Shoreward Flow Research Articles

Influence of offshore shoals on spatial variability of storm surge

Idealized bathymetries were subjected to idealized cyclones in order to measure the storm surge response to a range of shoals, under various storm conditions, for the purpose of informing the development of bathymetry thinning algorithms. Nine bathymetries were considered, including eight large, offshore shoals and a featureless reference domain. Six storm realizations (two different sizes/intensities and three different landfall directions) were used as meteorological forcing. Shoal influence on surge was examined over the entire continental shelf throughout the surge event. Quantification of shoal impact on surge was performed prior to landfall, near the time of peak surge, and during surge recession. Three elevation difference contours (1.0, 0.5, and −0.5 m) were used to quantify the spatial variation in surge caused by the shoals at the three times examined. Positive contours (1.0 and 0.5 m) were used to show that a shoal increased surge locally and the negative contour (−0.5 m) to show that a shoal decreased surge locally. The influences of three shoal parameters (depth below mean sea level (MSL), cross-shore width, and distance from shore) were isolated and examined to see which characteristics caused the shoals to alter surge.In many cases, the shoals did not have much effect on the surge response. Only 13% of the samples used to quantify shoal impact on surge indicated any shoal influence whatsoever, and the shoals did not alter maximum elevation within the threshold considered (Δη≥|±0.5m|) anywhere in the domain. However, analysis of the elevation difference contours revealed multiple types of shoal influence. The shoals increased surge when they reduced setdown in storms with offshore winds prior to landfall and when they delayed surge recession after all storms. The shoals decreased surge when they impeded shoreward flow, delaying the arrival of surge. In cases when the shoals altered surge, 95% of the impact was caused by Shallow features (0.5 m below MSL). Also, there appears to be a maximum area over which a shoal can influence surge. All changes in surge greater than the threshold of Δη≥|±0.5m| occurred over the shoal or approximately between the shoal and the shore.The implications of our results for bathymetry thinning follow: In general, offshore shoals do not need to be resolved in bathymetry data sets used in surge modeling, especially if their impact on maximum elevation is the primary concern. Since a shoal's depth below MSL was an excellent indicator of whether it would impact storm surge, we recommend that feature depth below MSL be used as a bathymetry thinning criterion. Finally, a feature's maximum area of influence could be a useful property for determining whether it should be retained in a bathymetry dataset – if one's area of interest is outside of a feature's area of influence, the feature could be omitted regardless of its impact on surge.

Sequential webcam monitoring and modeling of marine debris abundance

The amount of marine debris washed ashore on a beach in Newport, Oregon, USA was observed automatically and sequentially using a webcam system. To investigate potential causes of the temporal variability of marine debris abundance, its time series was compared with those of satellite-derived wind speeds and sea surface height off the Oregon coast. Shoreward flow induced by downwelling-favorable southerly winds increases marine debris washed ashore on the beach in winter. We also found that local sea-level rise caused by westerly winds, especially at spring tide, moved the high-tide line toward the land, so that marine debris littered on the beach was likely to re-drift into the ocean. Seasonal and sub-monthly fluctuations of debris abundance were well reproduced using a simple numerical model driven by satellite-derived wind data, with significant correlation at 95% confidence level.

Determining rates of sediment accumulation on the Mekong shelf: Timescales, steady-state assumptions, and radiochemical tracers

Thirty-two kasten cores, collected from the proximal Mekong continental shelf, have been analyzed for their excess 210Pb distributions in an effort to establish rates of sediment accumulation over the past 100 years. The length of the cores varied from 0.5 to 3m, and stations sampled topset, foreset, and bottomset beds (water depths 7–21m). Apparent excess 210Pb sediment accumulation rates ranged from > 10cm/y (no down-core decrease of excess activity over 300cm core length) near the Song Hau river mouth, to 1–3cm/y in topset and foreset beds within 20–50km of the river mouth, to rates as low as 0.4cm/y in cores from bottomset beds. The 210Pb sediment accumulation rates yield an overall sediment burial rate of 6.1 × 1013g/y for the proximal deltaic deposits, which corresponds to 43% of the total modern Mekong sediment burial on the southern Vietnam shelf (1.4 × 1014g/y; based on our 210Pb and seismic data and 210Pb data from the literature). This shelf burial rate is in reasonable agreement with current long-term estimates of Mekong River sediment discharge (1.3–1.6 × 1014g/y) from the literature. The inventory of excess 210Pb in the proximal Mekong deltaic deposits indicates that the shoreward flow of offshore water (entrained during river/ocean mixing) is approximately twice the flow of the Mekong freshwater discharge. Organic-carbon 14C ages were measured on 10 cores from the proximal Mekong delta and compared to 210Pb sediment accumulation rates in the same core. The 210Pb accumulation rates in all 10 cores were considered to be more robust and accurate than the 14C geochronologies, primarily because of down-core variations in the source of organic carbon deposited on the seafloor (old terrestrial carbon versus younger marine carbon). Variations in the source of organic carbon accumulating in the seabed were resolved by measuring the δ13C value of the seabed organic carbon.

