Current Research at North River Farms - Phase 2

Methods Used to Monitor Bi-directional Flow

Through years of monitoring experience and data analysis, members of the research team have learned many techniques to improve the accuracy of continuous flow measurements. This is critical in a tidal system where traditional methods such as developing a stage/discharge curve will not work. The flow in a tidal stream can change direction up to 5 times per day. The water level can vary over 3 ft during one tidal cycle at the downstream monitoring station. This change in water level is shown in Figures 1 and 2. These pictures were taken on the same day about 4 hours apart.

Figure 1 (left). Picture of the downstream monitoring station in Fall 2010 at mid-tide. The water level is lower than shown here during low tide. This shows the flume for flow monitoring, the housing for all of our equipment, and the bridge for manual stream gaging. Figure 2 (right). Picture of the downstream monitoring station in Fall 2010 at high tide. This picture was taken about 4 hours earlier than Figure 1.

Trapezoidal flumes have been installed in the restored salt marsh to serve as a constant cross section for flow calculations. With knowledge of the flow cross section area and the velocity, the flow rate can be calculated. For more information on the equipment used to calculate flow go to the Flow Monitoring Equipment page.

One challenge of using flumes in a salt marsh is dealing with the flow events that are above the height of the flumes. This can occur on a daily basis due to tidal flows and the low elevation of salt marshes. There are also extreme events that can be caused by large rain events, extreme tides, or storm surges. It is impossible to plan for every flow event, but the accuracy of the flow measurements would be unacceptable if daily events were not captured. To capture the regular events, a geotextile fence has been used to direct the flow into the flume. This geotextile reduces the flow that occurs outside of the flume to reduce measurement errors.

Figure 3 (left). Constructing the flume at the downstream monitoring station. Figure 4 (right). The smaller flume at the upstream monitoring station just after construction was completed.