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Section 3: Hydrology Policies and Standards

TxDOT uses the drainage practices and design standards described in this manual for designing drainage facilities and flood control works associated with transportation projects. They are also used for evaluating the design, construction, and performance of projects in TxDOT’s right-of-way.

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General Guidelines

The designer should keep the following in mind when using this manual:

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  • The standards and methods in this manual are aimed at sound planning and design; they are guidelines rather than steadfast rules.
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  • The design standards in this chapter represent minimums. Alternatives that meet a higher standard than presented herein must be approved by the TxDOT District Hydraulics Engineer (DHE).
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  • Exceptions to these design criteria may be allowed by TxDOT when they are in the best interest of the public and the alternative will be equivalent to the normally accepted method.
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  • Errors in peak flow estimates, whether due to computational errors or errors in judgment, may result in a drainage structure that is either undersized, which could cause drainage problems, or oversized, which costs more than necessary. On the other hand, any hydrologic analysis is only an approximation. Although some hydrologic analysis is necessary for all highway drainage facilities, the extent of such studies should be commensurate with the hazards associated with the facilities and with other concerns, including engineering, economic, social, and environmental factors.
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  • Design details are the responsibility of the design engineer and will be determined by good engineering practice.
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Third Party Studies

If third party studies (previous studies by others) are available and appropriate for the project area, TxDOT may, but is not required to, use these studies. In some circumstances, TxDOT may be required to use third party studies, such as for compliance with the Federal Emergency Management Agency’s (FEMA) National Flood Insurance Program (NFIP.) The use of third party studies should be cleared with the DHE.

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Hydraulic Design for Existing Land Use Conditions

All drainage facilities shall be designed for existing land use conditions. Drainage design must include capacity to convey runoff from all existing adjacent properties. The district office may consider runoff from future land use conditions at its discretion after consulting with and receiving approval from the DHE.

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Effect on Existing Facilities

TxDOT drainage projects must be designed so that they do not have a negative impact on existing facilities either upstream, neighboring or downstream of the project. A channel’s hydraulic grade line must be determined to evaluate the risks and potential impacts of flooding. The designer must define the upstream and downstream water surfaces until the effects of the new drainage facility match the pre-project hydraulic grade line. The proposed project must not induce flooding during the event for which it was designed.

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