CANAL FLOW MEASUREMENT USING A BUBBLER WATER LEVEL SENSOR

INTRODUCTION

On the Grand Valley Project near Grand Junction, Colorado, enhanced flow monitoring is helping operators manage their canal system. Since spring of 1995 a double bubbler system has been used to provide valuable flow monitoring information to canal operators. Water level is measured in a rated section of the canal where flow rate can be computed from the measured level. Canal water level and flow data are telemetered via telephone lines from the site to the headquarters office. This flow monitoring system has been relatively simple and inexpensive, yet a dependable tool.

THE GRAND VALLEY PROJECT - HIGHLINE CANAL

The Grand Valley Project is in west-central Colorado near Grand Junction, Colorado. The project first delivered water in 1917 and now supplies water to about 42,000 acres of land along the Colorado River. Project works include a diversion dam, a powerplant, two pumping plants, two canal systems totaling 90.1 miles, 106 miles of laterals, and 113 miles of drains. The Government Highline Canal is on the north side of the river and extends from the Grand Valley Diversion Dam a distance of 55 miles to the west. It has a diversion capacity of 1,675 ft3/sec, but more than half of this water is delivered into other major canal systems that branch off near Highline Canal's upstream end. About 720 ft3/sec maximum flow continues down Highline Canal into the irrigated project area served by the Grand Valley Water Users Association.

The project's primary goal has always been delivery of water to agricultural water users, but environmental concerns have become increasingly important in recent years. Farm irrigation practices are changing in order to reduce salinity in the Colorado River, and more water must be left in the river to support the fish population. These changes are requiring more flexibility in delivery flows and more precise management of the canal system.

THE SITE

Indian Wash Flume is a 112-foot-long, concrete rectangular flume that carries Highline Canal flow over the Indian Wash natural drainage channel. (See photo 1.) This site is at the entrance to the Grand Valley Water Users Association's service area, about 16 miles downstream from the canal headworks. Because it is below the other canal system diversions, the flow at this point is a measure of the service area's present water supply. This supply is balanced against water orders and spills to determine water availability and to manage daily canal flow adjustments.

Photo 1 -- Indian Wash Flume flow measurement site.

Indian Wash Flume is a rated section where water level indicates the flow rate within reasonable accuracy. The section was rated with a portable flow meter prior to installation of the bubbler device, yielding a rating curve that is thought to be within 5-7% accuracy. With a maximum flow of 720 ft3/sec, total flow can be computed to within about 30-40 ft3/sec from the measured water level. However, flow changes can be predicted much more accurately than this, and day to day canal operations are based on flow change rather than total flow. With an accurate water level measurement device, daily changes in water level should indicate flow change within only a few cubic feet per second.

THE BUBBLER DEVICE

Bubbler systems are used to measure water level by detecting the pressure required to force air through a submerged tube. The tube is mounted with the end of the tube below the water surface being measured, and the air emerges from the bottom of the tube as a stream of bubbles. The air flow rate is relatively small--just enough to prevent water from backing up into the tube--so the pressure required to push air through the tube is equal to the pressure at the tube's outlet. This pressure is proportional to the water depth above the bottom of the tube.

A big advantage to bubblers is the location of the pressure sensor. The sensor, or transducer, is at the air source on the canal bank instead of being under water. Submerged transducers can have problems with corrosion, clogging, freezing, and physical damage from debris, animals, or people. The only submerged part of a bubbler system is the air tube, which is inexpensive to replace if it becomes damaged.

The Campbell Scientific bubbler system installed at Indian Wash Flume is a self-calibrating "double bubbler" with two submerged tubes, installed so that the two orifices are separated by a fixed vertical distance. A single transducer measures pressure in each of the tubes as well as the atmospheric pressure. This technique compensates for temperature effects and long-term drift in the transducer, producing a more accurate measurement. The bubbler uses nitrogen instead of air. Nitrogen is supplied from a high-pressure bottle with a regulator to maintain a constant output pressure. The bottle holds up to 250 ft3 of nitrogen at 2000 psi.

The bubbler system is controlled by a Campbell Scientific CR10 measurement and control module. The CR10 controls nitrogen flow by opening and closing valves in a manifold assembly and determines which of the two bubbler tubes is used for pressure measurement. Based on the pressure transducer's output, the CR10 computes water level and flow data and stores these data for use by canal operators. It also monitors battery voltage and temperature data. The system calibrates and collects data every minute, then integrates these values every 15 minutes and stores the 15-minute averages in memory. The CR10 can store up to 6 weeks of data. All electrical components run on solar power from a solar panel that charges a sealed rechargeable battery. Photo 2 shows the bubbler system arrangement.

Photo 2 -- Bubbler system component arrangement.

