Estimating Sediment and Phosphorus Loading from Continuous DataJan. 10th 2015
Measurement of sediment and phosphorus transport in rivers often requires manual sample collection and analysis. Over the past decade, technologies, such as acoustic Doppler current profilers (ADCP), have been developed that allow for continuous in-stream data collection that can be used to estimate sediment and phosphorus loads. The Vermont Water Resources and Lake Studies Center and the Vermont Department of Environmental Conservation funded research conducted on the Rock River in Highgate Springs, Vermont. Dr. Breck Bowden and Elissa Schuett, in the Rubenstein School of Environment and Natural Resources at the University of Vermont, used data from an ADCP instrument installed at the U.S. Geological Survey gaging station on the Rock River. The ADCP has been collecting data since fall 2010. The researchers collected water samples during storm events and at baseflow throughout the year from October 2011 to September 2013, capturing a range of stream flows during different seasons. Acoustic Doppler current profilers measure velocity and discharge by sending an acoustic signal beam from the instrument, which then bounces off of particles in the water. The signal is returned to the instrument, providing information on the speed of the particle, and therefore the speed of the water can be determined. Discharge is then calculated from the speed and volume of the water. From this data, a linear regression relationship between the signal and the concentration of sediment and phosphorus in the water sample can be developed. The relationship can then be used to estimate the concentration from the continuous data, allowing for an improved understanding of the transport of sediment and phosphorus throughout a storm event and from different seasons. The Rock River is a largely agricultural (41.4%) watershed that flows into Missisquoi Bay in Lake Champlain. At the location of the gaging station and sample collection, the Rock River is a small and hydrologically flashy stream. Manual sampling during a storm event is difficult in this type of stream because an event can be over before personnel arrive to collect samples; missing important data points in calculating accurate phosphorus load estimates. Use of continuous acoustic data from the current profiler allows for a more complete picture of phosphorus dynamics occurring in the river, such as capturing potential stream bank failures that release sediment and phosphorus. Such events may not be noted during intermittent manual sampling. From the developed relationships for the Rock River, estimates of sediment and phosphorus were calculated from the continuous data. The load of phosphorus for the entire study period from the ADCP data was compared to that calculated by the state of Vermont Department of Environmental Conservation and fell within the confidence interval of the state model, suggesting that this method is able to reliably estimate loads. The instrument does have a cost to purchase and maintain, and effort is required to develop a relationship for calculating load estimates. However, once calibrated, minimal continuous effort would be required. Use of continuous data from an ADCP can prove to be more cost effective over the long term and captures more detailed information about specific storm events and short-term dynamics that are not revealed in intermittent sampling.