Steep Bottom Slope Research Articles

Physical Model Test for Wave Overtopping for Vertical Seawall with Relatively Steep Bottom Slope for the Impulsive Wave Condition

Wave overtopping rate is one of the most important design parameters for coastal structures. In this study, the physical model tests for measuring the wave overtopping have been conducted with the foreshore slope in front of the seawall. The bottom seabed for the coastal road area was fabricated at the wave flume for two areas in the East sea areas. The wave overtopping rate was measured for various water depths and wave conditions in each coastal area. In particular, the impulsive wave conditions were compared with the previous research and the similar trends of wave overtopping was observed. It could be known that the effect of foreshore slope was significant and should be concerned for applying theses formula like EurOtop.

Influence of environmental variability on harbour porpoise movement

The harbour porpoisePhocoena phocoenais a small marine predator with a high conservation status in Europe and the USA. To protect the species effectively, it is crucial to understand its movement patterns and how the distribution of intensively used foraging areas can be predicted from environmental conditions. Here, we investigated the influence of both static and dynamic environmental conditions on large-scale harbour porpoise movements in the North Sea. We used long-term movement data from 57 individuals tracked during 1999-2017 in a state-space model to estimate the underlying behavioural states, i.e. whether animals used area-restricted or directed movements. Subsequently, we assessed whether the probability of using area-restricted movements was related to environmental conditions using a generalized linear mixed model. Harbour porpoises were more likely to use area-restricted movements in areas with low salinity levels, relatively high chlorophyllaconcentrations and low current velocity, and in areas with steep bottom slopes, suggesting that such areas are important foraging grounds for porpoises. Our study identifies environmental parameters of relevance for predicting harbour porpoise foraging hot spots over space and time in a dynamic system. The study illustrates how movement patterns and data on environmental conditions can be combined, which is valuable to the conservation of marine mammals.

The rise of an apex predator following deglaciation

AbstractAimSea otters (Enhydra lutris) are an apex predator of the nearshore marine community and nearly went extinct at the turn of the 20th century. Reintroductions and legal protection allowed sea otters to re‐colonize much of their former range. Our objective was to chronicle the colonization of this apex predator in Glacier Bay, Alaska, to help understand the mechanisms that governed their successful colonization.LocationGlacier Bay is a tidewater glacier fjord in southeastern Alaska that was entirely covered by glaciers in the mid‐18th century. Since then, it has endured the fastest tidewater glacier retreat in recorded history.MethodsWe collected and analysed several data sets, spanning 20 years, to document the spatio‐temporal dynamics of an apex predator expanding into an area where they were formerly absent. We used novel quantitative tools to model the occupancy, abundance and colonization dynamics of sea otters, while accounting for uncertainty in the data collection process, the ecological process and model parameters.ResultsTwenty years after sea otters were first observed colonizing Glacier Bay, they became one of the most abundant and widely distributed marine mammal. The population grew exponentially at a rate of 20% per year. They colonized Glacier Bay at a maximum rate of 6 km per year, with faster colonization rates occurring early in the colonization process. During colonization, sea otters selected shallow areas, close to shore, with a steep bottom slope, and a relatively simple shoreline complexity index.Main conclusionsThe growth and expansion of sea otters in Glacier Bay demonstrate how legal protection and translocation of apex predators can facilitate their successful establishment into a community in which they were formerly absent. The success of sea otters was, in part, a consequence of habitat that was left largely unperturbed by humans for the past 250 years. Further, sea otters and other marine predators, whose distribution is limited by ice, have the potential to expand in distribution and abundance, reshaping future marine communities in the wake of deglaciation and global loss of sea ice.