Modeling glacial flow on and onto Pluto’s Sputnik Planitia

Observations of Pluto’s surface made by the New Horizons spacecraft indicate present-day N2 ice glaciation in and around the basin informally known as Sputnik Planitia. Motivated by these observations, we have developed an evolutionary glacial flow model of solid N2 ice that takes into account its published thermophysical and rheological properties. This model assumes that glacial ice flows laminarly and has a low aspect ratio which permits a vertically integrated mathematical formulation. We assess the conditions for the validity of laminar N2 ice motion by revisiting the problem of the onset of solid-state buoyant convection of N2 ice for a variety of bottom thermal boundary conditions. Subject to uncertainties in N2 ice rheology, N2 ice layers are estimated to flow laminarly for thicknesses less than 400–1000 m. The resulting mass-flux formulation for when the N2 ice flows as a laminar dry glacier is characterized by an Arrhenius–Glen functional form. The flow model developed is used here to qualitatively answer some questions motivated by features we interpret to be a result of glacial flow found on Sputnik Planitia. We find that the wavy transverse dark features found along the northern shoreline of Sputnik Planitia may be a transitory imprint of shallow topography just beneath the ice surface suggesting the possibility that a major shoreward flow event happened relatively recently, within the last few hundred years. Model results also support the interpretation that the prominent darkened features resembling flow lobes observed along the eastern shoreline of the Sputnik Planitia basin may be the result of a basally wet N2 glacier flowing into the basin from the pitted highlands of eastern Tombaugh Regio.

Numerical investigation of wave-induced flow in mound–channel wetland systems

Coastal wetlands are an important ecosystem in nearshore regions, but they are also significant in affecting the flow patterns within these areas. Wave-induced flow in wetlands has complex circulation characteristics because of the interaction between waves and plants, especially in discontinuous vegetation. Here, a numerical investigation is performed to analyze the wave-averaged flow in vegetated mound–channel systems. Different water levels, vegetated conditions, and mound configurations are studied with the COULWAVE (Cornell University Long and Intermediate Wave) Boussinesq model.Model simulations show rip currents in the mound–channel systems, whose strength varies with different mound separation distances. The relative influence of vegetation depends on both mound configuration and water level. Approximately a 15% change in significant wave height results as waves propagate over the vegetated mounds, while up to a 75% decrease in the mean shoreward flow speed through vegetation is observed. In addition, vegetation influences the spatial distribution of mean water level within the wetlands. Dimensional analysis shows that rip current strength and primary circulation size depend on mound spacing, water depth, wave height, and vegetation cover.

Circulation on the West Antarctic Peninsula derived from 6 years of shipboard ADCP transects

Over the past 30 years, shelf circulation on the West Antarctic Peninsula (WAP) has been derived from hydrographic data with a reasonable level of confidence. However, with the exception of a very few drifter tracks and current-meter timeseries from moorings, direct velocity measurements have not previously been available. In this article, shelf and shelf-edge circulation is examined using a new velocity dataset, consisting of several years of acoustic Doppler current profiler transects, routinely collected along the ship tracks of the R/V Gould and the R/V Palmer since the fall of 1997. Initial processing and quality control is performed by Dr. Teresa Chereskin and Dr. Eric Firing, who then place the data in an archive accessible by public website, resulting in the broad availability of the data for a variety of uses. In this study, gridded Eulerian means have been calculated to examine circulation on the shelf and slope off the South Shetland Islands, in Bransfield Strait, and on the shelf and slope south of these regions, including Marguerite Bay and the adjacent shelf and shelf-edge. Shelf-edge flow is northeastward in the study area from the offshore of northern Alexander Island to Smith Island, while a southward flowing shelf-edge feature, probably the shallow component of the polar slope current, appears between Elephant Island and Livingston Island. The shallow polar slope current appears to turn shoreward to pass through Boyd Strait between Smith and Livingston Islands. In Bransfield Strait, there is cyclonic circulation. The previously identified northeastward-flowing South Shetland Island jet is strong and present in all seasons, with a large barotropic component not revealed by the hydrography-based velocities derived in the past. On the shelf seaward of Adelaide, Anvers and Brabant Islands, the strong along-shelf Antarctic Peninsula coastal current flows southwestward, with strongest velocities in winter (June–September) off Anvers and Brabant Islands, but stronger in summer (December–March) off Adelaide Island. Seaward of Marguerite Bay, there is seaward flow in the upper 400 m of the water column over the southwest bank of Marguerite Trough, strongest in summer, and shoreward flow near the northeast bank and adjacent shallower shelf areas.

