Model for the genesis of coastal dune fields with vegetation
Research highlights
► Vegetation plays a major role for the genesis of coastal dune fields. ► The width of the non-vegetated backshore controls interdune spacing. ► The maximum vegetation cover density determines the appearance of trailing ridges. ► The growth rate of the vegetation controls the barchan-parabolic transition. ► Foredunes develop when vegetation grows fast enough.
Introduction
Sand dunes appear very commonly along coasts (Pye, 1983, Hesp et al., 1989, Nordstrom et al., 1990, Arens, 1996, Hesp, 2002, Bailey and Bristow, 2004, Levin and Ben-Dor, 2004, Hesp and Martinez, 2008, Provoost et al., 2009). They are formed by the aeolian transport of sand from the beach, and may assume different forms depending on the wind directionality, on the amount of sediment available for transport and on the growth of vegetation. In non-vegetated fields where the wind regime is unidirectional, barchans and transverse dunes are the characteristic dune shapes (Bagnold, 1941). However, in coastal areas where humidity is typically high, aeolian transport competes against the growth of vegetation. The transformation of a barchan into a parabolic dune due to the growth of plants on the dune arms is a well-known consequence of this competition and has been studied by many authors (Tsoar and Blumberg, 2002, Durán and Herrmann, 2006a, Baas and Nield, 2007). Indeed, a quantitative understanding of the dynamics of coastal dune fields as function of the local environmental conditions is of crucial importance for coastal management. In this work we present insights gained from a physically based model for aeolian transport and dune formation, into the genesis and dynamics of dunes evolving with growing vegetation.
Dunes cover extensive areas along the northeastern Brazilian coast, which is characterized by highly unidirectional wind regime – governed by easterly strong trade winds – and by the presence of vegetation (Jimenez et al., 1999, Barbosa and Dominguez, 2004, Durán et al., 2008, Levin et al., 2009, Tsoar et al., 2009). Currently active dunes, which comprise barchans, barchanoids and sand sheets, migrate on top of older generations of stabilized bedforms (Levin et al., 2009), and may advance more than several kilometers inland. In most areas, active dunes are separated from the coast by a deflation plain of 500 m to 2 km wide. Rates of dune migration vary significantly over the year (Jimenez et al., 1999, Levin et al., 2009). Indeed, average wind power displays a strongly seasonal behavior: most aeolian activity is restricted to the dry season between August and December, whereas the other half of the year wind velocities are mostly below threshold (Jimenez et al., 1999). Rainfall, inversely correlated with wind power, vanishes in the dry period and reaches maximum values of 400 − 600 mm month− 1 (with an average of about 250 mm month− 1) in the wet season. Time-series of wind speed and direction, rainfall and evaporation rates for the dune fields in north-eastern Brazil have been presented in previous works (Jimenez et al., 1999, Parteli et al., 2006, Levin et al., 2009).
The dune areas along the Brazilian coast are of particular economical interest since they are a tourist attraction and constitute potential aeolian parks (Floriani et al., 2004). The competition between landward dune migration and vegetation growth results in different types of dunes as illustrated in Fig. 1, Fig. 2, Fig. 3. Near the city of Natal, at 5°49′ S, 35°11′ W, shore-parallel dune ridges covered with vegetation (“foredunes” (Hesp, 2002)) constitute one characteristic dune morphology (c.f. Fig. 1). Due to the dense vegetation cover, the sand transported onto the continent accumulates in the form of large barriers that can, in some cases, reach heights of several tens of meters (Hesp, 2002). More to the south, at Pirambu (Fig. 2), and along the Ceará coast, where winds are typically stronger (Levin et al., 2007) barchans and parabolic dunes are observed. Far to the northwest we find the largest dune field of Brazil. Covering approximately 270 km2, the region known as “Lençóis Maranhenses” has an annual rainfall of about 2000 mm and is the coastal dune field with the strongest sand-moving winds in northeastern Brazil (Levin et al., 2007). Transverse dunes and chains of barchans extending over several kilometers alternating with freshwater lagoons that form in the rainy seasons (c.f. Fig. 3) are the dominant dune morphologies at Lençóis (Gonçalves et al., 2003, Parteli et al., 2006, Levin et al., 2007).
