Monitoring Equipment

Water-level

Sensors have been installed as each impoundment to monitor surface water and groundwater levels. The water-level monitoring is done using one of three devices. Each of these devices was selected to monitor water levels to 0.01 ft. Digital encoders systems were used to monitor surface water levels at locations that require two or more water level monitoring devices in close proximity (Fig. 7-5b). Typically, this would be an upstream and downstream site on either side of a dike. The digital encoders are linked to a datalogger that can collect the data from as many as four encoders. The encoders are mounted in 8-in diameter PVC-pipe stilling wells and connected to the datalogger by buried cable.

The other two water-level monitoring devices are vibrating-wire type pressure transducers. The first type, a levelogger (Fig. 7-5a), consists of a temperature compensated pressure-transducer with a datalogger mounted inside a ¾ in-diameter stainless-steel case. This device measures absolute pressure and the data must be corrected for variations in atmospheric pressure. Atmospheric pressure is obtained from a local weather station and an additional levelogger retained as a backup unit and used to collect atmospheric pressure data. The levelogger can be mounted in 2-in diameter wells for monitoring groundwater or surface water. These devices are used in groundwater wells where above-ground monitoring equipment interferes with grove management and at remote locations inside impoundments where frequent maintenance is not possible. The leveloggers are suspended inside the well using stainless steel leader wire. The data from these devices are downloaded using a laptop PC with the manufacturer-supplied infrared interface.

The third water-level monitoring device also is a temperature compensated pressure transducer with a single-channel datalogger. This device, WL14, is a relative pressure-transducer, which is open to the atmosphere and does not require atmospheric pressure compensation. The WL14 is constructed with the sensor at one end and the datalogger at the other end of a 15ft cable. The WL14 can be mounted in a 2-in diameter groundwater or surface water well. These devices can be used in similar locations as a levelogger with the exception that a separate structure may be built to support the datalogger away from the monitored location. This provides some advantage in wetland monitoring that the vegetation immediately around the monitored location is not trampled when the WL14 is serviced. Both pressure-transducer water-level monitoring devices can retain several weeks of water-level data. This is appropriate for use in remote locations.

Pumped Flow

Inflow to the impoundments is primarily through grove throwout pumps. The throwout pumps consist of a diesel engine attached to a vertical axial-flow pump located near the bottom of the collector ditch. The water is pumped laterally through the dike into the impoundment. Typically the pumps are actuated using a float to turn the pump on and off. The float system typically works on a 1 to 1.5 foot difference between turn-on and shut-off. The pumps range from 18-inch to 36-inch diameter. It is not practical to place flow meters in these discharge pipes to measure discharge. The flow is monitored by measuring the pump revolutions, and the flow is calculated using a pump curve developed by the pump manufacturer. To complete the calculation it is necessary to know the upstream and downstream water levels, which are measured separately. Each pump is calibrated using a flow meter to correct variance from the manufacturer's pump curve due to wear and installation.

The pump revolutions are monitored using a magnetic reed-switch mounted on the pump frame and a magnet mounted on the pump (Fig. 7-5c,d). On direct drive pumps the rpm sensor is mounted on the pump head cover. On V-belt and pulley drive units the sensor is mounted on the pulley cover above the pump head. A data logger (Fig. 7-5e&f) counts the revolutions and records the total revolutions every five minutes. Based on the pump curve this is converted into flow using a spread-sheet program.

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