8.2. MEASUREMENT TOOLS AND TECHNIQUES
493
sediment concentration determined using the pump sampler technique is
an average over the sampling time.
More modern and sophisticated electronic instruments for measuring
suspended sediment in the laboratory were discussed by Wessels et al
(1990). These instruments utilize acoustic and optical techniques to estimate sediment concentration over a small volume and at a high sample
rate. Air bubbles cause difficulty for the acoustic instruments because they
absorb sound and can be mistaken for sediment particles. Wessels, et al.
(1990) included several references for additional background information on
these instrument developments.
8.2.8
Movable-Bed Bathymetry Measurement
In movable-bed models, it is important to document the bed evolution
with periodic measurements of model bathymetry. One method is to use
manually operated pointers mounted on a traversing cart. This manual
procedure is very laborious and time consuming, and efforts should be made
to automate the procedure if at all possible.
Three-dimensional models in basins require a large volume of water, and
it is not practical to drain the facility every time intermediate bathymetry
measurements are needed. Furthermore, the draining process itself can
cause strong water currents that may alter the bed features. Intermediate
bed surveys can be made from a supporting grid suspended over the model,
and the soundings can be referenced to the grid coordinates.
Three-dimensional bed elevation contours can be documented by slowly
draining the basin water to known depths and marking the position of the
waterline on the exposed portions of the bed using string. Photographing
the contours from overhead provides a record of the bathymetric change;
and if necessary, the string locations can be determined using conventional
surveying techniques. Sharp (1981) noted that movable-bed bathymetry
can also be recorded and quantified using stereo-photography methods.
The task of profiling movable beds in wave flumes and basins has been
made easier by the development of a variety of automated profiling instruments. For example, an “infrared profiler” consists of a servo-driven staff
that moves vertically to maintain the bottom end of the staff a short distance above the sand bed. As the probe is traversed horizontally, changes
in bed elevation are sensed by changes in the angle of an infrared beam
that is reflected off the bottom. The servo-system responds by moving the
probe in the right direction to follow the bed. The vertical position of the
shaft is monitored by a potentiometer, which can be sampled by the com
Puter controlling the instrument. Horizontal coordinates are provided can
be provided by a “string potentiometer” or a number of other methods.
493
sediment concentration determined using the pump sampler technique is
an average over the sampling time.
More modern and sophisticated electronic instruments for measuring
suspended sediment in the laboratory were discussed by Wessels et al
(1990). These instruments utilize acoustic and optical techniques to estimate sediment concentration over a small volume and at a high sample
rate. Air bubbles cause difficulty for the acoustic instruments because they
absorb sound and can be mistaken for sediment particles. Wessels, et al.
(1990) included several references for additional background information on
these instrument developments.
8.2.8
Movable-Bed Bathymetry Measurement
In movable-bed models, it is important to document the bed evolution
with periodic measurements of model bathymetry. One method is to use
manually operated pointers mounted on a traversing cart. This manual
procedure is very laborious and time consuming, and efforts should be made
to automate the procedure if at all possible.
Three-dimensional models in basins require a large volume of water, and
it is not practical to drain the facility every time intermediate bathymetry
measurements are needed. Furthermore, the draining process itself can
cause strong water currents that may alter the bed features. Intermediate
bed surveys can be made from a supporting grid suspended over the model,
and the soundings can be referenced to the grid coordinates.
Three-dimensional bed elevation contours can be documented by slowly
draining the basin water to known depths and marking the position of the
waterline on the exposed portions of the bed using string. Photographing
the contours from overhead provides a record of the bathymetric change;
and if necessary, the string locations can be determined using conventional
surveying techniques. Sharp (1981) noted that movable-bed bathymetry
can also be recorded and quantified using stereo-photography methods.
The task of profiling movable beds in wave flumes and basins has been
made easier by the development of a variety of automated profiling instruments. For example, an “infrared profiler” consists of a servo-driven staff
that moves vertically to maintain the bottom end of the staff a short distance above the sand bed. As the probe is traversed horizontally, changes
in bed elevation are sensed by changes in the angle of an infrared beam
that is reflected off the bottom. The servo-system responds by moving the
probe in the right direction to follow the bed. The vertical position of the
shaft is monitored by a potentiometer, which can be sampled by the com
Puter controlling the instrument. Horizontal coordinates are provided can
be provided by a “string potentiometer” or a number of other methods.