As part of a sediment monitoring project (SSC - Suspended Sediment Concentration), we observed the Laak, a small river in Beerse (Belgium) fed from an urbanised area, during a rainy period last May. While it was no surprise that the rain would cause a change in discharge, the rapid and significant variation in sediment concentration during the increased discharge was unexpected.
First rain
21:00h: Start of Measurement
21:30h: Heavy rain begins, and it takes almost 90 minutes before the discharge increases significantly. Meanwhile, sediment measurements already show a rise in sediment concentration since 22:25h.
23:30h: The rain stops, but the discharge continues to rise until 03:30. The sediment concentration, having peaked at 02:10, quickly returns to its initial value.
Second rain:
The increase in the measurement value around 06:30 seems to precede the rain shower that becomes visible on the river's surface around 06:40.
Notably (but not surprisingly), we first observe an increase in sediment concentration, followed by a rise in discharge. This increase levels off quickly and then rapidly decreases, even with sustained high discharge.
Variability of sediment sources and particle sizes
The rise in sediment concentration in the river is quickly processed, with the extra particles passing through in a short time span. Eight hours after the start of the rain, the measurement returns to a baseline level that is only slightly higher than before the rain.
The sediments in the Laak come from various sources and consist of a wide range of particle sizes. This variability adds to the complexity of measuring and monitoring sediment concentrations.
Challenges for sensors: speed, accuracy, drift and low maintenance
Given the rapid variations in sediment concentrations, it is crucial that the sensors used have a wide dynamic range. They must be able to measure both very low and very high sediment concentrations within a short time span of a few minutes. This imposes high demands on the sensor technology used, which must be both robust and accurate.
This accuracy must not be compromised by sensor drift between maintenance intervals. Due to the unpredictability of rain showers, anticipating critical measurement moments is practically impossible. The sensor must maintain its accuracy at all times, even when the last maintenance was months ago.
The sensor must maintain its accuracy at all times, even when the last maintenance was months ago
Another challenge is the accessibility of measurement locations. Not all locations are next to a road, and often they are much harder to reach, making sensor maintenance difficult. Therefore, a low maintenance frequency is also an important factor to consider when choosing suitable measurement equipment.
The cost and burden of short maintenance intervals is underestimated. While bi-weekly maintenance intervals are common for turbidity sensors (even for those that have wipers!), migrating to sensors with a one-per-season maintenance schedule, makes the same team of operators able to maintain 6 times as many sensors!
Moving from a bi-weekly maintenance schedule to a once-per-season schedule, means the same number of operators can maintain 6 times as many sensors!
The Olpas Tentacle sensor family: Resistant to fouling and drift, without wipers
The Olpas Sediment Load sensor offers a solution to these challenges. This sensor is known for its exceptionally long maintenance interval, exceeding 100 days, and often requires maintenance even less frequently than seasonal intervals. This makes the Olpas Tentacle sensor family ideal for monitoring sediment concentrations in open water, where high accuracy and low maintenance costs are important.
The same similar robust sensor technology is used for sensors that measure in even more challenging conditions: measuring Total Suspended Solids (TSS) in sewers and storm overflows and measuring sludge concentrations in iron slurry.
Conclusion
Measuring sediment concentrations in a dynamic and urbanized river like the Laak requires advanced sensor technology that can respond quickly and accurately to rapid variations. The Olpas Tentacle sensor, with its global reputation for long maintenance intervals, provides an excellent solution to these challenges: it provides accurate and reliable measurements while also allowing very low maintenance cost.
Bonus: A curious heron visiting our sensor ;-)
