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Summary A comprehensive set of experiments was carried out to investigate the effect of the fractal architecture of granular aggregates on the free-fall acceleration through a still water column. Test aggregates were first generated numerically with a method that allowed to control the fractal dimension d and, next, three stochastic replicates were lithographically fabricated for each of six values of d ranging between 1.9 and 2.7. The recorded position, velocity and acceleration served to analyze their dynamics in the Reynolds and Galilei number space, and to calculate the momentum rate of change and the intensity of drag (viscous and impact) and inertial forces (added mass and Basset–Bousinnesq). Analysis of these forces highlighted a strong dependence on d ; additionally, integration of these forces in the particle momentum equation allowed to identify an additional resistance R x that showed a strong correlation with d . A correlation analysis of R x with various scaling laws combining velocity and acceleration suggested that R x could be described by a nonlinear drag force and a force intermediate between drag and inertia. It was therefore concluded that irregular granular fractal aggregates accelerating in water are subject to highly complex and nonlinear hydrodynamic effects caused by surface roughness and volume porosity, and that these effects have tight connection with the internal and external fractal characteristics of the aggregates.

Summary It has been a long lasting interest to understand the laws that underlie water movement on Earth with the purpose to control it and make use of it. It is now recognized that water is not exclusive to the Earth but, rather, is a major constituent of the matter in the universe and may be a proxy for life in other planets as well. In this paper, we reflect on the need to expand the boundaries of the current water sciences from the description of mere terrestrial hydrological processes to a comprehensive inclusion of the physical laws and processes pertaining to water dynamics off Earth. Are we at the edge of a new scope of hydrology?

Summary We describe the development and testing of a mechanistic model (BFLOC) to predict the average size of sediment aggregates in nutrient-rich aqueous ecosystems. The original capability of BFLOC is to couple turbulence-induced flocculation of suspended minerals and micro-organisms with the nutrient-related dynamics of aggregate-attached micro-organisms. The model, calibrated and validated against the average floc size recorded at two stations in the Belgian North Sea [Fettweis, M., Francken, F., Pison V., Ven den Eynde, D., 2006. Suspended particulate matter dynamics and aggregate sizes in a high turbidity area. Marine Geology 235, 63–74], closely captured site conditions and significantly clarified interpretation of field measurements. Modeling results indicated that an accurate prediction of time-varying floc sizes was possible only by taking into account the organic fraction of the suspended particle matter and the micro-organism colonization of the floc micro-environment. BFLOC showed that the floc excess density strongly correlated with the floc biomass volume, while the settling velocity strongly correlated with the floc mineral volume. We noticed that the settling velocity was poorly correlated with the total floc volume (and floc size), suggesting a revision of current methods that assess suspended matter deposition uniquely on the basis of the floc size. Additionally, various hypotheses tested with BFLOC suggested that the effect of aggregate-attached biomass on aggregation and breakup rates was very small when it was accounted for with a first-order description. More generally, the sediment and biomass parameters found here were nearly site independent suggesting that the mechanistic approach of BFLOC was relatively robust.

Flocculation of suspended cohesive sediment, well-known to impact the floc size distribution and vertical fluxes, and cause morphodynamic changes of marine and riverine environments, is modelled by means of a population balance equation that implements a novel description of floc geometry: the capacity dimension of fractal flocs, normally assumed constant over the population, has recently been argued to change during flocculation. Our experiments have shown that a power-law function of the dimensionless floc size can conveniently be used to describe these changes. This description of floc capacity dimension is used to explore in detail the extent to which the geometrical properties of flocs affect aggregation and breakup processes, and contribute to shaping their size distribution. A comparison of experimental floc size distributions from settling column test with computed distributions for two hypotheses of floc capacity dimension (i.e., constant and variable) and two hypotheses of flocculation reactions (i.e., semi-stochastic and deterministic) are shown. This suggests that the use of variable rather than constant floc capacity dimension, and the use of semi-stochastic and asymmetric reactions rather than deterministic and symmetric, result in better predictions of the floc size distribution in the environmental conditions herein analysed.

This paper presents an approach to process and analyse digital images of mud flocs recorded via optical techniques. Grey-scale images of mud flocs consist of bright spots on a dark background, the recordings being characterised by a bimodal intensity (luminosity) distribution. First, we propose a fast algorithm to separate such two-intensity regions to obtain black and white images via Gaussian-curve fitting. This algorithm is evaluated in its capability to recognise the mass distribution and perimeter segmentation of mud flocs. Next, we characterise these images of mud flocs by using the concepts of multifractality, entropy and symmetry. This approach gives indication that a complex behaviour rises in a population of mud flocs, which evolves far from equilibrium. Multifractal scaling and differentiation into dominant and rare geometrically featured flocs are inferred to mark the evolution of a population of cohesive sediment flocs. The mathematical tools and method of analysis presented here are applied to a small set of flocs. Despite this limitation, the results obtained here seem promising, and application of this approach to a wider data set is expected to advance the interpretation of flocculation and floc morphodynamics of suspended cohesive sediment.