Sheltering the Shore via Nearshore Oblique Seabed Bars

2016 ◽  
Author(s):  
Louis-Alexandre Couston ◽  
Mir Abbas Jalali ◽  
Mohammad-Reza Alam
Keyword(s):  
Field Measurements ◽  
Buffer Zone ◽  
Coastal Protection ◽  
High Momentum ◽  
Frequency Detuning ◽  
Resonance Mechanism ◽  
Broadband Spectrum ◽  
Parallel Direction ◽  
Wave Energy Flux ◽  
Incident Waves

Periodic seabed undulations, such as nearshore sandbars, are known to reflect incoming surface waves of twice the wavelength by the so-called Bragg resonance mechanism. In view of this property, longshore seabed-mounted bars were proposed long ago as a means of coastal protection against the high momentum of incident oceanic waves. Many theoretical, computational, experimental and field measurements were conducted to understand their effectiveness in shielding the shore. The idea, nevertheless, proved impractical when Yu and Mei (JFM 2000, [1]) showed that due to an inevitable finite reflection from the shoreline, energy can get trapped in the area between the shoreline and the patch of bars eventually resulting in a much higher wave energy flux impinging the shoreline. Here we propose an arrangement of oblique bars that shelters the shore by diverting, rather than reflecting, shore-normal incident waves to the shore-parallel direction. A protected buffer zone is thus created at the shoreline. We show that this novel arrangement can very efficiently shelter the shore, is almost insensitive to the distance between the bottom corrugations and the shoreline, is relatively robust against frequency detuning, and will discuss that it can be designed to protect the shore against almost the entire broadband spectrum of incident waves.

Shore protection by oblique seabed bars

2017 ◽  
Vol 815 ◽  
pp. 481-510 ◽  
Author(s):  
Louis-Alexandre Couston ◽  
Mir Abbas Jalali ◽  
Mohammad-Reza Alam
Keyword(s):  
Multiple Scale ◽  
Coastal Protection ◽  
Bragg Resonance ◽  
Shore Protection ◽  
Resonance Mechanism ◽  
Small Frequency ◽  
Protection Efficiency ◽  
Parallel Direction ◽  
Incident Waves ◽  
Incident Wave Energy

Shore protection by small seabed bars was once considered possible because seafloor undulations strongly reflect surface waves of twice the wavelength by the so-called Bragg resonance mechanism. The idea, however, proved ‘unreliable’ when it was realized that a patch of longshore seabed bars adjacent to a reflective shore could result in larger waves at the shoreline than for the case of a flat seabed. Here we propose to revamp the Bragg resonance mechanism as a means of coastal protection by considering oblique seabed bars that divert, rather than reflect, shore-normal incident waves to the shore-parallel direction. We show, via multiple-scale analysis supported by direct numerical simulations, that the creation of a large protected wake near the shoreline requires a bi-chromatic patch to deflect the incident waves to the shore-parallel direction. With two superposed sets of oblique seabed bars, the incident wave energy becomes efficiently deflected far to the sides, leaving a wake of decreased wave activity downstream of the patch. We demonstrate that the shore protection efficiency provided by this novel arrangement is not affected by reflection of leaked waves at the shoreline, and that it is relatively robust against small frequency detuning.

Subharmonic resonance of proposed storm gates for Venice Lagoon

1994 ◽  
Vol 444 (1920) ◽  
pp. 257-265 ◽  
Keyword(s):  
Experimental Evidence ◽  
Incident Wave ◽  
Laboratory Tests ◽  
Venice Lagoon ◽  
Subharmonic Resonance ◽  
Horizontal Axis ◽  
Trapped Modes ◽  
Resonance Mechanism ◽  
Trapped Mode ◽  
Incident Waves

A recent design of storm barriers at the inlets of Venice Lagoon consists of a number of articulated inclined gates hinged on a horizontal axis on the seabed. In laboratory tests with normally incident waves the gates have been found to oscillate at half of the incident wave frequency and out of phase with their immediate neighbours. In this paper we identify the resonance mechanism by first showing the existence of trapped modes as a consequence of the articulated construction. Experimental evidence is shown for the trapped mode and its subharmonic resonance by normally incident waves.