Sensitivity of a Model Shelfbreak Front to the Parameterization of Vertical Mixing*

Recent observations have suggested that the trapped-front model of Chapman and Lentz is consistent with some aspects of the shelfbreak front in the Middle Atlantic Bight. The sensitivity of the model to the parameterization of vertical mixing is examined to determine which model features are robust and potentially observable and which are variable and less reliable. The basic frontal trapping mechanism, frontal location, surface intensified frontal jet with weak flow at the bottom, and detachment of the bottom boundary layer at the shoreward edge of the foot of the front are all insensitive to the parameterization of vertical mixing. On the other hand, the frontal shape and width and the cross-frontal circulation and momentum balances within the front change dramatically with the parameterization of vertical mixing. Constant mixing coefficients produce strong vertical mixing within the front, which results in steady shoreward flow in the bottom boundary layer there. Mixing coefficients that depend on the local stratification and shear produce weak vertical mixing within the front, which allows oscillating currents that may be frontal-trapped waves.

A numerical study of the seasonal variability in the circulation off central Chile

The Princeton Ocean Model is used to study the coastal ocean circulation off central Chile (between 30° and 45°S) and its seasonal transitions in response to monthly mean climatological wind stress and surface heat flux. The model reproduces many of the dominant circulation features, including an equatorward coastal current, coastal upwelling, and a poleward undercurrent. To get the correct seasonal variability in the undercurrent, it was necessary to prescribe the inflow of the undercurrent on the northern model boundary. Meanders and eddies form throughout the simulation period seaward of the coastal current in response to baroclinic instability of the flow. The flow associated with the meanders shows upwelling in the shoreward flow and downwelling in the seaward flow. This upwelling/downwelling pattern is explained by conservation of relative vorticity in the flow. The upwelling velocity associated with the meanders corresponds to a vertical migration of 15–30 m over a typical timescale of the meanders of 1–2 months. It is argued that this upwelling may be important for maintaining observed high phytoplankton biomass and primary production seaward of the coastal zone directly affected by wind‐driven upwelling.

Variability of suspended sand concentrations, transport and eddy diffusivity under non-breaking waves on the shoreface

Suspended sand concentration profiles and current speed measurements were made in 1.6 ± 0.4m water depth on the seaward side of a bar beyond the break point at Stanhope Lane beach, Prince Edward Island, Canada, during a 4 day period. Two mild storm events occurred and 7 min-long bursts of 4.4 Hz data were recorded approximately every hour. Concentration profiles, with a resolution of 5 mm and 7.5 mm, were obtained using a 3 MHz acoustic backscatter (ABS) instrument. Some data on bed form dimensions were obtained by divers between storm events. Multiple bed echoes from the ABS also provide further information on the ripple heights but no direct observations were available at times of higher waves. During the 4 day period the local position of the bed decreased by about 8 cm, erosion occurring in two short (⋟6 h) periods, one during each storm event. Mean currents at this location were weak (<0.1 m s −1). The resuspension coefficient γ 0, calculated from the concentration at 2 cm above the bed, decreased as the wave height increased (in the break-off range) supporting the observations of Vincent et al. [(1991) Marine Geology, 96, 1–18]. Large inter-burst variability was observed in the concentration profiles and in the eddy diffusivity and suspended transport fluxes computed from these profiles. This variability was due to the short length of record (⋟7min) relative to wave groups and to the location of the ABS relative to bed forms; to obtain consistent concentration and transport fluxes it is necessary to average many bursts over a time scale that is long compared to both groupiness and bed form mobility. Except at the beginning and end of events when wave conditions were changing rapidly, averaging over groups of bursts with similar wave conditions produced eddy diffusivity profiles characterized by a linear ɛ s gradient of (20 ±2.5) cm s −1 from the sea bed to 20 ± 5cm, beyond which ɛ s slowly decreased, and a suspended sand transport which was all below 20 cm and dominated by a jet-like shoreward flow in the lowest 3–4 cm above the sea bed. Bursts obtained when conditions were in, or close to, the equilibrium range of bed forms showed concentration profiles which were partly exponential (hence ɛ s was constant with height) and transport profiles with more structure. Sensitivity tests indicate that concentrations and eddy diffusivities may be underestimated by about 20% by using the modal size for the sand in suspension rather than a distribution of sizes. Towards the end of the second storm a “low suspension event” of ⋟6 h duration occurred and suspended sand concentrations about 5–10 cm decreased by 1–2 orders of magnitude while the near bed concentrations remained approximately constant, suggesting that the bed forms responsible for vortex ejection of sand higher into the water column had been wiped out and the sea bed had become (temporarily) flat. Wave heights did not change significantly but wave period increased from 4.8 to 6.2 s, decreasing the maximum bed shear stress. The low suspension period may have a transient response of the sea bed to changed hydrodynamic forcing prior to the subsequent growth of bed forms with a different wavelength.

