Predicting soil erosion and sediment yield at the basin scale: Scale issues and semi-quantitative models

IAEG Commission No. 1—Engineering Geological Maps—is developing a guide to hazard maps. Scientists from 17 countries have participated. This paper is one of a series that presents the results of that work. It provides a general review of GIS landslide mapping techniques and basic concepts of landslide mapping. Three groups of maps are considered: maps of spatial incidence of landslides, maps of spatial–temporal incidence and forecasting of landslides and maps of assessment of the consequences of landslides. With the current era of powerful microcomputers and widespread use of GIS packages, large numbers of papers on the subject are becoming available, frequently founded on different basic concepts. In order to achieve a better understanding and comparison, the concepts proposed by Varnes (Landslide hazard zonation: a review of principles and practice, 1984) and Fell (Some landslide risk zoning schemes in use in Eastern Australua and their application 1992; Landslide risk assessment and acceptable risk. Can Geotech J 31:261–272, 1994) are taken as references. It is hoped this will also add to the international usefulness of these maps as tools for landslide prevention and mitigation. Six hundred and sixty one papers and books related to the topic are included in the references, many of which are reviewed in the text.

Engineering geology maps: landslides and geographical information systems

Gully erosion susceptibility assessment by means of GIS-based logistic regression: a case of Sicily (Italy)

Performance assessment of individual and ensemble data-mining techniques for gully erosion modeling

A century of gully erosion research: Urgency, complexity and study approaches

The Souar lithologic formation in semi-arid Tunisia is undergoing severe gully erosion which is threatening soil and water resources. Soil conservation strategies have focused more on terracing than on gully control techniques, since the contribution of gully sediment yield in the overall soil loss from watersheds is unknown. The paper reports investigations into the sediment yield provided by head-cut as well as sidewall–floor erosion of first order gullies on gentle and steep slope catchments underlined by the Souar lithologic formation. We measured mean field sediment volumes evacuated by different headward reaches of 10 and 9 gullies located on gentle and steep slope catchments, respectively. Two equations between the length of the gully head cutting and the corresponding volume of evacuated sediment were established. The treatment with a Geographic Information System (Arc View) of air photographs of six flights from 1952 to 2000 allowed the calculation of the volume of sediment provided both by head cutting and gully sidewalls–floor erosion through the following up of gully extension in eight catchments during the five periods separating the dates of these flights. Total gully erosion was on average 1.66 m3 ha− 1 year− 1 for the gentle slopes and 5.603 m3 ha− 1 year− 1 for the steep slopes. Sidewalls–floor contribution in total erosion was on average 81.5% for the gentle slopes and 77.8% for the steep slopes. We found out that the mean annual rainfall resulting from 40 mm daily rainfall threshold explained better the variation of annual head cutting sediment yield for these five periods than any other annual rainfall resulting from lower daily rainfall thresholds. Two equations between these two variables were established both for gentle and steep slope catchments.

Slope and rainfall effects on the volume of sediment yield by gully erosion in the Souar lithologic formation (Tunisia)

The Fortuna geological formation dated from the late Oligocene to early Miocene is mainly composed of friable sandstone. Gully erosion on this substratum is developing complex shaped gullies where bank processes are as important as head advance by cutting and bed deepening by vertical incision. For irregularly shaped gullies quantitative information on gully erosion rates as related to gully head extension are scarce because bank active sides take many forms presumably depending on topography, climate and lithology. Our field cross section measurements of 4 gullies developed in the Fortuna Formation, (2 located on a steep slope area (> 15%) and the other two on a gentle slope (< 9%)), have shown that the geometry of these gullies is far from a conical gully shape. These field measurements were used to calculate gully reaches geometry and volume variation. No direct relationships between gully volume and length could be established for these gullies in the Fortuna formation. Nevertheless, since both gully top view area and maximum top width are increasing, as the gully reach length increases, relationships could be established between maximum top width and their equivalent width defined as the ratio between gully reach top view area and its length. Then, gully volume–length relationships could be established for each of the 5 maximum bank-full widths studied on the two gentle and two steep slope gullies. For any maximum top width, the gully volume–length relationship was found to be not unique but the volume is increasing linearly up to the 15 m and 45 m distance from the gully head and then logarithmically for steep and gentle slope gullies, respectively. The linear and logarithmic equations' coefficients were also found to be a function of gully maximum top width. These equations were then used to quantify gully sediment yields related to gully shape evolution for eight gullies tributaries of As Sahsafa wadis. For this purpose the top view areas and lengths of these eight gullies were digitized and monitored on rectified air photographs of 1962, 1974 and 2000 using a Geographic Information System (Arc View). Half of these gullies were located in an area with a gentle slope gradient and the other half with steep slope gradient.

