There are numerous types of settling tubes; the most common is the
visual accumulation tube (Colby and Christensen, 1956), of which there
are also several types. The type now used at CERC (the rapid sédiment
analyzer or RSA) Works in the following way:
A 3- to 6-gram sample of sand is dropped through a tube filled with
distilled water at constant température. A pressure sensor near the
bottom of the tube senses the added weight of the sédiment supported by
the column of water above the sensor. As the sédiment falls past the
sensor, the pressure decreases. The record of pressure versus time is
empirically calibrated to give size distribution based on fall velocity.
(Zeigler and Gill, 1959.)
The advantage of settling tube analysis is its speed. With modem
settling tubes, average time for size analyses of bulk lots can be about
one-fifth the time required by sieves.
It is often claimed that a settling tube also provides a physically
more realistic size analysis than a sieve, since the fall velocity takes
into account the hydrodynamic effects of shape and density. However,
this claim has not been documented, and may be questioned in view of the
limited knowledge concerning the fluid mechanics of a sand sample falling
in a settling tube - the lead particles encounter effectively laminar flow
the trailing particles encounter turbulent flow, and ail particles internet with each other.
Because of lack of an accepted standard settling tube, rapidly changing technology, possible changes in tube calibration, and the uncertainty
about fluid mechanics in settling tubes, it is recommended that ail settüng tubes be carefully calibrated by running a range of s amp les through
both the settling tube and ASTM standard sieves. After thorough initial
calibration, the calibration should be spot-checked periodically by running
replicate sand samples of known size distribution through the tube.
4.3 LITTORAL WAVE CONDITIONS
4.31 EFFECT OF WAVE CONDITIONS ON SEDIMENT TRANSPORT
• port
thp nrimary cause of sédiment transWaves arriving at the shore are
fnrther offshore, widening
waves break rurcnei
in the littoral zone. Higher
Changes in wave period or
the surf zone and setting more sand in mo
angie between the
height resuit in moving sand onshore °r °
jptprmines the direction of
crest of the breaking wave and the shore i
surf Zone, and usually the
the longshore component of water motion in
^^ knowledge about the
longshore transport direction. For
height, period, and direction
wave climate - the combined distributio adequate understanding of the
through the seasons - is required or
littoral processes of any spécifie area.
4-27
visual accumulation tube (Colby and Christensen, 1956), of which there
are also several types. The type now used at CERC (the rapid sédiment
analyzer or RSA) Works in the following way:
A 3- to 6-gram sample of sand is dropped through a tube filled with
distilled water at constant température. A pressure sensor near the
bottom of the tube senses the added weight of the sédiment supported by
the column of water above the sensor. As the sédiment falls past the
sensor, the pressure decreases. The record of pressure versus time is
empirically calibrated to give size distribution based on fall velocity.
(Zeigler and Gill, 1959.)
The advantage of settling tube analysis is its speed. With modem
settling tubes, average time for size analyses of bulk lots can be about
one-fifth the time required by sieves.
It is often claimed that a settling tube also provides a physically
more realistic size analysis than a sieve, since the fall velocity takes
into account the hydrodynamic effects of shape and density. However,
this claim has not been documented, and may be questioned in view of the
limited knowledge concerning the fluid mechanics of a sand sample falling
in a settling tube - the lead particles encounter effectively laminar flow
the trailing particles encounter turbulent flow, and ail particles internet with each other.
Because of lack of an accepted standard settling tube, rapidly changing technology, possible changes in tube calibration, and the uncertainty
about fluid mechanics in settling tubes, it is recommended that ail settüng tubes be carefully calibrated by running a range of s amp les through
both the settling tube and ASTM standard sieves. After thorough initial
calibration, the calibration should be spot-checked periodically by running
replicate sand samples of known size distribution through the tube.
4.3 LITTORAL WAVE CONDITIONS
4.31 EFFECT OF WAVE CONDITIONS ON SEDIMENT TRANSPORT
• port
thp nrimary cause of sédiment transWaves arriving at the shore are
fnrther offshore, widening
waves break rurcnei
in the littoral zone. Higher
Changes in wave period or
the surf zone and setting more sand in mo
angie between the
height resuit in moving sand onshore °r °
jptprmines the direction of
crest of the breaking wave and the shore i
surf Zone, and usually the
the longshore component of water motion in
^^ knowledge about the
longshore transport direction. For
height, period, and direction
wave climate - the combined distributio adequate understanding of the
through the seasons - is required or
littoral processes of any spécifie area.
4-27