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Universidad de Almería (2011)

Delineation of channel networks from digital elevation models

Afana Ashraf

Titre : Delineation of channel networks from digital elevation models

Auteur : Afana Ashraf

Université de soutenance : Universidad de Almería

Grade : Tesis Doctoral 2011

Résumé partiel
Since the early work of W.W. Davis in 1899, channel networks and their drainage basins have formed one of the main scientific endeavours to understand landscape evolution, mainly the geomorphological and hydrological functions and processes that control the actual earth shape and aspect. Stream network extraction and delineation is one of the main tasks to understand the above roles and processes. Based on manual methods and eye observations, earlier scientists delineated channel networks from either topographic maps or aerial photographs. In this case, manual delineation depends on relief contrasts and is highly subjective leading to considerable errors at high resolutions. An alternative approach is the automated extraction of drainage networks from Digital Elevation Models (DEMs), based on the wide notion of the hydrological properties of topographic surfaces. Whilst extraction of channel networks is broadly simple and direct, delineation of stream limits is still a matter of debate because of the inherent challenge of formalising overland flow with respect to surface features. In general, methods of stream limits definition propose the use of a constant threshold drainage area to define where channels begin in the landscape. However, such a threshold depends on the topographic complexity, and consequently the majority of these methods fail to perform consistently wherever the basin is made up of heterogeneous sub-zones, as they only work lumped. In this study, the critical threshold value has been defined by the analysis of dominant geometric and topologic properties of stream network formations. In addition, a recursive stratification process has been integrated in the model to detect homogeneous hierarchical sub-basins in relation to dominant intrinsic properties. Such approach (i.e. adaptive model) provides with the necessary critical thresholds in relation to DEM-data resolution and to diversity of dominant landforms. All these assumptions are based on a basic notion that “DEMs are self-contained structures which reflect the geomorphic and hydrologic processes that form them, and therefore encompass the necessary information to extract and delineate the channel networks by using algorithms and models capable of processing such information. While delineation of stream limits has received a considerable attention from scientists, validation of the achieved results is still in lagging behind. How and what to validate were between the several questions that opened debates between researchers. The complex structure of natural stream systems makes it somewhat complicated to adapt a particular approach over the others. In general, two main approaches for stream network validation are widespread between geomorphologists : quantitative and qualitative methods. The former uses geomorphometrical indices that describe stream network structure properties, extracted from different sources (e.g. topographic maps, DEMs, etc.) which are statistically compared. The latter involve field visits and visual interpretation of the resulting data, and its post comparison with information from other sources (e.g. orthophotographs, 3D structures, etc.). In the present work, emphasis has been placed on the quantitative approach, because of the direct effect of geomorphometrical parameters on hydrological and geomorphological models. The work has been carried out in various catchments with different lithology and geomorphic processes. The studied area comprises the Tabernas basin (SE Spain) representing a heterogeneous complex landscape, and the El Cautivo and Rambla Honda sub-basins representing respective homogeneous relief formations of different types and origins. High-resolution DEMs of 6 cm and 1 m were used to obtain the best detailed drainage network that the algorithm can generate at these homogeneous landscapes, whereas a medium-resolution DEM (30 m) was applied to the general heterogeneous landscape. The used DEMs are of diverse origins. The 6 cm DEM was obtained by laser scanning technique (LST), the 1 m DEMs were interpolated from isohyets and contour maps, whereas the 30-m DEM was obtained by photogrammetric restitution and interpolation. The wide range and origin of those analysed DEMs should provide deeper insights on errors and uncertainties effect on stream network delineation. In addition, detailed DEMs may allow for a direct quantitative comparison as well indirect qualitative ones.

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