Foredune Ridges Research Articles

Beach Washouts, an Adverse Effect of Artificial Dune Construction

At South Beach, Padre Island National Seashore, a new and unexpected maintenance problem develops when drainage channels are eroded across the beach after heavy rainfall. Hurricane Carla eroded extensive portions of the foredune ridge and left behind a broad hurricane beach. A nearly continuous foredune ridge, created by planting various grasses, extends along the coast seaward of an unvegetated dune field generated from the pre-hurricane foredune sediments. If the newly formed foredune ridge had been constructed as a discontinuous feature, which would allow gradual rather than channelized drainage to occur, the problem would have been reduced. In order to minimize transverse beach channel effects, reconstructed foredunes should approximate natural foredune morphology and should be stabilized with grass patches instead of continuous rows of plantings.

Biogenic Sedimentary Structures Produced by Crabs in Lagoon Margin and Salt Marsh Environments Near Beaufort, North Carolina

ABSTRACT Intertidal burrowing crabs produce distinctive biogenic sedimentary structures in protected lagoon margin and salt marsh environments in the vicinity of Beaufort, North Carolina. Distribution of crabs is determined primarily by substrate characteristics, salinity, and vegetation cover in the intertidal zone. Protected sand beaches and flats are heavily burrowed by the sand fiddler, Uca pugilator, which produces gently curved J-shaped and sharply curved L-shaped burrows 1 to 2 cm in diameter. Larger Y- and U-shaped burrows are formed by the ghost crab, Ocypode quadrata, in the backshore and foredune ridge zones above the high tide line. Salt marshes influenced by normal to near normal marine salinities are burrowed by U. pugilator, U. pugnax, and Sesarma reticulatum. U. pugnax and S. reticulatum burrow in muddy substrates, particularly along tidal creek banks, whereas U. pugilator is restricted to sandy substrate areas. U. pugnax burrows are 1 to 2 cm in diameter and have complex twisting and turning forms. The larger communal burrows of S. reticulatum consist of a complex of interconnected shafts with several oval-shaped surface openings. Panopeus herbstii is often found inhabiting these burrows. In salt marshes where the influence of fresh water is great, U. minax and U. pugnax are the dominant crabs. U. minax forms burrows 2 to 5 cm in diameter with ho ded entrances and long vertical shafts. Intertidal crabs are important agents of bioturbation in the environments that they inhabit. Their burrows have excellent potential for preservation in the sedimentary record. Recognition of the record of biogenic sedimentary structures produced by intertidal crabs would be of significant value in identifying paleoenvironments, fixing ancient lagoon margin and estuarine shorelines, and determining paleosalinities.

Burrows of the Ghost Crab Ocypode Quadrata (Fabricius) on the Barrier Islands, South-Central Texas Coast

ABSTRACT Burrows of the grapsoid crab Ocypode quadrata (Fabricius) are important biogenic sedimentary structures on Texas barrier islands. Variation in the shape, diameter, length, orientation, and areal density of the burrows can be used to define subenvironments of the beach and foredune ridge. An increase in diameter, length, and morphological complexity of the burrows together with a decrease in burrow abundance from the upper foreshore to the back edge of the beach differentiate beach subzones. Greater burrow size and abundance and a simpler shape distinguish O. quadrata burrows in the dune mounds from burrows in the interdune flats of the foredune ridge. A preferred burrow orientation descending to the northwest in the backshore contrasts with a random orientation of burrow in the foredune ridge. Although the preferred orientation is away from the shoreline, factors not directly related to shoreline trend may be responsible.

Lake Michigan Dune Development. 3. Lake-Level, Beach, and Dune Oscillations

During periods of several successive years when lake levels are several feet below the mean level, sand is blown from the wide beach into a new foredune ridge. But much of it may be eroded by waves during ensuing years of unusually high mean lake level, aggravated by storms. When the level again falls and the beach widens, the beginning of a new ridge may cut off the sand supply from the ridge behind it. This shore-dune cycle may be repeated several times in a few decades. Protracted lowering of the lake may also help underwater sand bars emerge as barriers; these may be permanently colonized by vegetation if net longshore deposition of sand is sufficient to prevent their destruction during later episodes of high lake level. The connections between beach and dune forms and lake level and between lake level and precipitation and temperature account for many details of shore-line history around Lake Michigan for the past century and possibly for earlier episodes of the late postglacial climatic history.