Use of terrain variables for mapping gully erosion susceptibility in Lebanon

Abstract

This paper predicts the geographic distribution and size of gullies across central Lebanon using a geographic information system (GIS) and terrain analysis. Eleven primary (elevation; upslope contributing area; aspect; slope; plan, profile and tangential curvature; flow direction; flow width; flow path length; rate of change of specific catchment area along the direction of flow) and three secondary (steady-state; quasi-dynamic topographic wetness; sediment transport capacity) topographic variables were generated and used along with digital data collected from other sources (soil, geology) to statistically explain gully erosion field measurements. Three tree-based regression models were developed using (1) all variables, (2) primary topographic variables only and (3) different pairs of variables. The best regression tree model combined the steady-state topographic wetness and sediment transport capacity indices and explained 80% of the variability in field gully measurements. This model proved to be simple, quick, realistic and practical, and it can be applied to other areas of the Mediterranean region with similar environmental conditions, thereby providing a tool to help with the implementation of plans for soil conservation and sustainable management.The study area was chosen because it represents the environmental diversity of Lebanon in terms of geology, soil, hydrography, land cover and climate. It covers 676 km 2 , or 6·5% of the total area of Lebanon. It extends 33 km from west to east across the middle of Lebanon ( Figure 1) and can be divided into two major geomorphic units, Mount Lebanon and the Bekaa.Mount Lebanon, which comprised 76% of the study area, runs parallel to the shoreline, dipping steeply seaward, with an east-west gradient of 7·5-10%. It can be divided into three major parts: the lower slopes (100-500 m altitude), the upper sloping plateaus (500-1500 m altitude) and the elevated crests (>1500 m altitude). The lower slopes, consisting of clastic and oolithic limestone, sandstone and clayey rocks of the Lower Cretaceous and Upper Jurassic formations (Dubertret, 1945), are dominated by bare soils and residential/commercial urban areas. The upper sloping plateaus are covered with coniferous (mainly Pinus pinea), oak (mainly Quercus calliprinos) and broadleaf (Quercus infectoria) forests and shrub lands on dolomites, limestone and dolomitic limestone rocks with patches of basalts, sandstone and clay materials. The elevated crests are covered by grass and herbaceous vegetation on limestone and marly limestone Cenomanian rocks and dolomitic limestone Jurassic rocks. Mount Lebanon is structurally affected by faults running parallel to one another, cutting in a SW-NE direction and separated from the Bekaa by the shed line of the Dead Sea Fault Zone and the 'Yammounah Fault' with a NE-SW strike. The Bekaa comprises the hills (1000-1500 m altitude; 6% of total area) located between the crests of Mount Lebanon and the Bekaa valley, and the valley bottom (500-1000 m altitude; 18% of total area). The h...

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

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The application of GIS based decision-tree models for generating the spatial distribution of hydromorphic organic landscapes in relation to digital terrain data

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