Bottom Orbital Velocities Research Articles

Bottom Drag Variations Under Waves and Currents: A Case Study on a Muddy Deposit off the Shandong Peninsula

Bottom drag coefficient is one of the key parameters in quantifying shelf hydrodynamics and sediment transport processes. It varies markedly due to dynamic forcing and bed type differences, so a set of empirical values have been used for beds of coarse material where bedforms are often present. In comparison, dramatically fewer such rule-of-thumb values are available for muddy beds. Here, we present results of variations in bottom drag as calculated from in situ measurements by bottom-mounted tripods that were placed across the top of a muddy deposit during two different deployments, one in summer and another in winter. A tidal asymmetry of bottom drag was observed, most likely caused by variations of local bed roughness. For hydrodynamically smooth (Re<2.3×105) flows, computed values of bottom drag coefficient were fairly scattered but still showed an overall decreasing trend with an increase in Reynolds number. The bottom drag coefficient for hydrodynamically rough or transitional flow was typically constant, while the averaged drag coefficient over all observation periods was 1.7×10−3. Smaller waves (bottom orbital velocity ub <0.1 m/s) had a very limited impact on the bottom drag coefficient. However, with an increase in ub, the wave–current interactions can decrease the time-averaged near-bed velocity and enhance turbulent kinetic energy, thus leading to an increase in the drag coefficient.

Beach strandings of Tehuelche scallop, Aequipecten tehuelchus, in Patagonia: Multi-scale processes and management implications

The magnitude and causal mechanisms of a massive beach stranding of Tehuelche scallops that occurred in November 2017 in San José Gulf, Argentina, were investigated with the long-term goal of improving the assessment and management of the scallop fishery. The biomass of scallops washed ashore and deposited over a 10-km stretch of coast was estimated by quadrat sampling and compared with the results of a scallop stock assessment survey conducted three months prior to the stranding event. The resulting estimate of total biomass loss was in the order of 200 t, representing 10% of the estimated total scallop biomass in the San José gulf. The stranding coincided with persistent strong southerly winds (13 m/s) blowing for 24 h in San José Gulf, and large-scale windstorms that affected the southern tip of South America. Surface waves predicted under such windstorm conditions could generate strong bottom orbital velocities at shallow waters (<10 m depth), sufficient to drag and transport ashore scallops by Stokes drift (600–2000 m in 24 h). Analysis of local wind data recorded over a 6.8-year period indicated that such windstorm conditions occurred with an average frequency of 7.7 times per year, implying that beach strandings could have a significant impact on the scallop resource and its fishery. The actual impact of windstorms would depend on the location, depth and size composition of scallop beds, shallow beds (<10 m depth) being more susceptible to stranding risks. The use of spatial harvest control rules, instead of the global total allowable catch used at present, could reduce the risks of yield loss by directing the harvest to the more vulnerable scallop beds.

Математическое моделирование волнового режима бухты Ай-Тодор

Investigation of wave regime of coastal water areas is practically important for safe shipping, constructing and using of coastal infrastructure objects. At the present paper statistical characteristics and space distribution of parameters of wind waves in the Ay-Todor Bay region placed on the Crimea South Coast of the Black Sea are obtained on the basis of mathematical simulation. Analysis of the wind wave parameters for the year diapason of 1979–2017 has shown that waves running from the east and having 0.5 m heights and 3.0–3.5 s mean periods are the most repeatable. The most durable storms are formed by waves coming from East–South-South-West sector in the period from December to January. The multiannual calm duration is equal to ~11 days. Estimation of extremal characteristics (height, period and length) of wind waves of different probability, which possible one time in a year, 5, 10, 25, 50 and 100 years are made. For the storm probable one time in 25 years the mean wave height near the boundary of the Ay-Todor Bay is equal to 4.5 m. For this storm the wave height of 1% probability reaches 9.6 m. For the directions of winds of 4 % regime probability which causes the most dangerous waves calculations of wave characteristics with high space resolution (~1.5 m) are carried out by using of SWAN model and nested grid technology. Fields of significant wave height, mean wave length, bottom wave orbital velocities and space distribution of wave breaking zones are analyzed. It is found that most intensive waves are generated by wind having south-east, east and south directions. Maximal bottom orbital velocities of wave current reaches of 2.5–3.5 m/s and occurs along the coast till 10 m depth. Model calculation of wave currents by using of SWASH model allowed obtained presence in the coastal zone near beach of local cyclonic eddy, which transport suspended sediments along its south edge from beach towards the sea.