scholarly journals AT-SEA EXPERIMENT AND SITE MONITORING ON A NEW COASTAL PROTECTION ENGINEERING METHOD-ARTIFICIAL MARINE FOREST

2018 ◽  
pp. 37
Author(s):  
Jing-Hua Lin ◽  
Cheng-Chi Liu ◽  
Kao-Shu Hwang ◽  
Ching-Ju Chen ◽  
Nai-Kuang Liang
Keyword(s):  
Wave Energy ◽  
Buffer Zone ◽  
Engineering Method ◽  
Coastal Protection ◽  
Floating Bodies ◽  
Floating Structures ◽  
Dense Forest ◽  
Site Monitoring ◽  
Sea Area ◽  
Protection Engineering

A new engineering method on the coastal protection, "Artificial Marine Forest (AMF)" proposed by Liang et al. (2004) is composed by lots of tautly-moored floating structures (TMFS). The concept of this method is to establish a buffer zone between the shoreline and the sea area by building a dense forest to reduce the wave energy and protect the coast. When deploying a larger number of cylindrical floating bodies in the offshore region, the scene is similar to a marine forest.

Sedimentation and equilibrium in wave-formed bars: a review and case study

1979 ◽  
Vol 16 (2) ◽  
pp. 312-332 ◽  
Author(s):  
Brian Greenwood ◽  
Robin G. D. Davidson-Arnott
Keyword(s):  
Sediment Transport ◽  
Hydrodynamic Instability ◽  
Field Measurements ◽  
Theoretical Models ◽  
Break Point ◽  
Rip Currents ◽  
Group Vi ◽  
Process Factors ◽  
Incident Waves ◽  
Bar Formation

A tentative classification of wave-formed bars is presented based on available evidence of both morphological and process factors. One specific group of bars (group VI), which has been described by numerous studies in the Great Lakes, the Mediterranean, and the Gulf of St. Lawrence, is examined in detail; models of bar formation and equilibrium, which may be relevant to this group, are reviewed and evaluated. These include (1) vortex action under plunging breakers; (2) interaction of incident waves with standing waves generated by reflection; (3) interaction of incident waves with standing edge waves; and (4) hydrodynamic instability of a uniformly long wave setup produced by incident waves.Results of field measurements of one set of crescentic group VI bars in Kouchibouguac Bay, New Brunswick, indicate that none of the theoretical models adequately explains the characteristics of these bars: (a) the stability of the bar form in an area of high longshore sediment transport; (b) sediment transport patterns, which reveal a continuous circulation of sediment through the bar; and (c) the occurrence of spilling rather than plunging breakers. A conceptual model of bar formation and equilibrium is proposed based upon: (a) landward sediment transport by shoaling waves, increasing towards the break point; (b) gradual reduction of transport rates associated with spilling breakers leading to a buildup of the bar; (c) maintenance of the trough through removal of sediments by longshore and rip current circulation; (d) seaward transport of sediment by rip currents; and (e) landward transport of sediments under reformed waves landward of the trough. The application of this model to other group VI bars, particularly straight bars, is discussed.

scholarly journals OPTIMIZING METHODS TO MEASURE HYDRODYNAMICS IN COASTAL WETLANDS: EVALUATING THE USE AND POSITIONING OF ADV, ADCP AND HR-ADCP

2011 ◽  
Vol 1 (32) ◽  
pp. 51 ◽  
Author(s):  
Erik Horstman ◽  
Thorsten Balke ◽  
Tjeerd Bouma ◽  
Marjolein Dohmen-Janssen ◽  
Suzanne Hulscher
Keyword(s):  
Intertidal Zone ◽  
Coastal Wetlands ◽  
Field Data ◽  
Field Measurements ◽  
Doppler Velocity ◽  
Coastal Protection ◽  
Short Term ◽  
Hydrodynamic Processes ◽  
Measurement Devices ◽  
Acoustic Doppler Current Profilers

