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Research Topics

Watershed & Watersheds: Turbidity Threshold Sampling

^ Main Topic | Caspar Creek Watershed Study | Fine Sediment in Pools Kings River Turbidity Threshold Sampling Study | CALFED |

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.

Instrumentation:
Data Logger and Sampling Logic
A programmable data logger is required to make the required 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 TTS 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 OBS-3 turbidity probe, under control of the program logic, collects 60 measurements in 30 seconds (mention of product names is not an endorsement by the USDA Forest Service). Next, the raw turbidity values are sorted and the median value is determined. We have found that these two operations effectively reduce outlier values. In the case of the DTS-12, the sampling frequency and period, and reported statistics, are controlled by the sensor's onboard processor. The program next collects 150 stage readings in three seconds from a pressure transducer and computes the mean stage. The mean stage is then compared against the minimum operating stage to determine if the turbidity probe and sampler intake are adequately submerged (stage is above "baseflow") to allow sampling. If the program logic determines that a sample is required, based on the rules discussed above, it activates an automatic water 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. The TTS logic, discussed above, has been developed for Campbell data loggers.

[Datalogger]

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The TTS program is executed from the Campbell CR10X data logger platform

Turbidity Probe
The OBS-3 turbidity probe, manufactured by D&A Instrument Company, is a backscatter nephelometer that emits infrared radiation (IR) into the water column. The distance the IR penetrates the water depends on the probe's optical configuration and the amount and type of sediment in suspension.  The penetration, or volume sampled, decreases with increasing concentration of material. The scattered IR returned to the sensor's detector is a function of particle size and shape and the number of particles in suspension. Comparisons made with different turbidimeters should be viewed with some skepticism due to inconsistencies in light sources, calibrations, and the sampled volume. Periodic calibration of the turbidity sensor in formazin standards is required to compensate for instrument drift and scratched optical surfaces. Sensors with a small viewing area (1 cm or less) reduce the chance that large debris will be viewed by the optics and allow for shallow deployment. Small viewing areas often do not provide adequate sampling volume and may produce noisy data. Large viewing areas (7 to 25 cm) have the opposite characteristics. A viewing area of 4 to 7 cm is a good choice.

[Turbidity Probe]

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OBS-3 backscatter nephelometer

[DTS-12]

[240KB]

DTS-12 digital backscatter nephelometer with wiper

Turbidity Probe Housing
The turbidity probe housing reduces contamination from organics by shedding debris. The housing, if properly designed, can reduce hydrodynamic noise caused by turbulence and the entrainment of air or re-suspension of sediment close to the sensor. The housing also protects the sensor from direct impacts by large submerged organic debris.

[DTS-12 and housing]

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The DTS-12 turbidity probe housing protects the sensor and reduces fouling

Sampling Boom
The 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.

[158KB]

Upper Jacoby Cr. boom in stormflow (housing,sensor, and intake submerged)

Pressure Transducer
The pressure transducer measures the head, or water pressure, at the sensor. The pressure transducer is mounted below the lowest expected water stage. A vent tube inside the cable, open to the atmosphere where the cable terminates, compensates for changes in barometric pressure. The pressure transducer is calibrated before installation by submerging the sensor to known depths and recording the voltage signal. The data logger uses this relationship to convert the sensor's voltage readings to depth. It is possible, with the proper placement and orientation of the pressure transducer housing, coupled with averaging of multiple readings, to eliminate the need to dampen wave pressure with a stilling well.

[Pressure Transducer]

[68KB]

The Druck 1830 pressure transducer measures stage

Automatic Water Sampler
Samples for laboratory analysis are collected by an automatic pumping sampler. The intake tubing runs from the sampler's pump to within close proximity of the turbidity probe on the boom. Both the intake tubing and cable for the turbidity probe are routed inside the boom to provide protection.  In some situations, locating the sampler intake and turbidity probe in different stream locations can increase the variability between the two measurements if the transported sediment is not adequately mixed. An ISCO pumping sampler is capable of collecting 24 samples under control of the TTS program. Sample volumes are set to approximately 350ml, or about 1/3 of available bottle volume. When the TTS program determines that all the rules have been met for collecting a threshold sample, the data logger triggers the sampler to collect one sample. The sampler's distributor arm then advances to the next empty bottle position and waits until the next signal from the data logger. Addition samples, via the TTS program, may be collected under control of field personnel to match depth-integrated manual samples or to increase the frequency of sampling under certain conditions. The bottles containing samples are removed for laboratory analysis at the same time that the data is transferred from the data logger. In situations where the transported sediment is predominantly coarse (>0.5mm), and the required lift (head-height of the sampler above the stream) is more than approximately 10 feet, the line speed of the water sediment mixture in the intake tubing may be inadequate to capture a representative sample of large sediment particles. The particles' momentum may be too great for the sampler to reverse, or their settling rate may be too great, permitting them to fall out of suspension before reaching the sample bottle.

[ISCO Side View][ISCO Top View]

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Side and top views of the ISCO 3700 automatic water sampler

Remote Sites:
Sites that have difficult access benefit from solar panels and telecommunications. For sites with adequate solar access, a small solar panel can provide all the necessary power to keep the battery fully charged. Telecommunications (land-line phone, cellular phone, radio RF, or satellite uplinks) allow for inspection of the data and reduce the number of site visits.

[128KB]

South Lake Tahoe station with solar panel and cell antenna

Direct comments or questions about Turbidity Threshold Sampling (TTS) to Rand Eads

Research is being conducted by:
  • Cumulative Effects of Forest Mgmt on Hillslope Processes, Fishery Resources, and Downstream Environments (RWU-4351)

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    Last Modified: Aug 28, 2008 06:46:45 AM