The FOEN operates a nationwide network to monitor the transport of sediment in bodies of water. This involves measuring the concentration of suspended load and turbidity in watercourses, as well as recording quantities of bed load material collected in selected traps. Data from these measurements can be obtained from the FOEN. In previous years, river deltas were also surveyed. Within pilot projects, suspended loads are additionally analysed for various chemical and physical parameters, including heavy metals and microplastics.
What is sediment transport?
Sediment refers to all solid substances that are transported or deposited in water. In rivers and streams, it occurs in three forms: as suspended load, as bed load, and as floating debris. Of these, suspended load typically makes up the largest component of sediment.
- Suspended sediment load consists of fine, lightweight particles that are stirred up and suspended in the water. It typically includes clay and silt, but in rivers with steeper gradients and in mountain streams, can also contain sand. In stretches of water with little current, and in lakes, the suspended load sinks to the bottom (settleable solids) forming sediment layers.
- Bed load, in contrast, consists of coarser material, such as sand, gravel, stones and even large boulders. It is transported along the bottom of streams and rivers by rolling, sliding or bouncing. It requires greater transport energy from the flowing water, such as when there is higher discharge (floods) or greater gradient (mountain streams).
- Floating debris is predominantly of organic origin, such as aquatic plants and driftwood, but can also include plastic waste on the surface of the water.
Sediment transport often increases significantly during floods, when large quantities of material can be transported to flooded areas within a short space of time. This can even alter the course of the waterway under certain circumstances.
Messnetz des Bundes für den Transport suspendierter Sedimente in Fliessgewässern – Geschichte und zukünftige Entwicklung (PDF, 1 MB, 15.09.2025)Wasser, Energie, Luft, 1/2021
Betrieb des Bundes-Messnetzes für den Transport suspendierter Sedimente in Fliessgewässern (PDF, 3 MB, 15.09.2025)Wasser, Energie, Luft, 1/2021
Feststoffbeobachtung in der Schweiz
This publication does not exist in English. It is available in other languages. 2005
The importance of monitoring sediment
Information about sediment transport is of great importance for ecological and economic reasons, as detailed below.
- Water protection and pollutant transport: Suspended sediment plays an important role in water quality as it can adsorb and accumulate numerous pollutants and contaminants, such as heavy metals and organic compounds. Consequently, it facilitates the transport of these substances within the water body, thus contributing to the spread of pollution. As such, suspended sediment acts as a reservoir, and – when it gradually releases these compounds back into the environment – as a secondary source of long-term environmental pollution. Knowing the physical and chemical properties of these particles and their pollutant load is therefore key to protecting the ecological integrity of aquatic ecosystems and assessing water quality.
- Protection of aquatic life: Fine sediment can adversely affect aquatic habitats. Gaps between coarser particles at the bottom of a water body allow good permeability for water and provide small niches for lithophilic and rheophilic species. The importance of this can be clearly seen with the example of trout, which lay their eggs in gravel, where they are well supplied with oxygen-rich water. However, if the spaces between the gravel become clogged with fine sediment, the flow of oxygen-rich water through the gravel is reduced, which means trout eggs may no longer receive enough oxygen and can die. Too much fine sediment can lead to the loss of important habitat for lithophilic and rheophilic species, leading to high spawning mortality and a decline in biodiversity. In addition, a high concentration of suspended sediment in the water reduces light penetration, which limits photosynthetic activity and, consequently, primary production. This results in reduced growth of algae and aquatic plants. Furthermore, the suspended particles absorb heat, leading to an increase in water temperature. High concentrations of suspended sediment can also harm invertebrates and fish by clogging their filtering and respiratory organs, and by inhibiting the development of eggs and larvae. Fine particles have a more negative effect than coarse particles, which is why monitoring the concentration of suspended sediment is essential for protecting aquatic biodiversity.
- Protection of groundwater: Fine sediment can also hinder the process of groundwater recharge. The process whereby the pores in the sediment at the bottom of a body of water become clogged is known as colmation. Colmation reduces the permeability of the riverbed, which in turn limits the replenishment of the underlying groundwater with river water. If too much groundwater is extracted, the groundwater level will drop. Conversely, fine sediment acts as a filtering layer, retaining particles, bacteria and some pollutants, and preventing them from entering the groundwater.
