USGS/Cascades Volcano Observatory, Vancouver, Washington
DESCRIPTION:
Sediment Sampling
From:
Dinehart, 1985,
Sediment Data for Streams Near Mount St. Helens,
Washington - Volume 2., Water Years 1981-83: USGS Open-File Report 85-632
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During the water years 1981-1983, the average annual suspended-sediment yield
measured at Toutle River at Tower Road was 74,000 tons/sq.mi. That amount was
greater than the sediment yield from any other nonglaciated drainage basin of
similar size in North America, including the high sediment-yield streams of
northern California. The sediment has been largely
derived from materials that were deposited on approximately one-third of the
512 sq.mi drainage basin by the May 18, 1980 eruption of Mount St. Helens.
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The Lewis River basin had erodible sediments emplaced by the same eruption.
Sediment yields for the Muddy River, Clearwater Creek, Pine Creek, and other
Lewis River tributaries have increased from pre-eruption levels. ...
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Sediment data were collected on a weekly or biweekly schedule by U.S.Geological
Survey personnel at gaging stations during normal river flow. Storm runoff
usually required a more frequent sampling schedule to adequately define the
change of suspended-sediment concentration with time, so samples were generally
collected at intervals of one hour or less during many storm periods. The field
techniques used for collection of sediment data are standard Geological Survey
procedures as described by Guy and Norman (1970).
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Types of Sediment Data Collected
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The types of sediment data that were routinely collected are listed below.
- Cross-sections of suspended-sediment samples collected by the EDI
(equal-discharge increment) method or by the EWI (equal-width increment) method;
- Single-vertical samples collected by Geological Survey personnel;
- Single-vertical samples collected by observers (private citizens
instructed by Geological Survey personnel in collection of sediment samples);
- Fixed-point samples collected by automatic pumping samplers;
- Bed material samples collected at several verticals (usually determined
by the EDI method) in conjunction with suspended-sediment measurements.
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Methods of Sediment Analysis
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Concentration analyses were performed by the Tacoma and Vancouver sediment
laboratories, and particle size analyses were performed by the Sacramento,
Tacoma, and Vancouver laboratories. Concentration and particle-size analyses
were performed according to methods described by Guy (1969).
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Sand-break Analysis:
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Sand-break analyses of suspended-sediment samples give the percent of sediment
by weight that is finer than 0.062 mm. Sand breaks were performed occasionally
for samples collected during October through December 1980. Beginning in 1981,
sand breaks were performed for all Mount St. helens samples where feasible.
Sand breaks were usually determined by wet sieving.
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Particle-size Analysis:
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Particle-size analyses give the percent of sediment by weight that is finer than
a given size class. Pipet analysis was used for the suspended-sediment
fraction finer than 0.062 mm. Wet sieving or visual-accumulation tubes were
used for the fraction coarser than 0.062 mm. Samples were analyzed in distilled
water and were chemically dispersed.
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Cross-section samples from suspended-sediment measurements were generally
separated into two representative sets for a specific time. In most cases, one
set was analyzed for particle size while the other was analyzed for sand break.
There are some instances where single-vertical samples were analyzed for
particle size.
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Bed material samples were usually analyzed by dry sieving the material, although
some wet sieve analyses were performed on bed material samples in 1981.
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The high concentration sediment samples from the March 19-20, 1982 lahar-runout
flow were analyzed in the same manner as suspended-sediment samples.
Observers' samples and automatic pumping samples were usually analyzed for
sediment concentration only (no particle size or sand break).
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08/01/97, Lyn Topinka