Quantifying biostabilisation effects of biofilm-secreted and extracted extracellular polymeric substances (EPSs) on sandy substrate

Microbial assemblages (<q>biofilms</q>) preferentially develop at water–sediment interfaces and are known to have a considerable influence on sediment stability and erodibility. There is potential for significant impacts on sediment transport and morphodynamics, and hence on the longer-term evolution of coastal and fluvial environments. However, the biostabilisation effects remain poorly understood and quantified due to the inherent complexity of biofilms and the large spatial and temporal (i.e. seasonality) variations involved. Here, we use controlled laboratory tests to systematically quantify the effects of natural biofilm colonisation as well as extracted extracellular polymeric substances (EPSs) on sediment stability. Extracted EPSs may be useful to simulate biofilm-mediated biostabilisation and potentially provide a method of speeding up timescales of physical modelling experiments investigating biostabilisation effects. We find a mean biostabilisation effect due to natural biofilm colonisation and development of almost 4 times that of the uncolonised sand. The presented cumulative probability distribution of measured critical threshold for erosion of colonised sand reflects the large spatial and temporal variations generally seen in natural biostabilised environments. For identical sand, engineered sediment stability from the addition of extracted EPSs compares well across the measured range of the critical threshold for erosion and behaves in a linear and predictable fashion. Yet, the effectiveness of extracted EPSs to stabilise sediment is sensitive to the preparation procedure, time after application and environmental conditions such as salinity, pH and temperature. These findings are expected to improve biophysical experimental models in fluvial and coastal environments and provide much-needed quantification of biostabilisation to improve predictions of sediment dynamics in aquatic ecosystems

Quantifying biostabilisation effects of biofilm-secreted and extracted extracellular polymeric substances (EPSs) on sandy substrate

Microbial assemblages (biofilms) preferentially develop at water–sediment interfaces and are known to have a considerable influence on sediment stability and erodibility. There is potential for significant impacts on sediment transport and morphodynamics, and hence on the longer-term evolution of coastal and fluvial environments. However, the biostabilisation effects remain poorly understood and quantified due to the inherent complexity of biofilms and the large spatial and temporal (i.e. seasonality) variations involved. Here, we use controlled laboratory tests to systematically quantify the effects of natural biofilm colonisation as well as extracted extracellular polymeric substances (EPSs) on sediment stability. Extracted EPSs may be useful to simulate biofilm-mediated biostabilisation and potentially provide a method of speeding up timescales of physical modelling experiments investigating biostabilisation effects. We find a mean biostabilisation effect due to natural biofilm colonisation and development of almost 4 times that of the uncolonised sand. The presented cumulative probability distribution of measured critical threshold for erosion of colonised sand reflects the large spatial and temporal variations generally seen in natural biostabilised environments. For identical sand, engineered sediment stability from the addition of extracted EPSs compares well across the measured range of the critical threshold for erosion and behaves in a linear and predictable fashion. Yet, the effectiveness of extracted EPSs to stabilise sediment is sensitive to the preparation procedure, time after application and environmental conditions such as salinity, pH and temperature. These findings are expected to improve biophysical experimental models in fluvial and coastal environments and provide much-needed quantification of biostabilisation to improve predictions of sediment dynamics in aquatic ecosystems

Morphological and Stratigraphical Signature of Floods In A Braided Gravel-Bed River Revealed From Flume Experiments