Sandbar dynamics in microtidal environments: Migration patterns in unprotected and bounded beaches

Novel observations of bar features and migration patterns at three Italian sandy beaches in Senigallia (central Adriatic Sea) and Terracina (central Tyrrhenian Sea) are presented. While two of these beaches are unprotected, one is artificially embayed through the presence of a concrete jetty. Bar locations and motions are analysed with video imagery data sets from coastal monitoring stations. Wave climate from the Copernicus oceanographic model is used to correlate hydrodynamics and beach morphology. Clear net offshore migration (NOM) patterns are identified at all beaches, although exhibiting different response scales. At the low-slope beach of Senigallia (mean surf zone slope of 0.009) bars are more sensitive to seasonal changes in wave climate, with offshore shifts in winter and inactivity in summer. Mean annual migration rates of up to 0.26 m/day and 0.09 m/day are observed for outer and inner bars, respectively. No significant response to single storms is observed, except during a single NNE storm. No distinct erosive or accretive trend for the shoreline is evidenced. At the steep beach of Terracina (mean surf zone slope of 0.018), conversely, bars are strongly responsive to single storm events, with displacements of up to 25–50 m across a single storm. The mean inter-annual migration rate is 0.08 m/day for the single/outer bar, whereas a slight onshore trend (-0.02 m/day) for the newborn inner bar is established. The presence of an artificial jetty at the Misa river mouth in Senigallia, finally, leaves the bar system, typically oscillating around an equilibrium position, susceptible to larger seasonal oscillations and strong offshore migration due to two ESE storm events. Numerical simulations of nearshore circulation for two storms from different directions predict the birth of longshore currents, higher ratios of significant wave height to water depth, and stronger bottom orbital velocities over bars in case of the ESE storm, suggesting a greater tendency for offshore-directed sediment transport.

Modeling of suspended sediment by coupled wave-current model in the Zhujiang (Pearl) River Estuary

A three-dimensional wave-current-sediment coupled numerical model is developed to understand the sediment transport dynamics in the Zhujiang (Pearl) River Estuary (ZRE), China. The model results are in good agreement with observed data, and statistics show good model skill scores. Numerical studies are conducted to assess the scenarios of suspended sediment in the ZRE under the effects of different forcing (river discharges, waves, and winds). The model results indicate that the estuarine gravitational circulation plays an important role in the development of estuarine turbidity maximum in the ZRE, particularly during neap tides. The increased river discharge can result in a seaward sediment transport. The suspended sediment concentration (SSC) in the bottom increases with both wave bottom orbital velocity and wave height. Because of the shallow water depth, the effect of waves on sediment in the west shoal is greater than that in the east channel. The southwesterly wind-induced wave affects the SSC more than those resulting from the northeasterly wind, while the northeasterly wind-driven circulation has a slightly greater influence on the SSC than that of the southwesterly wind. However, a steady southwesterly wind condition favors the increase of the SSC in the Lingding Bay more so than a steady northeasterly wind condition. If the other forcings are same, the averaged SSC under a steady southwesterly wind condition is about 1.1 times that resulting from a steady northeasterly wind.

Internal solitary waves control offshore extension of mud depocenters on the NW Iberian shelf

