The City of Cheyenne Board of Public Utilities (BOPU) provides water and sewer services to ~75,000 City of Cheyenne and surrounding rural residents. The BOPU’s highly complex, trans-mountain water basin system begins 110 miles west of the City, has a tunnel that travels underneath the Continental Divide, and delivers water across two mountain ranges before it reaches Cheyenne. Along this route, about 60 miles from the City, two BOPU pipelines travel underneath the Laramie River – one 26 inches in diameter and the other 30 inches, each with a water pressure of 600 pounds per square inch (PSIs).
The BOPU installed these pipelines in the 1960s, with various rehabilitations taking place throughout the subsequent decades. Over time, channel erosion, bank erosion, stream meandering and head-cutting caused significant, high-risk erosion over one of the pipes and threatened the other. As the capital city of Wyoming, this infrastructure is extremely important to Cheyenne – delivering 75% of its water and needing to be of the highest quality.
In brief, the BOPU was faced with water flow to 75,000 residents being disrupted and the entire system being permanently harmed. In need of precision, timeliness and expertise, the BOPU partnered with SEH for a number of strategic channel and bank improvements.
The SEH team provided HEC-RAS hydraulic modeling, revetment design calculations, construction plans and engineering estimates, as well as strategic public outreach to negotiate access agreements across private land and roads. Digging deeper, SEH created a bank stabilization design strategy that involved articulated concrete open cell blocks with revegetation, as well as additional bank stabilization using a riprap toe key. The team developed this design using a combination of articulated concrete blocks and riprap for bank protection, along with grouted boulder drops downstream from the pipe.
The team’s efforts also included permanent bend-way weirs – upstream of the pipe crossing – which were used to “train” the water flow to stay in the main channel and direct channel flows away from the bank. This strategy ensures the river no longer flows with full force along the bank, eliminating and preventing any threat of future channel or bank erosion.
This portion of the river is also a fish passage – with spawning beginning each October. With the understanding that they needed to be out of the river by the end of September to avoid disruption, the team developed a three-month construction plan and schedule. The team constructed a temporary diversion channel allowing bank stabilization to proceed in a dry environment. Following rehabilitation, the culverts diverting the flow were removed and the flow re-routed back to its original state – ensuring the bank and pipes were protected for the long term and the fish passage viable during spawning season.
Cheyenne Board of Public Utilities Laramie River Pipeline
Cheyenne Board of Public Utilities (Cheyenne, Wyoming)