WESP and CRIS on the beach near Egmond aan Zee.

While the WESP was primairly constructed for measuring beach and surfzone bottom profiles, it was necessary to construct a support cart to conduct sediment studies with the WESP. This support cart, called the CRIS (aka CRISP; there used to be a Platform mounted on top), is constructed to have as little hydrodynamic influence on the measurements. The large wheels of the WESP cause too much disruption to the natural watermovements and they stirr up a lot of sediment from the bottom. Both parameters are -of course- fundamental in the study of sediment movements! The CRIS will therefore be towed at a distance of about 30 meters behind the WESP. Well out of range of the disturbances.

A typical experiment with the CRIS would be run as follows:
The CRIS is connected to the WESP by means of two cables, which ensures the CRIS will follow the WESP better than when using a single one. An umbilical is connected to the WESP; we'll look at this later. The computers of the data aquisition system are started and the supply to the CRIS instruments is engaged. Then the WESP can be driven to a chosen location. The instrumentation is lowered to the desired heigth above the bed and the data aquisition is started. All data is stored and presented onm the WESP. After a given period (ranging from some 20 minutes to several hours) the instrumentation is raised again and WESP and CRIS can go to a new location. These kind of experiments will be conducted mostly in autumn and spring, when storms induce large waves and thus a lot of sediment movement.

Now i'd like to concentrate on the technical details of the CRIS instrumentation.

On the CRIS the main instruments are all mounted on a adjustable arm. Thus the position of the instruments can be set to a desired level above the seabed. (within the maximum 60 cm span of the arm.) A control box on the WESP enables the researcher to position of the CRIS' instrument arm . The desired level in centimeters above the seabed is simply keyed in and the control box adjusts the arm accordingly. The instrument arm was newly developed for the CRIS at our laboratory.
As mentioned before the CRIS, is connected through an umbilical to the WESP. The umbilical consists of two cables for the instrumentation's supply and datalink, a control cable for the adjustable arm, a hose for supplying an underwater sampling pump with air. A return hose which delivers pumping samples (water plus sediment) to the WESP.

On the CRIS there are the following instruments: Mounted on the adjustable arm: A five level Ultrasonic Sediment Transport Meter (USTM; Delft Hydraulics). Two electromagnetic waterflow sensors (EMF; Delft Hydraulics). Two optical backscatter sensors for sediment concentration (OBS; D&A). Additionally on the CRIS: A pressuresensor for measuring waterheigths (Keller). A combined tilt and compass unit for measuring attitude (FGlab). A pump for taking in-situ water-sediment samples (Alfa-Laval).

Installing the instrumentation on the CRIS.

Ready to be towed to sea...

The USTM.(Delft Hydraulics) The USTM is a sophisticated instrument, capable of measuring the sedimentconcentration and the velocity of the sediment in the X-Y plane. The velocity is derived from the doppler frequency shift of the reflected acoustic sound. Sediment concentration is derived from the amplitude of the reflected signal. One USTM sensor consists of a single acoustic source with a 4.5 Mhz operating frequency. This frequency is chosen to have optimum sensitivity to grainsizes of 50um to 500um (beachsand). Two receivers pick up the reflected signals. Five of these sensors are mounted in the vertical, to provide a profile of sedimentconcentration/velocity. The data from the USTM is directly sent to computers on the WESP at a 2Hz rate. One computer continuously stores all the incoming data to harddisk. A second laptop gives an on-line representation of the USTM signals. The EMF, OBS and pressuresensor. As the USTM is a new instrument, additional -well known- EMF and OBS's are placed close to the USTM sensors as a check for the USTM operation.

The EMF.(Delft Hydraulics)
An EMF measures the watervelocity in the X-Y plane. It's operating principle is based on the law, that any moving conductor in a magnetic field induces a voltage across the conductor. The active element in an EMF consists of an electric coil with electrodes surrounding it. When the coil is energized, a magnetic field is produced through the surrounding water. The electrodes receive the minute electrical voltage, which is proportional to the velocity. After signal amplification and noise filtering the signal can be handled by any data aquisition system. Furthermore by measuring the induced voltage at right angles the watervelocity can be measured simultaneously in the X-Y plane.

The OBS.
The OBS's operation is a simple reflection detector. It sends out a beam of infrared light; emitted by a IRED. The reflected light is picked up by a phototransistor. This signal is amplified. Within certain limits the measured reflection is proportional to the sedimentconcentration. The only drawback is, that the response of the instrument is detemined by the optical properties of the sediment and finer sediments give relatively a larger signal than coarser sediments. The instruments have to be calibrated with in-situ sedimentsamples.

The pressuresensor.(Keller)
The pressuresensor gives information about water- and waveheight and waveshapes. The watercolumn above the sensor induces a pressure on a diaphragm. This diaphragm is fitted with a straingauge bridge. The signal from this bridge is proportional to the applied pressure. For obtaining actual waveheigths it is necessary to do some additional calculations. The pressure variations induced by waves are dampened in the watercolumn.

Like the USTM all the other instrument data is stored at a 2Hz rate. The data of the EMF's, OBS and pressure sensor are stored in memorymodules on the CRIS. Every 15 minutes the computers on the WESP retrieve the data and store it to harddisk. A second laptop gives one minute averaged values of the instrumentsignals.
In the presentation to the user the USTM data is presented as short-term direct information, and the other instruments supply information over a longer period (tidal information).
The processing of the all the data collected during the experiments and the subsequent interpretation takes up several months!

View from the WESP

Umbilical with power/data and suction hose and compressed air to the WESP

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HTML Created June 18th, 1997 by Bas van Dam Updated Jan 29th, 2001