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