D7 Sediment Monitoring and Geomorphology

Topic

Sediment monitoring quantifies two things simultaneously: the solid material transported by a river and the morphological changes occurring on the slopes and within the river channels themselves.

Turbidimeters continuously measure the optical turbidity of the water; this turbidity is then converted—using river-specific calibration curves—into the concentration of suspended sediment being transported at any given moment.

Acoustic Doppler profilers simultaneously measure the flow velocity profile at various depths and the acoustic attenuation caused by suspended sediments; this dual measurement allows for the integration of the total solid discharge across the entire river cross-section, rather than estimating it from a single point.

Sediment traps, by contrast, directly capture material moving under gravity near the channel bed—including both bed load and coarser suspended material that fails to remain suspended throughout the water column.

On slopes, erosion pins measure soil surface lowering or accretion based on the change in the length of the pin exposed between successive measurements; this change in exposed length reveals the amount of soil lost or new sediment accumulated at that specific point on the slope.

Wire extensometers and satellite positioning sensors detect—with millimeter-scale resolution—surface displacement in unstable ground masses (landslides); meanwhile, repeat photography from fixed points allows for the documentation of more gradual morphological changes that occur too slowly for point sensors to capture. Ultimately, the sediment balance for the entire basin integrates the production, transport, and deposition of all such material through the sediment delivery ratio—the fraction of sediment generated within the basin that actually exits it rather than being deposited along the way. Thus, sediment monitoring—encompassing everything from the solid transport and morphological changes that open this document to the sediment delivery ratio that concludes it, and including tools such as turbidimeters, acoustic Doppler profilers, sediment traps, and erosion pins—enables the tracking and quantification of the material moved by an entire basin, from the highest slopes down to the river outlet, at every stage of its journey.

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