Hydrodynamic conditions and near-bottom sediment transport on the NW Iberian shelf associated with a 5-day storm in September 2014 were monitored. During the post-storm relaxation period, active bottom sediment transport by internal solitary waves (ISWs) on a mid-shelf mud depocenter, located in between 110 and 130 m water depth (WD), was observed. To explore the potential of internal waves in sediment transport and its link to development of mid-shelf mud depocenters, we apply a weakly nonlinear model based on the variable-coefficient Gardner equation to estimate the flow fields and bottom shear stress induced by shoaling of mode-1 long internal solitary waves. Shoreward propagation of the ISWs in three representative periods (pre-, intra- and post-storm) is simulated, respectively. Transformation of the internal wave, from a single sech2 shape characterized by negative polarity and small amplitude to a dispersive trailing wave packet with varying amplitude and inverse polarity, are satisfactorily reproduced. Model results indicate enhancement of the maximum orbital velocity of the ISWs during and after the storm on the outer shelf (130–220 m WD) including the seaward margin of the mud depocenter. Bottom shear stress consequently becomes strong enough (≥0.1 Pa) to winnow unconsolidated sediment and constrains the offshore extension of the depocenter. The enhanced bottom orbital velocity and the asymmetry in the excursion direction of mode-1 long ISWs in the post-storm period prove to be efficient in transporting fine-grained sediment across shelf. Our results suggest that mid-shelf mud depocenters are not necessarily areas under permanently calm conditions where fine-grained sediment can settle straightforwardly. They could also result from convergent sediment transport from both onshore and offshore directions, and sediment may go through numerous cycles of resuspension-transport-deposition before its ultimate lasting burial.

Sediment Dynamics in Wind Wave‐Dominated Shallow‐Water Environments

AbstractSediment dynamics driven by waves and currents in shallow‐water estuarine environments impacts many physical and biological processes and is important to the estuary‐wide sediment budget. Observational restrictions have limited our ability to understand the physics governing sediment entrainment and mixing in these environments. To this end, we use direct numerical simulation to simulate sediment transport processes in shallow, combined wave‐ and current‐driven flows. Simulations are run with depth‐averaged currents ranging from 0 to 9 cm/s, while wave conditions are held constant with a bottom orbital velocity and period of 10 cm/s and 3 s, respectively. Our results indicated that for wave‐dominated conditions, waves reduce vertical momentum fluxes and the associated bottom drag, thereby accelerating mean currents. Conversely, currents do not significantly affect the wave velocity field. However, they increase the bed shear stress and change the timing and duration of sediment entrainment throughout the wave cycle. Counterintuitively, these effects lead to lower suspended sediment concentrations near the bed for a portion of the wave cycle. By analyzing sediment fluxes, waves are shown to drive near‐bed sediment dynamics while currents control vertical mixing above the buffer layer, where downward settling is predominantly balanced by the current‐generated vertical turbulent sediment flux. In the absence of currents, sediment concentrations are negligible above the wave boundary layer because mixing is weak. We show that the time‐ and phase‐averaged sediment concentration profiles for wave and current conditions resemble the theoretical Rouse profile derived for equilibrium conditions in statistically steady, unidirectional turbulent channel flow.

Interconnected Accommodation Space Controls Between Sand-Charged Shallow Tidal Channels and Wind-Wave Truncated Tidal Flats During Latest-Holocene Sea Level Rise (~3.0 m) in a Large Mesotidal Wave-Dominated Estuary, Grays Harbor, Washington, USA