TELEMETRY AND REMOTE MONITORING

Data stored in the CR10 is transmitted via telephone lines to the Grand Valley Water Users Association headquarters office. A software package (PC208) at the headquarters calls the site every hour and stores data on a personal computer. This 1-hour, automatic polling interval is adjustable, or operators can poll the site whenever they wish. Additional software developed by Mike Steves at Reclamation's Western Colorado Area Office creates graphical displays that show canal flow versus time. Canal operators can easily observe present flow as well as recent flow history information.

An additional feature allows the Indian Wash site to be monitored from any telephone. The on-site telephone modem has a voice synthesizer, so canal operators can call up the site and hear an automated voice report with the most recent water depth, flow rate, battery voltage, and temperature in the equipment enclosure.

INSTALLATION AND COST

Because water level in the concrete flume section was to be measured, bubbler tubes were attached directly to the flume wall rather than being installed in a stilling well alongside the canal. As shown in photo 3, two 1/4-inch aluminum tubes were welded to a single aluminum strip which is bolted to the concrete flume wall. These rigid aluminum tubes hold flexible tubing that connects to the bubbler manifold. The flexible tubing (high pressure hose for truck air brakes) fits snugly inside the aluminum tubes with the bottom orifice protruding slightly. This arrangement supports the tubes and maintains the exact position of the bubble orifices, which is important. The flexible tubing runs through a conduit and is either attached to support structures or buried underground for most of it's length back to the bubbler manifold. The bubbler manifold, CR10, pressure transducer, modem, battery, and charger are in a weather-proof enclosure, mounted on a steel pole with the solar panel at the top. The nitrogen bottle and regulator are in a protective tank made from a section of steel pipe. (See photo 4.)

Photo 3 -- Bubbler tubes attached to concrete flume wall.

Photo 4 -- Nitrogen bottle, regulator, and steel tank enclosure.

Most of the bubbler system's components were purchased from Campbell Scientific, Inc. for a cost of $3240. This included the CR10 module, liquid level sensor with transducer, phone modem with voice synthesizer, power supply with charging regulator, solar panel, battery, and the enclosure. Other components were either purchased or fabricated locally. The 1/4-inch air brake hose was purchased from a truck parts supplier for 38 cents per foot. The aluminum bubbler tube arrangement and the steel tank to enclose the nitrogen bottle were fabricated at local welding shops. Including the nitrogen bottle and regulator, these miscellaneous parts added about $2000 to the cost of the Campbell Scientific equipment, bringing the total equipment cost to about $5200.

Installation was accomplished by three Reclamation employees in about 8 hours. Additionally, one Reclamation employee attended Campbell Scientific's 3-day training course and spent a couple of weeks developing the graphical software that displays flow monitoring information at the headquarters office.

LESSONS LEARNED

The double bubbler system at Indian Wash Flume has been in operation for two irrigation seasons. During the first 7-month-long season, there were a few problems regulating the nitrogen pressure. The single-stage pressure regulator on the nitrogen bottle didn't maintain a constant output pressure as the bottle's internal pressure decreased. As the output pressure decreased, water would back up into the nitrogen tubes and the system would need to be reset. The pressure setting had to be adjusted periodically and the nitrogen was used up faster than necessary. The nitrogen bottle had to be refilled twice during the first season. After the first season, the single-stage regulator was replaced with a dual-stage regulator that has worked much better. Output pressure has remained constant through a wide range of internal pressures (from 2000 psi to 300 psi), so nitrogen has been flowing at a small, constant rate (about one bubble per second) and a single nitrogen bottle has lasted the entire season.

Another problem resulted from a lightning strike that damaged the modem. Apparently, the electrical surge from a nearby lightning strike came into the modem through the phone line. The modem was sent to Campbell Scientific and was repaired for no charge within a week, but additional surge protection should be added to the phone line to prevent a recurrence. (The power system has surge protection.)

Other components have performed well. The air brake hose used for flexible bubbler tubing is very durable and has been problem-free despite being exposed and unprotected in places. The voice synthesizer feature provided with the modem, which cost an extra $300, has been very useful. A good example occurred recently, when a rain storm caused many water users to break from their water schedules and shut off their turnout flows prematurely. The water master had to quickly reduce headworks flow as canal levels increased, then wait for water levels to come back down where they belonged. He was able to telephone the bubbler system from his home at 11 pm and confirm that water levels were once again OK so he could sleep.

The bubbler system at Indian Wash Flume has been one positive step in a continuing process to improve water management at Highline Canal. Although local automatic control and data logging have been used for many years, this is the first remote, real-time monitoring equipment on the project. It's success has helped to promote additional modernization efforts that are currently underway.

Last Modified: February 14,1997
Bureau of Reclamation, Department of Interior