Possible Presence of Shear Instabilities at Steep Slopes during an Upwelling Event in the Gulf of Finland, Baltic Sea

Delpeche-Ellmann, N. and Soomere, T., 2018. Possible Presence of Shear Instabilities at Steep Slopes during an Upwelling Event in the Gulf of Finland, Baltic Sea. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 481–485. Coconut Creek (Florida), ISSN 0749-0208.The cooler water jets that are present during wind-induced coastal upwellings are known to become unstable after some time and develop into sub-mesoscale phenomena that contribute to the overall mixing. In this study we examine the possibility of vertical shear instabilities, in particular the Kelvin–Helmholtz instability existing in an area where steep bottom slopes are present during an upwelling event in the Gulf of Finland, Baltic Sea. The study utilizes a combination of in-situ sampling stations, satellite derived sea surface temperature (SST) data and the HIROMB model for the Baltic Sea. Examination of the velocity shear and stratification shows that it is possible for the KHI instabilities to exist most popularly at the thalweg section of the study area at a depth of 10–16 m. It is also at this exact location that the cross-shore jets were found to develop into meanders and filaments and the greatest gradient in the SST were observed. Thus these observations signal the possibility of the KHI developing in the vicinity of steep slopes and contributing to the overall mixing.

Seasonal changes in habitat suitability for adult shovelnose sturgeon in the lower Mississippi River

Shovelnose sturgeon Scaphirhynchus platorynchus are a large-river fish distributed throughout the Mississippi River basin, including the lower 1,533 km of the Mississippi River where riverine habitat has been and continues to be modified for navigation and is a potential site for development of instream hydrokinetic electric power generation. Information about habitat use and preference is essential to future conservation efforts. Shovelnose sturgeon have previously been found to select particular habitat types, and these selected habitats vary seasonally; although these past analyses do not consider the selected habitats in a landscape context. We used ecological niche factor analysis (ENFA) that uses distributions of telemetry locations and environmental variables to model habitat suitability in a landscape context. We recorded 333 locations of shovelnose sturgeon during July–December 2013 that included periods of relatively high and low river stages. The ENFA analysis indicated high-suitability locations were in or near deep water during both high and low river stages. During high river stages, high-suitability locations were near island tip habitat, deep water, and steep bottom slope and far from main channel habitat. During low stages, high-suitability locations were in or near deep water and main channel habitat and far from secondary channel and wing dike habitats. This landscape-scale analysis supports seasonal shifts in habitat use and provides insights that can be used to inform habitat conservation and management to benefit shovelnose sturgeon in the lower Mississippi River and possibly other large rivers.

Eddy dynamics over continental slopes under retrograde winds: Insights from a model inter-comparison

Mesoscale eddies are ubiquitous in the ocean and play a key role in exchanges across continental slopes. In this study the properties of wind-driven baroclinic turbulence are investigated using eddy-resolving process simulations, focusing on the case of retrograde winds that arises around the margins of the subtropical gyres. In contrast to a flat-bottomed ocean, over steep slopes eddies develop from baroclinic instabilities are confined to the top few hundred meters. Deeper in the water column baroclinic instability and vertical momentum transfer are suppressed, so wind-input momentum is exported toward the open ocean by eddies before traversing down to the ocean bed. Close to the sloping topography, eddy energy sourced from the upper ocean is converted to potential energy, steepening isopycnals and driving bottom-trapped prograde flows. This process is associated with upgradient lateral buoyancy fluxes and downgradient isopycnal potential vorticity fluxes, and cannot be reproduced via linear stability calculations.These properties of wind-driven shelf/slope turbulence are contrasted against simulations with flat bathymetry. The key differences described above hinge on the flow close to the steep topographic slope, which may be sensitive to the model’s vertical coordinate system. The simulations are therefore replicated using models that employ geopotential coordinates, terrain-following coordinates, and isopycnal coordinates. Quantitative inter-model discrepancies in the momentum and energy budgets are much more pronounced in the presence of a steep bottom slope. However, the key findings of this study are consistent across the models, suggesting that they are robust and warrant incorporation into parameterizations of eddy transfer across continental slopes.

