Published July, 2010
TECHNICAL REPORTS:TECHNICAL LANDSCAPEREPORTS AND WATERSHED PROCESSES
Increasing Precision of Turbidity-Based Suspended Sediment Concentration and Load Estimates John D. Jastram U.S. Geological Survey Carl E. Zipper* and Lucian W. Zelazny Virginia Polytechnic and State University Kenneth E. Hyer U.S. Geological Survey Turbidity is an effective tool for estimating and monitoring suspended sediments in aquatic systems. Turbidity can be measured in situ remotely and at fine temporal scales as a surrogate for suspended sediment concentration (SSC), providing opportunity for a more complete record of SSC than is possible with physical sampling approaches. However, there is variability in turbidity-based SSC estimates and in sediment loadings calculated from those estimates. This study investigated the potential to improve turbidity-based SSC, and by extension the resulting sediment loading estimates, by incorporating hydrologic variables that can be monitored remotely and continuously (typically 15-min intervals) into the SSC estimation procedure. On the Roanoke River in southwestern Virginia, hydrologic stage, turbidity, and other water-quality parameters were monitored with in situ instrumentation; suspended sediments were sampled manually during elevated turbidity events; samples were analyzed for SSC and physical properties including particle-size distribution and organic C content; and rainfall was quantified by geologic source area. The study identified physical properties of the suspended-sediment samples that contribute to SSC estimation variance and hydrologic variables that explained variability of those physical properties. Results indicated that the inclusion of any of the measured physical properties in turbidity-based SSC estimation models reduces unexplained variance. Further, the use of hydrologic variables to represent these physical properties, along with turbidity, resulted in a model, relying solely on data collected remotely and continuously, that estimated SSC with less variance than a conventional turbidity-based univariate model, allowing a more precise estimate of sediment loading. Modeling results are consistent with known mechanisms governing sediment transport in hydrologic systems.
Copyright © 2010 by the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. J. Environ. Qual. 39:1306–1316 (2010) doi:10.2134/jeq2009.0280 Published online 15 Apr. 2010. Supplemental data files available online for this article. Received 23 July 2009. *Corresponding author (
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oil erosion and sedimentation have been integral to landscape formation throughout geologic time. On a more immediate temporal scale, these processes can clog stream channels and foul aquatic habitats when accelerated by human activity. Accelerated sedimentation degrades water resources globally and is a major source of water-quality impairments throughout the United States (USEPA, 2009). Several studies have found that the economic impacts of sedimentation in the United States are in the billions of dollars annually (Clark et al., 1985; Pimentel et al., 1995). Thus, an improved capability to quantify sediment movement through hydrologic systems may result in improvements in management of water resources. Sedimentation is a major water-resource management problem because of its prevalence combined with inherent obstacles to effective measurement and characterization. Sedimentation processes are ubiquitous on the landscape and exhibit temporal and spatial variation, which hinders characterization. Sediment transport by streams varies markedly with streamflow and hence with time, creating the need to monitor in-stream concentrations either continuously or strategically to characterize transport over extended periods. In-stream suspended sediment concentrations (SSC) are sufficiently dynamic as to render both random and fixed-interval manual sampling schemes that are commonly applied to characterize dissolved pollutants as inadequate for estimating loadings over time (Thomas, 1985; Thomas, 1988; De Vries and Klaver, 1994; Philips et al., 1999). The majority of suspended sediments are transported during storm-induced periods of extreme high flows (Wolman and Miller, 1960), when few data are generally collected by manual sampling programs. Waterborne sediment concentrations vary spatially within stream channels, thus rendering measurement of in-stream transport, even at a given point in time, a time-consuming and challenging procedure, especially during the higher flows that are responsible for the majority of sediment transport. J.D. Jastram and K.E. Hyer, U.S. Geological Survey, Virginia Water Science Center, 1730 E. Parham Rd., Richmond, VA 23228; C.E. Zipper, Dep. of Crop and Soil Environmental Sciences, Virginia Polytechnic and State Univ., Blacksburg VA 24061; L.W. Zelazny, formerly Dep. of Crop and Soil Environmental Sciences, Virginia Polytechnic and State Univ., Blacksburg VA 24061. Assigned to Associate Editor Rai Kookana. Abbreviations: EWI, equal width interval; FNU, formazin nephelometric units; SSC, suspended sediment concentration; %