Chemical and biological structure and transport of a cool filament associated with a jet‐eddy system off northern California in July 1986 (OPTOMA21)

During the OPTOMA21 cruise, from July 7 to 19, 1986, the distributions of nutrient, pigment, bio‐optical, and physical variables were mapped in a jet‐eddy system located off Point Reyes and Point Arena, California. The goals of this mapping were to describe the three‐dimensional variability of the filament and its relation to the nutrient and phytoplankton distributions offshore, to examine the interaction between the filament and coastal water, and to estimate the transport of nutrients and phytoplankton by the jet system. Several cool filaments were distinguishable at distances of more than 35–50 km from the coast in satellite imagery during this period. The juxtaposition of these features as well as the presence of an offshore anticyclone and a cyclone south of the filament anchored to the coast at Point Arena led to complex patterns in all variables, aided by the apparent alongshore variability in the source of upwelled water. The seaward jet associated with the filament entrained coastal upwelled water, with low temperatures and high nutrient and pigment concentrations, so that the filament maintained its characteristic low temperature and high chlorophyll in the offshore zone. The filament extended ∼250 km offshore, where it made a cyclonic bend southeastward toward the coast. Salinity and nutrient concentrations generally increased across the filament into the cyclonic eddy circumscribed by the filament, although their fronts were not always coincident with temperature fronts. Chlorophyll fluorescence maxima were associated with the cool filament core, but elevated levels were also associated with the low‐salinity, slightly warmer water on the outer edge of the filament along its entire track. Subduction within the filament resulted in elevated chlorophyll concentrations extending to ∼100 m depth, nearly twice the estimated euphotic zone depth. Phytoplankton taxa within the filament were characteristic of those found within coastal upwelling regions. As was expected, the cyclonic eddy contained high salinity and high nutrient concentrations, but temperature at the surface was higher than was expected for the salinity, suggesting that this water may have been at the surface for several days to weeks. Chlorophyll concentration was low at the center of the eddy, in contrast to the water in the coastal upwelling region and within the filament, suggesting that there was little horizontal exchange between the cyclone and the cool filament and that the high‐salinity water may have originated from local (open ocean) upwelling or it may have been displaced seaward from a near‐coast origin several weeks earlier. The seaward jet associated with the filament resulted in a large transport of nutrients and phytoplankton biomass offshore, but the shoreward flow, downstream from the cyclonic bend, reduced the net transport to the offshore region. Shoreward nitrate transport was 85 to 90% of seaward transport, similar in proportion to the shoreward volume transport (∼90%) relative to the seaward volume transport. In contrast, shoreward chlorophyll transport was about 40% of seaward transport, indicating that the meandering jet may supply significant phytoplankton biomass to the offshore region. The meandering jet and filament system had a clear imprint on the chemical and biological structure of the region. This structure has implications to the fluxes of organic material in the region and it is probably significant in organizing the interactions among different trophic levels within the system.

