RheoTune Nautical bottom mapping

RheoTune Nautical bottom mapping

Introduction

Setting

The setting is a natural harbour and river estuary. The river and estuary are too shallow for large container ships to pass during low tide. The waterway and the berth pockets along the river are being dredged regularly. One of the techniques used in this area is Water Injection Dredging. During this process water is injected under low pressure in the muddy bottom. This mix of water and sediment forms a turbidity current flowing down the slope or is transported by the tide.

Problem

Deeper parts of the river and berth pockets had been filled with this suspension of mud. Even a month after the water injection, mud clouds were present, that had not been removed by the tide or current or were subject to sedimentation. With standard surveying techniques the top of  the suspension is observed instead of the actual bottom of the river. This observed level appeared above the contractual depths set by the Port Authority. Question arose whether the actual nautical depth is deeper than the observed depth or the contractual depth can be reached in the future due to possible settlement and or consolidation.

To investigate this layer of suspended material (silt, mud) several density measurements were executed to observe the magnitude and parameters density and yield strength.

Data acquisition

47 measurements were deployed along the access channel from the port towards the sea over approximately 7 km during two sessions on two days. Thirteen were located in several berth pockets, 32 in the river and a further 2 in a shallow, none dredged part of the bay.

The contact ‘water–suspension’ was also observed by the echo sounding and during a RheoTune measurement, this depth was checked. If the top of fluid mud observed by the Rheotune deviates (more than ± 10 cm) from depth recorded by the echo sounder, the pressure sensor of the RheoTune, responsible for the depth-data of the probe, was cleaned and the measurement redone at the same location.

Results

Two graphs for every location were made: A density graph and a yield strength graph (both vs. depth). Besides those graphs a table is made with the locations in coordinates, the different levels were a criterion is reached.

The RheoTune profiles show clearly a low density layer above the solid bottom. In the berth pockets (graph 1) the density and yield strength gradually increase. In the channel (graph 2) the yield strength and density show both an abrupt transition. Density at the top, yieldstrength at the bottom.

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