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Extraction of Thalweg Networks from Dtms: Application to Badlands N Extraction of thalweg networks from DTMs: application to badlands N. Thommeret, Jean-Stéphane Bailly, C. Puech To cite this version: N. Thommeret, Jean-Stéphane Bailly, C. Puech. Extraction of thalweg networks from DTMs: appli- cation to badlands. Hydrology and Earth System Sciences Discussions, European Geosciences Union, 2010, 14 (8), p. 1527 - p. 1536. 10.5194/hess-14-1527-2010. hal-00576902 HAL Id: hal-00576902 https://hal.archives-ouvertes.fr/hal-00576902 Submitted on 15 Mar 2011 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Hydrol. Earth Syst. Sci., 14, 1527–1536, 2010 www.hydrol-earth-syst-sci.net/14/1527/2010/ Hydrology and doi:10.5194/hess-14-1527-2010 Earth System © Author(s) 2010. CC Attribution 3.0 License. Sciences Extraction of thalweg networks from DTMs: application to badlands N. Thommeret1,2, J. S. Bailly3,4, and C. Puech2 1CNRS-UMR 8591, Laboratoire de Geographie´ Physique, 92190 Meudon, France 2Cemagref-UMR TETIS, 34093 Montpellier, France 3AgroParisTech-UMR TETIS, 34093 Montpellier, France 4AgroParisTech-UMR LISAH, 34060 Montpellier, France Received: 23 December 2009 – Published in Hydrol. Earth Syst. Sci. Discuss.: 1 February 2010 Revised: 7 July 2010 – Accepted: 24 July 2010 – Published: 10 August 2010 Abstract. To study gully spatial patterns in the badlands us- 1 Introduction ing a continuous thalweg vector network, this paper presents methods to extract the badlands’ thalweg network from a reg- The badlands are usually defined as an intensely dissected ular grid digital terrain model (DTM) by combining terrain landscape, where vegetation is sparse or absent, and on morphology indices with a drainage algorithm. This method which agriculture is useless (Bryan and Yair, 1982). A com- will delineate a thalweg only where the DTM denotes a sig- bination of various weathering and erosion processes affects nificant curvature with respect to DTM accuracy and relies the side slopes and channels at different spatial and tempo- on three major steps. First, discontinuous concave areas were ral scales. This results in the modelling of a particular land- detected from the DTM using morphological criteria, either scape. To understand the geomorphological and hydrological the plan curvature or the convergence index. Second, the functioning over highly eroded areas, different scale car- concave areas were connected using a drainage algorithm, tographies are needed. The badlands morphology consists of which provides a continuous, thick, tree-structured scheme. numerous gullies of variable size (from 10 cm to 100 m wide) We assumed that these areas were physically significant and organised in hierarchical networks. The high drainage den- corresponded to a gully floor. Finally, the thick path was re- sity and steep slope make fieldwork and detailed cartography duced to its main course and vectorised to obtain a thalweg difficult. Consequently, remote sensing is the most appropri- network. The methods were applied to both virtual and ac- ate source of data for mapping the badlands. Badlands map- tual DTM cases. The actual case was a LiDAR DTM of the ping provides essential information for studying the spatial Draix badlands in the French Alps. The obtained networks pattern of this environment. The quantitative description of were quantitatively compared, both with a network obtained the pattern contributes to a better understanding of the func- using the usual drainage area criteria and with a reference tioning and scale relationships (Horton, 1945). In addition, network mapped in the field. The CI-based network showed it allows for accurate comparisons between different eroded the great potential for thalweg network extraction. areas. Representing and characterising the gully network from remotely sensed data is not straightforward. One possibility is to represent the gullies by one of their constitutive ele- ments: thalweg networks. A thalweg is defined as the line joining the lowest points of the gully. In badlands topog- raphy, these networks are characterised by tree-structured topologies. To study the spatial pattern, digital indices describing the network geometry (fractal dimensions) and Correspondence to: N. Thommeret topology (as Horton ratios and Tokunaga’s indices) must be ([email protected]) computed for different extents and resolutions. These latter Published by Copernicus Publications on behalf of the European Geosciences Union. 