The late-Holocene (5–0 ka) record of accommodation space controls of tidal channel and tidal flat deposition in the shallow mesotidal wave-dominated Grays Harbor estuary (236 km2 surface area) was investigated in previously reported drill cores (n=15) and new vibracores (n=20), reaching 3–10 m depth subsurface. Continuous vibracore facies sequences (3–4 m depth subsurface) discriminate between tidal channel and tidal flat deposition and demonstrate responses of both depositional settings to interseismic uplift and coseismic subsidence (1±0.5 m vertical) from cyclic neotectonic forcing (200–800 yr recurrence intervals) in the Cascadia subduction zone. Vibracore channel samples, at 0.5 m or 1.0 m depth intervals, were analyzed for sediment grain size (sample n=124) and sand source mineralogy (sample n=67). The mean and standard deviation of sand size in the sand fraction is 175±x34 1σ µm. Sediment 14C dates (n=29) range from 376 to 6,579 median calyrBP and establish long-term sedimentation rates in subtidal channel accretionary banks (average 4.2 m ka-1), intertidal channel accretionary banks (average 3.7 m ka-1), and tidal flats (average 1.1 m ka-1). Tidal channel accretionary bank deposition largely reflects reworking of pre-existing estuary deposits. Long-term total basin sediment accumulation rates (232x106 m3 ka-1) are tied to rates of net sea level rise (1.0 m ka-1) or increasing basin accommodation space. In latest Holocene time (3–0 ka) littoral sand import (117x106 m3 ka-1) was about twice as large as the retention of river sand and mud in the estuary. The selective export of winnowed mud from the estuary provided the necessary accommodation space for the import of littoral sand in latest-Holocene time. Shallow intertidal settings in Grays Harbor (60% by surface area) are maintained by self-regulating conditions of channelized sediment import, wind-wave erosion of tidal flats, and tidal prism forcing of tidal channel discharge. Hind-casted wind-wave bottom orbital velocities (&amp;gt;20 cm sec-1) are sufficient to truncate tidal flat elevations to lower-intertidal levels, which maintain substantial tidal prism volumes (modern MLLW-MHHW ~6.1 x 108 m3) and associated tidal channel discharge in the shallow estuary. Net sediment deposition in the estuary is controlled by the interaction of limiting accommodation space controls in the tidal flats and tidal channels. The balance between sediment supply, energy of sediment transport/resuspension, and sediment export has survived small changes in relative sea level (1±0.5 m) from cyclic neotectonic forcing. However, the prehistoric (natural) balance could be altered by future anthropogenic impacts from sustained global sea level rise (&amp;gt; 1.5 m during the next century) or diminished wind-wave fetch distances, which could result from tidal flat diking/filling or uncontrolled spread of non-native invasive stabilizing sea grass (Spartina). In this regard, the susceptibilities of prehistorically-balanced sediment dynamics in Grays Harbor serve as warning for other similar mesotidal wave-dominated estuaries that could be impacted by future global sea level rise, changing sediment inputs, and/or tidal flat diking/filling, which could reduce intertidal habitat and associated ecosystem functions.

Assessment of the potential for dredged material dispersal from dumping sites in the Gulf of Gdańsk

PurposeEnvironment-friendly management of sites used for disposal of locally generated sedimentary material involves designation of an optimal dumping site location which will render the dredged material re-usable for beneficial purposes. The objective of this research was to determine whether wind, waves, and currents can induce transport of sediment from offshore dumping sites located at intermediate depths in the southern Baltic.Materials and methodsThe problem was addressed by exploring potential sediment transport from two sites located in the Gulf of Gdańsk at depths of about 20 m. A total of 29 combinations of hydrodynamic variables, representing the most extreme possible situations in the area, including eight theoretical uniform wind fields over the entire Baltic Sea from the W, NW, N, NE, E, SE, S, and SW sectors, the wind speed of 30 m s−1, as well as 21 historical extreme storms, retrieved from the HIPOCAS project database, were used.Results and discussionThe bottom velocities resulting from waves and currents at the dumping sites considered were computed using wave models (WAM, SWAN) and the M3D hydrodynamic model (based on the POM model). To estimate the velocities critical for bedload transport, formulae developed by Soulsby (1997) and Sawamoto and Yamashita (Proc Coastal Sediments 87:415–423, 1987) were used. The volumetric bedload transport was computed based on Meyer-Peter and Müller (1948). The model simulations demonstrated that, for the storm conditions analyzed, the current velocity in the area of the two dumping sites would be so low that it would practically not affect the magnitude of the bottom sediment transport. Thus, the resultant volume of bedload transported would be equal to that generated by the wave action. For the heaviest historical storm, the maximum transport is about 3 × 10−5 m2 s−1.ConclusionsUnder conditions of theoretical storms, the bottom orbital velocities would be higher and the resultant sediment transport would reach almost 7 × 10−5 m2 s−1 for northerly winds. However, this value is still very low compared with the volume of sediment being dumped. The findings of this study may prove useful in designation of future dumping sites.

Extreme bottom velocities induced by wind wave and currents in the Gulf of Gda\u0144sk

