Wm. J. Wiseman

Seasonal and interannual variability within the Louisiana coastal current: stratification and hypoxia

Abstract Ten years of mid-summer survey cruise data mapping the west Louisiana inner shelf, supplemented by data from occasional cruises along a single transect and time series data, is used to characterize the seasonal and interannual variability of stratification in the region. Persistent strong haline stratification is modulated by the intensity and phasing of river discharge from the Mississippi/Atchafalaya River system. This stratification is only destroyed during periods of intense wind mixing. The strong relationship between stratification variability and hypoxia is demonstrated.

Processes of marine dispersal and deposition of suspended silts off the modern mouth of the Huanghe (Yellow River)

The processes responsible for the transport and deposition of concentrated suspended silts over the delta front of the Huanghe were observed during three cruises and have been modeled numerically. Suspended sediment concentrations in the lower Huanghe average about 25 kg m−3 and exceed 200 kg m−3 during flood stage. Cruises were conducted during normal discharge conditions in spring 1985 and summer 1986, and during low-discharge storm-dominated conditions in autumn 1987. During the first two cruises, the shallow delta-front top (depth≤ 5m) was covered by a turbid water mass with suspended sediment concentrations of 1–10 kg m−3. Strong (∼1m s−1) parabathic tidal currents resuspended newly deposited muds and advected them alongshore. Near a break in slope, the turbid layers plunged beneath the ambient water and descended the delta-front slope as gravity-driven hyperpycnal underflows. In 1987 the hyperpycnal underflows occurred only during an intense strom that resuspended delta-front sediments to produce underflows with concentrations on the order of 100 kg m−3. We infer that gravity-driven underflows constitute the most important mode of suspended sediment transport across isobaths. Concentrated and channelized “point source” underflows, apparently associated with flood conditions, were not observed but were inferred from morphological evidence and were modeled numerically. Modeling results show that the Coriolis force and ambient momentum should cause appreciable curvature to the paths of underflows, while entrainment of ambient mass contributes to underflow decay. Early extinction of all underflow types is suggested by field and modeling results, and is considered to be responsible for extremely rapid delta-front deposition and for the fact that most of the sediments discharged by the Huanghe remain close to the mouth.

The Role of Stratification in the Deoxygenation of Mobile Bay and Adjacent Shelf Bottom Waters

Oxygen depletion in the shallow bottom waters of Mobile Bay, Alabama, and in adjacent nearshore and continental shelf waters, is shown to be directly related to the intensity of water column stratification. Low winds speeds are coincidental with the onset of water column stratification and the occurrence of hypoxic events. Hourly, daily, and seasonal changes in the relationship between percent oxygen saturation or oxygen concentration in the bottom waters and surface-bottom density differences indicate that the oxidized materials are recently formed, and not relic or overwintering carbon sources. The influence of density structure (water column stratification) in other oxygen-depleted coastal water masses is compared to Mobile Bay.

Plumes and Coastal Currents Near Large River Mouths

Theory and observations of river plumes are reviewed. The importance of the Kelvin number in characterizing anticipated plume behavior is stressed. In the absence of strong external forcing, a northern hemisphere plume will turn anticyclonically and attach to the coast, where it then merges into a coastal current. Observations and theory of such coastal currents are also reviewed, with emphasis on flows over shallow continental shelves. Major unresolved questions involve the processes controlling mixing of coastal current waters with ambient shelf waters and the dynamics of the plume in the region where it attaches to the coast.

Salinity variability within the Louisiana coastal current during the 1982 flood season

Analysis of current and salinity time series from early 1982 over the inner shelf near the Texas-Louisiana border indicates a rapid freshening of the coastal current waters in response to the discharge of the Mississippi River. Coherence between records is surprisingly poor, although cross-shelf advection of the salinity front by tidal advection appears to be an important local process.

