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

Vignaga, Elisa (2012) The effect of biofilm colonization on the stability of non-cohesive sediments. PhD thesis, University of Glasgow.

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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.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Biostabilization, sediment transport, non-cohesive sediments, flume, erosion, Eco-Hydraulics, PIV, tensile testing
Subjects: Q Science > QE Geology
G Geography. Anthropology. Recreation > GB Physical geography
T Technology > TC Hydraulic engineering. Ocean engineering
Colleges/Schools: College of Science and Engineering > School of Engineering > Infrastructure and Environment
Supervisor's Name: Sloan, Prof William T.
Date of Award: 2012
Depositing User: Miss Elisa Vignaga
Unique ID: glathesis:2012-3505
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 06 Jul 2012
Last Modified: 10 Dec 2012 14:08

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