The principal goal of this study is to get some preliminary insights about the intensity of water movement generated by wind waves, and due to the currents in the bottom waters of Gulf of Gdańsk, during severe storms. The Gulf of Gdańsk is located in the southern Baltic Sea. This paper presents the results of analysis of wave and current-induced velocities during extreme wind conditions, which are determined based on long-term historical records. The bottom velocity fields originated from wind wave and wind currents, during analysed extreme wind events, are computed independently of each other. The long-term wind wave parameters for the Baltic Sea region are derived from the 44-year hindcast wave database generated in the framework of the project HIPOCAS funded by the European Union. The output from the numerical wave model WAM provides the boundary conditions for the model SWAN operating in high-resolution grid covering the area of the Gulf of Gdańsk. Wind current velocities are calculated with the M3D hydrodynamic model developed in the Institute of Oceanography of the University of Gdańsk based on the POM model. The three dimensional current fields together with trajectories of particle tracers spreading out of bottom boundary layer are modelled, and the calculated fields of bottom velocities are presented in the form of 2D maps. During northerly winds, causing in the Gulf of Gdańsk extreme waves and most significant wind-driven circulation, the wave-induced bottom velocities are greater than velocities due to currents. The current velocities in the bottom layer appeared to be smaller by an order of magnitude than the wave-induced bottom orbital velocities. Namely, during most severe northerly storms analysed, current bottom velocities ranged about 0.1–0.15 m/s, while the root mean square of wave-induced near-seabed velocities reached maximum values of up to 1.4 m/s in the southern part of Gulf of Gdańsk.

Distribution of natural disturbance due to wave and tidal bed currents around the UK

The UK continental shelf experiences large tidal ranges and winter storm events, which can both generate strong near-bed currents. The regular tidal bottom currents from tides plus wind driven ‘benthic storms’ (dominated by wave-driven oscillatory currents in shallow water) are a major source of disturbance to benthic communities, particularly in shallow waters. We aim to identify and map the relative impact of the tides and storm events on the shallower parts of the North West European continental shelf.A 10-year simulation of waves, tides and surges on the continental shelf was performed. The shelf model was validated against current meter observations and the Centre for Environmental, Fisheries and Aquaculture Science (CEFAS) network of SmartBuoys. Next, the model performance was assessed against seabed lander data from two sites in the Southern North Sea; one in deep water and another shallow water site at Sea Palling, and a third in Liverpool Bay. Both waves and currents are well simulated at the offshore Southern North Sea site. A large storm event was also well captured, though the model tends to underpredict bottom orbital velocity. Poorer results were achieved at the Sea Palling site, thought to be due to an overly deep model water depth, and missing wave–current interactions. In Liverpool Bay tides were well modelled and good correlations (average R2=0.89) are observed for significant wave height, with acceptable values (average R2=0.79) for bottom orbital velocity.Using the full 10-year dataset, return periods can be calculated for extreme waves and currents. Mapping these return periods presents a spatial picture of extreme bed disturbance, highlighting the importance of rare wave disturbances (e.g. with a return period of 1 in 10 years). Annual maximum currents change little in their magnitude and distribution from year to year, with mean speeds around 0.04ms−1, and maximums exceeding 3ms−1. Wave conditions however are widely variable throughout the year, depending largely on storm events. Typical significant wave heights (Hs) lie between 0.5 and 2m, but storm events in shallow water can bring with them large waves of 5m and above and up to 18m in North West Approaches/North West Scotland (Sterl and Caires, 2005).The benthic disturbance generated by waves and currents is then estimated by calculating the combined force on an idealised object at the bed. The patterns of this disturbance reflect both regular tidal disturbance and rare wave events. Mean forces are typically 0.05–0.1N, and are seen largely in areas of fast currents (>1ms−1). The pattern of maximum force however is more dependent on water depth and exposure to long-fetches (>1000km) suggesting that it is dominated by wave events.

Wave exposure as a predictor of benthic habitat distribution on high energy temperate reefs

The new found ability to measure physical attributes of the marine environment at high resolution across broad spatial scales has driven the rapid evolution of benthic habitat mapping as a field in its own right. Improvement of the resolution and ecological validity of seafloor habitat distribution models has, for the most part, paralleled developments in new generations of acoustic survey tools such as multibeam echosounders. While sonar methods have been well demonstrated to provide useful proxies of the relatively static geophysical patterns that reflect distribution of benthic species and assemblages, the spatially and temporally variable influence of hydrodynamic energy on habitat distribution have been less well studied. Here we investigate the role of wave exposure on patterns of distribution of near-shore benthic habitats. A high resolution spectral wave model was developed for a 624 km2 site along Cape Otway, a major coastal feature of western Victoria, Australia. Comparison of habitat classifications implemented using the Random Forests algorithm established that significantly more accurate estimations of habitat distribution were obtained by including a fine-scale numerical wave model, extended to the seabed using linear wave theory, than by using depth and seafloor morphology information alone. Variable importance measures and map interpretation indicated that the spatial variation in wave induced bottom orbital velocity was most influential in discriminating habitat the classes containing canopy forming kelp Ecklonia radiata, a foundation kelp species that affects biodiversity and ecological functioning on shallow reefs across temperate Australasia. We demonstrate that hydrodynamic models reflecting key environmental drivers on wave exposed coastlines are important in accurately defining distributions of benthic habitats.

