Andreas Baas

Chaos, fractals and self-organization in coastal geomorphology: Simulating dune landscapes in vegetated environments

Complex nonlinear dynamic systems are ubiquitous in the landscapes and phenomena studied by earth sciences in general and by geomorphology in particular. Concepts of chaos, fractals and self-organization, originating from research in nonlinear dynamics, have proven to be powerful approaches to understanding and modeling the evolution and characteristics of a wide variety of landscapes and bedforms. This paper presents a brief survey of the fundamental ideas and terminology underlying these types of investigations, covering such concepts as strange attractors, fractal dimensions and self-organized criticality. Their application in many areas of geomorphological research is subsequently reviewed, in river network modeling and in surface analysis amongst others, followed by more detailed descriptions of the use of chaos theory, fractals and self-organization in coastal geomorphology in particular. These include self-organized behavior of beach morphology, the fractal nature of ocean surface gravity waves, the self-organization of beach cusps and simulation models of ripples and dune patterns. This paper further presents a substantial extension of existing dune landscape simulation models by incorporating vegetation in the algorithm, enabling more realistic investigations into the self-organization of coastal dune systems. Interactions between vegetation and the sand transport process in the model—such as the modification of erosion and deposition rules and the growth response of vegetation to burial and erosion—introduce additional nonlinear feedback mechanisms that affect the course of selforganization of the simulated landscape. Exploratory modeling efforts show tantalizing results of how vegetation dynamics have a decisive impact on the emerging morphology and—conversely—how the developing landscape affects vegetation patterns. Extended interpretation of the modeling results in terms of attractors is hampered, however, by want of suitable state variables for characterizing vegetated landscapes, with respect to both morphology and vegetation patterns. D 2002 Elsevier Science B.V. All rights reserved.

Formation and behavior of aeolian streamers

Received 5 December 2004; revised 31 March 2005; accepted 9 June 2005; published 7 September 2005. (1) Field experiments were conducted to determine the characteristic spatial and temporal dimensions and behavior of aeolian streamers and to identify the processes responsible for their formation. An instrument array that included anemometer towers, piezo-electric transport sensors (Safires), and hot-film probes was deployed to measure streamers and airflow dynamics on both a coastal dune and a desert sand mound in California. Aeolian transport occurs predominantly in the form of families of intertwining and bifurcating streamers, while under intense wind forcing, more complex patterns of nested streamers and clouds with embedded streamers develop. The streamers display a characteristic width of approximately 0.2 m and an average lateral spacing of about 1 m. These dimensions appear to be independent of mean airflow characteristics. The observations and measurements of streamers under different environmental conditions suggest that bed surface control in the form of differentiation in moisture content, grain size, or microtopography is not a necessary condition for the formation of streamers. The results show little correlation between possible streamwise vortices or burst sweep events and streamers. It is proposed that streamers are generated by near-surface gusts that originate from large eddies propagating to the ground from higher regions of the boundary layer. These elongated and stretched eddies scrape across the surface and initiate saltation along their path. This concept accounts for the characteristic size and spacing of streamers, their rapid propagation, and the fast response of saltation to wind speed fluctuations.

Modelling vegetated dune landscapes

This letter presents a self-organising cellular automaton model capable of simulating the evolution of vegetated dunes with multiple types of plant response in the environment. It can successfully replicate hairpin, or long-walled, parabolic dunes with trailing ridges as well as nebkha dunes with distinctive deposition tails. Quantification of simulated landscapes with eco-geomorphic state variables and subsequent cluster analysis and PCA yields a phase diagram of different types of coastal dunes developing from blow-outs as a function of vegetation vitality. This diagram indicates the potential sensitivity of dormant dune fields to reactivation under declining vegetation vitality, e.g. due to climatic changes. Nebkha simulations with different grid resolutions demonstrate that the interaction between the (abiotic) geomorphic processes and the biological vegetation component (life) introduces a characteristic length scale on the resultant landforms that breaks the typical self-similar scaling of (un-vegetated) bare-sand dunes.

Spatiotemporal Variability of Aeolian Sand Transport in a Coastal Dune Environment

Abstract Aeolian sand transport on beaches and in dune environments shows a great spatial and temporal variability that has important implications for modeling and monitoring of coastal systems. Yet there have been few quantifications or statistical characteristics of transport variability in natural environments. Transport variability can result from bed surface control in the form of differentiation in grain size, surface moisture, and microtopography, or can be induced by fluid forcing in the form of gusts, burst-sweep events, and streamwise vortices. A field experiment was conducted on a coastal dune near Guadalupe, California, to quantify transport variability over spatial scales of 0.1–4.0 m and temporal scales of 1–120 seconds. Results show that spanwise (lateral) variability increases with spatial scale and decreases with temporal scale. Minimum transport variability over the smallest distances and longest time scales is on the order of 30%, providing error margins for transportrate measurements and model extrapolations. Variability reaches a maximum level at spatial scales larger than roughly half the boundary-layer height. In relation to shear velocity, greatest variability is found near the transport threshold and smallest variability occurs during periods of high shear velocities.

