Measuring sediment transport by means of stand-alone tripod instrumentation.

By: Ing. D.B. van Dam

Introduction.

On this page I'd like to give a short insight in the internal workings of the tripod instrumentation constructed at our department for Physical Geography. These tripods are used to take in-situ measurements of sediment transports on near-shore coastal waters and rivers.

First some development history and acknowledgements:
By now we have some six years of experience of building these kind of tripods; the first tripod we used was constructed for us at the NIOZ institute at the island of Texel. From this fruitful cooperation we learned a lot about how to go about putting electronic equipment on waterdepths up to 25 meters (80 ft).
All the research is initiated and funded and supported by Ministry of Transport and Public Works; dep. RWS-RIKZ and RWS-RIZA. (And lets not forget all the great support from the technical people over at RWS-Den Oever!)

Over the past years we've added several new features to the basic tripod design. Additional instruments, we chose a alternate power supply and added increased datastorage. The latter for obvious reasons; researchers never have enough data!

I'd like to continue with first a general tripod description and then a description of the tripod instrumentation. Although we have a variety of instrument configurations, the one described here is most commonly used.

The use of autonomous instrumentation in coastal research.

To gain insight in processes that influence the formation of coastlines it is necessary to conduct in-situ measurements in a given study area.
Typical physical parameters of interest are watervelocity, sedimentconcentrations and waveheight. The data gathered in the in-situ measurements help the scientist in developing new models or tune existing ones.
One methode of measuring the physical parameters is by the use of autonomous tripods. The use of autonomous tripods has advantages over other methodes like ships, boys and poles.

• Measurements with a total duration of more than a month, covering spring-neap tide cycle.
  The use of ships is too costly for such prolonged periods.
• Flexible in positioning. Opposite to the use of poles, it's simple to reallocate the tripods to another position.
• Tripods can be placed in shallow water, where ships wouldn't be able to come at low tide.
• Able to measure under extreme weatherconditions.

• No direct feedback on the proper operation of the tripods.
• The need for large amounts of batteries for powering the electronics.
• Vulnarable to (fishing-)ships.

Technical description of a typical UU tripod.

The instruments are mounted via brackets on the central pole. This enables the free positioning of the instruments relative to the bed. The instruments are wired to an electronics container on top of the tripod.
A second container is filled with 256 standard D type alkaline batteries and will last for about one month.

Basic tripod: 1.5 to 2.5 meters height. 3.5 meters between each leg. legs are solid steel rod 5 cm diameter two containers on top of tripod between protective crash rings. Instruments mounted on a stainless steel central pole. A fully equipped tripod weighs about 700 kg. Instruments: 2 electromagnetic flowmeters (also 'EMF',Delft Hydraulics). 2 optical sediment concentration meters (also 'OBS',D & A). 1 pressure sensor (measures waterlevel, Keller) a combined tilt and compass (for measuring frame orientation)

Example technical design drawing of a tripod with containers.

The electronics container.

The instrument container is the center core of the instrumentation. It is made out of PVC tube (20cm diameter) On either end a flange is mounted. Both are closed by a lid by means of three bolts each. Between the lids and flanges are O-rings to seal the container. The containers can withstand waterdepths of up to 25 meters. One of the lids contains all the watertight connectors to the instruments, batterypack and one for connecting an external PC.

Inside the electronics container. (Hi-Res)

Components in the instrument container:

1. The central element in the instrumentcontainer is a CR10 datalogger (Campbell Scientific Ltd).
   This datalogger measures the signals from the instruments, controls their powersupply. The datalogger has very low powerconsumption and can easily be programmed by means of any external MS-DOS compatible PC using Campbell Scientific PC208 support software.
2. Single board PC Ampro; CoreModule/3SXi.
   A single board PC with harddisk is used to increase storage capacity.
3. A Campbell Scientific SM716 solid state storage module.
   Used for temporary storage of the data.
4. Small batterypack to supply the datalogger.
   This to ensure that the datalogger functions without the external batterycontainer present.
5. The circuitboards that work in conjunction with the probes on the instrument pole.
6. A central signal adapter circuitboard, which converts signal levels to suite the datalogger.
7. Two powersupply boards.
   One stabilizes the battery voltage to supply the instruments. Another supplies the single board PC.

All the electronic components are mounted inside a rigid cubical frame inside the container. This minimizes vibrations and the risk of components shaking loose; the tripods can be subjected to vigorous forces!
For service the various printed circuit boards (PCB) can be tilted from the frame, to conduct in-circuit testing. All PCB's are connected to the frame via a single connector. Tilted from the frame the PCB's can thus easily be removed for calibration, repair etc.

The typical operation of the UU tripod.

The datalogger program would operate as follows:
One minute to the hour the instruments are powered and -after the instruments have stabilized for a minute- a single registration of status parameters is stored, like date and time, frame attitude, battery voltages and temperature.
On the hour the measurements start. At a rate of 2 measurements per second current velocities, sedimentconcentrations and pressure (waterheigth) are stored in the solidstate memory module for a period of 34 minutes. This period is refered to as a burst measurement. The lenght of a single burst conforms to the need for a certain number of datapoints needed for analysis (like Fast Fourier Transformation etc.)
After the burstmeasurement the instruments are powered down. This is repeated every hour until the external battery is empty or the tripod is stopped by the user.
The solid memory module can hold upto eight hours of data. To enhance the storage capacity of the tripods a single board PC with a harddisk was added. Three times a day (after every eigth burst measurement) the datalogger fires up the PC. The PC copies the information of the solidstate memory module to harddisks and then clears the module. This process will take about 15-20 minutes.

After retrieval of the tripod, an external PC can be connected to the instrumenthousing and the data can be transferred from the tripod to the harddisk of the external PC via an Ethernet link. After a quick inspection and a change of batteries, it is possible to re-deploy the tripod.
Collecting data from the tripod.

Conclusion

Up to now we have had a very high rate of success. We have some eight similar tripods in use at present. With these tripods we had things working fine over 90 percent of the time. Failures nowadays mostly have external causes. (Mainly encounters with fishing-nets and damage done by extreme storms.)

In all the tripods have aided many researchers in the study of the near shore processes. Several Ph.D. theses and other publications have been produced based on tripod data.

HTML Created May 4th, 1998 by Bas van Dam Updated Aug 31st, 1998