Hydrodynamic impacts of vegetation in the intertidal zone are highly important to coastal protection. However, most studies on hydrodynamic impacts of vegetation in the intertidal zone are carried out in flumes. This results in a lack of field data for validating models that describe short-term hydrodynamic impacts of vegetation. The current research focuses on field measurements of flow patterns and waves in vegetated intertidal areas. Ample measurement devices are available to measure hydrodynamic processes in the field. Examples are: acoustic Doppler current profilers (ADCP), high resolution acoustic Doppler current profilers (HR-ADCP) and acoustic Doppler velocity meters (ADV). This study focuses on the differences in the performance of these devices, to determine which of them can be best deployed in a future fieldwork campaign in mangroves. Major points of attention in this comparison are the accuracy of the data and the potential disturbance of the measurements by the presence of vegetation. It is concluded that ADV’s perform very well in vegetated intertidal areas, while (HR-)ADCP’s show difficulties when deployed upward looking. Furthermore, ADV’s are preferred over (HR-)ADCP’s due to their ability of combining high frequent wave and current measurements and their convenient deployment.

Non-lithographic Nanofabrication Using Porous Alumina Membranes

2005 ◽  
Vol 900 ◽  
Author(s):  
K. Bhargavan Ram ◽  
L. Tian ◽  
Z Wu ◽  
Latika Menon
Keyword(s):  
Porous Alumina ◽  
Field Measurements ◽  
Aluminum Film ◽  
Perpendicular Anisotropy ◽  
Plasma Etch ◽  
Alumina Template ◽  
Alumina Membranes ◽  
Nanopore Arrays ◽  
Parallel Direction ◽  
Magnetic Nanopillars

ABSTRACTWe describe a fabrication method to prepare highly ordered Si nanopore arrays. A nanoporous alumina template of thickness ∼1μm is prepared by means of anodization of an aluminum film. The template has a highly ordered hexagonal array of pores of diameter ∼50nm. The template is detached from the aluminum layer and placed on a Si substrate. The nanoporous pattern is transferred onto silicon substrate by means of a dry plasma etch process. This produces an array of nanopores in silicon with a diameter of ∼50nm and depth of ∼300nm. We have used such an array to prepare Fe nanopillars inside the pores by means of thermal evaporation. Magnetization versus applied magnetic field measurements for the Fe nanoarrays, demonstrate large perpendicular anisotropy typical of high aspect ratio magnetic nanopillars. The value of coercivity is about 500Oe in the perpendicular direction and 40Oe in the parallel direction.

Velocity Field Measurements in a “Coastal Buffer Zone”

1997 ◽  
Author(s):  
Hiroshi Yagi ◽  
Hirofumi Hinata ◽  
Kazuo Nadaoka
Keyword(s):  
Velocity Field ◽  
Field Measurements ◽  
Buffer Zone

scholarly journals A Methodological Approach to Determine Sound Response Modalities to Coastal Erosion Processes in Mediterranean Andalusia (Spain)

2020 ◽  
Vol 8 (3) ◽  
pp. 154 ◽  
Author(s):  
Rosa Molina ◽  
Giorgio Manno ◽  
Carlo Lo Re ◽  
Giorgio Anfuso ◽  
Giuseppe Ciraolo
Keyword(s):  
Land Use ◽  
Energy Flux ◽  
Wave Energy ◽  
Buffer Zone ◽  
Aerial Photographs ◽  
Land Uses ◽  
Natural Areas ◽  
Erosion Processes ◽  
Class 1 ◽  
Wave Energy Flux

