I would like to acknowledge three research grants/contracts that are supporting my current research on this theme: Grant F/07 040/AP from the Leverhulme Trust; Grant NE/F014597/1 from the Natural Environment Research Council, UK, and the REFORM collaborative project funded by the European Union Seventh Framework Programme under grant agreement 282656.

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Plants as river system engineers

Preventing the off-site effects of soil erosion is an essential part of good catchment management. Most efforts are in the form of on-site soil and water conservation measures. However, sediment trapping can be an alternative (additional) measure to prevent the negative off-site
effects of soil erosion. Therefore, not all efforts should focus solely on on-site soil conservation but also on the safe routing of sediment-laden flows and on creating sites and conditions where sediment can be trapped. Sediment trapping can be applied on-site and off-site and
involves both vegetative and structural measures. This paper provides an extensive review of scientific journal articles, case studies and other reports that have assessed soil conservation efforts and the sediment trapping efficacy (STE) of vegetative and structural measures. The
review is further illustrated through participatory field observation and stakeholders’ interview. Vegetation type and integration of two or more measures are important factors influencing STE. In this review, the STE of most measures was evaluated either individually or in such
combinations. In real landscape situations, it is not only important to select the most efficient erosion control measures but also to determine their optimum location in the catchment. Hence, there is a need for research that shows a more integrated determination of STE at catchment
scale. If integrated measures are implemented at the most appropriate spatial locations within a catchment where they can disconnect landscape units from each other, they will decrease runoff velocity and sediment transport and, subsequently, reduce downstream flooding and sedimentation problems.

Soil conservation through sediment trapping: a review

https://people.cas.sc.edu/ajames/Research/Pubs/12%20Bishop%20et%20al.pdf

Geospatial technologies and digital geomorphological mapping: Concepts, issues and research

A review of the mechanical effects of plant roots on concentrated flow erosion rates

The cover provided by forest vegetation makes it possible to fight erosion efficiently. Furthermore, vegetation barriers can also play a major role by trapping eroded sediments and thus vegetation located downslope of a gully can be important. The objective of this study is to highlight the role of vegetation distribution in a marly gully in reducing sediment loss at the outlet. To do this, sediment yield in gullies with similar geological and geomorphological components, but with different vegetation cover and distribution, was studied for 2 years. Sediment traps installed at gully outlets permitted the distinction of active gullies from inactive ones. The results show that gully activity is not correlated with the percentage of total vegetation cover, but with the percentage of cover of low vegetation in the gully floor. This is due to sediment trapping processes and highlights the importance of the spatial distribution of forest vegetation in reducing sediment yield at the gully outlet. Thus, gullies with similar total vegetation cover can have very different activities, and inactive gullies exist that are only partially covered with vegetation. Inactive gullies with as low as 33% vegetation cover occur where vegetation is mainly present in the gully floor. The results also show that above 50% cover of low vegetation in the gully floor as a percentage of the gully floor surface, gullies are generally inactive.

Influence of vegetation distribution on sediment yield in forested marly gullies

The effect of plant species on erosion processes may be decisive for long-term soil protection in degraded ecosystems. The identification of functional effect traits that predict species ability for erosion control would be of great interest for ecological restoration purposes. Flume experiments were carried out to investigate the effect of the root systems of three species having contrasted ecological requirements from eroded marly lands of the French Southern Alps [i.e. Robinia pseudo acacia (tree), Pinus nigra austriaca (tree) and Achnatherum calamagrostis (grass)], on concentrated flow erosion rates. Ten functional traits, describing plant morphological and biomechanical features, were measured on each tested sample. Analyses were performed to identify traits that determine plant root effects on erosion control. Erosion rates were lowest for samples of Robinia pseudo acacia, intermediate in Achnatherum calamagrostis and highest in Pinus nigra austriaca. The three species also differed strongly in their traits. Principal components analysis showed that the erosion-reducing potential of plant species was negatively correlated to root diameter and positively correlated to the percentage of fine roots. The results highlighted the role of small flexible roots in root reinforcement processes, and suggested the importance of high root surface and higher tensile strength for soil stabilization. By combining flume experiment to plant functional traits measurements, we identified root system features influencing plant species performance for soil protection against concentrated flow erosion. Plant functional traits related to species efficiency for erosion control represent useful tools to improve the diagnosis of land vulnerability to erosion, plant community resistance and the prediction of ecosystem functioning after ecological restoration. Copyright © 2012 John Wiley & Sons, Ltd.

Plant root traits affecting the resistance of soils to concentrated flow erosion

Plant species can have a major effect on erosion dynamics and soil losses by retaining sediment transported during concentrated runoff. Identifying plant functional traits that influence and predict a species ability for sediment trapping is therefore of great interest, especially to improve management and restoration of degraded lands. Sediment trapping ability of four morphologically contrasted species, the broadleaf species Buxus sempervirens and Lavandula angustifolia, and the coniferous species Juniperus communis and Pinus nigra, were investigated with flume experiments. Six functional traits describing stem, leaf and the overall plant morphology, were measured on seedlings. Analyses were performed to compare species efficiency in sediment trapping and to identify traits related to the amount of sediment trapped. Sediment trapping (RTS) was the highest upslope of Lavandula and the lowest upslope of Juniperus. Principal component analysis showed that RTS was best correlated (positively) with canopy density, described by plant biomass and leaf area per unit volume of plant. Leaf area and plant roundness were also positively related to RTS but to a lesser extent. The results of this experimental study suggest that canopy completeness, leaf morphology and plant shape influence sediment retention by plants. Such knowledge may improve the diagnosis of land vulnerability to erosion and the prediction of ecosystem functioning after ecological restoration by the construction of bioengineering works in gully floors.