Mapping functional vegetation abundance in a coastal dune environment using a combination of LSMA and MLC: a case study at Kenfig NNR, Wales

The interactions between wind-blown sand transport, pioneer vegetation and succession vegetation in coastal dune fields play an important role in landform development and determine the balance between stabilization and re-activation of these aeolian landscapes. High-resolution mapping of vegetation communities across a dune field – in particular, the mixture of different functional plant types such as pioneer versus succession species – is critical for the calculation of landscape metrics that enable a rigorous and quantitative testing of numerical simulation models, as well as for informing targeted land management actions that maintain biodiversity and ecological functions. This article presents a method of using maximum likelihood classification (MLC) to inform linear spectral mixture analysis (LSMA) for quantifying sub-pixel abundance of sand, pioneer and succession vegetation in a coastal dune area in Wales, from archived imagery obtained from the Compact Airborne Spectral Imager (CASI) in 1997. LSMA is first applied to derive sub-pixel fractional abundances of soil, green vegetation (GV) and non-GV elements. An MLC is developed separately for automatically identifying pixels believed to contain a mixture of the two functional vegetation types, and this then serves as a basis for applying a transform that interprets the LSMA results in terms of sand and pioneer and succession vegetation communities. Very high resolution (0.1 m pixels) colour aerial photography, taken simultaneously with the CASI data, and field survey data from both 1997 and 2009 were used to aid the MLC and the transform algorithm and were also used for a limited validation exercise. The LSMA abundance maps achieved an overall accuracy of 82.7% (kappa coefficient κ = 0.78). The reduced MLC vegetation maps (four classes) achieved an overall accuracy of 98.2% (kappa coefficient κ = 0.96). Although it was not possible to validate the final pioneer and succession vegetation abundance maps quantitatively, a qualitative review of the results for selected locations within the dune field indicates the viability of applying MLC to help direct a transformation of LSMA abundance maps into functional vegetation abundance data.

Environmental impacts: Coastal ecosystems

This chapter examines the impacts of climate change on the natural coastal ecosystems in the North Sea region. These comprise sandy shores and dunes and salt marshes in estuaries and along the coast. The chapter starts by describing the characteristic geomorphological features of these systems and the importance of sediment transport. Consideration is then given to the role of bioengineering organisms in feedback relationships with substrate, how changes in physical conditions such as embankments affect coastal systems, and the effects of livestock. The effects of climate change—principally accelerated sea-level rise, and changes in the wind climate, temperature and precipitation—on these factors affecting coastal ecosystems are then discussed. Although the focus of this chapter is on the interaction of abiotic conditions and the vegetation, the potential impacts of climate change on the distribution of plant species and on birds breeding in salt marshes is also addressed. Climate impacts on birds, mammals and fish species are covered in other chapters.

Electronic Measurement Techniques for Field Experiments in Process Geomorphology

The processes that shape the Earths surface have a variation or rhythm that can be measured in the field using a number of instruments and techniques. This chapter describes electronic field equipment that has increased our capacity to investigate sediment transport and fluid flows in coastal, fluvial, and aeolian environments. This equipment includes: (1) anemometers, velocimeters, profilers, and hydrophones for Eularian flow measurement; (2) traps, impact sensors, and backscatterance sensors for sediment transport measurement; (3) drogues, particle image velocimeters, and electronic tracers for measuring Lagrangian flow and transport; and (4) erosion pins, distance sensors, and sonar altimeters for measuring bed elevation change. Modern deployment of these instruments and past applications are described with respect to the spatial and temporal scale of the processes being examined, and the methodological and interpretative limitations inherent to field deployments.

Earth Pulses in Direct Current

The Earth’s pulse is evident in a variety of geomorphic processes that shape its surface. This chapter describes how electronic instrumentation has dramatically increased our capacity to investigate sediment transport by wind and water and to relate processes to morphological change. A number of instruments and techniques for measuring fluid flow and sediment transport in fluvial, coastal, and aeolian environments are discussed, including current meters, current profilers, pressure transducers, optical backscatter sensors, anemometers, hot-film probes, photoelectric erosion pins, sediment traps, and saltation impact responders. The deployment of such instruments is placed in the context of scale, methodology, limitations, and interpretation of spatiotemporal records of measured processes.

Approaches to Modelling Ecogeomorphic Systems

Drivers of land degradation often co-occur and their effects are often non-additive because of internal system feedbacks. Therefore, to understand how drivers of land degradation alter ecogeomorphic patterns and processes, novel tools are required. In this chapter we explore different modelling approaches that have been developed to simulate pattern formation, and ecological and geomorphic processes. These modelling approaches reflect some of the best available tools at present, but notably, they tend to simulate only one or at best two components of the ecogeomorphic system. The chapter culminates with a discussion of these different modelling approaches and how they provide a foundation upon which to develop much needed ecogeomorphic modelling tools.

A prospectus for future geomorphological investigation of the Namib Sand Sea

The Namib Sand Sea in southern Africa offers an ideal location in which to consider general questions about the evolution of sand seas, about the fluxes of sand through contemporary dune fields and about the patterns of dune form that are created. This paper aims to provide a concise account of the approaches and techniques that are currently being used and will be used in the future to address these questions. The paper considers the techniques employed to investigate wind climate, the morphometry of the dunes, the internal structure of dune sediments, the age of the dunes and the potential to model dune development.