10 HydroGrAPHIc SurvEy
Fluid mud density measurement
saves harbour dredging costs
In just one outing, a
mud density probe
has saved Associated
British Ports (ABP) over
£70 000 in dredging
costs.
Mit nur einem
Außeneinsatz konnte
der britische
Hafenverband ABP
(Associated British
Ports) durch eine
Schlammdichtesonde
mehr als 70.000 GBP
Aushubkosten
einsparen.
En una sola salida, una
sonda de medición de
la densidad de lodos
ha ahorrado a las
autoridades portuarias,
Associated British Ports
(ABP), más de 70.000£
en costes de dragado.
I
t is easy to determine nautical depth
Fig. 1. A typical bathymetric survey produced
where the bottom of the water area is
by running the MudBug simultaneously with a
bedrock. In most cases, however, the
standard 33 kHz echo sounder.
‘bottom’ is composed of soft material
such as silt or fine sediments which gradually greatest depth, usually at a mud density of over
increase in density with depth. 1300kg/m
3
, while an echo sounder with a 210 kHz
Tidal currents or storms continually transducer will indicate the least depth, where the
erode cohesive sediments and carry them density is around 1060km/m
3
. This appears to be
into navigation channels. As the wave energy the case regardless of the consolidation of the silt.
dissipates, initially mobile suspensions stagnate It is difficult to obtain a useful profile of fluid
to form denser static suspensions, often called mud density by acoustic methods. Echo sounders
‘fluid mud’, or sometimes ‘slib’ or ‘sling mud’. respond to both the density and the acoustic
Such static suspensions can appear very velocity gradients of the medium. Any abrupt
suddenly after a storm. They may have two or change in density, eg at the interface between
more layers and may reach up to three metres in ‘dirty water’ and freshly settled fluid mud, will
depth, which can take them significantly higher give a signal, but progressive density changes
than the channel datum. do not register.As a result, an echo sounder will
Experience in the Netherlands and often return at least two signals: a ‘hard’ echo
elsewhere indicates that although manoeuvring from the denser, well-consolidated mud of the
characteristics may change somewhat, ships can seabed, and a ‘soft’ echo from the interface
still navigate safely through fluid mud containing between the seawater and the top of the fluid
up to about 15 per cent by volume of dry mud. With a 3.0m static suspension, the first
sediment, corresponding to a density of around echo would come from a depth of 19.5m below
1200kg/m
3
. The challenge is to detect the depth at chart datum in a channel maintained at -22.5m
which the fluid mud reaches this critical density. through which tankers drawing 20.7m regularly
A standard leadline will always indicate the pass. Thus the true navigable depth of a channel
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