City of San Marcos Sessom Creek Wastewater Capacity Improvements

Wastewater Line Construction to Prevent Frequent Overflows in the City of San Marcos

Project Purpose

An existing wastewater line running along Sessom Drive in San Marcos was identified as a system bottleneck and another section of the line experienced frequent overflows during wet weather conditions near the headwaters of the San Marcos River. To address this issue, KFA provided expert engineering services to the city of San Marcos. The project included over 2.5 miles of 15- to 30-inch diameter wastewater mains constructed through downtown San Marcos and Texas State University. Improvements to the system included removing and replacing portions of the existing wastewater line located within Sessom Drive and installing a bypass line along the remaining line. The improvements were constructed in three separate phases.

Project Approach

KFA provided preliminary engineering, design, bid, and construction services for all three phases of the project. A static model of the wastewater line was created to properly size the line using flows developed by the city of San Marcos’ zoning ordinance and GIS data obtained from the city. Phase I, located along University Drive and Sessom Drive, aimed to increase capacity by removing and replacing an existing wastewater line. Phase II, located downstream of Phase I and terminating at Interstate Highway 35, is a bypass line located within pavement to minimize impacts near the San Marcos River and nearby park. The final phase of the project is a bypass line located within North LBJ Drive and is located upstream of the Phase I improvements. Due to evidence of human habitation dating back 10,000 years, an archaeological survey was required to address these highly sensitive areas where increased concentrations of buried materials have been found. The San Marcos River is also home to many threatened and endangered species. As a result, the project required increased attention to minimize impacts to their habitat. An additional challenge was the protection of many large trees dedicated and planted by conservationist and The Heritage Association of San Marcos in the 1980’s. KFA addressed this challenge by adjusting the alignments based on a tree survey. As a result, only one tree was required to be removed.

Project Results

KFA designed a 3 phase solution for the removal and repair of the existing line as well as the construction of a parallel bypass line. The resulting 2.5 miles of pipeline is 15 to 30-inch diameter, with depths up to 26 feet. Project design and construction management/observation was required to avoid and monitor numerous environmentally and historically sensitive areas, as well as numerous large diameter trees along the alignment.