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Development and Demonstration of a Bidirectional Advective Flux Meter for Sediment-Water Interface (EPA/600/R-06/122) November 2006
A bidirectional advective flux meter for measuring water transport across the sediment-water interface has been successfully developed and field tested. The flow sensor employs a heat-pulse technique combine with a flow collection funnel for the flow measurement. Because the direction of flow was initially unknown, the heater was located in the center of the flow tube. Two thermocouples were symmetrically placed to both sides of the heater for temperature monitoring. For each measurement cycle, the heater generates and injects a heat-pulse to the center of the flow tube, the water flow inside the flow tube carries the heat-pulse down gradient, and the temperature is monitored at each thermocouple over time. In theory, the heat-pulse arrival time is inversely proportional to the flow rate. The bidirectional feature of the flux meter is realized through the temperature measuring capability on either side of the flow tube.
The system has automatic data acquisition, real time data display, and signal display/analysis capability. The instrument has undergone several calibrations to establish empirical relations between flow rate and heat-pulse travel time. Flow rate can be derived as a function of peak temperature arrival time, or as a function of first temporal moment of the heat-pulse. In the field operation, the flow across sediment-water interface is funneled through a dome to the flow tube; the rate of water flowing through the flow tube is measured. The advective flux through the sediment-water interface, in term of vertical Darcy velocity, is calculated by dividing the flow rate by the dome area. The dome serves as an amplifier to bring the generally low Darcy flow within a measurable range by the flow sensor. The larger the dome area, the smaller the flow it can detect.
The flux meter has undergone field tests at three very different settings. The first test site was a shallow turbulent stream at Santo Domingo, Nicaragua. The large magnitude and frequency of shifts between base-flow and storm-flow caused by rainfall events during our field deployment prevented us from obtaining reproducible data. Nevertheless, invaluable lessons were learned. The second test site was at Grand Calumet River, Hammond, IN, which is a slow moving river with fine texture organic rich sediment. And the final test site was at a large reservoir with deeper sediment at Lake Hartwell, SC. The last two field deployments were successful in that the flux meter was operational, reasonable characterization of flux were obtained, and bidirectional flow measurement capability was demonstrated.