Parabolic dunes are identified by vegetated dune ridges and vegetated deflated plains. Due to variable wind gusts, parabolic dunes are commonly unvegetated in troughs or dune swells where wind tunnels transport currents. Ripple alignment in association with the main dunes can also identify parabolic dunes. Ripples minuet accumulations of sand against the main dune swale. The heights of ripples are normally measured on a millimeter to centimeter scale. In
Humboldt Bay, the wind is predominately blowing in from the northwest. As a result, the dune ridges are formed parallel to the wind currents while ripples are formed perpendicular to the wind.
Wind Northern California coastal dune environments are subject to high velocity winds at all times throughout the year. This strong variable causes the morphology of the dune ecosystem to constantly change. Dunes can range in height from a meter to tens of meters tall creating elevation changes and
habitat complexities.
Invasive species can further armor dune ridges, creating linear dunes, and preventing naturalistic parabolic dunes from being created.
Sediment transport Sand granules are transported in three ways:
suspension,
saltation, and creep. Suspended grains are fine granules that can easily be picked up by wind and carried for variable distances. Most visitors to coastal beach environments can attest to having sand blown in their face or leaving with a gritty feeling on their skin. This is due to fine sediment suspended in the moisture rich air. When suspended sediment is returned to the ground, granules physically impact the grounded grains. Due to physics principles, the grounded grains are receiving energy from the once suspended sediment. This impact leads to the dislodgement of grounded grains or creep of coarser grains. Saltation is the movement of grains being picked up by the wind and dropped in a cycling repetitive motion.
Sources of sediment Coastal environments act as drainage outlets for freshwater river systems. As a result, sediment from tributaries and headwaters are deposited at the mouth of the river. Long shore transport is a linear current off the coastline that moves sediment. For Northern California, this current moves sediment in a northern direction. Therefore, sand and sediment constructing Humboldt Bay's thirty-four mile dune ecosystem, is a result of sediment deposition at a southern location. Sediment accumulation can also be a result of wave action. Wave currents occur in a swash and backwash motion. This continual wave action allows for the movement of sediment. The angles at which the swash and backwash occur, are associated with the off shore transport current as well as the change in winter and summer ocean currents.
Foredune ecology The vegetation analyzed at the Mad River County Beach showed an evolutionary change in the ecosystem as a result of several thriving invasive species. Upon arrival to the beach, it became visually apparent just how abundant the
Ammophila arenaria (European beach grass) species is. According to Pickart and Sawyer (1998),
Ammophila arenaria is described as being foredune engineers. As
Ammophila arenaria attaches and begins to grow on a relatively flat dune system, wind currents that push sand inland it allows the plant to accumulate and mound massive amounts of sand creating large foredune ridges. This shift is supporting the invasion of but not limited to,
Ammophila arenaria, Tanacetum vulgare (tansy) and
Bromus diandrus (ripgut brome). Since introduction of these invasive plants, scientists have recorded a severe displacement in native grasses and dune mat vegetation throughout California. A characteristic of
Ammophila arenaria and
Bromus diandrus entering an ecosystem, are elevated levels of nitrogen within the soil [8] . Unfortunately, the implementation of nitrogen into the soil, limits the growth and livelihood of other species such as
Layia carnosa and
Erysimum menziesii. Since the early 1900s,
Ammophila arenaria has been introduced into the California landscape to perform as a natural re-engineering feature to transform the beach landscape. Characteristically,
Ammophila arenaria invades the historically flat Californian foredunes transforming the ecology of the dune system. As wind currents push sand inland, the
Ammophila arenaria begins to accumulate massive amounts of sand creating large foredune ridges. The alteration of dune morphology affects native plants and animal species that rely heavily on the dunes for nourishment and habitat. ==References==