492 CHAPTER 8. LABORATORY MEASUREMENT AND ANALYSIS
problems being the waterproofing of the gauge and its connecting wires,
and protecting the strain gauge from damage.
If a solid object is held by one or more supports, the total force exerted
by the fluid on the object can be determined using “load cells” on each
of the supports. Load cells are instruments that give an output signal
that is directly related to the total applied force. For example, a simple
load cell could be constructed by attaching a strain gauge to thin strip
of metal. As axial tension is. applied to the metal strip, the strain gauge
registers strain which can be related to the total axial force. Arrangements
of multiple strain gauges oriented at different angles in a load cell can be
used to determine bending moments and torsion.
Most often load cells need to be calibrated. One way to do a calibration
is applying incremental loads up to the maximum load, then removing the
load to assure there is no hysteresis. Dynamic calibration requires more
sophisticated methods.
8.2.7 Sediment Transport Measurement
Sediment transport measurements in the laboratory can be divided into
two types based on the time scale of sediment motion. The first type is
a measure of the average sediment transport rate over a time span that is
many times greater than the wave period. In this case, the measurement
is not a direct measure of sediment transport, but rather a measure of the
change in bathymetry brought about by sediment transport over a finite
time period.
Onshore/offshore transport rates can be determined by measuring profiles and calculating the volumetric change that occurred between successive
surveys. Longshore transport rates can be inferred from the build-up of sediment at a longshore obstruction, such as a groin or jetty. A more direct
measurement of longshore transport rate can be obtained using traps at
the downdrift boundary of the model.
The second type of sediment transport measurement has a time scale of
a wave period or less, and these measurements are more like point samples
of the sediment transport. Making this type of sediment transport measurement is a difficult task; consequently, development of basic instrumentation
is still an ongoing endeavor. Most instrument development efforts seem to
be directed at measuring suspended sediment concentrations.
Perhaps the first reasonably successful tool for measuring suspended
sediment concentrations was the pump sampler. In this technique sediment
laden water is pumped through one or more tubes into sample bottles. The
sediment concentration of the sample is determined, and this value represents the concentration at the location of the intake tube. The suspended
problems being the waterproofing of the gauge and its connecting wires,
and protecting the strain gauge from damage.
If a solid object is held by one or more supports, the total force exerted
by the fluid on the object can be determined using “load cells” on each
of the supports. Load cells are instruments that give an output signal
that is directly related to the total applied force. For example, a simple
load cell could be constructed by attaching a strain gauge to thin strip
of metal. As axial tension is. applied to the metal strip, the strain gauge
registers strain which can be related to the total axial force. Arrangements
of multiple strain gauges oriented at different angles in a load cell can be
used to determine bending moments and torsion.
Most often load cells need to be calibrated. One way to do a calibration
is applying incremental loads up to the maximum load, then removing the
load to assure there is no hysteresis. Dynamic calibration requires more
sophisticated methods.
8.2.7 Sediment Transport Measurement
Sediment transport measurements in the laboratory can be divided into
two types based on the time scale of sediment motion. The first type is
a measure of the average sediment transport rate over a time span that is
many times greater than the wave period. In this case, the measurement
is not a direct measure of sediment transport, but rather a measure of the
change in bathymetry brought about by sediment transport over a finite
time period.
Onshore/offshore transport rates can be determined by measuring profiles and calculating the volumetric change that occurred between successive
surveys. Longshore transport rates can be inferred from the build-up of sediment at a longshore obstruction, such as a groin or jetty. A more direct
measurement of longshore transport rate can be obtained using traps at
the downdrift boundary of the model.
The second type of sediment transport measurement has a time scale of
a wave period or less, and these measurements are more like point samples
of the sediment transport. Making this type of sediment transport measurement is a difficult task; consequently, development of basic instrumentation
is still an ongoing endeavor. Most instrument development efforts seem to
be directed at measuring suspended sediment concentrations.
Perhaps the first reasonably successful tool for measuring suspended
sediment concentrations was the pump sampler. In this technique sediment
laden water is pumped through one or more tubes into sample bottles. The
sediment concentration of the sample is determined, and this value represents the concentration at the location of the intake tube. The suspended