By Scott Bryan, CAP Senior Biologist
The CAP is an engineering marvel. Over 330 miles in length, the concrete-lined canal transports approximately 1.5 million acre-feet of water each year to the people of Arizona.
During the initial design phase of the aqueduct, it was recognized that sediment in the canal could be a source of problems, potentially decreasing the efficiency and reliability of the water conveyance system. However, after a thorough evaluation of de-silting facilities, it was decided that costs for building large scale facilities would not be justified. Instead, engineers designed protective dikes and several hundred overchutes, culverts, and other structures to help effectively divert and/or drain stormwater from the surrounding basins.
Despite the extensive efforts of design engineers to protect the CAP, sediment still finds its way into the canal. It is not known if the bulk of the sediment comes from our source water (Lake Havasu), is blown in with the monsoons, or comes in with storm runoff. Due to high costs and complex water delivery issues, the canal is only drained on rare occasions. Therefore, sediment removal must be accomplished using specialized underwater techniques. Over the years, CAP has utilized a variety of these methods, including clamshell dredging, highline buckets, pump dredges, "mucking" with loaders, and "vacuuming" using divers. Although each method has had various levels of success, there is now considerable debate as to whether sediment removal is even necessary. CAP Operations indicates that water deliveries have not been affected by sediment deposition and water treatment facilities ensure that sediment is removed prior to delivering to end users. There is only anecdotal evidence that suggests the sediment creates a need for increased maintenance or replacement of parts. Until the debate is settled, sediment removal activities are only scheduled as conditions warrant. In 2018, CAP will begin a study to determine the sources of sediment, sediment transport, the impacts of sediment deposition, and potential sediment removal techniques.
In the meantime, CAP has been monitoring sediments at pumping plants and major turnouts, where CAP connects to delivery systems, to establish long-term patterns in sediment deposition. Since 2013, CAP has been using a Lowrance Chartplotter mounted on a boat to map the bottom of the canal. The boat is driven in concentric circles around the forebay, the area leading up to a pumping plant, to collect comprehensive sonar data. The data is uploaded to a cloud-based service and with the help of CAP’s GIS Department, maps of sediment deposition are created. The long-term data and maps are used to evaluate changes in sediment depth and patterns in sediment deposition. Along with input from field personnel, the data is then used to determine if sediment removal should be scheduled.
Sediment mapping is just another way CAP works to ensure a reliable, consistent water supply.