The Hidrovia Project

The Rivers Plate and Paraná in Argentina have much natural sedimentation. In 1993 the Argentine government issued a tender for dredging and signalization of these vital waterways. The concession runs from Punta Indio, at the outer border between the river estuary and the Atlantic Ocean, to the entrance of the port of Santa Fé, about 800km upstream. The authors describe the eleven years of survey work involved, including equipment employed.

In May 1995 a joint venture named Hidrovia S.A., between Belgian Jan De Nul N.V. and its Argentine counterpart, Emepa, started dredging and signalization works on the Hidrovia waterway. It implied an enormous amount of survey work. Work began in 1995 with two hopper-dredges and one cutter-dredge. At peak periods during dredging operations we were working with four hopper-dredges at a time. Each needed constant updating of its onboard digital terrain model. Constant progress surveys were therefore performed. Apart from these, contractual surveys had to be done.



Four Zones
The Hidrovia waterway consists of four zones: the River Plate zone, Paraná de las Palmas, Paraná Inferior and Paraná Medio.

As the first zone is in fact the River Plate estuary, it mainly consists of almost straight, dredged channels. The second zone is characterised by narrow bends with radii as small as 450m. The last two zones are situat-ed in the wider part of the Paraná River. The critical parts of these zones are not bends but shallower passes, mostly with dune formation. According to contractual condi-tions the contractor had to survey all critical parts periodically using a single-beam system with certain line spacing. These line-spacings were not enough to provide a sufficient volume of data for a reasonable DTM. Extra control surveys and progress surveys were needed for the dredging works, with less line spacing.



Positioning
In 1995 DGPS systems using differential signals via satellites were not far enough developed for use in survey and dredging operations. The most advanced systems then on the market and suitable for dredging works of this scope were the Sercel long- and medium-range stations with receivers. Three NDS200 long-range stations with two frequencies were installed along the river. One station was situated near the city of La Plata, about 50km south of Buenos Aires, and had to cover most of the River Plate area. Another station was installed in Zaraté, about 100km north of Buenos Aires, for coverage of the Palmas Area. The third station was installed in Rosario and had to cover the Inferior and Medio areas.

During cutter works in Canal Emilio Mitre, a 40km-long channel in front of Buenos Aires, an extra NDS200 medium-range station was installed in the vicinity of the city. All these stations had to be maintained and this was a time-consuming and costly business. During the frequent electrical storms no differential signal could be received. Another prob-
lem of these systems was the difficulty in reception of the differential signal by the NR103 receivers in some areas due to buildings or trees. The differential antennas had to be placed very high, and so extremely high masts were put on the survey vessels.

Chosen as backup system in 1997 were Omnistar satellite differential signals with Omnistar 3000 LR 8 receivers. This system solved the lack of differential signals during thunderstorms. In 1999 an external company was installing very long-range differential beacons for the agricultural industry. With these new beacons we were able to get rid of the expensive Sercel stations and change to Trimble receivers. The new Trimble AG132 receivers proved economical and reliable and provided us with stable submetric positions using the differential signals from these beacons. The receivers were also able to receive differential signals by satellite. The Omnistar receivers and satellite signals were replaced by a Racal differential signal by satellite as backup. Hidrovia will probably in the near future replace the present receivers with more modern ones; the present receivers are no longer available on the market and it will therefore become harder to obtain spare parts. The modern equipment will also provide us with an even more stable and more accurate position than we have now.



Bathymetry
From the beginning of the works, bathymetric surveys have been done using DESO 15s from Navitronic. The DESO 15s, and later some DESO 17s, have been and remain sufficient for the performance of standard single-beam surveys during the project. The equipment is reliable, not too complicated and it has served its purpose well for the last ten years. All the vessels have a 210KHz and a 33KHz sensor included in their set-up.

