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Section 4: Design Activities by Project Phase

The following section describes the hydraulic design activities typically expected to occur in each phase of a project. The descriptions are largely derived from the TxDOT Project Development Process Manual.

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Planning and Programming

One of the objectives of the planning and programming phase is to develop a planning-level cost estimate. Certain projects involving significant drainage-related challenges may require some initial hydrologic and hydraulic investigation in order to appropriately estimate the nature and approximate size of required drainage structures for estimating purposes. The DHE should be consulted during the planning and programming phase to assess whether drainage issues will pose significant challenges to the project.

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Preliminary Design

In the preliminary design phase, the DHE should expect to participate in the Design Concept Conference to provide general background information on hydrology and hydraulics, and to identify major drainage features and regulatory constraints.

Drainage-related agreements and contracts that TxDOT has with other agencies need to be identified and taken into consideration during the preliminary design phase. Identification of existing agreements also helps determine the possible need for additional agreements. Some agreements may need to be amended and the appropriate division can assist. The Right of Way (ROW) and DES are involved with coordination of existing drainage agreements and in determining the need for additional agreements.

The locations and sizes of proposed cross-drainage structures (bridges and culverts) must be determined early in the preliminary design phase because of their potential to affect the roadway profile and other elements of the preliminary design of the project. Preliminary hydraulics analyses for bridges will enable the determination of the bridge limits, span/girder type, span lengths, bent locations, and bent orientation. An important aspect of the hydraulic analysis at this stage is consideration of the NFIP and whether the project will cross an NFIP designated Special Flood Hazard Area (SFHA).

Since many of the design parameters for drainage structures are to be established during the preliminary design phase, it is necessary to conduct the bulk of the hydrologic and hydraulic analysis during this phase. These analyses will usually include, but may not be limited to:

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  • Field reconnaissance
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  • Collection of relevant data on the stream and watershed
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  • Gathering of relevant previous hydrologic studies by TxDOT and other entities
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  • Conducting required hydraulic surveys of existing structures and streams
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  • Obtaining available topographic mapping of the streams and floodplains
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  • Establishing the relationship between flood discharge and AEP through hydrologic analysis or by adopting previous hydrologic analyses
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  • Determining stream flood profiles for existing conditions through hydraulic modeling
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  • Determining required sizes of drainage structures to meet design criteria
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    • For bridges this includes establishing preliminary opening size, span lengths, pier locations and girder elevation
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    • For culverts preliminary design of opening size and profile is performed
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    • For storm drains this includes preliminary design of trunk alignment, size and profile
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  • Estimating stream flood profiles under proposed project conditions (potentially for multiple design alternatives) to determine project impacts
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  • Adjusting proposed structure designs as necessary to mitigate project impacts

All projects affecting a waterway used for navigation require coordination with the USCG and the USACE. Hydraulic investigations or design may also be required for ensuring compliance with the USCG and USACE regulations.

See the TxDOT Project Development Process Manual, Chapter 2 for further discussion of the preliminary design phase.

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Preliminary hydraulic studies are needed in the preparation of environmental documentation to evaluate the impacts of the proposed project on waterways and floodplains. Changes in water surface elevation, construction in channels, bridge construction methods, etc. commonly impact water resources. The identification of appropriate temporary and permanent stormwater quality best management practices may require input from the DHE and the District Environmental Quality Coordinator during the environmental documentation phase.

See the TxDOT Project Development Process Manual, Chapter 3 for further discussion of the environmental permitting and documentation phase.

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PS&E Development

The Design Concept Conference marks the beginning of Plans, Specifications, and Estimates (PS&E) preparation and occurs after most of the background data is gathered and the preliminary hydrologic/hydraulic analysis and design is complete.

As part of the detailed design process, stream crossing hydrology and hydraulics should be refined and finalized. Refinement is usually needed to reflect detailed field survey data, changes in basic design conditions or assumptions, or to reflect revised methodology if there has been a significant delay between schematic development and PS&E development.

The FHWA requires a bridge scour evaluation during the hydraulic design process for all bridges. The results of a scour evaluation may highlight the need for design adjustments such as increasing the opening size, deepening the foundations, adding pier or abutment protection, or incorporating other mitigation measures. Scour countermeasure design is to be performed or directed and approved by the Geotechnical Section of the Bridge Division (BRG).

In addition to bridge hydraulic design and scour evaluations, a number of other H&H tasks are required as a project design is being finalized. These tasks include, but are not limited to:

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  • Refining the hydraulic design of culverts to finalize sizes, invert profile, end treatment and outlet protection;
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  • Preparing final storm drain details including design of appropriate sized inlets at the proper spacing and lateral sizing;
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  • Preparing pump station details for projects involving pump facility construction: and
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  • Preparing or contributing to the development of Stormwater Pollution Prevention Plans (SW3Ps) after the roadway drainage design is completed

Finalized hydraulic calculation sheets and hydraulic reports should be reviewed by the district and then submitted to DES-HYD for review and approval before PS&E submittal.

See the TxDOT Project Development Process Manual, for further discussion of the PS&E development phase.

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