Purgatory Creek Channel Improvements Project Moves Forward

By Jay Srinivasan

The Purgatory Creek Channel Improvements project moves forward with the City of San Marcos awarding a contract to civil engineering firm Lockwood, Andrews & Newnam, Inc. (LAN) to start work on the project’s next design phase.

Purgatory Creek carries flow from the entire Purgatory watershed through San Marcos ultimately reaching the San Marcos River through Children’s Park at Bicentennial Park. With the existing Purgatory Creek channel conditions, the areas adjacent to the creek are prone to flooding and street crossings within the city limits are overtopped during storm events, disrupting traffic.

In April 2018, the City hired LAN to develop a preliminary design to mitigate flooding conditions on Purgatory Creek from Wonder World Drive to its confluence with the San Marcos River, a distance of approximately two miles. In addition to flood mitigation, the City saw an opportunity to improve the quality of the water entering the San Marcos River and create sustainable, multi-use trails for the public through this project.

Other firms selected as subconsultants to join LAN’s project team include Hicks & Company for permitting, Asakura Robinson for landscape architecture, and Lady Bird Johnson Wildflower Center for ecosystem restoration.

The project has been divided into two phases with Phase 1 beginning upstream from Johnson Avenue and ending at the confluence of the San Marcos River. Phase 2 improvements will extend from Johnson Avenue upstream to Wonder World Drive. LAN completed the preliminary design on Phase 1 in March 2019. The next phase of design on Phase 1 – the 30 percent design – is currently underway and is expected to be completed in December 2020.

“The City of San Marcos has had a history of major flooding events including within the Purgatory Creek watershed,” said John Espinoza, P.E, CFM, City of San Marcos project manager. “With the support of City Council and its citizens, we are taking a holistic approach to address flood mitigation and to improve water quality and public space/connectivity. The Purgatory Creek project is one of the many important projects the City is currently conducting and gives us a great opportunity to partner with our stakeholders including local foundations and civic groups.”

The improvements will incorporate design features to establish a stable, natural channel that will support a sustainable eco-system and reduce maintenance requirements. These include a meandering earthen pilot channel with riffle pool complexes at strategic locations to provide for aquatic habitats, invasive species management, tree protection, and selective native plantings. Near the San Marcos River, high flows will be diverted through a new diversion channel/spillway providing a park amenity for river access. The project will include the establishment of “riparian” and/or “grow zones” to provide the transition of flows between the overbank and channel areas.

Improvements to the Purgatory Creek crossings will include bridge/culvert modifications or replacements at South Mitchell Street, Jackman Street, Guadalupe Street, South LBJ Drive, and South CM Allen Parkway crossings. A low water crossing modification will be incorporated at the Comal Street crossing to improve creek connectivity. Street improvements will include modifications to tie in the existing roadways with the vehicular bridge upgrades at South Mitchell Street, Jackman Street, South LBJ Drive and South CM Allen Parkway.

The project will also feature trail, sidewalk and pedestrian connectivity improvements. The path will be pedestrian and bike-friendly with access for maintenance vehicles along the trail. Trail design will include wayfinding and illumination and consider ADA-compliant guidelines. Trail connectivity along the San Marcos River will be maintained with the installation of the spillway at the Children’s Park and connect to trails at the Purgatory Creek Natural Area located west of Wonder World Drive. Sidewalks will be replaced for the crossings at South Mitchell Street, Jackman Street and South LBJ Drive on both sides of the crossing. Sidewalks on South CM Allen Parkway will be added on the Children’s Park side of the crossing. Sidewalks at S. CM Allen Parkway and S. LBJ Drive will be extended further than the crossing. Approximately 4000 linear feet of sidewalk will be added as part of this project.

The project includes modification to existing utility systems (water, wastewater, etc.) impacted by the channel improvements. In addition, the project includes extensive environmental investigations, permitting and approvals from various state and federal agencies, and the coordination and approvals from the Texas Department of Transportation and the Union Pacific Railroad. The project will also require the acquisition of right-of-way and easements.