Abstract: Stability of Dredged Material Deposited at Ocean Dumping Sites Seaward of Columbia River Mouth

R. W. Sternberg, J. S. Creager, W. Glassley, J. Johnson

A two-part study was conducted over a 19-month period in a region seaward of the Columbia River where disposal of large quantities of dredged material has occurred during the last 40 years. The first part included repeated bathymetric surveys and sampling for distribution and seasonal variations of sediment texture and mineralogy throughout the study area and especially at designated disposal sites. Near-bottom hydraulic conditions (waves, tides, wind-generated currents, turbidity) also were measured at several sites and during all seasons. The second part was related to an experiment in which 559,000 cu m of material dredged from the Columbia River estuary was dumped at a newly designated site, which was monitored before, during, and after disposal. The objectives of the sedimentologic aspects of the study were to identify and map all deposits of dredged material and to recognize seasonal and long-term changes. The objectives of the hydraulic aspects were to document the ambient near-bottom conditions, and their effect on the deposit at the experimental site.

Sedimentologic data show that all deposits of dredged material can be identified relative to the surrounding sediments. They tend to maintain their identity for many years and disperse northward at approximately 0.5 km per year. Approximately 30% of the volume of material dumped since 1950 remains at the disposal sites.

At the experimental disposal site the volume of the bottom deposit was 70% of the total material dumped. Calculations of bedload-transport rates based on seasonal measurements of bottom currents suggest that 635 cu m of material (0.2% of the total) spread northward from the site per year at a rate of about 0.4 km per year. This is similar to the rates determined by the sedimentologic techniques. The coherent and complementary nature of the results emphasizes the value of combining both descriptive sedimentologic techniques and measurements of oceanic processes for monitoring and predicting the fate of dredged material.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC