Numerical simulation of breakages of concrete armour units using a three-dimensional fracture model in the context of the combined finite-discrete element method

Development of a three-dimensional fracture model for the combined finite-discrete element method

: Crescent City breakwater is located on the California coastline approximately 17 miles south of the Oregon border. In 1974 and again in 1986, dolosse were placed along a section of the breakwater particularly susceptible to damage. Monitoring of above water dolosse moment and breakage has been conducted since 1986. In August of 2004, a detailed monitoring survey was conducted of the dolos section of the Crescent City breakwater. The goal of this monitoring is to understand the long-term structural response of the dolosse. Detailed monitoring included ground surveys, aerial photography, photogrammetric analysis, and a broken armor unit survey. Results showed very little dolos movement has occurred since the end of the initial nesting period in 1988. Furthermore, no additional dolos breakage was observed since 1993. From 1995 to 1999, core samples were obtained from dolosse cast in 1986 and in 1974 in order to determine the in-situ strength of units in both non-loaded (near concrete cap) and highly loaded regions (near still water level) of the structure. The cores were subjected to standard laboratory concrete strength tests. Results indicated that the structural strength has increased considerably since construction for non-loaded units. However, concrete strength for units placed in highly loaded units near the still water level was near constant.

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Monitoring of Dolos Armor Units at Crescent City, California

The beach drainage can be included among the soft engineering methods aimed to counteract the shoreline retreat related to the sediment redistribution along the beach profile. The idea that the groundwater table plays a role in the mobilization of the sediment grains underlies the application of the beach drainage system (BDS) as a tool able to modify the natural dynamics of groundwater table at sandy beaches. Indeed, BDS consists of a series of alongshore buried drains aimed to lower the groundwater table. Due to the drainage, the thickening of the layer of dry sand makes the solid grains less prone to be mobilized by the action of the waves at the swash zone, where interaction phenomena between the percolating and the incoming water lens take place. This tool had a troubled history. Its first applications were encouraging. Since then, controversial performances of both experimental and prototype scale experiences have been observed around the world. This paper aims to present an up-to-date full review of the studies and experiments carried out so far, in order to provide the reader with a complete perspective on its strength and weakness as well as open challenges to be faced in the near future.

/pdf/beach-drainage-system-a-comprehensive-review-of-a-1p86o3rm.pdf

Beach Drainage System: A Comprehensive Review of a Controversial Soft-Engineering Method

Coastal ecosystems, such as seagrass meadows, have been heralded as a nature-based solution for coastal protection. However, the functions of seagrass meadows, including wave attenuation and sediment stabilisation, have typically been measured offshore and do not directly translate to coastal protection at the shoreline. In contrast, the protective effects of conventional submerged structures, such as breakwaters and artificial reefs, on adjacent shorelines have been well described and are predictable. Importantly, these artificial structures often cause erosion in adjacent areas. This study examines how seagrass may affect shoreline morphodynamics by making inferences from existing knowledge of conventional submerged structures. Both conventional submerged structures and seagrass meadows affect wave height and direction, yet this effect on longshore sediment transport has only been considered within the context of coastal defence structures. From the conventional literature on submerged structures, it is known that a constant rate of longshore sediment transport will result in a constant shoreline shape. Therefore, a change in the rate of longshore sediment transport, or gradient, generates changes in the shoreline shape and, hence, the shoreline morphodynamics. We propose that seagrass also has spatially heterogeneous effects on shoreline morphodynamics, with protection occurring in some locations and downstream erosion likely in others. Empirical and modeling research is required to test this hypothesis. In the meantime, this review suggests contexts under which seagrass may affect shoreline morphodynamics and provide shoreline protection. This research bridges the gaps in knowledge across marine ecology, physical oceanography and coastal engineering to overcome some of the challenges of interdisciplinary coastal science. 

Contextualising shoreline protection by seagrass using lessons from submerged breakwaters

Ten years of monitoring data are now available from the Crescent City dolos monitoring program. This extensive field data collection program was initiated in 1986, following the rehabilitation of the dolos section of the main stem of the Crescent City Outer Breakwater. The rehabilitation involved the placement of 680, fiber-reinforced, 38-tonne dolosse. Unique field monitoring techniques have been employed to collect data at Crescent City: movement data have been collected with ground-truthed, low-altitude (helicopter) photogrammetric survey techniques and stress data have been derived from strain gages mounted internally in select dolosse grouped together near the center of the dolos field. Results presented in this paper include 10 year histories on: wave climate, dolos movement/breakage, stress build up within dolosse, and concrete strength and fatigue. In addition, internal strain gage instrumentation verification test results are presented.

https://icce-ojs-tamu.tdl.org/icce/index.php/icce/article/download/5580/5253

A ten-year history of dolos monitoring at crescent city

: This report discusses wave flume tests of a newly designed ocean swell powered renewable energy (OSPREY) wave power generating caisson to assess its suitability and efficiency. Tests were carried out in a new wave flume commissioned by Applied Research and Technology (ART), of Inverness, Scotland. Data from both regular and irregular wave tests are shown. Wave transmission test results are plotted and compared with previous tests of a proposed rubble-mound alternative. Test results indicate that three-unit and four-unit OSPREY arrays allow significantly more wave energy transmission than the rubble-mound breakwater. In order to satisfy the basic requirements for wave sheltering at this site, it would appear that the OSPREY units must be placed closer together, integrated within the rubble mound, or spaced using integrated inactive caissons.

http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA324221&Location=U2&doc=GetTRDoc.pdf

William S. Appleton

Papers

A ten-year history of dolos monitoring at crescent city

Evaluation of Wave Transmission Characteristics of OSPREY Wave Power Plant for Noyo Bay, California.