"Proceedings, Federal Interagency Workshop,
"Sediment Technology for the 21'st Century,"
St. Petersburg, FL, February 17-19, 1998"

Pre-proposal on in situ Application of Infrared Nephelometers

By Jack Lewis

Pre-Proposal

Issue/Problem

Suspended sediment concentrations (SSC) in rivers and streams commonly vary over short periods (minutes or hours). To estimate sediment loads accurately requires sampling frequencies that are impractical to achieve with manual sampling. In tidal rivers or estuaries, SSC can vary with tides, wind, and runoff, so continuous monitoring is desirable for interpretive studies of tidal flux. Optical sensors that allow continuous measurement of turbidity at remote sites are commercially available but are not part of basic data and interpretive programs of FISP agencies. With calibration, turbidity probes can provide more detailed records of SSC and more accurate estimation of suspended sediment fluxes than traditional methods.

Beneficiaries

This technology will make it easier to detect the impacts of land use activities. Agencies needing detailed suspended sediment information for management decisions will benefit, as will researchers and users of basic sediment data and interpretive sediment studies.

Objective

We propose to complete the development of this technology to the point where it can be readily implemented by FISP agencies, and public and private institutions wishing to gather suspended sediment data. The primary steps necessary to reach this point are:

  1. Infrared nephelometers have proven reliable when kept clear of organisms and debris. A properly designed housing/shield needs to be developed (from existing prototypes) to exclude debris and organisms, and to protect the probe from damaging impacts in high energy environments. To facilitate calibration (as described in step 3 below), a mounting apparatus for a pumping sampler intake nozzle should be included in the housing configuration. Further investigation is also needed into mechanical, chemical, or electromagnetic means of preventing biological colonization of the optics.

  2. The probe/housing should be accessible for inspection and cleaning at all times. Bed-mounted and overhead support systems have been designed to facilitate access, and could be further developed as plans or products.

  3. For estimating SSC, nephelometers require calibration, and, for the best accuracy, re-calibration is desirable when particle composition or size distribution changes substantially. A system has been developed and applied in the field that uses real-time nephelometric turbidity to automatically control a pumping sampler to obtain calibration data. The system utilizes a single-board computer, programmed in a high level language, that requires the user to build an interface circuit for controlling external devices and to provide a weatherproof enclosure. To make the system practical for a wider user group, a manufacturer is needed to build and package these components for off-the-shelf use.

  4. A comprehensive document (designed after Techniques of Water-Resources Investigations of the USGS, Book 3) should be prepared to describe how to apply optical sensors to the measurement of SSC in different types of environments. This document would serve to transfer this technology from research programs in FISP agencies to operational programs. A training class and/or video could also be considered as a supplement.

Approach

FISP staff would consult with developers and users of this technology to engineer and test a debris-shedding housing/shield and mounting systems; and to provide plans and an implementation document. Interested FISP agencies could participate in manufacture of mounting systems from plans, and field testing. Manufacturers would be sought to produce a housing/shield and to package a data logger and interface circuit.

Funding

Funding and time table: (Total ~ $100,000, 1-2 years)

  1. Housing/shield.
    1. Sensor purchase ($2200)
    2. Housing design and refinement (2 months, $8000)
    3. Manufacture prototypes ($2000)
    4. Flume testing (2 weeks, $3000)
    5. Field testing (2 weeks, $1500)

  2. Mounting systems.
    1. Draw up existing designs (CAD software $500, 2 weeks labor $1200)
    2. Manufacture of prototypes ($2000 - 5000 each)
    3. Field testing of prototypes ($1500 each)
    4. Design refinements and example plans (2 weeks, $2000)

  3. Data logger/interface production
    1. Legwork and consultation (3 weeks, $3000)

  4. Implementation document
    1. Writing and review process (1 person-year, $50000)
    2. Production (??)

  5. Travel/consultation between Calif. and Miss. ($5000)

Personnel

Jack Lewis, Rand Eads and David Schoellhamer could consult with FISP staff. Other users of this instrument in FISP agencies could be included too. Instrument manufacturers may be supportive.

References

Buchanan, P.A., and Schoellhamer, D.H., 1996, Summary of suspended-solids concentration data, San Francisco Bay, California, water year 1995: U.S. Geological Survey Open-File Report 96- 591, 40 p.

Gippel, C. J., 1989, The use of turbidimeters in suspended sediment research, Hydrobiologia 176/177: 465-480.

Gippel, C. J., 1995, Potential of turbidity monitoring for measuring the transport of suspended solids in streams, Hydrol. Processes, 9, 83-97.

Levesque, V.A., and Schoellhamer, D.H., 1995, Summary of sediment resuspension monitoring activities, Old Tampa Bay and Hillsborough Bay, Florida, 1988-91: U.S. Geological Survey Water Resources Investigations Report 94-4081, 31 p.

Lewis, J., 1996, Turbidity-controlled suspended sediment sampling for runoff-event load estimation. Water Resour. Res., 32(7), 2299-2310.

Lewis, J., and R. Eads, 1996, Turbidity-controlled suspended sediment sampling. Watershed Management Council Newsletter 6(4): 1,4-5.

Schoellhamer, D.H., 1993, Biological interference of optical backscatterance sensors in Tampa Bay, Florida: Marine Geology, v. 110, p. 303-313.

Schoellhamer, D.H., 1997, Time Series of Trace Element Concentrations Calculated from Time Series of Suspended-Solids Concentrations and RMP Water Samples: 1995 Annual Report of the Regional Monitoring Program for Trace Substances, p. 53-55.


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