--> Abstract: When Dunes Walk Over Dune Walkovers, by Rob Conti; #90069 (2007)

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When Dunes Walk Over Dune Walkovers

Rob Conti
Texas General Land Office, Coastal Resources Program, P.O. Box 12873, Austin, Texas 78711-2873

Dune walkovers are constructed in coastal areas to allow pedestrians or small vehicles access to beaches from private residences, hotels, condominiums, public buildings or parking areas while walking or riding above the dunes, so that the dune vegetation remains undisturbed. Undisturbed vegetation is crucial to stabilizing dunes which, in turn, protect landward portions of coastal areas from storm surges associated with tropical storms and hurricanes. When the walkovers are constructed without considering the nature of the site and surrounding topography, the walkovers can become part of the dunes they were intended to protect, despite an otherwise adequate design.

Walkovers are generally built so that they are at least as high above the dunes as they are wide, to achieve a minimum height to width ratio of 1:1. This ratio allows sunlight and precipitation to reach the dune vegetation beneath the walkover, thus stabilizing the underlying areas. Several other parameters also need to be considered in designing and building walkovers. They include: employing construction methods that will minimize dune and dune vegetation disturbance; construction materials used and their vulnerability to degradation from exposure to salt-air and ultraviolet radiation; volume of anticipated pedestrian traffic; type of anticipated traffic (strictly pedestrian or small vehicles); grades and ramps for users with special needs; deck board spacing to accommodate water and accumulating sand drainage and light penetration; orientation to predominant winds; and the users’ safety.

Historic photographs of the construction phase of one specific dune walkover reveal that the walkover was built in a pedestrian path that became a topographic low—likely caused by wind deflation of the sand after the dune vegetation had been continuously disturbed. Upon completion of construction, the handrails of the new dune walkover were the same height as the tops of the surrounding dunes. After the surrounding sand filled in the adjacent topographic low, it migrated and started covering the overlying walkover. This happened in the time interval of about 8 years or less, from 1998 to 2006.

The migrating sand issue was not isolated to this particular walkover. A tour of the Texas beach from North Padre Island to northern Mustang Island revealed several other walkovers that are experiencing similar sedimentation problems. Once constructed in a pedestrian trough, efforts to abate the migrating sand by using vertical barriers attached to the handrails are temporary, at best. Interim measures taken to mitigate the migrating sand include: attaching plywood, silt fencing or geotextile to the handrail structures; shoveling deposited sand off the structure; or simply continuing to walk over the deposited sand. Rebuilding above the old/sand-covered walkover is a more permanent solution. A cursory examination of the walkovers indicated that those built over and parallel to undisturbed dune topography fared best with respect to remaining free of sand deposits, maintenance or replacement.

 

AAPG Search and Discover Article #90069©2007 GCAGS 57th Annual Convention, Corpus Christi, Texas