CTRMA US 183S Bergstrom Expressway

The Central Texas Regional Mobility Authority (CTRMA) is an independent agency created to improve the transportation system in the fast-growing Williamson and Travis Counties located in Central Texas. A key project for the region, the 183S Bergstrom Expressway serves as a critical infrastructure connector tripling corridor capacity, improving local travel times, and creating a nonstop, signal-free route to the airport.

The 8-mile project included three tolled lanes and three non-tolled general-purpose lanes in each direction from US 290 to SH 71. The project also added pedestrian and bicycle facilities, and aesthetic enhancements. Our team was contracted to prepare a preliminary drainage impact study for the project corridor. The drainage impact study assessed impacts at each outfall location, evaluated risks to assess potential mitigation, and assessed adequacy of right of way and easements. The team prepared the drainage section of the technical provisions of the design-build contract to establish drainage criteria and design requirements for the design-build developer.

The corridor transects five watersheds: Walnut Creek, Little Walnut Creek, Boggy Creek, Colorado River, and Carson Creek. Our team used the best available hydrologic and hydraulic models for these watersheds from the City of Austin and modified them for the project analyses. In addition, new models were created when those available from the City were not at an appropriate resolution for the study. Schematic bridge and culvert configurations were incorporated into the models, and differences between pre- and post-project peak flows and water surface elevations were assessed for locations upstream and downstream of the outfall crossings.

Our drainage engineers developed solutions to the following project challenges:

• Organizing Information: US 183S is a roadway that has been studied and reviewed frequently since the 1980s. As such, there was a large quantity of information to be reviewed and cataloged for our analysis, including as-built plans, proposed reconstructions that were never built, other drainage studies, the current schematic, City studies and data, and adjacent development plans. Once organized, this information provided valuable context for analysis.
• Existing Flooding Issues: The southern terminus of the project is the US 183S and SH 71 interchange, which is located within the main stem of the Carson Creek 100-year floodplain. The frontage roads and mainlanes are subject to flooding as culverts underneath US 183S and SH 71 are significantly undersized. While not within the scope of this project to resolve the extensive existing flooding issues at the interchange, our team worked with the project team to develop an interim approach that considered project constraints while preserving flexibility for future solutions to improve flooding at the interchange.
• Developing Adverse Impact Criteria: A challenge of all drainage studies and impact analyses is defining and developing consensus for the conditions that need to be considered to assess adverse impacts, as well as the criteria that need to be met to make a statement of “no adverse impacts.” Our team worked closely with the project team to develop language and specific requirements, some tailor to individual outfalls, for the technical provisions in an effort to reduce conflicts and during the design-build phase of the project.

The preliminary drainage study supported the project schematic, the Environmental Assessment document, and design-build procurement and development. In particular, the study provided a preliminary assessment of adverse impacts, recommendations for mitigation, and considerations for the final drainage study and evaluation that was prepared by the developer during the design-build phase. Our team supported the design-build developer procurement and later served in Deputy Design Manager and Drainage Oversight Lead roles.

Our tasks included leading the technical review process of all disciplines; working with all CTRMA GEC design oversight team to ensure each submittal met contract requirements; supporting public involvement; and providing construction design revisions reviews and resolution. During construction, we assisted the CTRMA and Owner/GEC oversight team with inspection, document control support, and construction phase engineering support. Our construction phase engineering support included review of design submittals, NDCs, and RFIs from the design-build contractor.