The effect of biofilm colonization on the stability of non-cohesive sediments

In the past decades, engineers have started to realize the importance of the interaction between vegetation, biota and water flow, in riverine and marine environments; a discipline that has been named “Eco-Hydraulics”. Scientists have valued this coupled phenomenon for much longer than their engineering colleagues. As early as 1970, marine researchers presented the evidence that colonies of micro-organisms might alter the stability of fine cohesive sediments (Neuman et al., 1970). However traditional models of sediments transport (e.g. Shields, 1936) have been derived using abiotic sediments and did not consider that most wet surfaces would soon be colonized by micro-organisms and their extracellular polymeric substances (EPS), a combination called “biofilm” (Lock, 1993). Scientists during the 1990s, after observing this phenomenon in the field, coined the term “biostabilization”. During this period they showed that colonies of cyanobacteria and diatoms coating fine sand or cohesive sediments can increase their stability by up to 960% compared to abiotic sediments (Grant and Gust, 1987; Dade et al, 1990; Paterson 1997). Only recently have engineers started to take into consideration the effect of such increased cohesion and adhesion due to biogenic forces within the sediment transport model (Righetti and Lucarelli, 2007); yet all of those studies have low applicability because they are linked to specific environmental conditions. Moreover no data are available on the effect of biofilm on larger sediments (e.g. coarse sand and gravel). The present thesis provides experimental data carried out in a flume laboratory pertaining to biostabilization of non-cohesive coarse sand and gravels at a scale representation of a real river system (from 0.2m to 1m). Four sediment substratum (glass spheres of D50 = 1.09mm and 2.00mm; sand of D50 = 1.20mm and gravel of D50 = 2.20mm) were colonized under unidirectional flow by a cyanobacterium (Phormidium sp.) for between 1 and 10 weeks. The increase in erosion threshold for biotic sediment is then investigated using a series of different methods ranging from traditional sediment transport techniques (e.g. Yalin, 1972), to image thresholding and particle image velocimetry (PIV) assessments of flow modification due to biofilm presence. Moreover, tensile strength analysis of ex-situ biofilm/substratum specimens will be presented to understand better the mechanical property of this composite material. Data indicates that: i) biostabilization of sediments in the range of coarse sand and gravel occurs (9%-150% more shear stress required to induce entrainment compared to abiotic sediments) but to a lower extent compared to critical entrainment thresholds for fine sand and cohesive sediments (Paterson, 1997); ii) flume experimentation can be employed to control specific variables affecting biostabilization and could help to unfold the complicated interactions between environmental variables, and the affect of flow on the growth and strength of biofilm colonization over sediments; iii) strong biofilm growth generated a more uniform velocity field, with reduction in shear stress (up to 82% compared with abiotic sediments) and decreases in roughness length of the bed (up to 94% compared to abiotic sediments); iv) Composite biofilm/substratum specimens presented a clear elastic behaviour when tensile tested; v) Conventional models of sediment transport (e.g. Wiberg and Smith, 1987) do not consider the presence of biofilm and will not work in the case of bio-mats smoothing the surface of the bed; hence the need for new models which include the biofilm elasticity and the bio-mat smoothing process. This thesis suggests two theoretical examples where the biofilm action is considered at a grain to grain and bio-mat scale.