14 research outputs found
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Observations of cross-shore sediment transport and formulation of the undertow
A new set of data from a large-scale sand bar migration experiment is presented. During this experiment, two sandbars were generated. One of the bar was generated by the action of undertow, and sediment moved offshore. The other bar was generated by the shoreward movement of sediment coming from the first bar. The principal mechanism responsible for shoreward movement is associated with effects of velocity asymmetry. Analysis of bathymetric surveys and suspended sediment concentration data reveals that suspended load contributed to a large extent to the formation of the first bar. Bed load was also important, and was moving in the same direction as suspended load. For the second bar, shoreward sediment movement occurred as bedload. Suspended load was moving in the opposite direction as bed load. This difference in sediment movement is explained by the predominance of the undertow in the suspended sediment flux. Two models were tested to reproduce the observed sediment transport, a wave-averaged (energetics) model and a wave-resolved model. After proper calibration, both models yielded satisfactory results. Calibration efforts highlighted the need for robust models of sediment pickup functions and sediment eddy diffusivity. They also showed the need for a deterministic undertow models. Formulations of the undertow are presented. These formulations are valid for all relative water depths, and include mean current advective terms. These formulations show that the forcing of the undertow is depth-uniform, assuming linear water wave theory. The model is tested against four datasets to evaluate the possibility of a deterministic model. Although a constant eddy viscosity would yield such a model, no universal parametrization could be determined
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Catching the Right Wave: Evaluating Wave Energy Resources and Potential Compatibility with Existing Marine and Coastal Uses
Many hope that ocean waves will be a source for clean, safe, reliable and affordable energy, yet wave energy conversion facilities may affect marine ecosystems through a variety of mechanisms, including competition with other human uses. We developed a decision-support tool to assist siting wave energy facilities, which allows the user to balance the need for profitability of the facilities with the need to minimize conflicts with other ocean uses. Our wave energy model quantifies harvestable wave energy and evaluates the net present value (NPV) of a wave energy facility based on a capital investment analysis. The model has a flexible framework and can be easily applied to wave energy projects at local, regional, and global scales. We applied the model and compatibility analysis on the west coast of Vancouver Island, British Columbia, Canada to provide information for ongoing marine spatial planning, including potential wave energy projects. In particular, we conducted a spatial overlap analysis with a variety of existing uses and ecological characteristics, and a quantitative compatibility analysis with commercial fisheries data. We found that wave power and harvestable wave energy gradually increase offshore as wave conditions intensify. However, areas with high economic potential for wave energy facilities were closer to cable landing points because of the cost of bringing energy ashore and thus in nearshore areas that support a number of different human uses. We show that the maximum combined economic benefit from wave energy and other uses is likely to be realized if wave energy facilities are sited in areas that maximize wave energy NPV and minimize conflict with existing ocean uses. Our tools will help decision-makers explore alternative locations for wave energy facilities by mapping expected wave energy NPV and helping to identify sites that provide maximal returns yet avoid spatial competition with existing ocean uses
Using vulnerability assessment to characterize coastal protection benefits provided by estuarine habitats of a dynamic intracoastal waterway
The existence of coastal ecosystems depends on their ability to gain sediment and keep pace with sea level rise. Similar to other coastal areas, Northeast Florida (United States) is experiencing rapid population growth, climate change, and shifting wetland communities. Rising seas and more severe storms, coupled with the intensification of human activities, can modify the biophysical environment, thereby increasing coastal exposure to storm-induced erosion and inundation. Using the Guana Tolomato Matanzas National Estuarine Research Reserve as a case study, we analyzed the distribution of coastal protection services–expressly, wave attenuation and sediment control–provided by estuarine habitats inside a dynamic Intracoastal waterway. We explored six coastal variables that contribute to coastal flooding and erosion–(a) relief, (b) geomorphology, (c) estuarine habitats, (d) wind exposure, (e) boat wake energy, and (f) storm surge potential–to assess physical exposure to coastal hazards. The highest levels of coastal exposure were found in the north and south sections of the Reserve (9% and 14%, respectively) compared to only 4% in the central, with exposure in the south driven by low wetland elevation, high surge potential, and shorelines composed of less stable sandy and muddy substrate. The most vulnerable areas of the central Reserve and main channel of the Intracoastal waterway were exposed to boat wakes from larger vessels frequently traveling at medium speeds (10–20 knots) and had shoreline segments oriented towards the prevailing winds (north-northeast). To guide management for the recently expanded Reserve into vulnerable areas near the City of Saint Augustine, we evaluated six sites of concern where the current distribution of estuarine habitats (mangroves, salt marshes, and oyster beds) likely play the greatest role in natural protection. Spatially explicit outputs also identified potential elevation maintenance strategies such as living shorelines, landform modification, and mangrove establishment for providing coastal risk-reduction and other ecosystem-service co-benefits. Salt marshes and mangroves in two sites of the central section (N-312 and S-312) were found to protect more than a one-quarter of their cross-shore length (27% and 73%, respectively) from transitioning to the highest exposure category. Proposed interventions for mangrove establishment and living shorelines could help maintain elevation in these sites of concern. This work sets the stage for additional research, education, and outreach about where mangroves, salt marshes, and oyster beds are most likely to reduce risk to wetland communities in the region
Wave energy net present value in million USD (black contour lines) over a 25-year life-span of wave energy conversion facilities and wave energy areas of interest (gray contours) modified from British Columbia Marine Conservation Analysis Atlas [20], [35].
<p>Underwater transmission cable landing points (?) are located in Tofino and Ucluelet. Power grid connection point (empty X) is located in Ucluelet.</p
Department of Fisheries and Oceans Canada statistical areas and fishing grounds for groundfish fishing (multiple gears), salmon trolling, and shrimp trawling.
<p>Fishing grounds are shaded by their importance (scale: 1–4); high values indicate more important grounds.</p
Net value (USD) of three commercial fishing layers and wave energy annual net value (thousand USD; black contour lines).
<p>Underwater transmission cable landing points (?) are located in Tofino and Ucluelet. Power grid connection point (empty X) is located in Ucluelet.</p
Degree of spatial overlap between areas of positive annual net present value from wave energy facilities and five categories of existing uses and ecological characteristics.
<p>Overlap is expressed by quartiles (very low, low, moderate and high) of the median and range (minimum to maximum) of the number of existing uses in 2 km<sup>2</sup> cells (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0047598#pone-0047598-g006" target="_blank">Figure 6</a>) that overlap with areas of positive net present value for wave energy. See text for further explanation and consult <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0047598#pone.0047598-British2" target="_blank">[30]</a> for the full list for each category.</p
Economic parameters for net present value (<i>NPV</i>) assessment for the Pelamis wave energy conversion device.
<p>Economic parameters for net present value (<i>NPV</i>) assessment for the Pelamis wave energy conversion device.</p
Overlap between areas of positive annual net present value over a 25-year life-span of wave energy conversion facilities (black contour lines; value in million USD) and five categories of existing uses or ecological characteristics (gray grids): A) ecological characteristics, B) shipping and transport, C) tenures and offshore energy, D) tourism and recreation, and E) commercial fisheries.
<p>The inset legend indicates the number of existing uses or ecological characteristics in the category that occur in a grid cell.</p
Compatibility analysis between wave energy and fishing annual net values, where A) values are given equal weight and B) fishing values are weighted 50 times that of wave energy values.
<p>Underwater transmission cable landing points (⊙) are located in Tofino and Ucluelet. Power grid connection point (empty X) is located in Ucluelet.</p