In the past decades, engineers have started to realize the importance of the interactionbetween vegetation, biota and water flow, in riverine and marine environments; adiscipline that has been named “Eco-Hydraulics”. Scientists have valued this coupledphenomenon for much longer than their engineering colleagues. As early as 1970,marine researchers presented the evidence that colonies of micro-organisms might alterthe stability of fine cohesive sediments (Neuman et al., 1970). However traditionalmodels of sediments transport (e.g. Shields, 1936) have been derived using abioticsediments and did not consider that most wet surfaces would soon be colonized bymicro-organisms and their extracellular polymeric substances (EPS), a combinationcalled “biofilm” (Lock, 1993). Scientists during the 1990s, after observing thisphenomenon in the field, coined the term “biostabilization”. During this period theyshowed that colonies of cyanobacteria and diatoms coating fine sand or cohesivesediments 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 engineersstarted to take into consideration the effect of such increased cohesion and adhesiondue 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 specificenvironmental conditions. Moreover no data are available on the effect of biofilm onlarger sediments (e.g. coarse sand and gravel).The present thesis provides experimental data carried out in a flume laboratorypertaining to biostabilization of non-cohesive coarse sand and gravels at a scalerepresentation 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 (Phormidiumsp.) for between 1 and 10 weeks. The increase in erosion threshold for biotic sedimentis then investigated using a series of different methods ranging from traditionalsediment transport techniques (e.g. Yalin, 1972), to image thresholding and particleimage velocimetry (PIV) assessments of flow modification due to biofilm presence.Moreover, tensile strength analysis of ex-situ biofilm/substratum specimens will bepresented to understand better the mechanical property of this composite material.Data indicates that: i) biostabilization of sediments in the range of coarse sand andgravel occurs (9%-150% more shear stress required to induce entrainment compared toabiotic sediments) but to a lower extent compared to critical entrainment thresholdsfor fine sand and cohesive sediments (Paterson, 1997); ii) flume experimentation can beemployed to control specific variables affecting biostabilization and could help tounfold the complicated interactions between environmental variables, and the affect offlow on the growth and strength of biofilm colonization over sediments; iii) strongbiofilm 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 thebed (up to 94% compared to abiotic sediments); iv) Composite biofilm/substratumspecimens presented a clear elastic behaviour when tensile tested; v) Conventionalmodels of sediment transport (e.g. Wiberg and Smith, 1987) do not consider thepresence of biofilm and will not work in the case of bio-mats smoothing the surface ofthe bed; hence the need for new models which include the biofilm elasticity and thebio-mat smoothing process. This thesis suggests two theoretical examples where thebiofilm action is considered at a grain to grain and bio-mat scale.
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The effect of biofilm colonization on the stability of non-cohesive sediments
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