Human occupation along coastal areas has been greatly increasing in recent decades and, in many places, human activities and infrastructures are threatened by erosion processes that can produce relevant economic and human losses. In order to reduce such impacts and design sound management strategies, which can range from the “no action” to the “protection” option, coastal managers need to know the intrinsic coastal sensitivity and the potential vulnerability and value of land uses. In this paper, in a first step, coastal sensitivity was determined by calculating the following: (i) the spatial distribution at the coast of the wave forcing obtained by using the ERA5 wave dataset and defined as the energy associated with the 50-year return period storm. Two storm conditions were considered, that is, one for the eastern and one for the western parts of the Andalusia Mediterranean coast, respectively, characterized by a height of 8.64–7.86 m and 4.85–4.68 m and (ii) the existence of a buffer zone, namely the dry beach width expressed as a multiple of the 20-year predicted shoreline position that was calculated using a dataset of aerial photographs covering a time span from 1956 to 2016. Coastal sensitivity values were divided into five classes with class 1 indicating the lowest sensitivity (i.e., the presence of a wide buffer zone associated with low wave energy flux values) and class 5 the highest sensitivity (i.e., a narrow buffer zone associated with very high wave energy flux values). In a second step, land uses were obtained from the official Land Use Map of the Andalusia Region, based on the results of the “Coordination of Information on the Environment” (CORINE) European Project. Such uses were divided into five classes from class 1 including natural areas (typologies “A” and “B” of the CORINE Project) to class 5 including very capital land uses (typologies “E1” and “E2”). In a third step, information concerning coastal sensitivity and land uses was crossed to determine the best mitigation strategies to cope with erosion processes. The “no action” option was observed at the westernmost area of Cádiz Province and at some areas from the west coast of Almería Province, where both coastal sensitivity and land use classes show low values; the “adaptation” option was recorded along more than one half of the coast studied, essentially at natural areas with high sensitivity and at urbanized areas with low sensitivity; and the “protection” option was observed especially at some areas from the center and eastern part of Málaga Province and at the easternmost areas of Almería Province, where both coastal sensitivity and land use classes presented high values.

Breach the dikes! How to design saltmarsh restoration schemes for mitigating coastal flooding.

2020 ◽  
Author(s):  
Joshua Kiesel ◽  
Mark Schuerch ◽  
Elizabeth K. Christie ◽  
Iris Möller ◽  
Tom Spencer ◽  
...  
Keyword(s):  
Water Level ◽  
Field Measurements ◽  
High Water ◽  
Sensitivity Analyses ◽  
Coastal Protection ◽  
Tidal Prism ◽  
Erosion Risk ◽  
Scheme Design ◽  
High Water Level ◽  
Storage Area

<p>Managed realignment (MR), a form of of nature-based coastal adaptation to reduce flood and erosion risk, involves the abandonment of existing sea defences and their relocation further inland. MR aims to (re)create intertidal habitats, such as saltmarshes, between the old and new lines of defence; as well as flood water storage. The newly created habitats dissipate wave energy and thus provide new natural coastal protection. However, the assessment of the success of MR is difficult, as restoration targets are often vague and data on project performance are scarce. The few studies that do exist show a lack of understanding about the effects of MR scheme design on high water level (HWL) attenuation and thus its coastal protection function.</p><p>Here we present the results of a 2-D hydrodynamic model, calibrated and validated against field measurements of equinoctial tides between August and October 2017, taken within, and seaward of, the Freiston Shore MR site, The Wash, eastern England. Using this model, we performed sensitivity analyses to explore whether or not, and how, the Freiston Shore MR scheme design affects HWL attenuation. For this purpose we changed the configuration of the old defence line and the breaches created within it for renewed tidal exchange and manipulated the digital elevation model of within-site topography. Specifically, we applied six scheme design scenarios (two scenarios with three breaches and varying MR areas, three single breach scenarios of different breach width and one bank removal scenario) and assessed High Water Level (HWL) attenuation rates for each scenario.</p><p>Our results show that scheme design, particularly storage area and number and size of breaches, of the Freiston Shore MR site had a significant effect on the site´s HWL attenuation capacity. When the tidal prism is varied by changing the number and size of breaches and the storage area kept constant, modelled HWL attenuation rates increased with decreasing tidal prism. However, largest HWL attenuation rates (> 10 cm km<sup>-1</sup>) were only obtained if the MR area was of sufficient size, therefore, it is only the larger sites which are exhibiting effective coastal protection. Consequently, the maximum modelled HWL attenuation rate occurred (up to 73 cm km<sup>-1</sup>) for the scenario with the largest area (142 ha).</p><p>The Mean High Water Depth (MHWD) from each of these scenarios explained most of the variation in HWL attenuation between the scenarios (R² = 0.996). This strong correlation may help to inform the construction of more efficient MR schemes with respect to coastal protection in the future.</p>

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