Factors and processes of permanent gully evolution in a Mediterranean marly environment (Cape Bon, Tunisia)

Abstract

Gully erosion is considered to be one of the most important soil erosion processes in Mediterranean marly environments, but its actual contribution to total soil loss is still under discussion. The objectives of this paper are: (a) to acquire the distributed value of erosion rate in a permanent gully developed on a marly substratum in a Mediterranean environment; and (b) to quantify the key factors responsible for the spatial and temporal differences in erosion rates observed within the gully. A permanent gully located in Cap Bon (northeastern Tunisia) has been intensively and regularly monitored over a 7-year period with electronic survey equipment (total station) to give five field topographic surveys, as well as hydrological measurements at the gully outlet. The net soil loss for the 7-year period comprised a denudation of 51 m 3 of sediment on the gully bank slopes, which corresponds to a mean soil loss of 61 m 3 ha -1 year -1 or 6.1 mm year -1 . Denudation was observed on bed units with a slope gradient greater than 20%, while the remainder showed deposition. By confirming the factors involved in gully evolution, and by refining the statistical link between factors and erosion rates within the gully, the results provide important information to predict gully erosion rates in Mediterranean marly environments.Key words soil erosion; topographic survey; gully monitoring; marl; Mediterranean environment Facteurs et processus impliqués dans l'évolution d'une ravine permanente située dans un environnement marneux méditerranéen (Cap Bon, Tunisie)Résumé Si l'érosion ravinaire est souvent considérée comme un processus prédominant en milieu méditerranéen marneux, sa contribution au flux total de sédiments reste très incertaine. Les objectifs de cet article sont (a) d'acquérir des taux d'érosion spatialisés au sein d'une ravine permanente développée sur un substrat marneux; et (b) de quantifier les facteurs clés expliquant la variabilité spatiale et temporelle des taux d'érosion observés au sein de la ravine. Pour cela, une ravine permanente située au Cap Bon (Nord Est de la Tunisie) a été suivie durant 7 ans à travers cinq campagnes de mesures topographiques intensives réalisées à l'aide d'une station totale et à travers des mesures en continu du débit à l'exutoire de la ravine. Durant ces 7 années d'observations, les flancs de la ravine ont subi une perte nette de sol de 51 m 3 , ce qui correspond à des valeurs d'érosion spécifique annuelle de 61 m 3 ha -1 an -1 ou 6,1 mm an -1 . Les portions du lit de la ravine dont la pente est supérieure à 20% ont subi une ablation, les autres un dépôt. En confirmant les facteurs impliqués dans l'évolution de la ravine et en affinant les relations statistiques liant facteurs et taux d'érosion, cette étude fournit des informations pratiques très utiles pour la prévision des taux d'érosion de ravines situées en contexte méditerranéen marneux.

A Spatiotemporal Multiscale Analysis of Runoff and Erosion in a Mediterranean Marly Catchment

“…Intra‐annual evolution of vegetation cover was reported by Raclot and Albergel (2006) and intra‐annual evolution of cracks by Inoubli et al (2016). An intensive topographic monitoring of a representative permanent gully (included in P2+G) was also conducted in Kamech between 2002 and 2009 using a total station (El Khalili et al, 2013). This monitoring enabled us to estimate the long‐term mean interannual gully erosion rate in Kamech at 80 t ha −1 yr −1 .…”

“…In this model, α and β were quantified through a combination of aerial photograph interpretation and field surveys, whereas long‐term topsoil ( E Topsoil ) and gully ( E Gullies ) erosion rates were assumed to be known and representative of any parts of the Kamech catchment. In this study, E Topsoil was set to the measured erosion rate between 2005 and 2012 at the experimental plot P1 (Es_P1, this study) and E Gullies was set to 80 t ha −1 yr −1 as estimated by El Khalili et al (2013). Erosion from dwellings and scrublands was assumed to be negligible because (i) dwellings are located in the northeastern part of the catchment, which is a very flat area, (ii) most of dwellings are sedimentologically disconnected from the catchment by dense vegetative hedges at their edges that are used as animal enclosures and field fence, and (iii) the scrublands are located on sandstone outcrops that represent <10% of the catchment area and don't show field evidence of important erosion activities.…”

Using kites for 3-D mapping of gullies at decimetre-resolution over several square kilometres: a case study on the Kamech catchment, Tunisia

“…This evaluation was based on a profile comparison of the kite DEM with a reference DEM derived from an intensive field topographic survey of a mid-size gully. This reference DEM was calculated on a 0.05 by 0.05 m grid from a very dense point data set acquired in 2009 using a total station that had an (X, Y, Z) accuracy better than 0.01 m (Khalili et al, 2013). Standard statistics on the deviation between the kite DEM and the reference DEM were derived on an elevation profile of a path composed of a series of line segments.…”

Local Factors Controlling Gully Development in a Mediterranean Environment

“…However, this method does not allow the analysis of the gully channel's development. The second method is based on the use of topographic data sets, DEM, digital surface models (DSM) or point cloud (PC) [46][47][48] and aims to define the evolution of the morphological characteristics of a gully over a period of time.…”