By Scott Manning
This month, the Middle Rio Grande Conservancy District (MRGCD) will conduct a study of the Corrales Siphon to determine what maintenance work will be needed in coming years. The Corrales Siphon is a 70 year old wooden, barrel-like pipe that runs beneath the Rio Grande, bringing irrigation water diverted on the east side of the river to the Corrales Main Canal on the west side. Ditch bank roads along the easternmost part of the Main Canal will be closed as work gets underway.
In 2016, the MRGCD, in a partnership with the U.S. Bureau of Reclamation performed maintenance work on the siphon that aimed at protecting the pipe from further damage. It is exposed in the river, and this threatens its stability on its western side. To address this concern, the MRGCD completed stabilization work on the siphon by depositing rip rap in the river next to the wooden culvert. Small boulders were placed downstream of the old wooden pipe to restore the eroded sediment surrounding the siphon. That also diverted the flow of the river away from the exposed western side of the siphon to the eastern side of the river. These changes helped to stabilize the siphon and prevent excessive strain on the culvert.
As part of the project, the MRGCD completed other maintenance efforts in the area. The district improved steel fencing and gating to improve access to the bosque at the location of the siphon.
These maintenance efforts came in around the projected budget of $200,000. The project was cost-effective in part because the Albuquerque Metropolitan Arroyo Flood Control Authority (AMAFCA) donated boulders for the maintenance project. In addition, the MRGCD sought the assistance of the Bureau of Reclamation because it has experience in completing large-scale construction efforts in water environments.
Hamman explained that the siphon required maintenance efforts because the flow character of the Rio Grande has changed over the past few decades. When the pipe was constructed, it was originally buried approximately 14 feet beneath the riverbed. The siphon is designed to endure an aquatic, submerged environment, but the pipe is not engineered to withstand the exposure and stresses of the river water pouring over it.
The siphon became exposed after years of erosion of the river bed as the Rio Grande continued to cut deeper. The flow of water in the Rio Grande changed when Cochiti Dam was constructed upriver. The dam reduced river flows and trapped sediment, and these changes caused the Rio Grande to degrade and to erode downward. This downward erosion is observable on the incised banks of the river.Some impacts of the 2016 maintenance efforts are observable. Before that, the exposed siphon created rapids on the western side of the Rio Grande. Now that the rip rap diverts the main flow of the river, the rapids have changed location but still are present.
The MRGCD has monitored the siphon in recent years of high and low runoffs, and it seems to be stable. But Hamman says that a full study of the siphon is in order. After the irrigation season ends this year, the MRGCD will conduct a camera-run of the siphon in which experts examine the interior of the pipe for damage. The MRGCD must also examine the pipe in areas of high flow concentration to assess stability.
The repairs completed in 2016 have given the MRGCD time to study the siphon and consider future maintenance efforts. Hamman expects that the siphon will require more work in the coming years.
Depending on the results of the study, the MRGCD will determine if the siphon should be repaired and maintained or replaced completely. Workers could stabilize the siphon using a method called slip lining. Slip lining involves pulling a heavy plastic tube through the siphon to stabilize the pipe from the inside-out. To further stabilize the siphon, the district would add more boulders. This project is the cheaper option, and Hamman estimates that maintenance and stabilization efforts would cost anywhere from $250,000 to $500,000.
If the MRGCD were to replace the pipe, it could invest in a pipe constructed of modern materials such as concrete or polyvinyl. And the new pipe would be buried deeper in the riverbed to avoid exposure. Despite these advantages, a complete replacement would be the more expensive option. Hamman estimates that a replacement project could cost over $10 million.
The MRGCD will have more information about the current condition of the siphon after it conducts the study this fall.