Using an optical settling column to assess suspension characteristics within the free, flocculation, and hindered settling regimes

Abstract

Purpose Instruments able to measure the settling velocity distribution (SVD) and investigate the flocculation behavior of suspensions for a wide range of concentrations and settling regimes are required to understand and model sediment transport in headwater catchments. Such knowledge will improve our water resource management capabilities. Materials and methods An optical settling column, equipped with a vertical array of optical sensors, was used to provide light transmission through a suspension during quiescent settling. A new method to determine the settling velocity and the propensity of suspensions to flocculate is proposed. Its reliability was evaluated based on settling tests for (1) noncohesive sediments, (2) cohesive sediments at medium (~1 g l −1 ) concentration in a natural and deflocculated state, and (3) a cohesive sediment at a very high concentration (~10 g l −1 ). This choice of sediments and concentrations allowed for the assessment of free, flocculated, and hindered settling regimes. Results and discussion The proposed data processing method provides measurements for a range of test conditions. The result showed that different populations of particles with different settling behaviors can be identified within the suspensions. In the case of noncohesive sediments, the proposed method provided SVD similar to those obtained with reference methods. The propensity to flocculate was zero as expected for inert material. The natural cohesive sediment at medium concentration exhibited a large range of SVD (10 −2 -10 −5 m s −1 ) and high propensity to flocculate. These were both reduced with the addition of a deflocculant. Identified particle behaviors were consistent with independent measurement of size distribution, microscopic, and erosion properties. In the hindered regime, a narrow SVD corresponded to the hindered front settling velocity (~10 −4 m s −1 ). Conclusions An optical settling column was able to provide reliable SVD and an evaluation of the propensity of particles to flocculate. The settling column was able to detect variations of the settling velocities with settled depth, thus highlighting that settling columns with a single measurement point may provide erroneous results by not accounting for the full spectrum of settling depth.

Assessment of erosion and settling properties of fine sediments stored in cobble bed rivers: the Arc and Isère alpine rivers before and after reservoir flushing

“…The percentiles describing the settling velocity distributions exhibited significant positive correlations with particle sizes (aggregated and dispersed) as well as negative ones with chemical properties (C and species). However, no significant correlations were observed between SCAF variables and SSC (Pearson correlation coefficient r <0.12), even though SSC is known to impact the settling regime of particles (Wendling et al ., ). One reason for this apparent independence of the settling properties with SSC could be the rather small values of SSC for alpine rivers (mean value of 0.9 g L ‐1 for ES and 1.9 g L ‐1 for SM).…”

“…On the contrary, this study was the first to integrate two novel field/laboratory technologies which make it possible both to assess the erodibility of cohesive fine‐grained river bed sediment (using U‐GEMS), and to immediately assess the settling characteristics (using SCAF – velocity and propensity to flocculate) of the eroded material collected at a known shear (at τ crit for example). This integration of methods is made more novel by the fact that there currently are no commercially available instruments that can assess the settling characteristics of samples in excess of grams per liter of water (Wendling et al ., ). Such high SSC frequently occur within alpine rivers with rain and snowmelt events and during flushing events such as that on the Arc River (Figure (a)).…”

“…To do so, instrumentation capable of measuring fine sediment erosion dynamics [UMCES‐Gust Erosion Microcosm System (U‐GEMS), Gust and Müller ()] was used in conjunction with assessing settling dynamics [System Characterizing Aggregates and Flocs (SCAF), Wendling et al . ()] of particles from core samples collected in river beds before and after a dam flushing operated in June 2016 on the Arc River. The latter aim also includes in‐field SCAF temporal measurements of suspended sediments during the dam flushing.…”

“…The settling dynamics of suspended sediment samples were determined using the System Characterizing Aggregates and Flocs (SCAF). We provide here a brief description of the device, but the reader should refer to Wendling et al (2015) for a more complete description of the measurement principle and the data processing. The SCAF is an optical settling column (20 cm high and 3.5 cm in diameter) equipped with 16 infrared (λ=980 nm) emitters and 16 diametrically opposed photosensors located every 1 cm ( Figure 5(a)).…”

“…Given the above significance of fine sediment transport in mountainous rivers and in view to improving sediment management during reservoir flushing, the aims of this study were to (i) demonstrate the utility of taking an integrated approach combining measurements of both erosion and settling dynamics and (ii) make a first assessment of the spatial and temporal variability of such properties within two alpine cobble bed rivers: Arc and Isère. To do so, instrumentation capable of measuring fine sediment erosion dynamics [UMCES-Gust Erosion Microcosm System (U-GEMS), Gust and Müller (1997)] was used in conjunction with assessing settling dynamics [System Characterizing Aggregates and Flocs (SCAF), Wendling et al (2015)] of particles from core samples collected in river beds before and after a dam flushing operated in June 2016 on the Arc River. The latter aim also includes in-field SCAF temporal measurements of suspended sediments during the dam flushing.…”