Current Research at North River Farms - Phase 2

Equipment Used to Monitor Bi-directional Flow

To calculate flow using the velocity-area method, the flow cross sectional area and water velocity must be known. The flumes provide a constant cross section for flow measurement, but the height of the water is needed to determine the flow cross sectional area. This is illustrated in Figures 1 and 2.

Figure 1 (left). The flow cross sectional area in a flume at high tide. A high water level means there is a large flow cross sectional area. Figure 2 (right). The flow cross sectional area in a flume at low tide. The lower water level has a smaller flow cross sectional area.

The velocity and water level are both measured using an area velocity meter. The area velocity meter uses a pressure transducer to compare the pressure on the sensor to the atmospheric pressure. From the difference in pressure the water level can be determined. The area velocity meter contains a Doppler velocity meter. The Doppler velocity meter measures the velocity of the water by sending a Doppler “beam” into the water and measuring the change in the “beam” when it returns to the sensor.

The area velocity meter will be collecting velocity and water level measurements at least 4 times per hour to create a continuous record of flow in the tidal marsh. To calibrate the velocity readings to the actual velocity, we will use a hand held current meter on a wading rod to measure the water velocity in the flume. The bridge over the flume will allow us to make the manual measurements without being in the stream and disrupting the flow.

Figure 3 (left). An area velocity placed in the flume for testing before permanent installation. Figure 4 (right). The upstream monitoring station with a bridge over the flume for use in manually taking stream velocity measurements.