The problem with single-beam survey is that it is very time-consuming, especially when a survey has to be performed with smaller line spacing, as in the case of progress surveys for the dredges. Another disadvantage is that in between the run-lines no bathymetric data will be avail-able. For these reasons, in 2004 tests were done with a Simrad EM3000 multi-beam system. The object of these tests was to see whether to adopt multi-beam systems for the Hidrovia project. The outcome of the tests was quite satisfactory, the results of the multi-beam surveys showing a big improvement on the single-beam surveys. Due to the fan array of the narrow beams (1.5°), 100% coverage can be obtained. A single-beam system covers only a fraction of this. The DTM generated from multi-beam data will therefore be nearer to the reality than a DTM generated from single-beam data (see Figure 3). As a result, volume calculations on multi-beam data are more accurate. The accuracy obviously also depends on the grid size chosen for the DTM. These DTMs generated from multi-beam data are also an improvement for the dredges. The depths you see on the survey monitor are the same as the depths you encounter with the dragheads.



Wide-swath Trials
The latest chapter in the survey history of Hidrovia consists in the testing of an alternative bathymetric system. Since the Hidrovia comprises large areas that have to be covered regularly, attention was drawn to wide-swath systems. The Jan De Nul Group had already gained experience with a GeoAcoustics Geoswath system in deep water. In May 2006 the Group decided to test the Geo-swath 250KHz version for its reputed shallow-water wide-swath capabilities. Some of the objectives of the trials were to answer the following.

• Is the Geoswath system able to save time over single-beam or other multi-beam systems (taking into account both survey and processing time)?


• Is the system able to survey in all areas of the Hidrovia project (in-cluded are different soil conditions, from very soft soils to compacted sand)?


• Is the Geoswath system able to provide data of the same resolution and accuracy as the more common Reson 8101 or EM3002?


• Is the system able to provide accurate data near working dredges?


• Is there a significant gap at the nadir?

The system was provided by Nautikaris and flown into Buenos Aires at the beginning of May 2006. The 250KHz system was installed on a 30-degree v-plate, with its altimeter and mini SVS. To provide sufficient accuracy for the expected wide swaths, an iXSEA Octans generation III was installed. Installation and calibrations were done swiftly in Buenos Aires, the capital of Argentina.

The trials stretched from Buenos Aires to Diamante, almost 600km upstream. The Geoswath provided three to twelve times the water depth, depending on soil conditions and water depths. The system performed remarkably well when encountering water depths of less than ten metres. The ability to take sounding of quay walls and riverbanks, up to the surface, was above expectations.

On average, the system provided a width of six to 7.5 times the water depth. This resulted in significantly less 'boat time'. In large surveys this proved to be halved in comparison with a single-head beam former. A slight downside to the system was that every survey had to be post-processed. Post-processing of the data with the GS+ software took about 30% of sailing time. The Jan De Nul Group’s standard multi-beam set-up consists of a beam former interfaced to QPS's Qinsy. Generally, surveys performed with the standard set-up do require little to no post-processing, which proves it is a simple and straightforward set-up; 'what you see is what you get'. The Geoswath set-up, on the other hand, requires more interpretation.

All areas along the 600 kilometres, which were mainly sandy, could be surveyed with the Geoswath system. It should be noted that the swath width had to be reduced during a survey in a channel having finer sediments. Data produced by the Geoswath appears to have higher standard deviation compared to the more common beam-formers. But since the data density is much higher, this poses no problem when working with mean depths.

During the trials the Geoswath was used to provide a trailer hopper suction-dredge with progress surveys. This proved no problem. Data could be collected even as close as 300m behind the dredger. Any closer to the dredger the wash from its propeller caused too much interference.

The data at the nadir proved to be rather sparse and more affected by noise. This is due to the nature of the system. To filter out the spikes and noise in the nadir area, a 1-m limit filter was used for both transducers. This resulted in gap of 2m under the transducer, possible to fill in using the system altimeter. If the area is of high interest extra lines should be sailed to cover the entire area.
The trials show that the Geoswath can offer an interesting alternative to other multi-beam systems. The wide swaths can save time in performing surveys. On projects with shallow waters the system by far out-performs a beam-former. An improved user interface should be provided.



Concluding Remarks
Unlike most other dredging jobs, where the entire contract is executed using the same equipment, the Hidrovia project undergoes a continuous evolutionary process in terms of improving survey performance. The huge areas to be surveyed and the high frequency of surveys performed push the survey team on the Hidrovia project to ever further innovation and improvements. So, frankly, we can repeat our old saying: You haven't seen our limits yet!