- Protection of technical installations and infrastructure: Dynamic sediment transport can pose a significant risk to infrastructure such as bridges, dams and other technical installations. Targeted monitoring allows hazards such as erosion and abrasion to be identified at an early stage, enabling protective measures to be taken. In reservoir management, the accumulation of sediment in the lake basin can reduce storage capacity and consequently the volume of water available for electricity generation. As around 59.6% of Switzerland's electricity is generated from hydropower (SFOE, 2024), it is essential that plant operators actively monitor their infrastructure. Turbines must also be protected against wear and tear, which is achieved using filters or other technical equipment (e.g. desanders) based on measurements of the concentration and particle size distribution of the suspended sediment. Monitoring data on suspended sediment can therefore help prevent damage due to wear and tear and ensure the long-term efficiency of installations.
- Siltation, sedimentation and boat navigation: Sediment transported by tributaries into lakes is deposited there and forms deltas. This natural process of silting and sedimentation can impair boat navigation in the long term. To keep waterways open, it is necessary to measure the quantities of sediment being transported and deposited. Sometimes dredging is required to remove any obstacles to boat navigation.
Conversely, the accumulation of sediment in deltas can lead to instability and landslides, particularly during floods. Such landslides can cause significant damage to infrastructure. Monitoring suspended sediment load is therefore essential for understanding developing deltas, assessing long-term risks and planning effective countermeasures.
- Assessing stability, erosion and soil loss in water catchment areas: The consequences of climate change, such as prolonged periods of drought, heavier rainfall, retreating glaciers and rising temperatures, are leading to increased erosion and soil loss in water catchment areas. This, in turn, can increase the transport of suspended sediment in bodies of water. By measuring suspended sediment and bed load, it is possible to detect erosion processes within a catchment area. This enables erosion rates to be compared across catchments, while also supporting the evaluation of erosion control measures.
Information on the individual monitoring networks
Bed Load Monitoring Network
The FOEN records the volume of bed load transported by bodies of water from around 100 bed load traps. The cantons and research institutes provide the FOEN with the volume values of bed load transported and deposited in the traps, which are estimated using measuring instruments or when the bed load traps are emptied.
Suspended Sediment Monitoring Network
The monitoring of suspended sediment began in the 1960s to document the condition of bodies of water and long-term changes, as well as to assess the effectiveness of legislation in the areas of water and environmental protection.
Over the years, suspended sediment has been recorded at 59 monitoring sites in watercourses. Currently, suspended sediment concentrations are monitored at 14 strategically located stations, situated on major rivers upstream of large lakes. Samples are taken twice a week from the surface of the centre of the river using a suspended sediment sampler developed by the FOEN. In addition, a more detailed cross-sectional profile of suspended sediment in the river is carried out once a year, in which samples are taken at various verticals and at different depths. This multidimensional sampling allows the distribution of suspended sediment concentrations across the river cross-section to be assessed.
The measured concentrations are used – together with the flow measurements and turbidity – to calculate the annual suspended sediment load. Additional chemical analyses are planned in the coming years in order to fully assess the chemical pollution and contamination of water bodies. These will also include testing for heavy metals and persistent, highly hydrophobic organic pollutants such as polycyclic aromatic hydrocarbons (PAHs), as these are often transported bound to suspended sediment. Since particle size affects how these pollutants are carried, the size distribution of the sediment particles is also determined. Suspended sediment also includes microplastics and tyre abrasion.
Turbidity Monitoring Network
The monitoring of turbidity (reduced transparency) in bodies of water is used, among other things, to estimate suspended sediment concentrations. Turbidity values are a non-specific indicator of this. The correlation between turbidity and suspended sediment concentration is not always linear due to the optical properties of suspended solids, for example their refractive index, shape and other characteristics.
The FOEN's 14 suspended sediment monitoring stations are equipped with optical probes to continuously monitor turbidity. Turbidity is measured using a nephelometric method, in which a probe emits a beam of light into the surrounding water. This light is scattered by particles in the water and only partially reflected back to the probe's measurement window. The intensity of the scattered light that reaches the measurement window is an indicator of the particle concentration in the water. For different concentration ranges, the probe can detect the scattered light at different angles relative to the emitted beam (i.e. at 90° and 140° for higher concentrations).