Simulating multi-scale oceanic processes around Taiwan on unstructured grids

We validate a 3D unstructured-grid (UG) model for simulating multi-scale processes as occurred in Northwestern Pacific around Taiwan using recently developed new techniques (Zhang et al., Ocean Modeling, 102, 64–81, 2016) that require no bathymetry smoothing even for this region with prevalent steep bottom slopes and many islands. The focus is on short-term forecast for several months instead of long-term variability. Compared with satellite products, the errors for the simulated Sea-surface Height (SSH) and Sea-surface Temperature (SST) are similar to a reference data-assimilated global model. In the nearshore region, comparison with 34 tide gauges located around Taiwan indicates an average RMSE of 13 cm for the tidal elevation. The average RMSE for SST at 6 coastal buoys is 1.2 °C. The mean transport and eddy kinetic energy compare reasonably with previously published values and the reference model used to provide boundary and initial conditions. The model suggests ∼2-day interruption of Kuroshio east of Taiwan during a typhoon period. The effect of tidal mixing is shown to be significant nearshore. The multi-scale model is easily extendable to target regions of interest due to its UG framework and a flexible vertical gridding system, which is shown to be superior to terrain-following coordinates.

Circulation in the Arctic Ocean: Results from a high-resolution coupled ice-sea nested Global-FVCOM and Arctic-FVCOM system

A high-resolution unstructured-grid global-regional nested ice-current coupled FVCOM system was configured for the Arctic Ocean and used to examine the impact of model resolution and geometrical fitting on the basin-coastal scale circulation and transport in the pan-Arctic. With resolving steep bottom slope and irregular coastal geometry, the model was capable of simulating the multi-scale circulation and its spatial variability in the Arctic Basin and flow through the Bering Strait, Fram Strait and Canadian Archipelago. The model-simulated annual-mean velocities were in good agreement with observations within the measurement uncertainty and variability due to insufficient sampling. The errors in the flow direction varied with the flow speed, larger in the weak velocity zone and smaller as the velocity increased. In the upper 50-m layer, the annual-mean circulation pattern was dominated by the wind- and ice-drifting-induced anticyclonic circulation in the Arctic Basin and a relatively strong cyclonic slope current along the edge of the continental shelf. In the deep 200–600-m layer, a relatively permanent cyclonic circulation occurred along the steep bottom slope. These annual-mean circulations accounted for ∼85% of the total kinetic energy variance. De-trending the mean flow, an empirical orthogonal function (EOF) analysis showed that the semi-annual and seasonal variability of the sub-tidal flow was dominated by the first and second modes that accounted for ∼46% and ∼30% of the total variance in the upper 50-m layer and ∼58% and 20% in the deep 200–600-m layer. Consistent with observations, the AO-FVCOM-simulated cyclonic slope flow was characterized by a large positive topostrophy. Sensitivity experiment results with various grid configurations suggested that the currents over slopes, narrow straits and water passages featured topographic and baroclinic frontal dynamical scales associated with bathymetric slope and internal Rossby deformation radius. Over the Arctic slope, since these two scales are in the same order, the along-slope current could be captured, as the cross-isobath model resolution was refined to resolve the steep bottom topography. Under this condition, there is no need to add Neptune forcing into the momentum equations. The accuracy of the estimation of the transport through the strait and narrow water passage was affected by the model resolution. In Fram Strait where the flow is characterized by strong lateral current shear resulting from the Atlantic inflow and Arctic outflow, the transport estimation could have a significant uncertainty due to both horizontal and vertical sampling resolutions.

High Resolution 3-D Finite-Volume Coastal Ocean Modeling in Lower Campbell River and Discovery Passage, British Columbia, Canada

The 3-D unstructured-grid, Finite-Volume Coastal Ocean Model (FVCOM) was used to simulate the flows in Discovery Passage including the adjoining Lower Campbell River, British Columbia, Canada. Challenges in the studies include the strong tidal currents (e.g., up to 7.8 m/s in Seymour Narrows) and tailrace discharges, small-scale topographic features and steep bottom slopes, and stratification affected by the Campbell River freshwater discharges. Two applications of high resolution 3-D FVCOM modeling were conducted. One is for the Lower Campbell River extending upstream as far as the John Hart Hydroelectric dam. The horizontal resolution varies from 0.27 m to 32 m in the unstructured triangular mesh to resolve the tailrace flow. The bottom elevation decreases ~14 m within the distance of ~1.4 km along the river. This pioneering FVCOM river modeling demonstrated a very good performance in simulating the river flow structures. The second application is to compute ocean currents immediately above the seabed along the present underwater electrical cable crossing routes across Discovery Passage. Higher resolution was used near the bottom with inter-layer spacing ranging from 0.125 to 0.0005 of total water depth. The model behaves very well in simulating the strong tidal currents in the area at high resolution in both the horizontal and vertical. One year maximum near bottom tidal current along the routes was then analyzed using the model results.

