Using Suspended-Solids Sensors for Stormwater Runoff Monitoring
Stormwater runoff can transport large amounts of sediment in minutes. A short, intense rainfall event may produce a sediment pulse that begins and ends between routine grab samples. By the time a crew reaches an outfall, the highest suspended-solids concentration may already have passed downstream.
Suspended-solids sensors provide a way to observe these changing conditions continuously. Installed in a pipe, channel, detention basin, or receiving water, an optical instrument can record changes in particle concentration at a frequency suited to fast-moving runoff events. When those measurements are paired with flow, rainfall, and laboratory samples, they support a much clearer understanding of pollutant loads and sediment behavior.
A successful program requires more than selecting a sensor and placing it in water. Site hydraulics, particle characteristics, fouling, calibration, telemetry, and maintenance all affect the quality of the final dataset. The following practices help environmental teams build a stormwater monitoring system that produces useful evidence rather than isolated readings.
Why Stormwater Needs Continuous Evidence
Runoff quality changes rapidly as rainfall intensity, land cover, and drainage conditions shift. The first flush from a construction site, roadway, industrial yard, or urban catchment may contain a high concentration of fine sediment and attached pollutants. Later runoff can be less concentrated, while a renewed burst of rainfall may remobilize material deposited in a channel or basin.
A single laboratory sample represents one moment. It may be valuable for compliance documentation, but it cannot describe the complete shape of a storm hydrograph or sediment concentration curve. Continuous or event-triggered sensing reveals when turbidity and suspended solids rise, how long the peak lasts, and whether concentrations track flow or occur before the maximum discharge.
These data support several practical decisions. Stormwater managers can evaluate erosion controls, compare treatment systems, estimate sediment export, identify problematic subcatchments, and document performance during permitted discharge events. Continuous monitoring can also reduce unnecessary site visits by alerting staff when a threshold has been exceeded.
How Optical Suspended-Solids Sensors Work
Most field suspended-solids instruments use optical measurements to detect particles in water. A light source illuminates the surrounding sample, and a detector measures scattered or transmitted light. As the number, size, shape, and optical properties of particles change, the measured signal changes as well.
The raw optical response is commonly reported as turbidity or a related signal. It is not automatically equivalent to total suspended solids (TSS) expressed in milligrams per liter. The relationship depends on the material being measured. Clay, silt, organic particles, algae, sand, and mixed urban sediment can produce different optical responses at the same gravimetric concentration.
For that reason, a stormwater program should establish a site-specific correlation between sensor output and laboratory TSS. Samples should cover low, moderate, and high concentrations and, where possible, multiple storm events. A regression model or other conversion method can then be developed, with its uncertainty documented rather than hidden.
Optical design also matters. Wavelength, detector geometry, path arrangement, measurement range, and compensation for ambient light influence performance. D&A Instruments describes several optical sensing technologies used in water-quality and sediment-monitoring applications, including systems intended for marine and freshwater environments. The correct configuration depends on the expected particle concentration, water depth, fouling pressure, and deployment location.
Designing A Field Monitoring Network
Begin with the monitoring objective. An outfall compliance station may need event-triggered measurements and dependable time stamps, while a research project may require high-frequency observations at several locations. A treatment-performance study may need sensors both upstream and downstream of a pond, wetland, filter, or sediment trap.
Site hydraulics should guide installation. In open channels, the probe should be placed where water is well mixed and sufficiently deep, but away from stagnant margins, sharp bends, hydraulic jumps, and areas where sediment settles. In pipes and culverts, the sensor must remain submerged under expected low-flow conditions and be protected from impact by debris. Installations in detention basins require attention to changing water level, sediment deposition, and access for retrieval.
Pair suspended-solids measurements with supporting observations. A tipping-bucket rain gauge can identify event timing, while a pressure transducer or area-velocity instrument can provide discharge. Water level alone may be adequate where a stable stage-discharge relationship exists. Conductivity, temperature, and pH can add context when runoff sources or treatment processes are being investigated.
Sampling frequency should reflect the speed of change. Five- or fifteen-minute intervals may capture many storm events, but a rapidly responding urban drainage system may need shorter intervals during rising flow. A practical strategy is to use a lower background interval and switch to more frequent logging when rainfall, stage, or turbidity passes a trigger.
Matching Sensor Configuration To Program Goals
Different monitoring situations place different demands on the instrument and its supporting equipment. A short-term construction-site study may prioritize portability and simple retrieval. A remote watershed station may require low power consumption, robust telemetry, and a housing that can remain deployed for months.
The table below provides a planning framework. Actual specifications should be checked against the intended instrument, water matrix, concentration range, and installation conditions.
| Monitoring need | Useful configuration | Main planning concern |
|---|---|---|
| Outfall event monitoring | Optical suspended-solids sensor, rain gauge, event logger, cellular or radio telemetry | Rapid level changes and debris |
| Construction-site discharge | Sensor with protective mounting, frequent logging, automatic alerts | Very high concentrations and fouling |
| Detention basin performance | Sensors upstream and downstream, level measurement, scheduled sampling | Stratification and changing probe depth |
| Watershed sediment export | Suspended-solids sensor, flow measurement, rainfall station, site-specific calibration | Converting concentration to reliable load |
| Research or model validation | Multiple synchronized sensors, high-resolution logging, laboratory samples | Time alignment and uncertainty analysis |
| OEM or integrated monitoring system | Optical sensing module, compatible data interface, custom enclosure | Mechanical, electrical, and software integration |
Concentration range deserves particular attention. A sensor that performs well in relatively clear receiving water may saturate during a muddy runoff pulse. Conversely, a high-range instrument may provide less useful resolution near background conditions. If the program spans both extremes, consider a measurement strategy that includes appropriate range selection, dilution for laboratory verification, or multiple monitoring locations.