The competition between vegetation growth and aeolian transport of sand in coastal areas can, thus, lead to different dune morphologies. Recent models based on cellular automata could shed some light into the morphology and dynamics of aeolian landscapes with vegetation growth (Nishimori and Tanaka, 2001, Baas, 2002, Baas and Nield, 2007, Nield and Baas, 2008a, Nield and Baas, 2008b). In the present work, we use a model for dune formation recently developed (Sauermann et al., 2001, Kroy et al., 2002, Schwämmle and Herrmann, 2005, Durán and Herrmann, 2006b) and applied with success in the investigation of different dune morphologies (Sauermann et al., 2003, Parteli et al., 2006, Parteli et al., 2009), in order to study the genesis and early stages of evolution of coastal dune fields with vegetation growth. In a recent work, the dune model, which incorporates a mathematical description of saltation sand transport and of the turbulent wind flow at the scale of dunes, has been extended to calculate the transition of a barchan into a parabolic dune due to vegetation growth (Durán and Herrmann, 2006a). The aim of the present study is to investigate the formation of a dune field starting from a flat hill of sand on the beach, as well as the dependence of the field morphology on the main parameters of the vegetation cover, the wind strength and the amount of sand available for transport.
In fact, little attention has been given by modelers to the early developmental stages of coastal dune fields. Field studies on formative processes of coastal dune fields have also been few, in part because most dune fields consist of “mature” bedforms that provide little insight as to dune morphodynamics during the genesis of the field (Kocurek et al., 1992). The dune fields of the northeastern Brazilian coast are some of the areas where one can observe the inception of coastal dunes competing with vegetation growth. Therefore, we start our research through using numerical modeling to reproduce the morphology of the dunes in these fields. The calculations aim to address specific questions regarding coastal dune formation, as for example the role of the initial sand volume and vegetation growth rate for the emergence and morphology of along-shore dune barriers or foredunes (Hesp, 2002), as well as the factors controlling dune height and interdune scaping in a dune field emerging with vegetation growth.
This paper is organized as follows. In the next section, we present a brief description of the dune model. The simulations are explained in detail, then, in Section 3. In Section 4 we present and discuss our results. Conclusions are given in Section 5.
Section snippets
The model
The dune model combines a quantitative description of the turbulent wind field over the terrain with a continuum saltation model, which encompasses the evolution of the sand surface due to erosion and deposition and also accounts for avalanches and flow separation at the dune's lee. In the model, the vegetation cover acts as a rough patch that modifies the wind field and is allowed to grow at a rate that depends on the local erosion and deposition. The model consists of solving the following
Simulations
Calculations are performed using open boundaries, a constant sand influx qin at the inlet of the dune field and a wind of constant upwind shear velocity u∗. The dune field has a length of 512 m transverse to the wind and 1024 m in the wind direction. We start with a transverse dune which has a Gaussian profile in the wind direction, a height of 1.5 m and a length of 80 m, and is placed at the beginning of the field. The dune surface is modulated with random fluctuations, the amplitude of which is
Genesis of a sand dune field: the role of sand influx and vegetation growth
We study, firstly, the evolution of the field without vegetation growth. An upwind shear velocity of u∗ = 0.38 m/s, which is a typical value for sand-moving winds in coastal dune areas (Tsoar et al., 2009), is taken, while the relative sand influx qin/qs is varied.
It is well known that a flat sand hill submitted to a unidirectional sand-moving wind and undersaturated flux is unstable and evolves into a dune that migrates downwind (Sauermann et al., 2001, Kroy et al., 2002, Andreotti et al., 2002).
Conclusions
The genesis of coastal dune fields was studied through using a model for aeolian saltation and dune formation that accounts for vegetation growth. The morphology and dynamics of dunes emerging from a sand patch under unidirectional wind was investigated. The results of the calculations may be summarized as follows:
- 1.
In the absence of vegetation growth, coastal dune fields are formed when there is a high influx of sand. The calculations shed light on the formative stages of coastal dunes emerging
Acknowledgments
This work was supported in part by FUNCAP, CAPES, CNPq (Brazilian agencies), by Swiss National Foundation Grant NF 20021-116050/1 and ETH Grant ETH-10 09–2.
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