Acoustic sediment flux profiler
Optical turbidity measurements are strongly influenced by the particle size and shape of suspended sediment. The distribution of particle size is usually not uniform and can change rapidly depending on flow behaviour and catchment conditions. As a continuous input parameter for calculating total suspended sediment loads, turbidity measurements therefore involve a degree of uncertainty. To improve the suspended sediment monitoring network and reduce uncertainties, the FOEN commissioned the Vienna University of Technology to further develop and improve the existing system. The project focuses on hydroacoustic measurement technology, specifically an acoustic sediment flux profiler (ASFP), which enables the suspended sediment concentration, particle size and flow velocity to be continuously monitored across entire river cross-sections. The ASFP is intended to significantly improve the accuracy and temporal resolution of measurements, better capture events with increased transport such as heavy rainfall, and reduce manual sampling and laboratory analyses in the long term.
Delta and lake bed monitoring
Monitoring the development of deltas where watercourses flow into lakes is important, as it can have an impact on the watercourses (changes in morphology, reduction in gradient) and on the lakes themselves (siltation, changes in the biosphere, drinking water abstraction). As a rule, the FOEN no longer carries out delta and lake bed surveys. However, bathymetric data are still available via swissBATHY3D, a digital terrain model that provides a detailed representation of the topography of Swiss lake beds.
Documents
Die Mengenmässige Erfassung von Schwebstoffen und Geschiebefrachten (PDF, 9 MB, 22.03.2013)GHO Mitteilung Nr. 2
Empfehlung zur Abschätzung von Feststofffrachten in Wildbächen I (PDF, 3 MB, 26.05.2009)GHO Mitteilung Nr. 4
Empfehlung zur Abschätzung von Feststofffrachten in Wildbächen II (PDF, 13 MB, 26.05.2009)GHO Mitteilung Nr. 4
Geschiebelieferung alpiner Wildbachsysteme bei Grossereignissen (PDF, 9 MB, 02.06.2009)Ereignisanalysen und Entwicklung eines Abschätzverfahrens. Dissertation von Eva Gertsch, Universität Bern, 2009.
Feststoffverlagerung in steilen Fliessgewässern (PDF, 4 MB, 26.05.2009)GHO Mitteilung Nr. 5
Anhänge zur Dissertation von Eva Gertsch (PDF, 20 MB, 02.06.2009)Factsheets zu den untersuchten Wildbacheinzugsgebieten; Anleitung zur GIS-basierten Herleitung der Inputparameter für das Geschiebeabschätzverfahren; Vorlagen der Hang-Beurteilungsmatrix und Gerinne-Beurteilungsmatrix des Geschiebeabschätzverfahrens
Monitoring von Feststofffrachten in schweizerischen Wildbächen (PDF, 564 kB, 07.04.2010)In "Wasser, Energie, Luft", Heft 1, 2010.
Ereignisanalyse der Hochwasser vom August 2005 in Bächen des Beobachtungsnetzes der GHO (PDF, 607 kB, 19.08.2009)Studie im Auftrag des BAFU, 2009
Die Anhänge zu dieser Studie sind zu beziehen über:
hydrologie@bafu.admin.ch
Abschätzung von ereignisbasierten Feststoffvolumina in Schweizer Wildbächen mit neuronalen Netzwerken (PDF, 3 MB, 31.05.2016)Masterarbeit von Jan Baumgartner, Universität Bern, 2016
Geomorphologische Auswirkungen von Geschiebesammlern auf den Unterlauf in Wildbächen (PDF, 8 MB, 31.05.2016)Eine Untersuchung an Geschiebesammlern der Datenbank Solid. Masterarbeit von Silvia Käser, Universität Bern, 2016.
Das Modell sedFlow und Erfahrungen aus Simulationen des Geschiebetransports in fünf Gebirgsflüssen der Schweiz (PDF, 19 MB, 03.06.2016)Synthesebericht der Eidgenössischen Forschungsanstalt für Wald, Schnee und Landschaft WSL, 2015.
Datenbank Solid - Konzeptoptimierung (PDF, 1 MB, 03.06.2016)Schlussbericht, Universität Bern, 2015. Im Auftrag des BAFU
Other literature references on this subject is listed in the document below. The studies and reports are available on request from the following address: hydrologie@bafu.admin.ch.
Last modification 15.09.2025