The data logger and communications package are part of the measurement system. Confirm that the logger can store raw readings, diagnostics, calibration information, and quality flags. Remote communications should be tested at the actual station, since low-lying drainage corridors, dense vegetation, and infrastructure can weaken cellular or radio signals.
Building A Reliable Calibration And Quality Program
Field readings should be checked against physical samples. Collect discrete samples across the full expected range, with extra attention to the rising limb and peak of storms. Record the exact sensor time, stage, flow condition, weather, and sampling location. A sample taken from a quiet edge while the sensor measures the main current may not represent the same water mass.
Laboratory analysis commonly involves filtering, drying, and weighing the retained material according to the selected method. The laboratory result should be paired with the sensor output collected at the same time. Teams should inspect whether the relationship is linear, whether separate curves are needed for different seasons or sources, and whether extreme values show evidence of optical saturation.
The distinction between field turbidity and laboratory analysis deserves careful treatment. The guidance on in-situ and laboratory measurements helps explain why readings made directly in the water may not match a controlled bench measurement. Differences in sample handling, settling, bubbles, temperature, instrument geometry, and particle distribution can all affect the comparison.
Routine quality assurance should include a pre-deployment inspection, post-retrieval check, cleaning record, clock verification, and review of diagnostic values. Flag data collected when the probe was exposed, buried, obstructed, or outside its operating range. Keep raw optical values where possible, even if the reporting system primarily displays converted TSS, because later validation may require them.
Managing Fouling, Debris, And Power
Stormwater is a difficult environment for optical instruments. Organic growth, oil films, fine sediment deposits, and trapped air bubbles can reduce or distort the signal. Wipers, copper components, mechanical guards, and suitable mounting angles can limit fouling, but no anti-fouling method eliminates maintenance.
Debris protection must balance safety with measurement quality. A cage can shield a sensor from branches and trash, yet it may collect leaves or alter local flow. Inspect the installation after major storms, especially when the site receives material from steep slopes, unpaved surfaces, or active construction. The mounting assembly should allow technicians to clean and remove the instrument without entering unsafe water.
Power planning should account for the monitoring interval, telemetry frequency, sensor cleaning mechanisms, and winter conditions. Solar panels may be effective at exposed sites but less dependable under tree cover or during extended storms. Battery capacity should be based on realistic low-light periods, not average conditions alone.
A maintenance schedule can be calendar-based, event-based, or a combination of both. High-sediment sites may require service after every major storm, while a cleaner receiving-water station may operate longer between visits. Remote alerts for low battery, abnormal readings, communication loss, or prolonged dry exposure can help teams focus fieldwork where it is most needed.
Turning Measurements Into Actionable Results
The goal of monitoring is usually a management decision, not a graph by itself. Establish thresholds tied to the program objective: a discharge limit, a treatment-performance target, an investigation trigger, or an alert for unusual sediment movement. Thresholds should account for sensor uncertainty and avoid treating every short-lived spike as a confirmed exceedance.
Load calculations require both concentration and flow. A concentration record without discharge data can show when water became muddier, but it cannot reliably quantify the mass exported from a catchment. Combining calibrated suspended-solids estimates with flow produces a time series of sediment load, subject to the quality of both inputs.
Data review should focus on patterns as well as individual values. Compare sensor response with rainfall intensity, stage, conductivity, maintenance records, and laboratory samples. A sudden increase that occurs during falling water level may indicate resuspension or a disturbed probe rather than new catchment runoff. Consistent lag between rainfall and turbidity can reveal travel time through a drainage network.
Reporting should preserve traceability. Document sensor model, installation depth, calibration dates, laboratory method, conversion equation, data gaps, quality flags, and maintenance actions. When results may support regulatory or contractual decisions, retain the original files and an auditable record of every processing step.
Practical Steps For A Stronger Program
A phased approach makes it easier to identify site-specific issues before committing to a large network.
- Define whether the program measures turbidity trends, TSS concentration, sediment load, treatment performance, or compliance conditions.
- Characterize the expected runoff range using historical records, reconnaissance visits, and preliminary laboratory samples.
- Select a sensor and mounting system that can tolerate the site’s concentration, depth, debris, fouling, temperature, and access conditions.
- Pair optical readings with rainfall, water level, flow, and synchronized laboratory samples.
- Establish written procedures for calibration, cleaning, data validation, threshold alerts, maintenance, and record retention.
Pilot deployments are particularly valuable at new stations. A few well-documented storm events can expose problems with mounting depth, telemetry, sediment deposition, or the assumed relationship between turbidity and TSS. The pilot results can then guide permanent installation and reduce the risk of collecting a large but poorly interpretable dataset.
For organizations adapting older D&A Instruments equipment or selecting a current replacement, Campbell Scientific provides product-management and contact information for supported systems. Reviewing the application requirements with the equipment provider can help align the optical sensor, logger, communications, and installation hardware before field deployment.
A well-designed suspended-solids monitoring program turns brief, difficult-to-observe runoff pulses into defensible environmental data. Contact Campbell Scientific to discuss the appropriate supported instrumentation and build a stormwater station around the site’s flow conditions, sediment characteristics, and reporting objectives.