Flow-induced uprooting of young vegetation on river bedforms

Riparian vegetation stabilizes sediment by its roots and henceforth impacts riparian morphodynamics. After germination or vegetative reproduction on river bars or islands, juvenile plants are exposed to a high risk of mortality due to uprooting by floods. We distinguish two main types of root erosion by flow. As Type I root erosion, we defined a flow induced drag mechanism, which causes a nearly instantaneous uprooting of mainly very young vegetation with not fully developed root system by pullout drag exceeding root resistance. Type II root erosion arises as a combination of bedform erosion resulting in a decreased anchoring resistance of the roots and subsequent Type I uprooting. This second type applies to later stages of root development and is a delayed process induced by sediment erosion of morphodynamic origin. In laboratory experiments we tested the validity of both mechanisms. We investigated the first Type of root erosion mechanism with static uprooting experiments with 1550 seedlings of Avena sativa and Medicago sativa grown in low-cohesive sediment in order to quantify the distribution of their anchorage forces for different sediment size and moisture conditions as well as for varying root structure. Furthermore, we measured root strength of Avena sativa seedlings and compared pullout and breaking force of young vegetation with identical root structure. Type II root erosion mechanism, which is driven by the reduction of root anchorage due to sediment erosion, was investigated in laboratory flume experiments. The intensity of sediment erosion that was required before uprooting occurred increased with increasing root length. The higher the flow, the less time was necessary to erode a seedling of certain root length. Thus, the duration that a given flow requiresto erode a certain root structure, can be associated to a certain vegetation maturity stage. Following, Type II root erosion results from the balance of timescales of both vegetation growth and flood occurrence and duration.

How Does Plant Diversity Affect Soil Aggregate Stability and Erosion Processes in Disturbed Alpine Ecosystems?

Plant species diversity is hypothesized to be among the most relevant factors for enhancing soil stability in disturbed sites at high elevation. Because a more diverse plant community may comprise a high number of plant species of different growth forms, plant functional groups, and root characteristics, the chance of including a species that fulfills a specific key function may increase, thereby ensuring ecosystem integrity. Although plausible, the contribution of plant diversity to soil stabilisation in general, and to erosion control on alpine ski slopes in particular, has scarcely been demonstrated. This thesis investigated the relationship between plant diversity and soil stability in disturbed alpine ecosystems. The main objectives were (i) to determine root traits of 13 alpine pioneer species growing on a machine-graded ski slope without the in uence of neighbors, in order to address mechanisms of belowground diversity effects (Chapter 2), (ii) to quantify the contribution of plant diversity to soil stability by testing the aggregate stability of the soil from machine-graded ski slopes and from adjacent undisturbed vegetation (Chapter 3), (iii) to compare aggregate stability data from both graded and un-graded ski slopes and the associated control sites (Chapter 4), and (iv) to investigate the relationship between interrill erosion, vegetation cover, and plant functional diversity by applying rainfall simulations (Chapter 5). Both field and laboratory experiments were conducted. Chapter 2. Belowground diversity in root characteristics of alpine plants: key traits for soil restoration. Alpine plant species were highly diverse in functional root traits. Three of the 13 studied species were significantly different in root length and spreading, plant age and tensile strength. The results show that belowground diversity can be enhanced substantially by employing a few plant species with specific root traits. This study suggests that restoration attempts should aim at selecting species with a high belowground diversity to achieve a high diversity in functional root types which can positively affect slope stability. Chapter 3. Higher plant diversity enhances soil stability in disturbed alpine ecosystems. Plant diversity, vegetation cover and root density positively influenced aggregate stability, which was significantly lower on ski slopes compared to control sites. Out of all the tested variables, plant diversity explained considerably more variance than abiotic soil parameters. Plant species associated with more diverse communities were from various functional groups, i.e. grasses, forbs and shrubs. A higher density of fine roots and, to a lesser extent, coarse roots positively influenced aggregate stability. This study showed that, in addition to re-establishment of a functional vegetation cover with a dense root system, a high number of plant species with different growth forms is important. Chapter 4. Soil aggregate stability on alpine ski slopes. Aggregate stability was lower on graded ski slopes, but not on ungraded ski slopes compared with control plots . Root length density, number of plant species and vegetation cover were all positively correlated with aggregate stability. However, more than 13 plant species, 80% vegetation cover and 0.011 g cm-3 root density did not cause a further increase in aggregate stability. In multiple regression analysis, we determined that the effect of the number of plant species on aggregate stability was more pronounced on graded and gravelly ski slopes than on un-graded and sandy slopes. This study provided evidence that high plant diversity plays an important role in soil aggregate stability at disturbed alpine sites. Chapter 5. Interrill erosion at disturbed alpine sites: effects of plant functional diversity & vegetation cover. Interrill erosion was reduced the most when vegetation cover was high: 60% vegetation cover reduced the sediment yield by 83% compared to unvegetated ground. At 60% vegetation cover, sediment yield was significantly reduced in the presence of three functional groups compared to one plant functional group. Furthermore, combinations of plant functional groups including grasses reduced the sediment yield more than combinations without grasses. This study supports the view that, besides the re-establishment of a closed vegetation cover, high plant functional diversity is relevant for reducing interrill erosion at disturbed alpine sites. In conclusion, the resistance of topsoil to water-induced erosion at disturbed alpine sites depends not only on the degree of vegetation cover, but also on the presence of a high number of plant species that are highly diverse in functional traits above and below ground.

