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Page Title: EXPERIENCE WITH TDS
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ERDC TN-DOER-I2
July 2000
vessel form (displacement curves). Draft measurements are usually taken with at least two pressure
sensors as shown in Figure 2, one mounted forward and one mounted aft on the underside of the
vessel (Rokosch 1989). These sensors measure the pressures (proportional to depth) experienced at
the underside hull locations.
EXPERIENCE WITH TDS
European Experience. The European TDS system (TDSS) was developed by the Dutch
Ministry of Transport, Public Works and Water Management (Rijkswaterstaat) for maintenance
dredging in navigation channels of the Port of Rotterdam. Its development was initiated to replace
the bin measurement method called the half-sphere and centrifuge that had been used previously as
a payment basis. This method involved lowering a half-sphere (designed to float in a material density
of 1,200 kg/m3) into the hopper at predetermined locations to determine the volume of material with
a density over 1,200 kg/m3. The additional hopper material above this measured interface was
sampled halfway in the remaining dredged material column, and the percentage of solids volume
was determined in a centrifuge. Although improved technology has allowed hopper dredges to
dredge material with densities over 1,200 kg/m3, this method of payment measurement did not
provide the contractor with an incentive to dig anything over 1,200 kg/m3 (van der Gouwe and Blok
1993). The TDSS determines, presents, and records the amount of TDS onboard the hopper dredge
on a continuous basis and per dredging cycle. The data collection system comprises three elements:
the dredge cycle data (including delays), the hopper load per dredge cycle, and positioning data in
real time (van Oostrum and van Rijn 1989).
TDSS field trials were conducted from 1985 to 1987, and the first dredging contract using it as a
payment basis was executed during the winter season 1988-1989 (Ottevanger and van Rijn 1992).
The experiences with this prototype system proved that the TDDS was a reliable and accurate
method for determining the amounts of dry solids dredged and transported (Rijkswaterstaat 1990).
Since that first contract, the system has been used in Rotterdam as a payment basis and has been
continuously modified to improve measurement accuracy and efficiency.1 Measurement of the
weight of solids in the hopper is one of the most effective methods for evaluating the amount of
dredging work being done. TDSS has undergone lengthy testing aboard various types of trailing
suction hopper dredges, and various studies (Rokosch 1989; Rullens 1993; Rullens, dAngremond,
and Ottewwanger 1994; and van der Gouwe and Blok 1993) have found the following:
TDSS requires no manual actions.
TDSS is more precise than the half-ball and centrifuge method.
TDSS is objective since it eliminated errors due to human fatigue caused by routine actions.
TDSS becomes an incentive for the contractor to improve performance.
The TDSS used in 1993 measures production to an accuracy of approximately 8 to 10 percent.
1
Personal Communication, 22 February 1999, Henk Keyser, Civil Engineer, Ministry of Transport, Public
Works and Water Management (Rijkswaterstaat) North Sea Directorate, The Netherlands.
4

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