Limanların Yüzer Dalgakıranla Modellenmesi

Floating breakwaters which have usually been constructed for marinas in the milder wave climate regions of Turkey can be an efficient alternative for the harbors with high water depth, steep bottom slope and poor soil conditions. This study presents a model for Candarli Container Terminal with a floating breakwater where the water depth is reaching to 40 m and the soil is too weak. However, breakwater of Candarli Container Terminal was designed as a combination of rubble mound breakwater with 800m length and piled breakwater with 1000 m length. According to the design, two rows of 90m length piles with a diameter of 1.778m will be driven with 0.2m distance between two piles. Although the construction of the breakwater has already been started in Candarli, in this study, floating breakwater scenario is studied to investigate the performance of the floating breakwater and the possibility of its usage for the harbors with moderate wave climate. The results of the model study show that the required wave agitation could be gained when a port is constructed through floating breakwater scenario

Dealing with the Bottom Intersection Problem in Isopycnic-Coordinate Models Using the Flooding Algorithm

In isopycnic-coordinate models, special algorithm is required to compute the horizontal pressure force on steep bottom slopes. The “extrapolation” algorithm which constructs an “underground” estimate of pressure force using extrapolation was used in previous studies to deal with the bottom intersection problem. However, extrapolation will definitely introduce some errors, and would not be the best way to deal with this problem. In this paper, the flooding algorithm which is traditionally used to simulate transient flooding of coastlines caused by tides or tsunamis is used in the isopycnic-coordinate model to deal with the bottom intersection problem. The two algorithms were compared in the test case with a two-dimensional isolated Gaussian topography. The results of comparison indicate that the flooding algorithm performs better in dealing with the bottom intersection problem especially with the situation where interfaces near the bottom topography are going upwards.

Abyssal Circulation Driven by a Periodic Impulsive Source in a Small Basin with Steep Bottom Slope with Implications to the East Sea

In the theory of source-driven abyssal circulation, the forcing is usually assumed to be steady source (deep-water formation). In many cases, however, the deep-water formation occurs instantaneously and it is not clear whether the theory can be applied well in this case. An attempt is made to resolve this problem by using a simple reduced gravity model. The model basin has large depth change compared for its size, like the East Sea, such that isobaths nearly coincide with geostrophic contours. Deep-water is formed every year impulsively and flows into the model basin through the boundary. It is found that the circulation driven by the impulsive source is generally the same as that driven by a steady source except that the former has a seasonal fluctuation associated with unsteadiness of forcing. The magnitudes of both the annual average and seasonal fluctuations increase with the rate of deep-water formation. The problem can be approximated to that of linear diffusion of momentum with boundary flux, which well demonstrates the essential feature of abyssal circulation spun-up by periodic impulsive source. Although the model greatly idealizes the real situation, it suggests that abyssal circulation can be driven by a periodic impulsive source in the East Sea.

Case study of a giant debris flow in the Wenjia Gully, Sichuan Province, China

The debris flow, which was triggered in the Wenjia Gully on August 13, 2010, is an extreme example of mass movement events, which occurred after the Wenchuan earthquake of May 12, 2008. This Earthquake triggered in the Wenjia Gully the second largest co-seismic landslide, which can be classified as a rockslide-debris avalanche. A lot of loose sediments was deposited in the basin. In the main so called Deposition Area II of this landslide, with a volume of 30 × 106 m3, flash floods can easily trigger debris flows because of the steep bottom slope and the relative small grain sizes of the sediments. The largest debris flow of August 13, 2010 destroyed the most downstream dam in the catchment during a heavy rain storm. The debris flow with a peak discharge of 1,530 m3/s and a total volume of 3.1 × 106 m3 caused the death of 7 persons, 5 persons were missing, 39 persons were injured and 479 houses buried. After three rainy seasons, only 16 % of the landslide-debris deposition was taken away by 5 large-scale debris flow events. Since the threshold for rainfall triggered debris flows in the Wenjia Gully and other catchments drastically decreased after the Wenchuan Earthquake, new catastrophic events are expected in the future during the rainy season.

