Over the past four years, Provo City has been planning an Aquifer Storage and Recovery (ASR) project, also known as a Managed Aquifer Recovery (MAR) project. The groundwater level below Provo has been steadily declining over the past 40 years from about 200 feet to 260 feet below the ground surface—a decline of 60 feet.
To help restore groundwater levels, the project will use untreated and treated Provo River water for infiltration and direct injection into the aquifer. The major features of the project include a new drinking water treatment plant (WTP), 36-inch diameter pipeline, an ASR pump station and pipeline, and discharge facilities in Rock Canyon.
In preparation for the new WTP, Hansen, Allen & Luce (HAL) has been helping Provo City rehabilitate the existing Provo River diversion located at 2230 North, known as the Mill Race Diversion. The water is currently diverted into the Mill Race Canal which is mostly enclosed, but includes some sections of open canal. The existing diversion structure consists of a concrete diversion dam with kick-plates for placement of stop logs, a radial or Tainter gate, a canal slide gate, and trash rack with 6-inch bar spacing. It appears the Mill Race Diversion was last upgraded in the mid-1970s.
When complete, the rehabilitation project will improve access for maintenance, allow easier operation, prevent taking of fish into the canal, and allow fish to pass upstream.
The existing trash rack was a slanted bar rack with 3/16-inch steel bars spaced 6 inches on center. The rack allowed large debris and potentially fish and other animals to enter the Mill Race Canal. It also required manually removing any debris trapped on the rack. The trash rack was replaced with four vertical fish exclusion screens, each 8 feet long and 4 feet high. The screens have 0.125-inch slot openings, selected to prevent juvenile fish from passing through. The area of the screens was designed to keep the approach velocity less than 0.4 feet per second to prevent trapping fish against the face of the screens. Adjustable baffles behind each screen (bottom photo at center) help equalize flow velocity; when tested at approximately 7,000 gallons per minute, the average velocity was 0.16 feet per second. The screens were designed with a brush cleaning system to remove debris. Log booms (bottom photo at right) were installed upstream to keep tree branches and large floating objects away. A pressurized wash system clears sediment buildup at the base of the screens.
During project design, HAL coordinated with Michael Slater, Regional Aquatics Program Manager at the Utah Division of Wildlife Resources, to determine appropriate design requirements. It was also requested that a fish ladder (top photo at right) be included to allow for passage of Brown Trout upstream of the diversion dam. The fish ladder was designed with a series of steps (15 inches high) and pools. An orifice opening was included in each step, with a slotted frame which will allow for blank panels or different sizes of orifice plates to be installed.
The project includes lining the existing canal pipeline with an HDPE pipe and enclosing open sections with HDPE pipe. Just over 600 feet of HDPE pipe was installed from the diversion to the proposed location for the new WTP.
The existing radial gate concrete structure was in adequate condition and did not require rehabilitation. The existing gate consisted of a corrugated metal plate face and metal frame. Gate seals were either missing or non-existent, which meant the gate leaked severely. The gate was replaced with a stainless-steel gate and frame. Side wall rubbing plates and side and bottom gaskets were installed with the new gate. The gate operator was also upgraded, and an electric motor actuator was included to allow for remote operation.
The existing slide gate was constructed of a single steel plate with steel side channels and a steel threaded rod with handwheel. The gate was replaced with a self-contained stainless-steel slide gate and equipped with an electric motor actuator for remote operation. The gate was installed on a new concrete headwall and mounted to a wall thimble cast into the headwall.
Other appurtenances were included in the project to help with controls and monitoring. A submerged area/velocity flow meter using ultrasonic and doppler technology was installed in the 42-inch Mill Race Pipe downstream of the slide gate. The flow meter will be used to provide feedback to the control panel which will modulate the slide gate to maintain the flowrate setpoint. Submersible level sensors were installed on the front side and back side of the screens. The differential water level will be used to signal the brush cleaning system to start a cleaning cycle.
Several unforeseen issues arose after the area was dewatered and excavation began: an irrigation return flow ditch, a large concrete footing adjacent to the radial gate, and significant erosion on the surface and below the concrete apron.
The irrigation return flow ditch was discovered when the vegetation was cleared from the site. This return flow ditch flows through the golf course and residential ponds north of the site. It was decided that there would be a potential for contamination of this water which would not be prudent to allow through the screens and into the future WTP. The ditch was routed around the backside of the screen structure through a pipeline and discharged to the river downstream of the dam.
The large concrete footing found adjacent to the radial gate created a construction issue with the fish ladder. The fisher ladder was designed to use the east wall of the radial gate structure. This concrete “footing” was a large unformed block of unreinforced concrete filled with cobble rock. Rather than try and break out this block of concrete and potentially reduce the structural integrity of the radial structure, the fish ladder was re-designed to shift it away from the concrete block.
The biggest issue was discovered after the area was dewatered and the east half of the existing diversion dam was exposed. The surface of the lower apron had multiple spots where the concrete had been eroded away to create holes which ranged from about 6 inches to 12 inches deep. Also, as the fish ladder area was excavated, a large amount of seepage was found coming under the diversion dam into the excavation. After further investigation, the construction team found that the underside of the structure had severe erosion created caverns as deep as two feet or possibly more.
To prevent further erosion under the dam, sheet piles were driven along the downstream face to a depth of 20 feet. Holes were drilled on top of the lower apron and concrete was pumped to fill the caverns created by the erosion. The surface holes were repaired by saw cutting around the edge, adding dowels epoxied into the concrete, and then filled with a Sika grout product. After the screen structure was completed, the dewatering system was changed to allow for these repairs to be completed on the west side of the diversion dam. The total amount of concrete placed under the dam was approximately 80 cubic yards.