Artificial recharge of groundwater has become a necessity in Orange County, California, where natural recharge is insufficient to satisfy water demands. The area is part of a semi-arid coastal plain with fewer than 20 in. of rainfall each year, and the Orange County Water District is supplementing nature with artificial recharge to supply 70 percent of the water needs of its more than two million residents. As part of the program, the District has expanded its recharge capabilities with the Santa Ana River Inflatable Rubber Dam Project.
Two inflatable rubber dams with bypass facilities to divert river flows to off-river recharge basins have been constructed. These dams, both of which are 2.1 meters in height and stretch 97.5 meters across the Santa Ana River, are among the largest single span systems of this type in North America. They allow the District to recharge as much of the river flows as practically possible by capturing excess flows that would otherwise be lost to the ocean.
Nearly 25 million cubic meters of stormwater captured annually for recharge will supply the water needs of 100,000 of the population within the service area. In the first operating year (1993), with only the first dam completed, almost 17 million cubic meters of water with an estimated value of $2 million, were saved. The cost to construct the dams and appurtenant facilities was $4.7 million. Design work included comparison of technologies previously used to control headwater surface levels, and evaluation of design considerations for debris control, metering, flow control, hydraulic conveyance, and energy dissipation.
The District extracts groundwater from an extensive aquifer below a rapidly developing urban area. To replenish the reserves, it uses over 1,500 acres of land along the Santa Ana River for percolation basins. These are maintained in the river itself, and several off-river basins also are in use for retention and recharge. The off-river basins have a combined volume of over 33 million cubic meters.
Sand levees in the river are used to create a serpentine path for the water, as well as providing sufficient head to divert approximately 7.1 cubic meters per second (cms) of water to the off-river basins. When flows in the river exceed 14.2 cms, however, the sand levees collapse and are scoured away and the water flows unimpeded to the ocean. It was envisioned that inflatable rubber dams could replace some of the sand levees to increase the diversion capacity to 14.2 cms into the off-river basins. This would increase the capture of the larger flows and reduce the time and effort needed to replace the sand levees after storm flows have receded.
The project, which is illustrated in the figure, consists of one inflatable rubber dam (the Imperial Highway installation) and bypass facilities at the headworks of the recharge basins, and a second similar installation (known as Five Coves) located approximately three miles downstream.
In addition to the use of the rubber dams, two other aspects of this project are significant. First, the bypass at the Imperial Highway dam incorporates a relatively new trashrack design with self-cleaning debris removal equipment. Second, since space for the bypass facilities was limited, complex hydraulic problems for bypass flow control, metering, and energy dissipation had to be solved.
Inflatable Rubber Dam Design
The multiple uses of the Santa Ana River required the evaluation of several diversion structure alternatives for the river. This is the largest river in southern California and urban development along its course has made it an important water supply resource and flood control channel. The U.S. Army Corps of Engineers has been engaged in a flood control improvement project on the river since 1989, and reviewed the preliminary and final designs to ensure compatibility between water diversion and flood control activities on the river. The simplicity of the rubber dam concept, along with its proven reliability, were key considerations in evaluating its suitability as a diversion structure.
Alternatives investigated included steel gates with variations on the operating mechanism. Conditions that inhibited the use of steel gates were the corrosive environment at the sites (approximately 19 kilometers from the ocean with the major component of the base river flows being wastewater effluent); the high sediment transport capability of the river (silt could clog gate operators and abrade steel gates and hinges); additional maintenance (compared to rubber dams); and cost. While inflatable rubber dams also have deficiencies, including increased exposure to vandalism and a limited history (the oldest example in North America is less than 25 years old), their advantages justified their selection.
A typical rubber dam installation consists of a reinforced concrete foundation constructed across a riverbed with a rubber bladder anchored to the foundation. In the Santa Ana River project the two bladders extend 97.5 meters across the river bed without need for intermediate piers or abutments. The bladder is inflated and deflated through connected air piping. Most contemporary rubber dams use air for inflation, but water may be suitable where hydraulic conditions are more demanding.
Structural design of the rubber dam foundation and anchoring system is straightforward, with one caveat: the seepage and related uplift pressures created by the new reservoir must be reviewed for their effect on the stability of the riverbed and local riverbed structures. A typical rubber dam foundation has upstream and downstream cutoff walls to increase the ground water seepage path and reduce uplift pressures exerted by the ground water. Seepage analyses for the Santa Ana dams indicated that the structural stability of river drop structures near the proposed dams could be adversely affected. The locations of the foundations were adjusted to allow relief of the uplift pressures upstream of the drop structure, however, and structural stability was maintained.
A key benefit of this type of dam is the ability to serve as a reliable, low-maintenance adjustable-crest weir. In the Santa Ana project the dam/weir creates a shallow diversion pool that provides a hydraulic gradient to convey the water into the adjacent off-river recharge basins. Each dam can be fully inflated or fully deflated to completely block, or completely open, the flood channel-or can be set to operate at intermediate heights.
When a dam is inflated, river flows are diverted to the off-river recharge basins, or bypassed around the dam and back into the river. If the flow is such that the water level is rising and will overtop the dam, the bladder will automatically deflate. (These dams are capable of withstanding overflows up to 0.6 meters above the top). Internal air pressure is relatively low, and associated operating equipment is minimal. Redundant electrical and mechanical controls ensure virtually failsafe operation under emergency conditions-a capability essential in gaining approval from the Corps of Engineers.
A secondary benefit is the adaptability for automation. The labor intensive efforts of constructing and maintaining sand levees and diversion dikes have been replaced by a 30-minute inflation task that can be performed by a local or remote push-button. Design provisions were made to permit future installation of telemetry equipment for remote monitoring and operation of the dams and most of the related ground water recharge facilities. However, the District intends to gain practical experience with this flood control system before getting into automation.
