Sediment Budget
A sediment budget employs conservation of mass to quantify sediment sources, sinks, and pathways in a littoral cell
environment. Moreover, a sediment budget can be used to quantify the effects of changing sediment supply on the coastal system and to
understand the large-scale morphological responses of the coastal system. Throughout the last several thousand years the Columbia
River has supplied sand to nearby bays, coastal dunes, the continental shelf, and the continental slope and submarine canyons
and fans.
By quantifying the amount of sand that occupies each of these environments, the Study is attempting to better understand the
Columbia River dispersal system and better predict the response to changes in the system, whether they are natural or human-induced. To
help separate natural from human-induced changes in the littoral cell, the sediment budget is being calculated for pre-historic
periods as well as for historic and recent periods. By examining the sediment budget from various time periods, it is possible to compare
the natural evolution and variability in the system to changes forced by human intervention.
The Columbia River is the primary source of sand to the littoral cell. The figure below illustrates the dispersal of Columbia River sediment throughout the littoral cell.
Sediment-transport pathways and patterns of sediment accumulation in the littoral cell are not static, but change over geological,
historical, and seasonal time scales. Some changes in the sediment budget are the result of natural cycles such as long-term changes in
sea level, or of short-term fluctuations such as in wind and wave directions. Other changes in pathways and sinks in the sediment
budget are the result of human influences, such as the construction of jetties, or dredging practices.
Although research on the sediment budget is continuing, several observations can be drawn from the Study’s analyses:
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The majority of Columbia River sediment has accumulated on the continental shelf, in the deep sea, and in the estuaries. The amount in the beaches, by comparison, is smaller.
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Average sediment supply from the Columbia River was apparently much greater for the last 10,000 years than it is now, possibly reflecting the contribution of several unmitigated volcanic eruptions, erosion of glacial deposits, and extreme floods.
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Early historical shoreline accretion rates are much greater than pre-historical rates, and in general, greater than recent accretion rates. The timing of the rapid accretion in the early part of the century and the alongshore variation in the beach accretion suggest changes in the ebb-tidal deltas after jetty construction as the primary cause.
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Ebb-tidal deltas and the inner shelf may act as temporary sources of sediment to the beaches.
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Sediment supply from the Columbia River to the estuary has likely been reduced over the last several decades due to reduction in transport capacity resulting from flow regulation, and possible direct trapping behind dams.
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The volume of dredged material placed at the mouth of the Columbia River is large compared to long-term changes in the tidal-delta complex.
