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. 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.  Turbidity Threshold Sampling 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 a reliable mechanical method for removing optical fouling, such as a wiper, are beneficial in most streams. Careful design of the turbidity probe's housing and mounting hardware can reduce fouling and impacts 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 stream bank 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.

Instrumentation:

A programmable data logger is required to make the sampling decisions. For remote locations, it is important that the data logger has low power requirements in order to preserve the battery's capacity. The Turbidity Threshold Sampling program only requires input information about stage and turbidity to decide what actions to take. Wake-up intervals are either set at 10-minutes for small, flashy watersheds, or at 15-minute intervals for larger basins. At the beginning of each wake-up interval, the turbidity sensor, under control of the program logic, returns the median turbidity value. The program next collects stage readings from a pressure transducer and computes the stage based on the sensor's calibration constants. The stage is then compared against the minimum operating stage to determine if the turbidity sensor and pumping sampler intake are adequately submerged to allow sampling. If the program logic determines that a sample is required, based on the rules discussed above, it activates a pumping sampler to collect one sample. Other instruments, such as tipping bucket rain gages and water temperature probes, may be connected to the data logger to provide additional information. Finally, all pertinent records are written to data logger memory.

A sampling boom positions the turbidity probe and sampler intake at the appropriate position and depth in the stream. Since the boom is articulated, large floating organic debris can, on impact, lift the vertical arm of the boom to the surface and pass underneath. Increasing water velocity and depth pushes the vertical boom arm downstream, raising the turbidity sensor higher in the water column. A counterweight prevents the boom from rising to the water surface. The highest probability of contamination by organics, and resulting loss of data, occurs during flood stages when organic material is recruited from flood plains. A bank-, cable-, or bridge-mounted retrievable boom is desirable for all but the smallest streams to allow debris removal during high flows. The depth of the turbidity probe can be adjusted as needed to position the probe above the zone of bedload transport and below the water surface. Changing the depth of the turbidity probe can change the ratio of coarse and fine particles sampled by both the turbidity probe and sampler intake.