Investigating parabolic and nebkha dune formation using a cellular automaton modelling approach

Abstract

Vegetation plays an important role in shaping the morphology of aeolian dune landscapes in coastal and semi-arid environments, where ecogeomorphic interactions are complex and not well quantified. We present a Discrete ECogeomorphic Aeolian Landscape model (DECAL) capable of simulating realistic looking vegetated dune forms, permitting exploration of relationships between ecological and morphological processes at different temporal and spatial scales. The cellular automaton algorithm applies three simple rules that lead to selforganization of complex dune environments, including nebkhas with distinctive deposition tails that form in association with mesquite-type shrubs, and hairpin (long-walled) parabolic dunes with trailing ridges that evolve from blowouts in association with vegetation succession. Changing the conditions of simulations produces differing landscapes that conform qualitatively to observations of real-world dunes. The model mimics the response of the morphology to changes in sediment supply, vegetation distribution, density and growth characteristics, as well as initial disturbances. The introduction of vegetation into the model links spatial and temporal scales, previously dimensionless in bare-sand cellular automata. Grid resolutions coarser than the representative size of the modelled vegetation elements yield similar morphologies, but when cell size is reduced to much smaller dimensions, the resultant landscape evolution is dramatically different. The model furthermore demonstrates that the relative response characteristics of the multiple vegetation types and their mutual feedback with geomorphological processes impart a significant influence on landscape equilibria, suggesting that vegetation induces a characteristic length scale in aeolian environments. This simple vegetated dune model illustrates the power and versatility of a cellular automaton approach for exploring the effects of interactions between ecology and geomorphology in complex earth surface systems. Figure 3. Relationship between values of U * and c/q (solid blue line) and the range of appropriate slab heights (1/7·5 to 1/13) for use in the model. This figure is available in colour online at www.interscience.wiley.com/journal/espl 730 Figure 4. Flow diagram of the DECAL model. Yellow shaded sequence indicates the algorithm of an individual slab displacement. Green section represents the annual vegetation growth response. This figure is available in colour online at Figure 8. Nebkha dune formation under varying sediment flux (top) and speed (bottom) conditions compared with the standard conditions used for Figure 6; transport direction left to right. This figure is available in colour online at

Vegetation in Drylands: Effects on Wind Flow and Aeolian Sediment Transport

“…For instance, surface changes resulting from wind erosion can be linked to subsequent vegetation growth/dieback within a coupled CA framework (e.g., [2,25,26,33,174]). This approach is rooted in the concept of 'landscape connectivity' [175,176], which considers dryland landscapes as a series of conduits for processes (e.g., fire, wind and water propagation) that link vegetation growth, aeolian processes and external forcing factors through various biotic and abiotic feedbacks [41,177,178].…”

“…The Discrete ECogeomorphic Aeolian Landscape model (DECAL) [2,25,174], later extended by Yan and Baas [179], provides an approach for simulating the dynamics of vegetation-dependent dunes such as parabolic dunes and nebkhas, by incorporating feedbacks between sedimentation balance and vegetation growth. DECAL introduces a length scale to resultant landforms that remains otherwise dimensionless in bare-sand CAs.…”

“…Wind regime and sediment supply are known to largely control the formation of bare-sand dunes (e.g., [20][21][22][23]), but the complicating effects of vegetation, which form part of a wider, multi-scaled interplay of biophysical and anthropogenic factors, are less well understood [24][25][26].…”

Aeolian dynamics of beach scraped ridge and dyke structures

“…Although CA modelling is effective at creating realistic dune forms under a range of conditions such as wind direction (Zhang et al, 2012) and vegetation (Nield and Baas, 2008), the simple assumptions determining the models outcome cannot yet be used to predict morphological change in reality (Zheng, 2009). Furthermore the two-dimensional nature of flow modelling conducted in CA modelling does not permit users to adequately investigate lateral patterns of deposition or erosion that may occur.…”

Aeolian Sediment Transport Integration in General Stratigraphic Forward Modeling

“…A large number of numerical models have been developed to simulate the physical processes involved in saltation [11][12][13][14][15], (McEwan and Willetts [16]), and [17][18][19][20][21][22]. Recently, advances in numerical modeling have facilitated investigation on interaction between soil vegetation cover and aeolian transport [23][24][25][26][27][28][29]. In evaluating the ecogeomorphic landscape evolution under different wind regimes, these models make a significant breakthrough in the understanding of wind and soils interactions.…”