Plant functional traits and species ability for sediment retention during concentrated flow erosion

L'objectif de l'etude est d'analyser l'efficacite des ouvrages de genie biologique pour le piegeage et la retention de sediments au sein de petites ravines inscrites dans des marnes, dans les Alpes francaises du Sud. Nous avons suivi durant trois annees (2002, 2003 et debut 2004) l'effet de soixante ouvrages de 1,2 m de large, constitues de boutures de saules : 29 cordons sur fascines (equipement comprenant un simple alignement de boutures) installes en 2002 et 31 cordons avec garnissages sur fascines (equipements lineaires et surfaciques avec disposition de boutures en deux dimensions) mis en place en 2003. Ces ouvrages ont ete construits tous les 2 m dans le lit de cinq ravines experimentales. Il s'agit d'analyser la reprise des boutures et de mesurer les quantites de sediments pieges par chaque equipement. Les resultats montrent que, si les boutures ont parfois largement souffert de la secheresse due au climat mediterraneen, elles ont permis sur certains ouvrages le developpement d'obstacles vegetaux efficaces pour pieger des sediments. En une annee et par ouvrage, les cordons sur fascines ont piege en moyenne 0,06 m3 de sediments et au maximum 0,19 m3. Les cordons avec garnissages sur fascines ont piege en moyenne 0,11 m3 de sediments et au maximum 0,29 m3.

Efficacité des ouvrages de génie biologique pour le piégeage des sédiments dans des ravines incisées dans des marnes (Alpes du Sud, France)

Les MNT a tres haute resolution spatiale sont desormais accessibles pour des secteurs fortement disseques comme les badlands. Ces representations numeriques de l’altimetrie sous une forme spatialisee sont porteuses d’utilisations multiples et nouvelles pour la connaissance, le suivi et la modelisation des zones erodees. Nous explorons ici des MNT a tres haute resolution spatiale, c’est a dire avec des resolutions de quelques centimetres a quelques metres et avec des qualites de restitution en Z (altitude) meilleures que le metre. Apres une breve presentation des techniques d’acquisition de ces MNT a partir d’images stereoscopiques ou du Lidar et des precisions qui leur sont associees, on rappelle quelles sont les transformees de l’altitude utiles pour la morphologie. Afin de montrer quelques-unes des possibilites qu’ouvrent ces nouvelles donnees, diverses applications, menees sur les petits bassins versants marneux de l’Observatoire de Recherche et d’Experimentation de Draix (Alpes, France), sont detaillees. Elles ont trait, d’abord, a l’extraction du reseau des talwegs a partir de MNT a 2 m (et de MNT degrades a 4, 8, 20 metres) et au calcul de sa dimension fractale pour les resolutions a 2 m et 8 m ; puis au reperage automatique de ravines sur criteres morphologiques simples afin d’aider a la localisation d’amenagements anti-erosifs. On montre enfin les potentialites des topographies precises par Lidar terrestre pour reperer d’etroites rigoles, suivre leur evolution, ou etablir des bilans d’erosion sur versant alors qu’une precision d’ordre centimetrique est requise.

MNT à très haute résolution dans les modelés fortement disséqués : des données aux tests d'application

Resume La maitrise de l'erosion par la vegetation forestiere, pour la prevention et la lutte contre les risques torrentiels et l'envasement des retenues hydroelectriques, reste une preoccupation importante des amenageurs dans les Alpes francaises du Sud. Les etudes en cours, menees sur plusieurs sites des Alpes-de-Haute-Provence, visent a mieux comprendre l'influence de la vegetation forestiere sur la production sedimentaire de terrains marneux, a differentes echelles spatiales : celles du versant de ravine, de la ravine elementaire et du bassin versant. Il en ressort que la vegetation forestiere joue non seulement un role de protection des sols par couverture et fixation, mais egalement un role de piegeage et de fixation des sediments a l'amont des barrieres qu'elle constitue. Les resultats montrent, de ce fait, l'importance de la distribution spatiale de la vegetation forestiere sur une ravine, notamment des strates herbacees et buissonnantes, pour une reduction significative de la production de sediments a son exutoire. L'interet de ces resultats reside dans leur application a la gestion du risque torrentiel et au controle de l'erosion, dans un objectif d'optimisation des travaux de genie ecologique, ainsi qu'a la cartographie du risque torrentiel.

Influence de la végétation forestière sur la formation de dépôts sédimentaires en terrains marneux/ Influence of forest vegetation on sediment delivery in marly terrain

Freddy Rey

Papers

Plant root traits affecting the resistance of soils to concentrated flow erosion

Plant functional traits and species ability for sediment retention during concentrated flow erosion

Efficacité des ouvrages de génie biologique pour le piégeage des sédiments dans des ravines incisées dans des marnes (Alpes du Sud, France)

MNT à très haute résolution dans les modelés fortement disséqués : des données aux tests d'application

Influence de la végétation forestière sur la formation de dépôts sédimentaires en terrains marneux/ Influence of forest vegetation on sediment delivery in marly terrain