Near-shore wave induced setup along Kalpakkam coast during an extreme cyclone event in the Bay of Bengal

An enhanced sea level during an extreme weather event can result from cumulative effects of reduced sea-level pressure, storm surge, tidal effects and wave induced setup. A numerical experiment was conducted to evaluate sensitivity of model resolution and bottom slope on wave induced setup for an arbitrary coast. Further, an assessment of wave setup at coastal Kalpakkam was investigated, in response to passage of a Category-4 cyclone ‘NARGIS’, in the central Bay of Bengal. The sensitivity experiments cover an arbitrary region of 5km×5km, with varying horizontal resolutions and beach slopes, and subjected to uniform wind forcing and JONSWAP wave spectra prescribed at the open boundary. The computed wave setup using SWAN model reveals that model resolution is not sensitive for gentle bottom slopes, unlike the case with steep bottom slopes. Findings from sensitivity experiments were then applied to evaluate wave setup at three different locations along Kalpakkam coast, for the Cyclone NARGIS event. The study shows that wave setup was highest for a steep slope (1:20), reducing to almost 50% for gentle slopes (1:80). It is inferred that wave setup is an important parameter that needs to be incorporated in an operational storm surge forecasting system.

Validation of current acoustic dead‐zone estimation methods in lakes with strongly sloped bottoms

Inland water bodies contain extremely steep‐sloped bottoms compared with those typically occurring during marine vertical surveys. These steep bottom slopes can cause high acoustic dead zones, biasing our estimates of living organisms. The studies so far have used the assumption that the first contact between the acoustic wavefront and bottom will be at the point where the radial ray from the transducer is normal to the bottom (90°), which we refer to as the normal ray assumption (NRA). Nevertheless, as acoustic energy dramatically decreases laterally due to the beam's pattern, this assumption may not be fulfilled further from the acoustic axis. It is reasonable to believe that the methods assuming the NRA can fail at quite steep slopes. We installed a calibration benthic gillnet of known height at sites with different bottom slopes, ranging from 12° to 50°. The gillnet's float‐line served as a good visible marker above the monitored lake bottom and was successfully used for measuring the acoustic dead‐zone height empirically. By comparing the observed and modeled dead zones based on the NRA, we can show that the current methods for their estimation are invalid at quite sloping angles. We conclude that the current dead‐zone estimation methods are not always applicable for surveying inland water bodies with extremely steep bottom slopes. Installing a simple calibration net as an off‐bottom marker can provide help for in‐situ dead‐zone measurements.

An unstructured-grid, finite-volume sea ice model: Development, validation, and application

[1] A sea ice model was developed by converting the Community Ice Code (CICE) into an unstructured-grid, finite-volume version (named UG-CICE). The governing equations were discretized with flux forms over control volumes in the computational domain configured with nonoverlapped triangular meshes in the horizontal and solved using a second-order accurate finite-volume solver. Implementing UG-CICE into the Arctic Ocean finite-volume community ocean model provides a new unstructured-grid, MPI-parallelized model system to resolve the ice-ocean interaction dynamics that frequently occur over complex irregular coastal geometries and steep bottom slopes. UG-CICE was first validated for three benchmark test problems to ensure its capability of repeating the ice dynamics features found in CICE and then for sea ice simulation in the Arctic Ocean under climatologic forcing conditions. The model-data comparison results demonstrate that UG-CICE is robust enough to simulate the seasonal variability of the sea ice concentration, ice coverage, and ice drifting in the Arctic Ocean and adjacent coastal regions.

