Maximum Sampling Depths and Transit Rates for Suspended Sediment and Water-Quality Samplers
In Reply Refer To: January 31, 1994
Mail Stop 415
OFFICE OF SURFACE WATER TECHNICAL MEMORANDUM 94.05
Subject: Maximum Sampling Depths and Transit Rates for Suspended
Sediment and Water-Quality Samplers
There have been several questions recently about the acceptable
transit rates for sediment samplers and why depth integrating
samplers are limited to sampling of water depths less than about
15 feet. The following discussion explains reasons for the
restrictions as well as presenting the limits on transit rates and
depths for some sampler/nozzle combinations which have not been
readily available in the past for some sampler/nozzle
combinations.
For a sediment sampler to collect a representative volume of the
surrounding medium, the water-sediment mixture must move through
the nozzle undisturbed. To move into the nozzle undisturbed, the
water must not change directions to enter the nozzle, which
implies that it must enter the nozzle at the same velocity as it
is moving in the stream at the point through which the nozzle is
passing. When the velocity is undisturbed upon entering the
nozzle, the condition is characterized as isokinetic. All depth-
and point-integrating samplers used by the U.S. Geological Survey
(USGS) that have rigid sample containers sample isokinetically
only if pointing directly into the flow and if they are used
within certain ranges of depths. Depth-integrating samplers also
operate isokinetically only when the vertical transit rate is
maintained within a given range.
A sampler will not operate properly if used at too large a depth.
For example, if the volume of the water-sediment mixture is to be
two-thirds of the total container volume for a depth-integrated
sample, the sampler should be one-third full as it reaches the
bottom of the river so that it will have room for the water
collected on the return trip. At the bottom of the river,
therefore, the volume of air in the sampler must be at least two-
thirds of the total volume. At sea level the pressure of the air
in the bottle before it enters the water is about 34 feet of water
(atmospheric pressure). Boyle's gas law states that the product
of the pressure and volume is a constant, so equating the products
at the surface and bottom gives an expression for the maximum
pressure (depth) for the sampler:
34 Vs = P 2Vs/3
in which Vs = the volume of the sample container (which cancels
out) and P equals the maximum pressure, in feet of water, for the
sampler. Solving for the pressure gives P = 51 feet. Subtracting
atmospheric pressure (34 feet) leaves 17 feet as the compression
limit of the sampler. If the sampler is lowered below the
compression limit it will intake too much water and not sample
isokinetically. The compression depth limit varies for various
depth-integrating samplers, as shown in Table 1, depending on the
maximum percentage of useful volume. It also varies with sample
container size and volume of the pressure compensating chamber for
point-integrating samplers. The values given in Table 1 are for
sea level conditions. The maximum depth decreases about 1 foot
for every 1000-foot increase in elevation.
There are two factors which control the maximum vertical transit
rate for a depth-integrating sampler. These factors include:
approach angle and the compression rate. The approach angle is
determined as the ratio of vertical velocity of the sampler (rate
at which it is lowered or raised) to the mean stream velocity. If
the sampler is lowered or raised at a rate exceeding 0.4 times the
mean flow velocity, the intake velocity will be less than the
stream velocity (FISP 1952). The maximum vertical transit rate
for any depth integrating sampler, therefore, should not exceed
0.4 times the mean stream velocity of the section.
The compression rate, which is related to the compression limit,
may restrict the vertical transit rate to less than 0.4 times the
mean stream velocity. As the sampler is lowered through the water
column, the increased water pressure compresses the air in the
sampler. If the sampler is lowered slowly, the incoming water
more than takes up the space created by the compression of the air
and the excess air exits through the exhaust vent. If the sampler
is lowered too rapidly, however, the incoming water does not
compress the air within the sampler fast enough so the pressure on
the inside of the sampler is less than the hydrostatic pressure
outside the sampler. When this occurs the intake velocity
increases above the stream velocity. Water may even enter the
sampler through the air exhaust vent. If the sampler is raised
too rapidly, the compressed air inside the bottle will not escape
fast enough through the exhaust vent and the intake velocity will
be less than the mean stream velocity. The compression-rate limit
is a function of the size of the nozzle and sample container. For
large bottles with small nozzles it can limit the vertical transit
rate to less than 3 percent of the mean stream velocity. Table 1
lists the maximum transit rates that should be used with most USGS
samplers for various combinations of nozzles and container sizes.
Edwards and Glysson (1988) discuss the proper use of the samplers
and transit rate ratios for some of the more common combinations
used by the USGS. Because of small allowable vertical transit
rates and difficulty of maintaining a slow transit rate, the
Office of Surface Water does not recommend using the 1/8-inch or
3/16-inch nozzle on the D-77 sampler with a 3-liter container.
References:
Edwards, Thomas K. and Glysson, G. Douglas, 1988, Field Methods
for Measurement of Fluvial Sediment, U.S. Geological Survey Open-
File Report 96-531, 118 p.
Federal Interagency Sedimentation Project, 1952, The design of
improved types of suspended-sediment samplers - Interagency
Report 6: Minneapolis, Minnesota, St. Anthony Falls Hydraulics
Laboratory, 103 p.
Charles W. Boning
Chief, Office of Surface Water
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