Interactions between waves, sediment, and turbulence on a shallow estuarine mudflat

Measurements were collected on a shallow estuarine mudflat in northern San Francisco Bay to examine the physical processes controlling waves, turbulence, sediment resuspension, and their interactions. Tides alone forced weak to moderate currents of 10–30 cm s−1 in depths of 0–3 m, and maintained a background suspension of 30–50 mg L−1 of fine sediment. In the presence of wind waves, bottom orbital velocities spanned 20–30 cm s−1, suspended-sediment concentrations (SSC) at 15 and 30 cm above the bed (cmab) increased by 1–2 orders of magnitude, and vertical gradients in SSC were strong enough to produce turbulence-limiting stratification, with gradient Richardson numbers exceeding 0.25. Simultaneously, turbulent stresses (decomposed from wave motions) increased by an order of magnitude. The apparent contradiction of energetic turbulence in the presence of strong stratification was reconciled by considering the turbulent kinetic energy (TKE) budget: in general, dissipation and buoyancy flux were balanced by local shear production, and each of these terms increased during wave events. The classic wave-current boundary layer model represented the observations qualitatively, but not quantitatively since the velocity profile could not be approximated as logarithmic. Rather, the mean shear was elevated by the Stokes drift return flow and wind-generated surface stress, which diffused sediment upward and limited stratification. Our findings highlight a pathway for waves to supply energy to both the production and destruction of turbulence, and demonstrate that in such shallow depths, TKE and SSC can be elevated over more of the water column than predicted by traditional models.

Variability of Sand Mobility Surrounding Cylinder Object Freely Resting on the Seabed under the Action of Typhoon

In sandy sediments, scour and fill is the key process contributed to mine burial. The scour processes surrounding the cylinder mines freely resting on the sandy seabed under the 12-hr combined action of tidal currents and wind-generated waves, especially over typhoon events are numerically simulated using the DRAMBUIE model. The East China Sea is a good case study due to the dominant impact of summer typhoon events on sediment transport and scour. The numerical results show that the scour depth generally increases with time under the combined current and wave stresses exerted on the seabed, while the depth of the scour pit depends on infill once the currents subside. There is a positive relationship between the scour depth and the bottom orbital velocity after experiencing 12-hr wave action including storm waves, while the relation is not linear. The experimental results also display an elevated trend for scour depth with the increase of orbital velocity. The numerical results reveal a surprising phenomenon: the mobility of sand altering with the increasing bed shear stress larger than the certain threshold, which is also manifested as the curves of scour depth with the different grain size might cross each other. For laboratory experiments, the variability of sand mobility does not occur, likely because typhoon storm waves cannot be reproduced in the flume. More numerical tests indicate that the intersection will be triggered by the division of critical Shields parameter. The preliminary analysis suggests that the phenomenon never documented is likely generated from the error of empirical formulae.

Simulating sediment transport processes in San Pablo Bay using coupled hydrodynamic, wave, and sediment transport models

