Amargosa Creek Underground Conveyance

The Promenade at Amargosa Creek is a 545,000 square foot development project that will contain 1.2 million square feet of store and restaurant space encompassing a total of 157 acres.

“In order to prepare the land for development, we needed to enclose Amargosa Creek and infill the original open channel to provide a buildable site,” said Jon Cantrell with the City of Lancaster. “Amargosa Creek is a natural earthen channel that bisects the site and during large storm events, the creek alignment widens beyond its original dimensions. This uncontrolled event prevents the site from development without significant creek modifications.”

The City needed an underground conveyance channel that would be easy to maintain, not erode, provide for infiltration into local watersheds and provide for a buildable site. Originally, the City considered using reinforced concrete box (RCB) structures to underground the creek; however, they realized that standard RCBs may be cost prohibitive and prevent water recharge.

After researching cost effective and long-lasting channel alternatives, the City worked with Penfield & Smith, the engineering firm on the project, to design both precast concrete and metal plate arches for the project.

A 3,705 foot, 28-ft. x 10-ft. SUPER-SPAN™ plate structure was used for the majority of the channel alignment. Future plans for the site include the construction of a medical center, thus a portion of the channel required a precast structure to support the building loads. A 535 foot long, 28-ft. x 10-ft. CON/SPAN^® Bridge System was used where the channel crossed under roadways and future building sites.

“The invert of these arch culverts was lined with ArmorFlex^® articulating concrete porous pave blocks to allow for the stormwater runoff to infiltrate through while also providing maximum scour protection,” said Brianna Daniels, Senior Engineer with Penfield & Smith. “They also provided for great infiltration and easy maintenance.”

The ArmorFlex blocks provide for a firm surface to accommodate City maintenance equipment via a maintenance ramp that was fenced and landscaped to provide safety and pleasant aesthetics. Cass Construction installed 71,268 square feet of 50L blocks and 36,932 square feet of 70L blocks.

“We were very comfortable with the SUPER-SPAN, CON/SPAN and ArmorFlex products throughout the design phase,” said Ray Hunt, Capital Engineering Manager with the City of Lancaster. “Our main concern was getting a solid contractor through the bid process and Cass did a great job for us. The products were installed like we expected and it went without a hitch.”

“The speed with which the products were installed, as well as outstanding project coordination with the City’s Public Works Department, were extremely beneficial in limiting the exposure inherent to working within a federally regulated waterway during the rainy season,” added Matt Goubeaux, Estimator / Project Manager with Cass Construction. “In fact, the project was completed not only on time, but a few weeks ahead of schedule.”

The undergrounding of the earthen channel also brought up many environmental concerns and right-of-way issues that required the involvement and coordination of several government agencies, utility companies and property owners.

“In addition to hydraulic analyses and FEMA coordination, two Streambed Alteration Permits through the Department of Fish and Game (DFG) were prepared,” said Cantrell. “We then designed and constructed mitigation improvements including landscaping the side slopes of the Creek with native plant material.”

Construction for the Amargosa Creek Underground Conveyance Project began in June 2009 and was finished by December 2009. The system is now fully operational from a flood control standpoint, and the City is currently completing the final landscaping. The SUPER-SPAN, CON/SPAN and ArmorFlex products have already proven successful as the current water flow has now been controlled and there has been no scouring, a factor which was critical for successful implementation. The City was also acknowledged by local water purveyors for their porous invert design, allowing for local water recharge.