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Section 2: Stream Channel Planning Considerations and Design Criteria

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Location Alternative Considerations

The planning phase for a highway section usually involves consideration of a number of alternate highway locations, which often require construction across or along streams and floodplains. During the planning phase, evaluate the effects that location alternatives would have on stream systems. (See the Project Development Process Manual for more details.) Include a preliminary hydraulic study of the various alternatives because the type and cost of drainage facilities required can determine location selection. As project development proceeds, you may find that locations selected without adequate hydraulic consideration to floodplain encroachments and extensive channel modifications are unacceptable.

Consider the environmental effects, risks, and costs of required drainage facilities in the final selection of an alternative. Analysis of alternative alignments may reveal possibilities for reducing construction costs, flood damage potential, maintenance problems, and adverse environmental impacts.

Detailed information and survey data are seldom available for an in-depth hydraulic study during the planning phase; however, it is possible to ascertain basic requirements and consequences of a particular location or alignment and the relative merits of alternatives. Topographic maps, aerial photography, stream gage data, floodplain delineation maps, and a general knowledge of the area often provide the basis for preliminary evaluations of alternatives.

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Phase Planning Assessments

Consider the following factors:

Assessments may require the cooperative efforts of Area Office designers and Division personnel as well as others with experience on similar projects or specialized expertise in the particular area. Design all projects to comply with Federal and State regulations. As such, it is necessary to consider the implications of the following:

Refer to the Project Development Process Manual for more information on the above regulations.

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Environmental Assessments

Consult the Chapter 3, Environmental in Project Development Process Manual for environmental concerns. (See Texas Parks and Wildlife Department (TPWD), Clean Water Act (CWA), in the Environmental Procedures in Project Development Process Manual.) Consider stream channel modification only after examining all other alternatives. Regulatory requirements invoked by stream channel modifications can be substantial.

Consider the U.S. Fish and Wildlife Service (USFWS) review requirements where review may result in recommendations to avoid, minimize, or compensate for the adverse effects to wildlife habitat.

Refer to Chapter 3, Environmental for more information. It is prudent to plan measures to avoid, minimize, or compensate for stream modifications.

Justify the selection of a stream modification alternative. Consult with resource agencies early in design planning, and include these consultations in the Environmental Assessment Statement (EAS) or Environmental Impact Statement (EIS) with supporting documentation. (See Chapter 3, Environmental, in the Project Development Process Manual for more details.) The EA should also contain compensation plans for replacing any removed habitats. Avoid or minimize adverse effects, or implement mitigation plans to the best of your ability when transportation projects impact riparian corridors as described in the Fish and Wildlife Coordination Act (FWCA). (See Chapter 3, Environmental, in the Project Development Process Manual for more details.) If the department cannot offer mitigation for riparian corridor impacts, offer an explanation as justification in the environmental documentation.

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Consultations with Respective Agencies

During the planning phase, contact Federal, State, and local agencies in regard to plans or land uses such as the following that could affect the highway drainage design:

Consult the four agencies having regulatory authority over navigation and construction activities in waters of the United States and agencies with special expertise, such as in the limits and classification of wetlands, for preliminary information that may affect location decisions. The four agencies are as follows:

See References for contact information.

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Other Agency Requirements

An increasing number of federal and state permits are required for construction activities that may involve navigation and water quality. Program application for permits and approvals by Federal and State agencies having reulatory authority over streams early in the project development process. See Chapter 2, Section 2, Federal Laws, Regulations, and Agencies Governing Hydraulic Design.

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Stream Channel Criteria

Stream channel criteria include the following:

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  • Evaluate the hydraulic effects of floodplain encroachments for the peak discharges of the design AEP and the 1% AEP on any major highway facility.
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  • Avoid relocation or realignment of a stream channel wherever practicable.
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  • Match the cross-sectional shape, plan-view, roughness, sediment transport, and slope to the original conditions insofar as practicable.
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  • Include some means of energy dissipation when velocities through the structure are excessive or when the original conditions cannot be duplicated.
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  • Provide stream bank stabilization, when appropriate, to counteract any stream disturbance such as encroachment. Stabilize both upstream and downstream banks, as well as the local site. Refer to “Stream Stability at Highway Bridges,” FHWA-IP-90-014 for guidance.
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  • Provide a sufficient top width with access for maintenance equipment for features such as dikes and levees associated with natural channel modifications. Provide turnaround points throughout and at the end of these features.
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