1528 N. Thommeret et al.: Extraction of thalweg networks from DTMs indices are very sensitive to noise, especially for small tree thalweg extraction (Tarboton and Ames, 2001; Molloy and structures (Dodds and Rothman, 2001), so the network rep- Stepinski, 2007; Tarolli and Dalla Fontatana, 2009). resentations must fit the data information to provide stable In this paper, we present a method that combines an ex- results. isting drainage algorithm and a morphological index to de- Airborne or high-resolution satellite sensors supply the lineate the thalweg network. Compared with the plan cur- source data for the representation of the thalweg. We need an vature, the convergence index seems to be a good choice automatic and easily repeatable method with the least subjec- due to its ability to underscore the morphological features, tive parameter possible to extract the thalweg network. Even such as valleys. This method attempts to extract thalwegs ob- if manual image interpretation is possible and provides con- jectively, considering the significant landforms included in a sistent networks, the result depends on the operator, so it is DTM. This report is divided into four main sections: Sect. 2 not reproducible (Molloy and Stepinski, 2007). Moreover, presents the study areas and the data; Sect. 3 describes the this operational and fastidious technique is difficult to apply method proposed to extract and assess thalweg network from to extensive areas. On the other hand, landforms can be ex- DTMs; Sect. 4 presents the results obtained with both virtual tracted directly from digital terrain models (DTMs). Indeed, and actual DTM; and Sect. 5 discusses the method and the DTMs are spatial, digital representations of the relief that result and compares them with another extraction method. allow one to stake out morphological features. Nevertheless, extracting features such as thalwegs from a DTM raises some fundamental issues, such as which methods permit extraction 2 Material of complete and stable networks from spatial data. The most-used methods are based on regular grid DTMs. 2.1 Virtual catchment and simulated data Various drainage algorithms, including D8 (O’Callaghan and Mark, 1984), kinematic routing algorithm (Lea, 1992), FD8 First, several virtual catchment DTMs were used to assess (Quinn et al., 1991) or D∞ (Tarboton, 1997), offer the pos- the benefit of the proposed method. The use of virtual catch- sibility of computing drainage networks all over the grid sur- ment DTM allows for the representation and comparison of face. However, the transition from one drainage flow path networks extracted using different methods, while the ini- to a vector thalweg network is not self-evident (Martz and tial shape of the valley is known and controlled. Our virtual Garbrecht, 1995; Tarboton and Ames, 2001; Turcotte et al., DTMs are simple shapes embedded into tilted planes. The 2001). Most of the time, the process uses a unique contribut- chosen shapes are either virtual valleys or idealised mathe- ing area threshold beyond which thalwegs, valleys or hydro- matical shapes. The virtual valleys are presented as various graphical networks are depicted. Because this criterion has confluences (mainly varying angles), width and network pat- no direct physical significance, a relevant question to ask is terns. The most interesting mathematical shape used is the where to start the thalwegs upstream? Some authors have corrugated sheet. proposed using topological or physical reasoning to estab- lish the threshold. Montgomery and Dietrich (1994) have 2.2 Draix badlands catchments and data suggested distinguishing dissected areas, where fluvial trans- port influence is dominant, from undissected areas, where The study area is located in the Southern Alps in France, at diffusional, hillslope transport processes are dominant, to lo- approximately 6◦20′ E and 44◦08′ N. The test site represents cate channel heads. Tarboton et al. (1991) rely on search- one of the upper Durance catchment’s badland areas. These ing the smallest scale, measured in terms of support area, badlands were formed from the highly erodible, black, Juras- where the constant drop property breaks. This property as- sic marls. In addition, the Mediterranean climate and pre- sumes that the average drop is approximately constant along cipitation served to intensify the active weathering and ero- the Strahler order. The authors argue that this point repre- sion processes. This particular test site, known as the Draix sents the physical transition from channel erosion to hillslope experimental catchments, is a field laboratory for the study erosion. However,
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