Environment Field Note: The Development of Coastlines, Sandy Beaches, Fårö, and Ulla Hau in Gotland, Sweden
The mechanisms which control the appearances of coastlines are really quite simple. Imagine a pile of hay and iron nails being repeatedly acted upon by strong gusts of wind. The blades of hay would naturally be removed from the pile faster and more frequently because they are less resistant to the forces acting on them than the nails. The same principle can be applied to erosion and geology. In the same way that certain materials are more resistant to movement based on their weight, certain minerals and rocks are more resistant to erosion based on their chemistry. Most igneous and metamorphic rocks (and some sedimentary rocks such as limestones) are particularly resistant to ‘wave attack.’ Conversely, rocks composed of unconsolidated sediments or many bedding planes, such as sandstones, glacial deposits, or shales, are much more easily eroded by waves. If a coast is composed of rocks without uniform resistances to erosion, which it invariably is, eventually some portion of the coastline will recede inland; typically in more place than one. An area where the coast seems to have extended out to sea is most often just the resulting effect of that area being more resistant to erosion than its former surroundings. Reality is not always so straightforward, though. In cases such as Gotland, apparent movement of the ‘littoral zone’ can be caused by ‘isostatic rebound.’ The ‘littoral zone’ refers to the entire sedimentary formation (including those sediments not directly affected by surface waves) rather than just those grains that lie above sea level. When a massive object such as a glacier compresses a portion of earth’s surface, the lithosphere responds by bending further into the asthenosphere. Depending on the physical properties of the rocks below and the pressures applied to them, surrounding areas will sometimes respond by rising in response to another area sinking. Conversely, when a glacier melts and the weight above the lithosphere decreases, the previously down-bended crust ‘rebounds’ and returns to its original elevation (see fig. 1). It is speculated that this is the mechanism responsible for Gotland’s inland sands and the relative drop in sea level.
To beach or not to beach:
While it is crucial to understand how materials respond to forces generally, it is equally important to be able to contextualize those materials and forces according to their setting. ‘Bathymetry’ refers to the study of ocean, sea, or lake floors; essentially underwater topography. Much in the same way that the elevation of a mountain or dune affects wind’s erosional effect on it, the structure of an ocean floor will dramatically affect the extent and behavior of water erosion. A beach/littoral zone can essentially be divided into two parts; the ‘offshore’ and the ‘nearshore zones’ (see fig. 2). Though both the offshore and nearshore zones are components of the broader littoral zone, they interact very differently with wave action. The offshore leads into the nearshore zone and is typically smoother with a gentler slope. This is because deeper ocean waters are less turbulent and have more exposure to sediments, thereby allowing for greater attrition. The nearshore zone, however, is often shallower, choppier, and has a much steeper slope. Nearshore zones are divided into three parts: 1) the breaker zone, 2) the surf zone, and 3) the swash zone. The breaker zone refers to the area in which the bathymetry of the sea floor changes abruptly from a steady slope to a steeper one. It is at this point that waves are uplifted as their velocity is redirected vertically by the sea floor. The slope is quite minimal in the surf zone which allows gravity to act on the waves again, causing them to subside momentarily. The bathymetry of the sea floor changes dramatically at the swash zone as slope steepens and the waves’ behavior becomes more erratic. The ‘transition zone’ is the area between the surf and swash zones in which “the return flow of the swash collides with the incoming surf bores, creating high turbulence, a broad energy spectrum, and a bimodal sand-size distribution” (Komar, 14).
It is also worth mentioning that littoral zones mature and evolve; generally becoming smoother with time (see fig. 3). The sedimentary composition and sorting of beaches is largely based on these factors nonetheless. Littoral zones with greater tidal ranges sometimes develop separate high-tide and low-tide beaches, in which sediments are sorted according to weight. Low-tide beaches generally exhibit finer-grained sand while grains on high-tide beaches are coarser. Low-tides often produce finer-grained sands because the wave velocity is low enough for the heavier (i.e. coarser) grains to settle out before reaching the shore and because their more frequent exposure to lower sediments allows additional time for attrition. High-tide waves can carry their suspended loads further up the beach due to the additional gravitational pull exerted on them by the moon. It is also not uncommon to find coarser materials far up along a beach after a storm due to waves’ increased velocity, turbulence, and carrying capacity during such events.
The process of sedimentary saltation in water is not terribly unlike the process of Aeolian erosion. The migration of underwater sediment is largely controlled by bathymetry and force, much in the same way that topography, geometry, force, and geography are major controls on Aeolian sedimentary motion. In the northwestern portion of Fårö is a 144-acre deposit of felsic (qtz-feld/sand) sediment of which Ulla Hau, Gotland’s gigantic U-shaped dune, is a part (see fig. 4,6). Beginning in the 1700s a massive sediment deposit was formed and transported by northerly winds (which drove it south) around the island at about three feet per year. This created a rolling dune/desert which was destructive to the ecosystems around it until a man named Marcus Larsson managed to ‘bind’ the Ulla Hau deposit and prevent it from coating the entire island. This parabolic dune encloses an area of about 1.3km (called the ‘deflationsyta’) and its highest point is about 15m above its surrounding environment. Despite the 50-acre sedimentary coverage, Ulla Hau fosters a unique ecosystem including pine trees and gräshoppsstekeln (“ant-lions”), which are an insect species unique to the area (see fig. 5). These U-shaped dunes are produced when migrating sediment essentially becomes snagged on vegetation in two or more locations. Parabolic dunes are special because unlike others, the arms of horseshoe-dunes follow the bulk of the formation and the crests point upwind as they migrate. Aside from that quality parabolic dunes are similar to crescent dunes in that they are governed by a consistent wind direction; unlike star dunes, for instance (see fig. 7).
“Ancient Shorelines of Lake Bonneville.” GeoCaching. N.p., 13 May 2013. Web. 16 May 2013.
“Geography: Depositional Landforms.” BBC News. BBC, 2013. Web. 16 May 2013.
Komar, Paul D. Beach Processes and Sedimentation. Englewood Cliffs: Prentice-Hall, 1976. Print.
McKee, Edwin D. “Parabolic Dune, Blowout Dune.” A Study of Global Sand Seas. Michigan Institute of Technology, 1979. Web. 16 May 2013.
“Types of Dunes.” Types of Dunes. USGS, 29 Oct. 1997. Web. 16 May 2013.
“Ullahau.” Länsstyrelsen Gotlands Län. Naturreservat, 2000. Web. 16 May 2013.
“Ulla Hau.” Gotlands Största Portal. Guteinfo, 8 Oct. 2009. Web. 16 May 2013.