Abstract San Pablo Bay is a tidal sub-embayment of the San Francisco Estuary which is dominated by broad shallow shoals bisected by a deep channel. Under the conceptual model of sediment transport in San Pablo Bay proposed by Krone (1979) , sediment typically enters San Pablo Bay during large winter and spring flows and is redistributed during summer conditions through wind wave resuspension and transport by tidal currents. Numerical simulations of sediment resuspension due to wind waves and the subsequent transport of this sediment by tidal currents show that sediment concentrations and fluxes throughout the channel–shoal system result from a complex temporal and spatial interaction of the waves and tides. The three-dimensional UnTRIM San Francisco Bay-Delta Model was coupled with the Simulating WAves Nearshore (SWAN) wave model and the SediMorph morphological model, to develop a three-dimensional (3D) hydrodynamic, wind wave, and sediment transport model of San Francisco Bay and the Sacramento–San Joaquin Delta. The coupled model was validated using water level, velocity, wind waves and suspended sediment data collected in San Pablo Bay, and then used to quantify the spatial and temporal variability of sediment fluxes on the extensive shoals in San Pablo Bay under a range of tidal and wind conditions. The model validation shows this modeling system can accurately predict hydrodynamics, waves, and suspended sediment concentration in San Pablo Bay. The predicted bottom orbital velocities are elevated across the shoals during large wave events regardless of tidal stage, but near low tide orbital velocities are increased even under low to moderate waves. Sediment fluxes between the shoals and the deeper channel are highest during spring tides, and are elevated for up to a week following wave events, even though the greatest influence of the wave event occurs abruptly.

Coherence of river and ocean conditions along the US West Coast during storms

The majority of water and sediment discharge from the small, mountainous watersheds of the US West Coast occurs during and immediately following winter storms. The physical conditions (waves, currents, and winds) within and acting upon the proximal coastal ocean during these winter storms strongly influence dispersal patterns. We examined this river–ocean temporal coherence for four coastal river–shelf systems of the US West Coast (Umpqua, Eel, Salinas, and Santa Clara) to evaluate whether specific ocean conditions occur during floods that may influence coastal dispersal of sediment. Eleven years of corresponding river discharge, wind, and wave data were obtained for each river–shelf system from USGS and NOAA historical records, and each record was evaluated for seasonal and event-based patterns. Because near-bed shear stresses due to waves influence sediment resuspension and transport, we used spectral wave data to compute and evaluate wave-generated bottom-orbital velocities. The highest values of wave energy and discharge for all four systems were consistently observed between October 15 and March 15, and there were strong latitudinal patterns observed in these data with lower discharge and wave energies in the southernmost systems. During floods we observed patterns of river–ocean coherence that differed from the overall seasonal patterns. For example, downwelling winds generally prevailed during floods in the northern two systems (Umpqua and Eel), whereas winds in the southern systems (Salinas and Santa Clara) were generally downwelling before peak discharge and upwelling after peak discharge. Winds not associated with floods were generally upwelling on all four river–shelf systems. Although there are seasonal variations in river–ocean coherence, waves generally led floods in the three northern systems, while they lagged floods in the Santa Clara. Combined, these observations suggest that there are consistent river–ocean coherence patterns along the US West Coast during winter storms and that these patterns vary substantially with latitude. These results should assist with future evaluations of flood plume formation and sediment fate along this coast.

Glider observations of sediment resuspension in a Middle Atlantic Bight fall transition storm

In October of 2003 a fleet of autonomous underwater gliders began a time series of transects across the New Jersey shelf. The gliders are equipped with a conductivity‐temperature‐depth sensor, and some carry optical ECO‐sensor pucks. The physical‐optical data are used to examine storm‐induced sediment resuspension. There are two types of storm response found. In summer, the seasonal stratification limits midshelf sediment resuspension to below the pycnocline even during hurricanes. In contrast, winter storms suspend sediment throughout the full water column. The transition between summer and winter seasons starts with surface cooling that preconditions the shelf for rapid mixing during fall storms. The mixing storm of October 2003 was a classic northeaster. Early in the storm when waves were high, sediment resuspension was limited to below the pycnocline. After the pycnocline eroded through growth of the bottom boundary layer, particles immediately filled the full water column. The spectral ratio of backscatter indicated that the particles were likely similar materials both before and after the stratification was eroded. The backscatter profiles in the bottom boundary layer decay with distance from the bed at rates consistent with theory but with variable slopes. The reduced slope of the backscatter profiles increased after stratification was lost, which is consistent with an increase in vertical transport or turbulent mixing. Wave bottom orbital velocities during this time were decreasing, and the glider vertical velocities showed no enhancement consistent with Langmuir cells. Enhanced mixing was related to the interaction of the surface and bottom boundary layers while the stratification was eroded, and the observed variability in the resuspension during the event was also due to the tide.

