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During initial phases of landform evolution, the surface of sediment bodies is commonly not yet equilibrated with the environmental conditions and is thus liable to rapid changes by accelerated geomorphic activity. Hydro-geomorphic processes acting during these early development phases often lead to the emergence of surface structures that constitute the boundary conditions for further ecosystem development, and of linear erosion features that act as pathways for water, solute and sediment transport, and thus can increase the system's connectivity. Studies on initial landform evolution are often limited to relatively small-scale experiments or rely on modeling exercises. Larger-scale experiments can be carried out in constructed hydrological catchments with well-known and documented initial conditions. The central aim of this work was to describe the hydro-geomorphic surface structure evolution during the initial years of ecosystem development, its dependence on initial and boundary conditions; and on its relations to hydro-geomorphic connectivity. The data are from the 6-ha, constructed hydrological catchment 'Hühnerwasser'. Hydro-geomorphic development was quantified using a combined analysis of digital elevation models and aerial photographs. Empirical results were compared with those obtained with a numerical landscape evolution model. Phases of rill network growth, contraction and stabilization could be characterized. Results show influences of initial morphology, precipitation characteristics, and developing structure-process-interactions on rill network geometry and connectivity increase in the catchment. The results suggest that phases of hydro-geomorphic surface structure evolution can be related to the spatial organization of surface flow patterns during initial phases of landform development.
During initial phases of landform evolution, the surface of sediment bodies is commonly not yet equilibrated with the environmental conditions and is thus liable to rapid changes by accelerated geomorphic activity. Geomorphic processes acting during these early development phases depend on the initial sediment composition and the subsurface and surface structures of the geosystem; at the same time, these processes lead to landscape structures that constitute boundary conditions for further ecosystem development.
For systems such as soils or watersheds, the initial distribution of sediment mass and composition at the starting point of the development are mostly not well-known. The development of such systems, however, has frequently been assumed to depend on boundary and initial conditions including the solid phase. Here, an experimental watershed of about 6 ha surface area was artificially-created in order to imitate an initial stage just at the beginning of the geo-ecosystem development. However, sediment homogeneity could not be achieved for this large-scale field experiment, and the 3D spatial distribution of structures and components remained uncertain. The objective of this study was to quantify and reconstruct the initial sediment distribution and the first development stages using aerial photographs, point information, and a 3D model. The watershed was made of coarse-textured sediments of quaternary origin with a low permeable clay liner as the bottom boundary. A 2D horizontal digital elevation model (DEM) of the surface and a DEM of the subsurface clay layer are used to construct a 3D triangulated numerical grid of the catchment’s initial spatial structure using the 3D-GIS software GOCAD. Physical and chemical soil properties obtained from borehole samples are assigned to this model and interpolated onto the 3D grid. The volumetric changes in space and time are quantified and related to material properties to obtain the mass changes. Correlations between terrain attributes, sediment properties, and mass changes are explored specifically in regions of differing source materials. A temporal sequence of surface DEMs is obtained from photogrammetric, high-precision ground based laser scanning and airborne laser scanning data. 3D models of elevation change are constructed from these DEMs. The processes following the initial state are mainly characterized by runoff-induced erosive mass relocations related to the distribution of surface and subsurface structures and sediment properties. The results suggest that both sediment structures and mass translocations differ between the western and eastern parts of the watershed and that correlations between initial surface structures and subsequent mass changes exist.
Many processes in a developing catchment can only be fully understood if the spatial
heterogeneity is adequately described. Process-based modeling allows reproducing
specific key structural elements of catchments. Constructed catchments are ideal test
objects because of the well-known spatial conditions. In our study, we (i) modeled the
entire process chain of catchment construction, (ii) established a consistent sediment mass/
volume balance between the excavated outcrop and the generated catchment model, and
(iii) provided consistent three-dimensional distributions of texture and bulk density for
deriving soil hydraulic properties. Data are from the 6 ha Hühnerwasser catchment (Chicken
Creek, Lower Lusatia, Germany), which was constructed from overburden sediments in an
open-cast lignite mine. Uncertainty about the parent material origin was captured in a
multiscenario approach based on a geological model of the outcrop site and the simulation
of excavation procedures. Generated distributions are incorporated in a gridded threedimensional
volume model (GOCAD), and results were evaluated by semivariogram
analysis and by quantifying point-to-point deviations. Filling and compaction are modeled
based on three-dimensional volume analysis and mass balances of dumped sediments.
Heterogeneities reflect both the batch-wise sediment variation and the succession of
excavated geological units. The scale of the generated spatial models can be adjusted for
three-dimensional hydrological catchment modeling while three-dimensional sediment
mass balances can be used for analyzing soil and ecosystem development processes.
A structure model describes the boundary geometries and the three-dimensional solid phase distributions of the catchment. In contrast to an established ecosystem, feedback relations can be assumed for an initial geosystem between the development of spatial structures and the water and element fluxes. The objective was to develop a hydrological catchment model as a three-dimensional spatial database of the solid phase that allows an integrative analysis, a periodical mass balance, and the generation of distributed model parameters for the developing system. Data were from the constructed catchment “Chicken
Creek.” The initial texture and bulk density distributions were generated by imitating sediment dumping, segregation, and compaction. Boundary geometries and changes in surface topography due to erosion and sedimentation processes were quantified on the basis of digital elevation models (DEMs) derived from aerial photographs. The catchment was visualized with the three-dimensional software GoCad; the “emergence” of the water table and other structures (e.g., soil horizons, root zone) could be spatially assigned and quantified to identify regions with specific processes. A combination of three-dimensional catchment with time-dependent two-dimensional surface models allowed generating the development of spatially distributed erosion–deposition patterns that formed new initial surfaces. The three-dimensional distributed solid phase structure of the catchment allowed for a more direct comparison with observations using minimal invasive methods. The mass balance of the solids could be related with pedologic development and transfer to other geopedologic systems achieved by adapting sediment transport and deposition in the descriptions of the model. This model may help improve the integrative analysis of hydrological catchments.
The formation of erosion rills and gullies is a critical step in land surface development, but possibilities to study initial unaffected surface development under natural conditions and with well-defined initial and boundary conditions are rare. The objective of this study was to characterize rill network development from ’point zero’ in the artificially-created catchment ‘Hühnerwasser’. To ensure unaffected development, the study was largely restricted to the analysis of remotely-sensed data. We analyzed a series of photogrammetry-based digital elevation models (DEMs) for 10 points in time, over a period of five years and beginning with the initial state. The evolving erosion rill network was quantitatively described based on mapping from aerial photographs. DEMs and rill network maps were combined to specifically analyze the development of morphometry for different parts of the network and to characterize energy dissipation and connectivity. The restriction to remote-sensing data did not allow for analyzing specific processes governing rill network development, nevertheless, two major development phases could be characterized. We observed a phase of growth of the rill network along with variations in drainage patterns during the first two years of development and a subsequent phase of reduction of its area along with comparably stable patterns. Region-specific analysis of morphometry indicates that, besides effects of changing sediment characteristics and vegetation cover development, locally evolving hydro-geomorphic feedback cycles influenced this development. Results show an increasing similarity of overall statistical characteristics (e.g. drainage density) for two parts of the catchment, but a persistent influence of initial conditions on specific rill geometry. The observed development towards higher orderliness and increased connectivity is consistent with experiments and concepts on drainage network evolution across scales; however, we did not observe major influences of rill piracy and cross grading or a reduction of energy dissipation with network development.
The structure of a hydrological catchment is determined by the geometry of the boundaries and the spatial distribution
of soil and sediment properties. Models of the 3D subsurface structure and the soil heterogeneity have
often been built based on geostatistical approaches and conditional simulations for spatial interpolation between
measurements. Here, an alternative model was proposed that generated 3D subsurface structures by imitating basic
structures resulting from mass distribution processes. Instead of directly assuming stochastic variations of the subsurface structure, the present approach assumed stochastic variations in parameters of the process-based algorithms of the generator models. The constructed hydrological catchment “Hühnerwasser” located in the Lower Lusatia region of Brandenburg, Germany, was used as an example for the development of such a 3D structure generator model. Boundary geometries and changes in the surface topography due to erosion and sedimentation processes were quantified on the basis of digital elevation models (DEMs) derived from aerial photographs and terrestrial laser scanning information. Basic sediment properties came i) from a geological model of the parent material at the outcrop site, ii) from actual soil
sample measurements on-site, and iii) based on stochastic texture variations. Sediment distributions were generated
according to construction processes such as sediment dumping, particle segregation, and soil compaction.
The resulting internal structures reflect the formation of spoil cones and surface compaction by machinery. The
simulated 3D model scenarios of soil texture and bulk density distributions were incorporated in a gridded 3D
volume model using the 3D software tool GoCAD (Paradigm Ltd.). This 3D distributed solid phase structure of
the catchment allowed for a more direct comparison with observations using minimal invasive methods. By including
structural changes over time (e.g., derived from DEM’s), spatially-distributed solid mass balances can be
inferred based on the comparison of 3D structure models for subsequent time steps, which is a basic information
for studying soil development. The 3D structure model as spatial database of the solid phase allows an integrative
analysis, a periodical mass balance, and a mechanistic process-based generation of complex realizations of
distributed hydraulic model parameters for the exemplary catchment.