CR 258 Extension from US 183 to Sunset Ridge

Williamson County, Texas

American Structurepoint provided PS&E plans for the CR 258 Extension, located in Precinct 2 of Williamson County. It consisted of extending the existing CR 258 at Sunset Ridge Drive to US 183 and included TCP design, grading, roadway design, drainage structures, SW3P, a Contributing Zone Plan (CZP), and pavement marking and signing. American Structurepoint’s engineers made adjustments to the roadway profile, ditches, and culverts to avoid utility relocation of the three existing petroleum product lines crossing the project’s path, resulting in approximately $2 million savings in relocation costs.

The project included a drainage impact study for mitigation of increased flows. An environmentally friendly solution of using ponding in the roadside ditches as a means of detaining stormwater saved the county substantial costs compared to designing and building a more expensive drainage solution, a pond. Five cross culverts were proposed for the new CR 258 project and a CZP was prepared since the project was in the Edward’s Aquifer Contributing Zone. Deliverables included a drainage impact study and a drainage report.

This project was undeveloped and a majority was new alignment. There were two existing culverts and the project replaced the existing two and added five more culverts. To perform the hydrologic analysis, a complex hydrology model was created using HEC-HMS to determine how the drainage areas combined along the project at each outfall. After modeling existing and proposed conditions, American Structurepoint determined the flows were very different. The model had common junctions for existing and proposed, and we compared the flows to identify where the flow was diverted. For the proposed design, there was flow being diverted at the request of the property owner resulting in increased flows and the client and FPA were notified and an impact study was performed. We revised the design to show that there were no impacts downstream of the project.

Seven cross culverts on CR 258 extension were design and outfall into Dyeus stream. We performed complex hydraulics using HEC-RAS and identified the impacts to the 100-year water surface elevation (WSEL) in the stream, a FEMA Zone A. To mitigate the increases in WSEL to the downstream subdivision, we used an innovative approach by using a ditch as a detention facility. We proposed an area where flow is stored within the ditch and outfall through a concrete weir structure. We added detention to the HMS model and included the stage storage discharge for each culvert. We calculated the approximate storage required to reduce the flow and identified the best ditch grading option with the least impact to right-of-way and was successful in reducing the flow at the outfall.