CONTINUOUS IN-SITU MEASUREMENT OF TURBIDITY IN KANSAS STREAMS

Continuous, in-situ measurements of turbidity to estimate suspended-sediment concentrations are being made at stream monitoring sites throughout the United States. Considerations for selecting instrumentation, proper installation, methods for verifying sensor performance, and collection of point in-situ data that are representative of the channel cross section need to be well thought out for the data to be of acceptable quality. Experiences and specific examples for selected monitoring sites in Kansas are discussed. Choosing an Instrument: There are many turbidity/optical backscatter probes suitable for continuous insitu measurements. Sensors can measure turbidity values ranging from 0 to 1,000 nephelometric turbidity units (NTU), with some capable of measuring up to 4,000 NTU. Most turbidity probes conform to ISO method 7027 or GLI Method II. Currently, the only method approved for measuring turbidities > 40 NTU in stream source water is ISO method 7027. Some manufacturers offer features that help improve the quality of the data and extend the time between maintenance trips. Such probes are equipped with mechanical wipers or shutter technology that activate prior to a measurement and keep the sensor clear of interference. Probes that are SDI-12 (serial data interface at 1200 baud) compatible are easily installed at U.S. Geological Survey (USGS) stream-gaging stations that have data-collection platforms (DCPs), and the data can be displayed on the World Wide Web in real time. Daily review of real-time turbidity data is essential for timely troubleshooting of equipment malfunctions. Installation: Several factors need to be considered prior to installation of a turbidity sensor as a surrogate for determining suspended-sediment concentrations in streams. First, a monitoring site that represents the area of interest and is located at a cross section of the stream that is well mixed needs to be selected. In Kansas, the USGS has selected mostly sites with existing stream-gaging stations. Adding a turbidity sensor to an existing stream-gaging station has several advantages: (1) continuous flow data are available for load calculations, (2) the equipment infrastructure for logging and transmitting the data is in place, and (3) sample collection is possible at all flow regimes. For ungaged installations, site selection for the turbidity sensor should be based on the same criteria for choosing the location of a stream-gaging station (that is, accessibility during all flow regimes, total flow is confined to one channel, the general course of the stream is straight within a few hundred feet of the stream, etc.. Rantz and others, 1982). After the site is selected, the type of installation needs to be determined. In Kansas, the USGS has successfully used two types of installations, horizontal bank (or fixed) and vertical suspension. Bank installations have been limited to sites with small drainage areas. This type of installation has failed at sites with large drainage areas because, during extended periods of high flow, floating debris damages the equipment and high sediment concentrations fill the protective plastic pipe with mud and silt to the point that the turbidity probe becomes extremely difficult to retrieve. Most of the USGS turbidity monitoring sites in Kansas use a vertical suspension installation from the bridge deck to the stream. Vertical suspension is the most adaptable and convenient for installation and maintenance. The installation is made up of a turbidity probe, 10 feet of plastic pipe, a chain, a 12-volt winch, and sometimes a radio transmitter. The pipe and turbidity sensor typically are suspended behind a bridge pier so that the sensor is protected from debris. The pipe and turbidity sensor are tethered from the bridge deck using the chain. The DCP inside the gage house logs data every 15 or 30 minutes, either directly from the sensor or via radio communication from the sensor. The DCP then transmits the logged data every 4 hours via satellite for display of the data on the World Wide Web. A watertight aluminum box encloses the transmission equipment and is mounted to the bridge rail using clamps so that no holes are necessary in the bridge rail. The winch is used to raise the pipe to the bridge deck for servicing or repairing the sensor. The versatility of this type of installation is