TxDOT Hydraulic Modeling Guidance
Establishing best practices for 1D unsteady and 2D modeling
The Texas Department of Transportation develops and maintains multiple design manuals to make sure analysis and design are performed consistently and according to engineering best practices across the state. These manuals include the Roadway Design Manual (RDM), Bridge Design Manual (BDM), and the Texas Manual on Uniform Traffic Control Devices (TxMUTCD).
Our KFA drainage team had the opportunity to help TxDOT develop a new chapter for one of the design manuals we use the most here at KFA, the Hydraulic Design Manual (HDM). Our stormwater experts worked with the Hydrology and Hydraulics (H&H) Section of the TxDOT Design Division from 2018 to 2020 to develop a new chapter on 1D unsteady and 2D modeling guidelines.
The new chapter provides guidance and recommendations for the application of one-dimensional (1D) and two-dimensional (2D) unsteady hydraulic modeling for transportation projects. Steady state 1D hydraulic models are commonplace and have been industry-standard for many years across the United States. For transportation projects, steady state 1D models are typically used to analyze riverine systems and storm drain hydraulics; however, these models can be limited in analyzing more complex flow patterns due to the fundamental assumptions that flow is temporally static in quantity and can only move in a single direction. Unsteady 1D and 2D models can be used to gain a better understanding of hydraulic phenomena outside of the capability of steady state 1D models, such as storm drain overflow, open channel overflow, routing, storage, and varied flow directions. This chapter provides guidance for when to apply unsteady 1D or 2D hydraulic modeling methods as well as general best practices for model development.
KFA worked with TxDOT to create this new chapter of the Hydraulic Design Manual, which introduces designers to the concepts of 1D unsteady modeling and 2D modeling. The chapter discusses the commonalities and differences between 1D steady state modeling and 1D unsteady and 2D modeling principles. It also presents general theory and equations for riverine and storm drain systems. The chapter is intended to educate designers on new hydraulic modeling capabilities and design responsibilities.
As part of this effort, KFA led the team through development of tools to assist in choosing an appropriate modeling approach, including a hydraulic method selection flow chart and several example scenarios. The guidelines are “software neutral,” generally avoiding favoring or specifying one software model over another as both the public and private development of 2D modeling packages have experienced and will continue to experience rapid changes. Rather, the material focuses on education and supporting of selecting appropriate modeling approach for the design scenario and best practices for development.
To complete this project, the KFA team performed four major tasks:
- Conducted peer agency outreach including a literature review, an online survey, and phone interviews to learn about 2D model usage from other DOTs and public agencies.
- Developed guidelines and recommendations for 1D and 2D unsteady modeling, which were formatted for inclusion in the TxDOT Hydraulic Design Manual.
- Prepared an FHWA Everyday Counts Report, which compared inputs and outputs of unsteady and 2D hydraulic modeling software packages for common projects. Software packages evaluated included SRH-2D, XPSWMM, HEC-RAS 2D, HEC-RAS (steady and unsteady), CivilStorm, and ICM.
- Developed and led a two-day training course to 17 TxDOT hydraulics engineers that covered common processes in ArcGIS and 2D model development in HEC-RAS.
“This was a large project that took a large team to complete. The team and resources were managed excellently.” – Ab Maamar-Tayeb, PE, CFM, TxDOT Design Division H&H Section
As of 2022, the new HDM chapter on 1D unsteady and 2D modeling is currently under review by TxDOT for publication. Once incorporated into the HDM, it will help the drainage design community and TxDOT reviewers by providing guidance and best practices for using current modeling platforms.