Purgatory Creek Channel Improvements – City of San Marcos

San Marcos Texas

​LAN performed a watershed scale drainage study, modeling, improvement evaluation, and preliminary engineering design for a two-mile section of a flood control improvement project on Purgatory Creek, a major urban stream in San Marcos that outfalls to the San Marcos River. Purgatory Creek is characterized by frequent, high-risk flood events and depending on hydraulic conditions, Purgatory Creek overflows into Willow Creek which leads to significant increased flows downstream.

LAN provided a Preliminary Engineering Report (PER) with design recommendations to reduce flood risk for approximately 200 homes located along 6,100 LF of Purgatory Creek, which flows through the middle of the city. As part of this analysis, LAN delineated the existing and proposed floodplains to determine cost benefits and overall flood risk reduction to support allocation of CDBG-DR funding.

The channel design included a 180-foot-wide natural channel with a meandering pilot channel, and a high-flow overflow spillway to outfall into the San Marcos River and serve a dual-purpose as a park amenity for river access and natural water play area. A pedestrian trail was incorporated that includes numerous roadway and trail connections to tie into existing city infrastructure along with wayfindings that will make the trail more pedestrian friendly.

These solutions will reduce water surface elevations, improve water quality, reduce ongoing erosion and sedimentation issues, and provide a holistic approach to flood mitigation while also serving to add public amenities to the channel.

At a glance

Large diameter wastewater relocations
Spillway and flow diversion structure serves dual-purpose as park amenity
Individual 404 Permit
Environmental restoration in collaboration with City’s Habitat Conservation Plan


6,100 LF of natural channel design
1+ mile of trails connecting isolated pocket parks to the San Marcos River linear park system
Four road crossings featuring new bridge upgrades
Union Pacific Railroad (UPRR) crossing and coordination


Civil engineering
Hydrologic and hydraulic modeling
Floodplain delineation
Natural channel design
Permitting and regulatory requirements


City of San Marcos

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