Pillot Gulley (K140) Natural Channel Design Stabilization

SWCA completed the design for a channel stabilization project for a 2,200-foot reach of Pillot Gully, a tributary to Cypress Creek and part of a linear greenway park system. SWCA developed both preliminary design alternatives and the final design for the project. The channel has undergone extensive downcutting and lateral erosion in response to straightening and over-excavation as part of flood control improvements, and extensive watershed modification from urban/suburban development. A large concrete grade control structure was installed in the 1950s and had failed within the last 10 years, resulting in exaggerated downcutting and several active head cuts moving upstream. SWCA was retained to develop a channel stabilization and restoration solution using NCD methodologies.

The final design focused on a Rosgen Priority 2 restoration, where the incised and entrenched stream is fully restored using NCD methods at its existing elevation with a nominal geomorphic (bankfull) floodplain bench and the original gully walls are over-excavated and laid back to a stabilized form to convey a larger, design flood event (e.g., 100-year event). The NCD stream restoration included constructed boulder riffles and log and boulder cross vanes, NCD grade control structures, and the use of toe wood for meander bend bank stabilization. HEC-RAS models were developed and used to both confirm “no-rise” conditions as a result of the project relative to regulatory flood elevations, as well as to evaluate both in-stream stream power and shear stress to confirm the validity of the design relative to sediment transport and erosion prevention. SWCA also provided permitting support for the project, including CWA Section 404/401 permitting through the USACE Galveston District and will also be providing engineering construction oversight.