Modeling the Effects of Wave Skewness and Beach Cusps on Littoral Sand Transport

Abstract A process-based numerical modeling system is utilized for predicting littoral sand transport. The intent is to examine conditions slightly more complex than linear waves impinging upon a plane beach. Two factors that we examine are wave skewness and longshore varying bathymetry. An empirical model is used for calculating the skewed bottom wave orbital velocity. The advection of sediment due to the skewed wave velocity is larger and in the direction of the waves, opposite to the results with sinusoidal wave velocities, due to the increase in the bottom shear stress under the wave crests. The model system is also applied to bathymetry containing beach cusps. When the wave field has relatively weak longshore wave power, the currents and the littoral transport exhibit significant longshore variability, thereby altering the overall mean littoral transport.

Calculating wave-generated bottom orbital velocities from surface-wave parameters

Near-bed wave orbital velocities and shear stresses are important parameters in many sediment-transport and hydrodynamic models of the coastal ocean, estuaries, and lakes. Simple methods for estimating bottom orbital velocities from surface-wave statistics such as significant wave height and peak period often are inaccurate except in very shallow water. This paper briefly reviews approaches for estimating wave-generated bottom orbital velocities from near-bed velocity data, surface-wave spectra, and surface-wave parameters; MATLAB code for each approach is provided. Aspects of this problem have been discussed elsewhere. We add to this work by providing a method for using a general form of the parametric surface-wave spectrum to estimate bottom orbital velocity from significant wave height and peak period, investigating effects of spectral shape on bottom orbital velocity, comparing methods for calculating bottom orbital velocity against values determined from near-bed velocity measurements at two sites on the US east and west coasts, and considering the optimal representation of bottom orbital velocity for calculations of near-bed processes. Bottom orbital velocities calculated using near-bed velocity data, measured wave spectra, and parametric spectra for a site on the northern California shelf and one in the mid-Atlantic Bight compare quite well and are relatively insensitive to spectral shape except when bimodal waves are present with maximum energy at the higher-frequency peak. These conditions, which are most likely to occur at times when bottom orbital velocities are small, can be identified with our method as cases where the measured wave statistics are inconsistent with Donelan's modified form of the Joint North Sea Wave Project (JONSWAP) spectrum. We define the “effective” forcing for wave-driven, near-bed processes as the product of the magnitude of forcing times its probability of occurrence, and conclude that different bottom orbital velocity statistics may be appropriate for different problems.

Northeast storms ranked by wind stress and wave-generated bottom stress observed in Massachusetts Bay, 1990–2006

Along the coast of the northeastern United States, strong winds blowing from the northeast are often associated with storms called northeasters, coastal storms that strongly influence weather. In addition to effects caused by wind stress, the sea floor is affected by bottom stress associated with these storms. Bottom stress caused by orbital velocities associated with surface waves integrated over the duration of a storm is a metric of storm strength at the sea floor. Near-bottom wave-orbital velocities calculated by using measurements of significant wave height and dominant wave period and the parametric spectral method described in Wiberg and Sherwood [Wiberg, P.L., Sherwood, C.R. Calculating wave-generated bottom orbital velocities from surface wave parameters. Computers in Geosciences, in press] compared well with observations in Massachusetts Bay. Integrated bottom-wave stress (called IWAVES), calculated at 30 m water depth, and a companion storm-strength metric, integrated surface wind stress at 10 m (called IWINDS), are used to provide an overview of the strength, frequency, and timing of large storms in Massachusetts Bay over a 17-year period from January 1990 through December 2006. These new metrics reflect both storm duration and intensity. Northeast storms were the major cause of large waves in Massachusetts Bay because of the long fetch to the east: of the strongest 10% of storms ( n=38) ranked by IWAVES, 22 had vector-averaged wind stress from the northeast quadrant. The Blizzard of December 1992, the Perfect Storm of October 1991, and a December 2003 storm were the strongest three storms ranked by IWAVES and IWINDS, and all were northeasters. IWAVES integrated over the winter season (defined as October–May) ranged by about a factor of 11; the winters with the highest integrated IWAVES were 1992–1993 and 2004–2005 and the winter with the lowest integrated IWAVES was 2001–2002. May 2005 was the only month in the 17-year record that two of the nine strongest northeast storms ranked by IWINDS occurred in the same month or year; these were also the only storms of the nine strongest northeast storms to occur in the spring.