Hypoxia and the Physics of the Louisiana Coastal Current

The Mississippi River is the largest river in North America. It drains 2.979 106 km2, including 41% of the lower 48 United States and parts of two Canadian provinces. The effluent from this system is debouched into the Gulf of Mexico through the modern birdfoot delta of the Mississippi River and through a secondary outlet, the Atchafalaya River delta. Under the influence of the Coriolis force, a significant portion of these waters flows westward along the Louisiana coast as the highly stratified Louisiana Coastal Current (Wiseman and Kelly, 1994). Each summer, hypoxic or, occasionally, anoxic conditions develop beneath the associated halocline. (Hypoxia is defined operationally as dissolved oxygen concentrations less than 2 mg/1 (Rabalais, et al. 1991).)

Submarine slope processes on a fan delta, Howe Sound, British Columbia

Modern side-scan sonar technology was used to resurvey the site of the Howe Sound submarine slide described by Terzaghi. Chutes, hummocky topography, and subparallel scarps indicative of submarine mass movement are widespread. Submarine slope processes appear to be far more important to the development of this coarsegrained fan delta than suspected.

Satellite observations of the circulation east of the Mississippi delta: cold-air outbreak conditions

Abstract Examination of 12 years of Landsat multispectral scanner images shows a recurrent pattern of westward flow immediately south of the Mississippi-Alabama barrier islands under northerly winds. Such flow patterns are also seen under similar conditions in imagery from the Advanced Very High Resolution Radiometer (AVHRR) of the NOAA-series satellites. The flow enters Chandeleur Sound between Ship Island and the northern end of the Chandeleur Islands. It appears to be driven by northerly winds, which force water south through the ChandeleurBreton Sound, drawing water in from the shelf region south of the Mississippi-Alabama barrier islands. These observations on circulation can be simply explained assuming linear dynamics. These two operational satellite systems are accumulating valuable records of coastal circulation patterns under clear-sky conditions.

SUSPENSION AND TRANSPORTATION OF FLUID MUD BY SOLITARY-LIKE WAVES

The equations describing conservation of mass, momentum and energy in a turbulent free surface flow are derived for a controle volume extending over the whole depth. The effect of the turbulent surface oscillations are discussed but neglected in the following analysis, where the equations are applied to the energy balance in a surf zone wave motion. This leads to results for the wave height variation and the velocity of propagation. The results cannot be reconciled completely with measurements and the concluding discussion is aimed at revealing how the model can be improved.A three-dimensional morphodynamic model of sequential beach changes Is presented. The model Is based on variations in breaker wave power generating a predictable sequence of beach conditions. The spectrum of beach conditions from fully eroded-dissipatlve to fully accreted reflective is characterised by ten beach-stages. Using the breaker wave power to beach-stage relationship the model Is applied to explain temporal, spatial and global variations In beach morphodynamlcs.The agents of initial damage to the dunes are water, which undermines them, and animals (including man) which damage the protective vegetation by grazing or trampling. Of these, man has recently assumed predominant local importance because of the popularity of sea-side holidays and of the land-falls of certain marine engineering works such as oil and gas pipelines and sewage outfalls. The need is therefore increasing for active dune management programmes to ensure that under these accentuated pressures, the coast retain an equilibrium comparable with that delicately balanced equilibrium which obtains naturally at a particular location.

Coastal Breakup in the Alaskan Arctic

During observations of breakup along the Alaskan Arctic Coast, river flooding of the frozen nearshore zone, sea ice breakup, and beach thaw were examined. Spring river flooding, generated by earlier inland melt, accompanies arrival of temperatures above 0°C on the coast. The extent of flooding over the nearshore ice is related to total flood discharge and coastal morphology. Along wave-controlled barrier-island coasts, flooding and bed load are confined to lagoons, whereas on fluvial-dominated coasts, floodwater and sediment spread across lobate delta fronts and offshore shoals. During this time, marine influence is minimal as a result of protection afforded by sea ice cover. Sea ice melt continues through summer, and the final coastal sea ice breakup and ice dispersion depend on offshore Ekman transport, breakup of the offshore pack ice, and local bathymetry. The coastal ice breaks up 4 to 8 weeks after initiation of melt. Melt of ice and snow within the beach generates beach collapse and resultant unique arctic beach features, whereas flow of tundra snowmelt across the beach produces micro-fans and micro-deltas.