The asymmetric distribution of chlorophyll associated with a coastal upwelling center

A well-developed upwelling plume was observed off the California coast near Point Conception and Point Arguello (∼34°20′N latitude, 120°35′W longitude) during April 1983. The core of the upwelling plume was cool, nutrient-rich, and low in phytoplankton biomass. The distributions of temperature and nutrients were relatively symmetric about the offshore axis of the cool core of the upwelling center. However, the distribution of chlorophyll was asymmetric about the center with the highest abundance observed on the southern (equatorward) edge of the feature. Comparison of the near-surface currents measured by Doppler acoustic profiling, with the distribution of temperature, nutrients, and chlorophyll, suggests that the phytoplankton asymmetry is determined by the horizontal velocity field. High offshore velocities on the poleward side of the center advect the upwelled water offshore before accumulation of biomass can occur. Lower velocities, more variable current direction, and possible shoreward flow on the equatorward side of the upwelling center permit a longer residence time for the upwelled water near the upwelling center allowing accumulation of phytoplankton biomass to occur.

Shelf and slope circulation induced by fluctuating offshore forcing

A linear model which includes continuous vertical stratification, arbitrary cross‐shelf bottom topography, and bottom friction is used to examine the response of shelf and slope waters to fluctuating offshore forcing in the form of a specified pressure field. For forcing which is periodic in the alongshore direction and in time, the response varies dramatically with frequency. For periods of less than about 10 days, the response is dominated by near resonances with free coastally trapped waves. These are not pure resonances because of the mismatch between the forcing structure and the free wave structure. For periods of greater than about 10 days, the velocity response decays away from the forcing with a scale determined by the projection of the forcing onto the flat‐bottom baroclinic modes, typically the first baroclinic Rossby radius. If the continental slope is encountered within this scale distance, then the flow is altered and creates a bottom‐trapped, alongshore velocity maximum seaward of the shelf break. Increased stratification enhances bottom trapping and inhibits flow across the slope, moving the maximum in alongshore velocity seaward, thus reducing the alongshore flow near the shelf break. The shelf response is always weak and barotropic. Furthermore, the slope response is largely independent of the shelf geometry, suggesting that narrow shelves appear to be more easily influenced by offshore forcing only because their coasts are closer to the forcing. The model is used to simulate the response to a Gaussian‐shaped anticyclonic eddy translating uniformly in the alongshore direction. The eddy flow is blocked by the topography and becomes squashed at the shelf break. Some shelf water is entrained creating a weak shelf circulation cell. The shoreward flow in the eddy cannot move onto the shelf and instead forms an alongshore jet near the shelf break. The jet extends away from the eddy in the direction toward which free coastally trapped waves propagate, but the alongshore velocity within the jet is opposite to the alongshore velocity within the eddy.

East China Sea tide currents

The East China Sea is a broad continental shelf over which there are large tides and tidal currents. During the joint China-U.S.A. Marine Sedimentation Dynamics Study, a large number of current measurements were made across the continental shelf along approximately 30°N. There is general agreement between these observations and Choi's (1980) tidal model of the area, which supports his assumed boundary conditions and numerical procedures. The semi-diurnal tidal current near the mouth of the Changjiang transports water southward at maximum ebb and northward following maximum flood. Because the tides rotate clockwise, southward flow is followed by shoreward flow, confining the initial river plume and sediment deposition to the coastal region south of the river mouth. Six hours later the remnants of the initial plume move to the north and seaward. The southward discharge on ebb results in initial and greatest sedimentation rates on the south sides of the river mouths.

Circulation in central Long Island Sound

Current meter records from 28 stations are used to define the flow of water near the bottom of central Long Island Sound. Records were made at two of the stations for over 1 year and for 10 days or more at most of the others. Tidal and nontidal flow components are separated. Random fluctuations of up to 10 day's duration occur in the nontidal flow; they are not directly influenced by wind, rainfall, river runoff, or variations in sea level along the shore. Salinity observations show the presence of well-defined surface and bottom water layers. Mixing between these is confined to shore side zones where the water is less than 10 m deep and to shoals where strong turbulence is generated. The current meter data show the bottom water at depths greater than 20 m to be flowing upstream at a rate that decreases toward the head of the estuary. At depths less than 20 m there is a shoreward flow of bottom water toward the mixing zone. The salinity and current data are used to construct a circulation model for the sound. The large-scale flow is apparently due to gravitational convection associated with salinity differences. However, response to changes in the freshwater inflow is delayed by about 2 months because of the large volume of surface water relative to the freshwater supply.