The alluvial deposits of braided rivers consist of channel-form bounding surfaces due to channel cutting and migration. It is unknown, however, to what extent these bounding surfaces result either from more gradual channel processes or from large but infrequent flood events. Field measurements of morphodynamically formed bounding surfaces associated with floods are not currently available. Flume experiments are useful here since concurrent observations of channel migration and the formation of bounding surfaces can be made whilst conditions are controlled. We report on two flume experiments with the objective to determine the effects of floods on i) channel cutting and migration and ii) alluvial architecture of braided gravel-bed rivers. One of the experiments has a constant bankfull discharge, and the other is run with a schematized long-duration low flow and short-duration high flow but otherwise similar conditions. High-resolution digital elevation models (DEMs) are used to monitor channel migration and to create the three-dimensional braid-belt architecture. The data demonstrate that the flood events result in longer bars and more frequent chute cutoffs. The DEMs show neither deeper channels nor bar aggradation during multiple flood events compared to a steady bankfull discharge. Consequently, the braid-belt architecture that forms during these large floods does not significantly differ from the stratigraphic architecture that forms during more frequent bankfull flows. This implies that these events cannot be differentiated based on stratigraphy. Furthermore, we find that between 10% and 40% of the mean channel depth is preserved. This can be used to characterize braid-belt architecture for purposes of reservoir engineering and provides error bounds to reconstruct paleochannel dimensions from stratigraphy

Morphological and Stratigraphical Signature of Floods In A Braided Gravel-Bed River Revealed From Flume Experiments

The alluvial deposits of braided rivers consist of channel-form bounding surfaces due to channel cutting and migration. It is unknown, however, to what extent these bounding surfaces result either from more gradual channel processes or from large but infrequent flood events. Field measurements of morphodynamically formed bounding surfaces associated with floods are not currently available. Flume experiments are useful here since concurrent observations of channel migration and the formation of bounding surfaces can be made whilst conditions are controlled. We report on two flume experiments with the objective to determine the effects of floods on i) channel cutting and migration and ii) alluvial architecture of braided gravel-bed rivers. One of the experiments has a constant bankfull discharge, and the other is run with a schematized long-duration low flow and short-duration high flow but otherwise similar conditions. High-resolution digital elevation models (DEMs) are used to monitor channel migration and to create the three-dimensional braid-belt architecture. The data demonstrate that the flood events result in longer bars and more frequent chute cutoffs. The DEMs show neither deeper channels nor bar aggradation during multiple flood events compared to a steady bankfull discharge. Consequently, the braid-belt architecture that forms during these large floods does not significantly differ from the stratigraphic architecture that forms during more frequent bankfull flows. This implies that these events cannot be differentiated based on stratigraphy. Furthermore, we find that between 10% and 40% of the mean channel depth is preserved. This can be used to characterize braid-belt architecture for purposes of reservoir engineering and provides error bounds to reconstruct paleochannel dimensions from stratigraphy

Biogeomorphology, quo vadis? : On processes, time, and space in biogeomorphology

Biogeomorphology has been expanding as a discipline, due to increased recognition of the role that biology can play in geomorphic processes, as well as due to our increasing capacity to measure and quantify feedbacks between biological and geomorphological systems. Here, we provide an overview of the growth and status of biogeomorphology. This overview also provides the context for introducing this special issue on biogeomorphology, and specifically examines the thematic domains of biogeomorphological research, methods used, open questions and conundrums, problems encountered, future research directions, and practical applications in management and policy (e.g. Nature based solutions). We find that whilst biogeomorphological studies have a long history, there remain many new and surprising biogeomorphic processes and feedbacks that are only now being identified and quantified. Based on the current state of knowledge, we suggest that linking ecological and geomorphic processes across different spatio‐temporal scales emerges as the main research challenge in biogeomorphology, as well as the translation of biogeomorphic knowledge into management approaches to environmental systems. We recommend that future biogeomorphic studies should help to contextualise environmental feedbacks by including the spatio‐temporal scales relevant to the organism(s) under investigation, using knowledge of their ecology and size (or metabolic rate). Furthermore, in order to sufficiently understand the ‘engineering’ capacity of organisms, we recommend studying at least the time period bounded by two disturbance events, and recommend to also investigate the geomorphic work done during disturbance events, in order to put estimates of engineering capacity of biota into a wider perspective. Finally, the future seems bright, as increasingly inter‐disciplinary and longer‐term monitoring are coming to fruition, and we can expect important advances in process understanding across scales and better informed modelling effort

