Technical article
Dipl.-Ing. David Mauro, Geospector Munich
To ensure their retention and settling function, stormwater treatment facilities must be checked regularly – usually annually – for sediment inflow. If deposit thicknesses or volumes are exceeded, they are then cleaned out at great effort and expense.
Accurately recording sediment inflow is difficult, especially for facilities with a permanent pool, and is usually done only by spot checks. Combined with complex basin geometries, this approach can lead to grossly incorrect results and is therefore often unsuitable as a basis for decisions and planning. The following shows the typical pitfalls using an example from real-world practice and presents a solution for more accurate surveys.
Sediment thickness is usually measured with a sounding rod. In the simplest case, the rod is driven from the water surface down to the bed; when it is pulled back in, the material adhering to it directly shows the sludge thickness. However, the water surface of basins with a permanent pool is usually difficult to access; only rarely, with favorable geometries or from jetties, can the deeper areas of the facility down to the bed also be reached from the bank.
Boats therefore often have to be used, which is cumbersome and requires extensive safety measures, in order to obtain even coverage of the entire facility area. The measurements usually remain limited to a few points, and their positions are documented only schematically.
Once the supposedly representative samples have been taken, they are usually aggregated into an average sediment thickness by simple averaging and then converted into a sediment volume by multiplying by the water surface area. This approach is perfectly appropriate for larger facilities and ponds. For small basins and ponds with complex bed profiles and sloping banks, however, gross errors can occur that go far beyond the usual level of measurement uncertainty. This particularly affects settling basins, which usually have these characteristics.
This insight emerged recently during actual facility surveys using the alternative method presented below, which delivers exact results. A comparison with surveys from recent years that had been carried out using the averaging method showed deviations of up to 100% of the calculated sediment volume. As a result, either unnecessary rehabilitation work may have been initiated in the past or work that was actually necessary may have been omitted (the volume deviations can be both positive and negative). This prompted the author to publish this article to raise awareness of the issue.
The old and newly measured sediment thicknesses from the facility surveys were quite comparable (short interval between them); the error therefore had to lie in the distribution of the samples and the averaging evaluation. To illustrate this, here is an abstracted but real survey example of a typical settling basin:
Originally, five reasonably evenly distributed soundings were taken within the water surface (Figure 1). The calculation using the averaging method (Table 1) gives a volume of 202 m³. However, the geometry of the basin and the sediment inflow (Figure 2) result in an actual volume of 76.2 m³!
Looking at the basin geometry, it becomes clear that the soundings, which at first appear reasonably evenly distributed over the area, in reality give greater weight to the deep areas of the basin with higher sediment loads – the mean value is therefore no longer representative.
A possible first attempt at improvement is to add further soundings, especially in the edge areas (Figure 3, left). As a final correction, the actual sediment area is used instead of the water surface area (Figure 3, right).
Table 1: Volume calculation with step-by-step improvement of the result
| Calculation | Mean (sediment) thickness | Area | Volume = area × mean thickness |
|---|---|---|---|
| Original measurement (Figure 1) | 1/5 × (3 × 1.50 m + 2 × 0.28 m) = 1.01 m | Water: 10 m × 20 m = 200 m² | 200 m² × 1.01 m = 202 m³ |
| Extended measurement (Figure 3, left) | 1/9 × (3 × 1.50 m + 6 × 0.28 m) = 0.69 m | Water: 10 m × 20 m = 200 m² | 200 m² × 0.69 m = 137 m³ |
| With sludge area (Figure 3, right) | 1/9 × (3 × 1.50 m + 6 × 0.28 m) = 0.69 m | Sediment: 16 m × 6.40 m = 102.40 m² | 102.40 m² × 0.69 m = 71 m³ |
This value finally corresponds to the actual volume with sufficient accuracy. The following conclusions can be drawn from this example:
In practice, implementing these recommendations is likely to involve considerable effort or even be impossible. An alternative approach therefore suggests itself, which is usually much faster and cheaper and at the same time delivers more exact volumes.
The above conclusions lead to the following approach:
In contrast to the individual manual soundings of the averaging approach, all measurements are carried out comprehensively, in detail and automatically using specialized drone sensors (air, water, underwater) (Figure 4).
Depending on requirements, these basic data can then be further processed into 2D and 3D plans, condition reports, visualizations, etc.
Thanks to the latest drone and sensor technology, this approach is actually faster and more economical than conventional methods – while offering unrivaled precision.
With the Geospector Sludge Check, we survey the sediment surface and bed of your stormwater retention facilities area-wide by drone – without a boat and without the need for an existing baseline survey.
The full article is also available as a PDF download (in German).