Two major manufacturers of inflatable rubber dams were consulted throughout the design process (Bridgestone and Sumitomo). While their operating schemes appeared virtually identical, there were differences in the fabric, anchoring systems, hydraulics, and equipment costs. Key considerations in determining suitability of one type versus another included the operational hydraulics (e.g., depth and frequency of dam overtopping, trailrace depth on the downstream face, adjustment range for the dam crest); potential for in situ dam fabric repairs; and the need for special coatings to protection against vandalism, caustic conditions, or overflow of potentially damaging debris.
Both designs were found to be acceptable for this project, and the specifications written allowed competitive bidding between the two manufacturers. Each was selected as low bidder on one of the two facilities, and their products have improved the reliability and safety of the river diversion facilities. Earth moving equipment works the river bed to rebuild levees and diversion dikes less frequently. Also, the District can improve the quality of diverted water by deflating the dams and allowing the silt- and debris-laden "first flush" of stormwater to bypass the recharge basins.
Trashracks Needed for Continuous Flows
Self-cleaning trashracks installed at the bypass intakes of the Imperial Highway inflatable dam permit uninterrupted water diversion operations during heavy river flows. Slugs of debris and vegetation would instantaneously plug the entrances to three 1.2 by 1.2-meter diversion conduits, critically reducing their capacity at times when water was most available. The trashrack and self-cleaning rake system selected was evaluated using the research and practices of two of the largest agencies responsible for water resources, the U.S. Bureau of Reclamation (USBR) and the U.S. Army Corps of Engineers.
In 1990, the USBR had conducted a study of 85 sites that were experiencing debris problems. Manual and self-cleaning trashracks were evaluated. The most common types of debris reported were moss and aquatic weeds, floating debris and driftwood, windblown weeds, and sediment. All are found in the Santa Ana River. Based on study data and specific Santa Ana information, several performance criteria were developed:
- The raking beam/lifting mechanism had to be located behind the rack (downstream)
- Raking teeth must extend through the rack from the rear, and debris separation from the teeth must be reliable
- The number of moving parts below water must be minimized.
- The raking mechanism drive had to be load sensitive to reduce jamming and motor burnout
- The raking mechanism must clean the full height of the rack, particularly the invert, from bottom to top.
Various types of racks and raking systems were evaluated to identify equipment that could match (or exceed) these criteria. The evaluations included catenary, hoist-and-carriage, indexing hydraulic, and traveling screen racks and raking systems, and they concluded that a customized system installed at a Corps dam on the Cumberland River in Tennessee would be suitable. The Cheatham Dam system uses chain-mounted teeth to rake between the bars of the rack and carry debris over the top of the rack. The raking chain is driven from the top by a hydraulic motor, an arrangement which provides the capability to handle heavy point loads or awkward loads that might jam the mechanism. Also, this design uses standard steel shapes, chains, and mechanical components that are typically stocked by industrial suppliers, which eases repair and modification tasks.
The Cheatham Dam evaluation revealed that this type of self-cleaning trashrack had been successful in a number of installations. The design was subsequently chosen for the Santa Ana project, and acquired from the Duperon Corporation of Michigan. Debris is removed at the rate of over 1200 pounds per hour by the system during high-flow storm events. It has eliminated the manual operations that had required ten-member crews to work non-stop round-the-clock in all weathers for several days. Savings in water and labor costs per storm event are estimated to be $10,000.
Bypass Facility Design
Most rubber dams operate as diversion facilities that do not allow water to pass downstream while the dam is inflated, or do allow the flow to pass by overtopping the dam. The District's use of sand levees to develop percolation basins in the Santa Ana River, however, requires accurate and reliable methods to measure and control the flows allowed to pass downstream of each dam. The river hydraulics and the design of the levee protection defined the upper limit of the bypass control capacity to be 14.2 cms. The bypass facilities therefore must control, transport, meter, and dissipate the energy of 14.2 cms of Santa Ana River flows.
Various approaches to designing the bypass were considered. They focused on operational and constructability issues that would affect performance and reliability, particularly three factors:
- Remote monitoring and operation would require maximum flexibility in hydraulic characteristics
- The sediment load of the bypass flows can be extremely heavy
- The site is spatially limited, and any facilities extending beyond existing levees would encroach on the percolation or recharge basins.
To achieve maximum flexibility and deal with occasionally heavy sediment loads dictated that typical flow metering methods such as weirs, flumes, and propeller meters could not be used. Flow measurement would have to be accomplished using ultrasonic technology. This method provides an unobstructed flow path, creates zero head loss, and requires a shorter upstream segment with uniform flow to achieve the desired accuracy. The bypass conduits were designed to operate under full flow conditions with downstream flow control.
The height of the box culverts designed for the bypass flow was limited so the diversion pool could submerge the intake and prevent the formation of vortices that might interfere with the accuracy of the flow measurement. This also allowed the box culverts to be conveniently buried in the levee directly under the sloping end footing of the rubber dam. Bypass flows are controlled by vertical slide gates that provide a bottom outlet at the downstream end of the culvert, allowing sediment to pass unobstructed. The design of the "hidden" bypass conduits was approved by the District and the Corps. The minimal hydraulic operating constraints of the bypass facilities were important in controlling the record Santa Ana River flows that occurred in the 1993p;1994 rainy season, and the Accosonic ultrasonic flow meters have performed accurately and reliably.
About the Authors:
Michael R. Markus, P.E., is engineering manager for the Orange County Water District, Fountain Valley, California. Curtis A. Thompson, P.E., is a project engineer with ECI, a consulting firm in Englewood, Colorado. Matt Ulukaya, P.E., is a project engineer with the Orange, California, consulting firm of Holmes & Narver.