Data from an optical turbidity sensor deployed at the stream station were recorded at 15-minute intervals by a data logger and uploaded every hour to the U.S. Geological Survey (USGS) database (Anderson, 2005; Wagner, 2006). Suspended-sediment samples were collected using an ISCO automatic sampler (suction pump), equal width increments or grab sampling techniques (Edwards, 1999). The use of an optical sensor to continuously monitor turbidity provided an accurate estimate of sediment fluctuations without the collection and analysis costs associated with intensive sampling (Office of Surface Water Memorandum 2016.07; Rasmussen et al., 2009). Turbidity was used as a surrogate for suspended-sediment concentration (SSC), which is a measure of sedimentation and siltation. Regression models were developed between SSC and turbidity using turbidity data from the optical sensor and the SSC data collected from the suspended-sediment samples.
For Gap Creek near Nunley, AR, instantaneous turbidity measurements began on April 26, 2018 and ranged from 0.1 to 778 Formazin Nephelometric Units (FNU). A total of 52 suspended-sediment samples were collected beginning June 25, 2018 and ranged from 1 to 1380 milligrams per liter (mg/L) across a range of streamflow from 0.54 to 2020 cubic feet per second (cfs). There were 39 suspended-sediment samples used in the final model after removing quality assurance/quality control samples, samples corresponding with gaps in turbidity data, samples associated with sampling/fouling errors, and extraneous samples from the same storm event which would result in overfitting of the model.
