Articles & Case Studies

Groundwater Dataloggers Prevent Flooding of St Mark's Square in Venice

Posted: Wednesday 28th October 2009

The company S.P.G. s.r.l. has begun to install severalSTSDLN 70 groundwater dataloggers (Figure 2) from the Swiss pressure sensor manufacturer Sensor Technik Sirnach AG on its site on St Mark's Square in Venice (Figure 1) in 2003. These loggers are specially designed for the specific requirements and, above all, they have the characteristic to withstand several days of flooding, sinceSt Mark's Squareis periodically flooded during the high tide. The opening of the site, introduced by the Water Supervisory Authority, was one of the key measures for protecting the lagoon and the city ofVeniceagainst inundation.

Forced lowering of the groundwater level

The engaged consortium Venezia Nuova planned to rebuild the wharf opposite St Mark's Square with innovative technical features. The requirement was to monitor the course of the groundwater, which moved gradually from the area around the site to the buildings behind it.

The monitoring was an appropriate means to show that when moving from the zone, subjected to the forced lowering of the groundwater level (using a cofferdam, within which the necessary pumping-out took place, in order to be able to work below the water level), in the direction of the buildings, this forced lowering does not turn out too high, so that it cannot cause any damage to the buildings. Therefore, level dataloggers from STS were installed on the request of the customer, for the continuous measurement of the variations of the groundwater level inside the piezometers. The loggers were arranged one behind the other, so that they were forming three parallel lines, when moving step by step towards the Doge's Palace and other historic buildings.

Comparison with the tide recorder

The groundwater measurements, recorded by the dataloggers, were referred to the absolute value of the average sea level for the final analysis, in order to be able to compare them to the data from the tide recorder of Punta Salute. The following records of the course of the water level on one of the profiles and the tide, which are combined in a single record for each measuring period, were very helpful for observing more closely the overall course. This also applies to the evaluation and realisation of analyses and interpretations of the course of the groundwater level in comparison with the tidal values and the work carried-out on the site.

Application examples for the pressure transmitters

The datalogger was installed on the third line (Figure 3). Since this line is farthest away from the pumping station, the tidal influence on the course of the groundwater level is quite low.

Figure 4 shows the final phase of the site with the visible external pile planking in the foreground, one end of the cofferdam, on the left, and the buildings, on the right. The record PZ11 (Figure 5) refers to the final phase without pumping-out. The measurements at the piezometer 11 were taken in the district of San Marco.

Interpretation of the recorded groundwater measurements

In general, the measurements recorded during the monitoring period show a clear, linear, unnatural, constant course, which was influenced by the pile planking on the canal-side. During the forced operation, this planking led to a lowering of the natural piezometric water level which, normally, is only regulated by the tidal variations. Only locally, at the points marked on the following combined records, one can recognise periods, where the interruption of the pumping-out has caused a natural revival of the variations of the groundwater level (marked with a red circle), and in some cases (marked with a yellow circle) the period was too short to enable a natural revival.

Operation with and without forced lowering

In some cases, it is clearly visible how, in the absence of forced lowering operation, the course of the groundwater level in the piezometers follows almost exactly the course of the tide, since they work more or less as tide recorders. The only difference is a lag of several hours and variations, which appear the less clearly, the farther one moves away from the shore and closer to St Mark's Square. The effect of the distance from the shore was also measured during the forced pumping-out, namely via hypothetical typical sections, where the following measurements were taken by sampling one after the other at different times of the day:

Distance from the shore Height above average sea level

7.8 m PZ1 0.047 m

4.2 m PZ2 -0.725 m

1.2 m PZ3 -1.047 m

-0.1 m Tide 0.590 m

0 Height of shore 0.930 m

Situations during the forced operation, where the tide in the canal can freely vary, while the curve remains constant in the shore area, with a lower level in the first row, where the effects of the pumping-out are greater, and the trend towards a gradual increase, the further one moves towards the buildings. Figure 6 shows the groundwater level for the above-mentioned day and time of the day. In this case, the conditions are "forced" because of the pumping on the site.

Groundwater datalogger measures three parameters

The STS DLN 70 groundwater datalogger enables the simultaneous measurement of level, temperature and electrical conductivity in the range from 0-50 cm H2O to 0-250 m H2O, -5 to +50C and 0.020 to 20 mS/cm. If necessary, the user can retrofit a remote data transmission unit at any time. The logger (Figure 2) is characterised by a simple, user-friendly operation, a large memory for 500,000 measured values and a probe diameter of only 24mm or 10mm.

The connector versions offer the possibility of cable extension and more flexibility. New software functions can be updated by the user without a troublesome return. The standard lithium battery with a very long life can be easily replaced on the site. Data can be transmitted in ASCII or XML format in standard software like Excel and further processed there. Variable storage intervals depending on pressure or time allow a variety of measurements. The use of various materials such as stainless steel, titanium, polyurethane, polyethylene or Teflon cables ensures a high media compatibility for a wide range of applications such as dumping grounds, hazardous waste, pumping tests, high-water messages, and measurements at rain overfall basins. Ask for detailed datasheets with interesting application notes for the STS DLN 70 groundwater dataloggers by ticking the reader service number.

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