EPFL2010

Effect of stone cover on rainfall-driven soil erosion with negligible surface roughness

David Andrew Barry, Alessandro Brovelli, Seifeddine Jomaa

Numerous studies have shown that, for soils with sparse vegetation cover, stones/rock fragments are an important factor controlling the hydrological response and soil erosion yields. Additionally, it has been observed that this effect is complex and ambivalent depending on the features of the stones (fraction of soil surface covered, stone size, shape and emplacement, i.e., resting on the soil surface, partially or totally embedded in the soil). The aim of this study was to ascertain whether cumulative soil erosion is proportional to the area of soil exposed under the condition of constant and uniform precipitation onto an initially smooth soil surface. Precipitation-driven soil erosion with surface stone cover and negligible roughness was tested in laboratory flume experiments under controlled conditions. The surface coverage was adjusted by placing stones onto identically prepared surfaces in laboratory flumes. Three experiments were carried out with different stone coverage (20 and 40%), rainfall intensity (28 and 74 mm h-1) and initial moisture content (6.52-24.30%), but with a fixed slope (2.2%). The laboratory results showed that the cumulative mass eroded depends on the cumulative runoff, and that soil erosion is proportional to the soil surface area exposed to raindrops. Additionally, the results showed that this relationship is controlled to a smaller extent by the soil’s initial moisture content and bulk density. Three sets of previously published field data on precipitation-driven soil erosion were analyzed to ascertain whether the same behavior occurs under field conditions. For this case it was found that precipitation-driven erosion is not proportional to the area exposed. The data sets showed consistently that prediction of cumulative eroded mass using only the stone coverage over predicts the measured eroded sediment (when plotted as a function of cumulative runoff). In contrast to the laboratory experiments, the field experiments are characterized by non-uniform initial surface roughness, and heterogeneous stone size and spatial distribution. The data are consistent with the conclusion that soil erosion is controlled by several factors including area of soil exposed, surface roughness, stone size and spatial distribution, and surface soil aging. However, the presented laboratory results show clearly that, for soils with negligible surface roughness, erosion depends on (i) the area of soil exposed to rainfall and (ii) the cumulative runoff, and (iii) that it is only slightly dependent on other soil variables.

2011

Evidence and modelling biomass selection by floods in a riverbed

Benoît Crouzy, Paolo Perona

Riparian and in-bed vegetation growth and erosion dynamics are strongly coupled with river hydrologic and morphodynamic processes. Many field observations documented the engineering role of vegetation and its contribution to build, stabilize and control erosion and deposition on gravel bars. Yet unclear, is how do the interplay between the hydraulic time scale (arrival time of flood disturbances) and the vegetation germination and growth rates determines the statistical distribution of the surviving vegetation and of that being removed by flooding events. Soon after germination on exposed gravel bars, young vegetation can be removed by even moderate floods without substantial channel reworking, root anchoring being almost negligible at this stage of growth. Hence, understanding survival and uprooting dynamics of early germinated seedlings or rejuvinated woody debris is key to unravel their role in the future evolution of alluvial forms. In this paper, we provide experimental evidence and a model on how the statistics of young biomass up- rooted by flow depend on stream power and on the above- and below-ground characteristics of the biomass growing in the alluvial sediment. Particularly, we present results of recent laboratory experiments where periodic flow disturbances of constant magnitude have been run on sand flumes with vegetation growing in situ at a rate that is comparable to the interarrival time of disturbances. This allows to access the regime of competition between vegetation development and flood disturbances. While non-eroded plants continue to grow in the successive runs, flow erosion acts preferentially on plants that have a weaker root system. Eventually, the statistic of the uprooted biomass turns out to be time independent, leading to suppose the existence of a biomass selection mechanism operated by floods in alluvial floodplains. We then propose a simple time and state dependent stochastic model to quantitatively explain this conjecture. Eventually, our minimalist model can be solved analytically, and to a certain extent it helps shedding light on which components of the hydrologic and vegetation processes and related time scales control the erosion mechanism of early germinated vegetation.

2011

How Does Plant Diversity Affect Soil Aggregate Stability and Erosion Processes in Disturbed Alpine Ecosystems?

EPFL2010

Evidence and modelling biomass selection by floods in a riverbed

Benoît Crouzy, Paolo Perona

2011

Effect of stone cover on rainfall-driven soil erosion with negligible surface roughness

David Andrew Barry, Alessandro Brovelli, Seifeddine Jomaa

2011