participating communities. It is important to acknowledge FEMA floodplains in the planning phases of a project and accommodate them in design. Early coordination with the community's NFIP administrator is essential to identify and avert potential problems. See Chapter 5, Federal Emergency Management Agency (FEMA) National Flood Insurance Program (NFIP) Compliant Design of Floodplain Encroachments and Minor Structures.Anchor: #i1006871
All streams change with time. Planning, roadway and bridge design engineers should be conscious of stream morphology and be aware that methods are available for quantifying natural changes and changes that can occur as the result of stream encroachments and crossings. See Chapter 7, Channels.Anchor: #i1006945
Procedure to Check Present Adequacy of Methods Used
Methods to analyze the hydrology and hydraulics at bridge sites continue to improve. In many cases, a method used in the original analysis is no longer an appropriate method. The following steps should be used to examine the adequacy of the method:
- Examine the adequacy of the analysis for the original crossing design before undertaking major reconstruction or replacement.
- If the method originally used is no longer appropriate, recalculate the analysis for these crossings using an appropriate one.
- Reconsider the risk of failure of the existing structure,
including the following:
- increased traffic volumes
- changed traffic service requirements
- increased highway construction and maintenance costs
- liability for damages to property that could be attributed to the highway crossing.