United States Department of Transportation
i
Dynamic revetments for coastal erosion in Oregon : final report.
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2005-08-01
[PDF-4.33 MB]
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Abstract:Gravel beaches have long been recognized as one of the most efficient forms of "natural" coastal protection, and have been suggested as a form of shore protection. "Cobble berms," "dynamic revetments" or "rubble beaches" involve the construction of a gravel beach at the shore, in front of the property to be protected. These structures are effective in defending properties because the sloping, porous cobble beach is able to disrupt and dissipate the wave energy by adjusting its morphology in response to the prevailing wave conditions. Dynamic revetments are much easier and cheaper to construct than a conventional riprap revetment or seawall. They are also aesthetically pleasing compared with "hard" engineered solutions. There remain, however, unanswered questions about their design particularly along the high-energy Oregon coast--the sizes and types of gravel to be used, their slopes and crest elevations, the volume of material to be included in the berm, and where the material may be obtained to construct such features. This study involved an examination of the morphological and sedimentary characteristics at 13 naturally occurring gravel beach study sites along the Oregon coast. Heights of the gravel beaches ranged from 5.7 to 7.1 m (19-23 ft), while the slopes of the beaches varied from 7.7 to 14.1 deg. Mean grain-sizes were found to range from 30 to 128 mm, and were classified as well sorted to moderately well sorted. However, a comparison of these parameters among stable versus eroding gravel beaches revealed no clear discernible pattern. A key difference in the stability of the gravel beaches was the volume and width of gravel contained on the beach, with beaches containing larger volumes of gravel [>50 cu m/m (538 cu ft/ft)] and larger widths [>20 m (66 ft)] being the most stable. Based on this analysis, a crest elevation of ~7.0 m (23 ft), mean grain-size of no less than 64 mm, and a beach slope of 11 deg was recommended in future designs of dynamic revetments for the Oregon coast. While numerous quarry sites were identified that could supply crushed rock for the building of a dynamic revetment, rounded gravels were more difficult to locate and tended to be located farthest from the coast, increasing the costs that would be incurred to transport the material.
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