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Research Topics Water & Watersheds

Turbidity Threshold Sampling

A bridge-mounted boom suspends a turbidity probe in a pond.

Turbidity threshold sampling is an automated procedure for measuring turbidity and sampling suspended sediment. The basic equipment consists of a programmable data logger, a turbidimeter mounted in the stream, a pumping sampler, and a stage-measuring device. The data logger program uses turbidity to govern sample collection during each transport event.

The Importance of Automated Data Collection

The ability to collect useful information about suspended sediment transport and water discharge is dependent on the timing and frequency of data collection during storms. All river systems, particularly smaller watersheds that respond very quickly to rainfall, benefit from automated data collection.

In rain dominated regions most suspended sediment is transported during a small number of events. Although it is possible to rely solely on manual measurements, important storm flows are usually infrequent and difficult to predict. 

When they do occur, trained personnel may not be available to collect the required information.  Infrequent, systematic manual sampling will not provide adequate information to make credible suspended sediment load estimates under these conditions. As of yet, there is no reliable method to directly measure suspended sediment concentration in the field. 

Usually water discharge is not a good predictor of sediment concentration for rivers and streams that transport the bulk of their sediment load as fines because the delivery of sediment to the channel from hillslopes, roads, and landslides is highly variable.

For rivers that transport mostly sand, water discharge and concentration may be more closely coupled if transport depends mainly on stream power to mobilize in-channel sources that are not easily flushed from the system.  However, in streams transporting fine sediment, a sampling scheme that employs a parameter such as turbidity, that is well correlated to suspended sediment concentration, can be expected to improve sampling efficiency and load estimation.

Turbidity threshold sampling collects physical samples that are distributed over a range of rising and falling turbidities (Lewis and Eads: 1996, 1998 and 2000). The resulting set of samples can be used to accurately determine suspended sediment loads by establishing a relationship between sediment concentration and turbidity for any sampled period and applying it to the continuous turbidity data.

How Turbidity Threshold Sampling Works

Turbidity is an optical measure of the number, size, shape, and color of particles in suspension. A number of manufacturers offer turbidity probes that can be deployed on a continuous basis in streams. The optical properties of sediment, mainly size and shape, have a large influence on the magnitude of the turbidity signal.

For instance, sand particles return a much lower turbidity signal for a given concentration than silt and clay particles of the same concentration.  TTS utilizes turbidity thresholds, points at which physical samples are collected, distributed across the entire range of expected rising and falling turbidities. Contamination of turbidity probe's optics by debris, algae, or macroinvertebrates can lead to a noisy, or progressively increasing, turbidity signal.

Sensors with reliable optical wipers, such as the DTS-12, manufactured by FTS, can reduce optical fouling and are recommended to improve data quality. Careful design of the turbidity probe's housing and mounting hardware can reduce fouling from large organic debris.

Turbidity thresholds are selected by taking into consideration the maximum expected turbidity value for a stream, the range of the turbidity probe, and the number of desired physical samples based on the magnitude of the storm. In our experience, using a square-root scale to distribute the thresholds provides an adequate pairing of turbidity-concentrations to produce acceptable regressions.

For the smallest storms, three or four samples should be adequate, while large events may produce 5 to 15 samples. Different sets of thresholds are used when turbidity is rising and falling, with more thresholds required during the much more prolonged falling period. The user can fine-tune the distribution of thresholds to maximize efficiency.

A set of rules, in addition to the predefined turbidity thresholds, aids in reducing sampling during short duration turbidity spikes, ensures that a "startup" sample is collected at the beginning of a storm, and defines reversals in turbidity. The rules permit continued sampling when turbidity levels exceed the turbidity probe's range, and they allow collection of non-threshold, manually triggered samples to be paired with depth-integrated samples or to augment sample numbers if desired.

Closely spaced turbidity measurements produce interesting trends in sediment transport such as spikes superimposed on the storm turbidigraph that often indicate landslides or streambank failures upstream. In the case of nested watersheds, the timing and magnitude of these sediment pulses may provide additional information about cumulative effects, or dilution, downstream. Authenticity of these turbidity spikes is confirmed when physical samples taken during the spikes have higher concentrations than surrounding samples.

Last Modified: Aug 29, 2016 11:24:24 AM