Preservation of meandering river channels in uniformly aggrading channel belts

Channel belt deposits from meandering river systems commonly display an internal architecture of stacked depositional features with scoured basal contacts due to channel and bedform migration across a range of scales. Recognition and correct interpretation of these bounding surfaces is essential to reconstruction of palaeochannel dimensions and to flow modelling for hydrocarbon exploration. It is therefore crucial to understand the suite of processes that form and transfer these surfaces into the fluvial sedimentary record. Here the numerical model ‘NAYS2D’ is used to simulate a highly sinuous meandering river with synthetic stratigraphic architectures that can be compared directly to the sedimentary record. Model results highlight the importance of spatial and temporal variations in channel depth and migration rate to the generation of channel and bar deposits. Addition of net uniform bed aggradation (due to excess sediment input) allows quantification of the preservation of meander morphology for a wide range of depositional conditions. The present authors found that the effect of vertical variation in scouring due to channel migration is generally orders of magnitude larger than the effect of bed aggradation. This explains the limited impact bed aggradation has on preservation of meander morphology. Moreover, lateral differences in stratigraphy within the meander belt are much larger than the stratigraphic imprint of bed aggradation. Repeatedly produced alternations of point bar growth followed by cut-off result in a vertical trend in channel and scour feature stacking. Importantly, this vertical stacking trend differs laterally within the meander belt. In the centre of the meander belt, the high reworking intensity results in many bounding surfaces and disturbed deposits. Closer to the margins, reworking is infrequent and thick deposits with a limited number of bounding surfaces are preserved. These marginal areas therefore have the highest preservation potential for complete channel deposits and are thus best suited for palaeochannel reconstruction

Quantifiable effectiveness of experimental scaling of river- and delta morphodynamics and stratigraphy

Laboratory experiments to simulate landscapes and stratigraphy often suffer from scale effects, because reducing length- and time scales leads to different behaviour of water and sediment. Classically, scaling proceeded from dimensional analysis of the equations of motion and sediment transport, and minor concessions, such as vertical length scale distortion, led to acceptable results. In the past decade many experiments were done that seriously violated these scaling rules, but nevertheless produced significant and insightful results that resemble the real world in quantifiable ways. Here we focus on self-formed fluvial channels and channel patterns in experiments. The objectives of this paper are 1) to identify what aspects of scaling considerations are most important for experiments that simulate morphodynamics and stratigraphy of rivers and deltas, 2) to establish a design strategy for experiments based on a combination of relaxed classical scale rules, theory of bars and meanders, and small-scale experiments focussed at specific processes. We present a number of small laboratory setups and protocols that we use to rapidly quantify erosional and depositional types of forms and dynamics that develop in the landscape experiments as a function of detailed properties, such as effective material strength, and to assess potential scale effects. Most importantly, the width-to-depth ratio of channels determines the bar pattern and meandering tendency. The strength of floodplain material determines these channel dimensions, and theory predicts that laboratory rivers should have 1.5 times larger width-to-depth ratios for the same bar pattern. We show how floodplain formation can be controlled by adding silt-sized silicaflour, bentonite, Medicago sativa (alfalfa) or Partially Hydrolyzed PolyAcrylamide (a synthetic polymer) to poorly sorted sediment. The experiments demonstrate that there is a narrow range of conditions between no mobility of bed or banks, and too much mobility. The density of vegetation and the volume proportion of silt allow well-controllable channel dimensions whereas the polymer proved difficult to control. The theory, detailed methods of quantification, and experimental setups presented here show that the rivers and deltas created in the laboratory seem to behave as natural rivers when the experimental conditions adhere to the relaxed scaling rules identified herein, and that required types of fluvio-deltaic morphodynamics can be reproduced based on conditions and sediments selected on the basis of a series of small-scale experiments