The average gullies length variation for the whole study period (1962–2000) were 11.74% and 10.32% whereas the average top view areas variation was 23.57% and 29.4% for gentle and steep slope gullies, respectively. Gullies volume changes during the periods 1962–1974 and 1974–2000 were then derived from the variation of gullies length and top view areas photo-measurements on these 8 tributaries using the relationships established from the four measured gullies. On gentle slopes, the average gully sediment yields during 1962–1974 and 1974–2000 periods were 3.98 and 2.28 m3 ha− 1 year− 1, respectively. On steep slopes, these values are 4.82 and 2.47 m3 ha− 1 year− 1.

Sediment yield from irregularly shaped gullies located on the Fortuna lithologic formation in semi-arid area of Tunisia.

Soil conservation strategy in the semi-arid areas of central Tunisia has focused on the construction of numerous hill reservoirs. However, limited rainfall combined with severe soil erosion is jeopardizing the usefulness of hill reservoirs such that there is an urgent need to undertake further studies into the location and efficiency of small dams. The paper reports investigations into the relationship between location of the dam and the underlying lithology of the catchment areas with the dam efficiency. A gully erodibility factor has been proposed for nine groups of lithological formations identified and a watershed lithologic number was computed as the product of seven watersheds for which 1:50,000 scale lithological maps were available. A significant linear relationship was determined between the watershed lithologic number and the efficiency of the small dams, indicating that an accurate lithological survey prior to small dam planning can be used to indicate the dam's likely efficiency and to plan any necessary soil and water conservation works in the catchment areas.

Relationship between small dam efficiency and gully erodibility of the lithologic formations covering their watershed

L'erosion ravinante reduit de plus en plus les terres cultivables en Tunisie Centrale en les transformant en badland. Le developpement des ravins est variable dans l'espace et dans le temps suite a la variabilite des parametres physiques et naturels qui regissent l'accroissement des ravins. Nous nous sommes proposes dans cette recherche d'etudier l'effet des formations lithologiques sur le developpement des ravins dans le bassin versant de 1500 ha d'Oued Maiez situe en Tunisie Centrale. Les cartes du reseau hydrographique de 1963 et 1989 ont eteetablies a partir des photographies aeriennes au 1/12,500 non redressees. La prospection du terrain nous a permis de constater que la nature lithologique des differentes unites geologiques et superficielles identifiees comme relativement homogenes dans le bassin versant est un facteur determinant dans le developpement des ravins. Ainsi, les argiles gypseuses de la formation Cherichera (Lutetien-Priabonien) presentent un chevelu de ravins dense alors que les sables fins de l'Oligocene inferieur sont incises par des ravins en forme de gorges etroites dont la profondeur depasse souvent les 5 m (Fig. 7). Nous avons alors etabli un leve cartographique detaille au 1/12,500 de ces differentes unites qui sont au nombre de 8.

Importance du facteur lithologique sur le développement des ravins du bassin versant d'Oued Maiez en Tunisie Centrale

In Tunisian semi-arid areas, human pressure, torrential rains and low vegetation cover are the essential factors of gully erosion. In these areas, gullies naturally exhibit complex morphologies. Hence they are difficult to measure. This type of erosion, in its most severe form, threatens cultivated lands by the extension of badlands as well as water resources by water reservoir sedimentation. For a long time, the objective of most gully erosion studies was the morphological characterization of the gullies in order to properly understand erosion processes. Gradually, the technological advancement in sensors and platforms for aerial image acquisition made it possible to achieve more detailed mapping of gullies. During the last decade, low altitude aerial platforms have experienced the strongest development in acquiring high-resolution aerial photographs and generating associated digital elevation models (DEMs), in particular with the rise of structure from motion algorithms use in geosciences. Such DEMs meet the need for mapping at the sub-meter scale as well as the capability of studying the gullies in three dimensions. In previous studies, a DEM and the corresponding orthophotography were produced at very high resolution (6.2 cm for the DEM, 3.1 cm for the orthophotography). The 3D reconstruction was performed from overlapping images taken from a consumer grade camera hung down a kite. Such experiments, producing unusual aerial datasets, are poorly reviewed in the literature. Therefore, the objective of this paper is to assess the accuracy of such DEMs obtained from overlapping aerial photographs taken from kite platforms.

https://hal.archives-ouvertes.fr/hal-01780728/document

Mohamed Habib Snane

Papers

Slope and rainfall effects on the volume of sediment yield by gully erosion in the Souar lithologic formation (Tunisia)

Sediment yield from irregularly shaped gullies located on the Fortuna lithologic formation in semi-arid area of Tunisia.

Relationship between small dam efficiency and gully erodibility of the lithologic formations covering their watershed

Importance du facteur lithologique sur le développement des ravins du bassin versant d'Oued Maiez en Tunisie Centrale

Assessment of kite born DEM accuracy for gullies measuring