Experimental study of dam-break flow in cascade reservoirs with steep bottom slope

Dam break can cause a significant disaster in the downstream, especially, in a valley with cascade reservoirs, which would aggravate the disaster extent. The experimental studies of the dam-break flow of cascade reservoirs are few and far between at the present. Most of related studies concern the failure of a single dam.. This article presents an experimental study of the characteristics of an instantly filled dam-break flow of cascade reservoirs in a rectangular glass flume with a steep bottom slope. A new method was used to simulate the sudden collapse of the dam. A series of sensors for automatic water-levels were deployed to record the rapid water depth fluctuation. The experimental results show that, the ratio of the initial water depth of the downstream reservoir to that of the upstream reservoir would greatly affect the flood peak water depth in the downstream reservoir area and in the stream channel behind the downstream dam, while the influence of the dam spacing is insignificant. In addition, the comparison between the single reservoir and the cascade reservoirs shows some difference in the dam-break flow pattern and the stage hydrograph at the corresponding gauging points.

Sigma-coordinate numerical model for side-discharge into natural rivers

Due to large topography slopes in natural rivers, pollutant concentration embodies a property of three-dimensional distribution when wastewater is discharged from effluents along the bank. With the sigma coordinate along the vertical dimension fitted to both the moving free surface and the bed topography, a three-dimensional numerical model was developed in the present work to address pollutant transport processes in the above-mentioned cases. To avoid the reduction in accuracy caused by spurious diffusion in the case of steep bottom slopes, a formula for horizontal diffusion in the sigma coordinate system was derived. A case study for the side discharge into a straight open-channel flow shows that numerical results are verified well by experimental data. Furthermore, the present model is also verified by the simulation of discharging wastewater from Fuling Phosphorus Factory effluent into the Three Gorges Reservoir and the agreement between the numerical simulation results and field observation data is satisfactory. The change of the mixing zone scope in the water surface versus the layers along the vertical dimension was also discussed in detail. The study shows that a more realistic calculation for pollutant discharge has been provided by the present model than by the depth-average model which predicts an unrealistically smaller mixing zone.

Observation and Simulation of Surface Waves in Two Water Supply Reservoirs

Observations and model predictions of progressive surface waves were made for Cannonsville and Schoharie Reservoirs, located in southeastern New York State. These reservoirs are deep with steep bottom slopes and relatively small fetch. The Donelan/Great Lakes Environmental Research Lab model, a parametric second-generation wave model was applied to these reservoirs assuming deep water throughout the domain. This assumption was based on the relatively small waves and steep bottom slopes, resulting in a very narrow region of wave interaction with the bottom along a lee shore. Previous applications of this model have been for water bodies with larger fetch. Observations of wave characteristics were made near the shoreline at two sites in Cannonsville and one site in Schoharie using submerged pressure sensors, from which the height of larger waves was determined. Model hindcasts were made for the observation periods with model inputs being wind speed and direction and water surface elevation. The model performed well in simulating significant wave height determined from observations. Some implications of the use of wave model simulations to predict sediment resuspension in these and other deep lakes and reservoirs are discussed.

An Effective Algorithm to Reduce Horizontal Pressure Gradient Errors in σ-Coordinate in EFDC Hydrodynamic Model

Abstract Environmental Fluid Dynamics Code (EFDC) is a three-dimensional sigma coordinate hydrodynamic model coupled with a water quality model. The EFDC model has been used worldwide in environmental modeling of surface water hydrodynamics and transport. The EFDC model employs a vertical sigma-coordinate transformation to deal with irregular water depth. This coordinate transformation introduces additional terms in horizontal pressure gradients. It is well known that direct numerical discretization of the full sigma-transformation of the horizontal pressure gradient produces numerical errors when a steep bottom slope exists. Errors in pressure gradient calculations can cause errors in velocity field and ultimately can result in spurious transports. In this study, an effective numerical algorithm is presented to reduce numerical errors induced by the horizontal pressure gradient term in the sigma coordinate. The transformed pressure terms in the sigma coordinate are discretized in sigma grids into the sim...