Turbid Plume Research Articles

Teaching unit for blood pressure screening and control for secondary schools: Jim Glick, Health Education Division, State of Washington Department of Social and Health Services, Olympia, Washington

To address the relative importance of shrimp trawling on seabed resuspension and bottom characteristics in shallow estuaries, a series of disturbance and monitoring experiments were conducted at a bay bottom mud site (2.5 m depth) in Galveston Bay, Texas in July 1998 and May 1999. Based on pre- and post-trawl sediment profiles of 7Be; pore water dissolved oxygen and sulfide concentration; and bulk sediment properties, it was estimated that the trawl rig, including the net, trawl doors, and “tickler chain,” excavate the seabed to a maximum depth of approximately 1.5 cm, with most areas displaying considerably less disturbance. Water column profile data in the turbid plume left by the trawl in these underconsolidated muds (85–90% porosity; <0.25 kPa undrained shear strength) demonstrate that suspended sediment inventories of up to 85–90 mg/cm2 are produced immediately behind the trawl net; an order of magnitude higher than pre-trawl inventories and comparable to those observed during a 9–10 m/s wind event at the study site. Plume settling and dispersion caused suspended sediment inventories to return to pre-trawl values about 14 min after trawl passage in two separate experiments, indicating particles re-settle primarily as flocs before they can be widely dispersed by local currents. As a result of the passage of the trawl rig across the seabed, shear strength of the sediment surface showed no significant increase, suggesting that bed armoring is not taking place and the trawled areas will not show an increase in critical shear stress.

The missing link in the compliance chain: Jan Gilmore, Sharon Horn Holguin, High Blood Pressure Council of Los Angeles, California

To address the relative importance of shrimp trawling on seabed resuspension and bottom characteristics in shallow estuaries, a series of disturbance and monitoring experiments were conducted at a bay bottom mud site (2.5 m depth) in Galveston Bay, Texas in July 1998 and May 1999. Based on pre- and post-trawl sediment profiles of 7Be; pore water dissolved oxygen and sulfide concentration; and bulk sediment properties, it was estimated that the trawl rig, including the net, trawl doors, and “tickler chain,” excavate the seabed to a maximum depth of approximately 1.5 cm, with most areas displaying considerably less disturbance. Water column profile data in the turbid plume left by the trawl in these underconsolidated muds (85–90% porosity; <0.25 kPa undrained shear strength) demonstrate that suspended sediment inventories of up to 85–90 mg/cm2 are produced immediately behind the trawl net; an order of magnitude higher than pre-trawl inventories and comparable to those observed during a 9–10 m/s wind event at the study site. Plume settling and dispersion caused suspended sediment inventories to return to pre-trawl values about 14 min after trawl passage in two separate experiments, indicating particles re-settle primarily as flocs before they can be widely dispersed by local currents. As a result of the passage of the trawl rig across the seabed, shear strength of the sediment surface showed no significant increase, suggesting that bed armoring is not taking place and the trawled areas will not show an increase in critical shear stress.

Turbidity Dispersion in Lake Superior Through use of Landsat Data

Consecutive Landsat images of a red clay turbidity plume due to runoff from Nemadji River, Wisconsin, are used in computation of the horizontal eddy diffusivity for Lake Superior. Dispersion of turbid areas ranging from 2.5-5.4 km in diameter and having average velocities of 0.4 cm/sec-6.1 cm/sec are investigated. The coefficient for horizontal eddy diffusivity ranges from 0.6-1.5 m 2/sec.

Remote Sensing of Turbidity Plumes in Lake Ontario

Space imagery and high-altitude photography provide an effective method of monitoring the dynamics of turbidity plumes in Lake Ontario. The Niagara River plume, as much as 200 sq miles (520 km²) in area, is by far the largest turbidity feature in the lake. Plume analyses corroborates the presence of a prevailing eastward flowing longshore current along the entire south shore of the lake. This current is most persistent at the Oswego River outlet but it is quite variable in the Rochester embayment, where rapid shifts in water movement were occasionally detected. The position of the spring thermal bar, a zone of maximum density water corresponding to the 4°C isotherm, was located near the Niagara River outlet in images obtained during April, 1973.

Turbidity Sources in Lake Superior

Landsat data and measurements of turbidity profiles with depth, suspended solids and particle sizes are used in analysis of red clay turbidity plumes in western Lake Superior for a variety of wind conditions. The measurements allow for an estimate of the relative contribution to lake turbidity arising from shore erosion, runoff, and bottom sediment resuspension. The major source of turbidity is due to erosion, yielding an estimated 2.3 × 10° metric tons of material removed per year from the shores of Douglas County, Wisconsin. This is equivalent to a shore recession rate of nearly 1.2 m/year. During the months of May through November when all three turbidity sources contribute, 70% of the total suspended load observed in the lake is due to shore erosion, 10% is due to precipitation runoff from the rivers (with major input in this category coming from the Nemadji River), and 20% of the load is due to resuspension.

Distribution of Surface Turbidity Off Mississippi Delta

Aerial and surface observations around the Mississippi Delta show a great quantity of suspended sediment in the water close inshore. The amount of material in suspension, controlled chiefly by volume of river flow and distance from source, in general decreases to a small value a few miles off shore, but under certain conditions turbid water extends to sea for distances of at least 65 miles as long plumes off the mouths of the important passes. Most of these plumes have relatively sharp boundaries. The direction of seaward distribution of the turbid plumes is controlled chiefly by the wind and to less extent by semi-permanent currents, tidal currents, and by river flow. The intensity of surface turbidity generally can be correlated with the rate of deposition in any area.