V. Müller, H. Eden and
D. Vorrath
Eden, Vorrath & Partner, Germany
EDEN,
VORRATH & PARTNER
Schauenburger
Strasse 116
24118
Kiel
Germany
+49
431 560 6666
+49
431 560 6660
E-mail:
info@e-v-und-partner.de
www.e-v-und-partner.de
Abstract:
Estimation of near – bottom sediment dynamics has become of growing importance
for an effective dredging management and/or design of harbour and coastal
structures. But remote sensing of the near – bottom flow and sediment
phenomena is a difficult and complex measuring task. Often remote sensing
attempts were made to determine some parameters of near bottom sediment dynamics
by a combination of fan
out sounding systems with measurements of the flow by ADCP - technology. With
direct measurement technologies it is difficult or cost-intensive to reach a
synoptic result of the near – bottom fluid – sediment interactions
especially at tidal dominated regions.
By combination
of an innovative echo sounding technology, the DSLP® – method ‘Detection
of Sediment Layers and Properties’ with ADCP –
measurements and LOG_aFlow® , a hydrodynamic data interpolation
technology, near – bottom sediment dynamics will be estimated more accurately.
Determination of both, flow determined erosion/sedimentation – probabilities
and thickness of the suspension layers, is a necessary requirement to calculate
sedimentation as well erosion areas in means. These combined measuring and data
evaluation technologies allow to give a forecast on the trend of bathymetry and
should be an effective tool for dredging management.
Keywords:
DSLP®-method, ADCP, LOG_aFlow®, dredging, near-bottom
sediment dynamics
Strongly rising costs of dredging operations for waterway maintenance as well as port, coastal and offshore development lead to efforts for more effective sediment and dredging management tools. One way to minimise the dredging costs is, determining the sediment sources and flow causes for establishing areas of dominated erosion or sedimentation and to design an “engineering answer” following. Here, knowledge of the near – bottom sediment dynamic is important but mostly, it remains unknown. From ADCP - measurements for the flow velocity field it is possible to get information about the bottom stress distribution and together with some sediment parameter (i.e. grain size distribution, critical shear stress) an erosion or sedimentation probability can be calculated. But for determining a real sedimentation probability it is also necessary to get information about the amount of suspended sediment particles in the fluid and especially near the bottom. Attempts to use the strength of the back-scatter signal from ADCP-systems as an indication for the concentration of particles potentially available for sedimentation were mostly not successful. This is due to the high acoustic frequencies used by the ADCP-systems, necessary to exploit the Doppler effect. The back-scatter signal reaches a high level at low and medium concentrations of small particles in the fluid volume. But these particles are mostly not potentially available for sedimentation. So the combination of ADCP-based particle concentration with the sedimentation probability estimated from the flow field gives mostly not a accurate information about the real sedimentation areas. So knowledge about the near – bottom transient concentration of particles potentially available for sedimentation is necessary.
Commonly used echo sounding systems have
detection problems, because they are based on fixed frequencies and an
evaluation of received sound intensity. By reaching an adjusted intensity level
a depth horizon is detected. The existence of frequency–dependent and
frequency–independent interaction processes between sound waves and material
results in a splitting up of the depth horizons of the different used acoustic
frequencies. Unfortunately the detected depth horizons of the different
frequencies are generally not corresponding to the
real interfaces in the stratification and also a distinction between fluid and
solid state is not possible. All remote sensing technologies for bathymetric
surveys (single-
and multi-frequency echo sounder systems, fan out sounding systems, sediment
echo sounder systems) can’t distinct between suspensions (fluid state) and
consolidated sediment layers (solid state).
It should be annotated here, that bedload transport also can be seen as a near bottom flow of suspended sediment. The only way to get more accurate results for the near – bottom sediment dynamics seems to use direct measurement technologies at the moment. But with these direct measurement technologies the main advantage of remote sensing technologies is lost – the temporal and spatial covering of the whole area. High effort is necessary to reach such striven for synoptic result of the near – bottom fluid – sediment interactions especially at tidal dominated regions by direct measurement methods.
A remote sensing
technology for determining near – bottom sediment dynamics, consisting of (1)
the DSLP® (Detection of Sediment-Layers and Properties)– method, a
new and innovative echo sounding technology,
(2) conventionally ADCP – measurements, (3) LOG_aFlow®, a
ADCP hydrodynamic data interpolation in space and time, has been established.
This information system for sediment dynamics closes the above described lack of
former remote sensing technologies for near – bottom sediment dynamics.
Our new DSLP®-method
(Detection of Sediment Layers and Properties) is an
innovative acoustic method that delivers correct
results of interfaces in a complex stratification of suspensions and sediments
(Eden, Müller, Vorrath, 1998, 1999, 2000, Seefeldt et. al., 1999).
The DSLP® – method is independent of utilised acoustic
frequencies and provides an unambiguous, definite physical high-resolution
analysis of the acoustical interaction of sound wave and targets (i.e. suspended
matter and sediment types).
With the DSLP®–method
it is possible to detect suspensions in a wide range of concentrations (e.g.
fluid mud, suspended sediments, sand suspensions etc.) and in wide range of
thickness (3 cm up to several meters). The physically proven detection of the
fluid-solid interface between the suspension and consolidated sediments has an
accuracy of 3 cm.
Physical state
and material stratification can be proved by the kind of interaction between
sound wave and the material itself. A suspension (fluid), a mixture of particles
with water, is always a continuum of scattering particles. So scattering is the
only physical interaction process in fluid state. Besides, an acoustically
detected stratification in a suspension belongs on a natural concentration
profile (orange depth range on the left side of Fig. 1). The interface between
fluid and solid state can always be clearly detected by changing an acoustic
property independent from the used frequencies – the debut
of reflection at the whole frequency range with the same significance
(interface orange – grey on the left side of Fig. 1). This is the acoustic
definition of the fluid–solid interface. The solid state is detected
acoustically as a composite material with properties of scattering and
reflection. With the DSLP®-method, a combination of physically
proven detection of the depth dependence of sound–material interaction
processes with an extreme data density, it is possible to reach a depth
resolution of 3 centimetres.
So, contrary to
commonly used echo sounding systems, the DSLP®-method allows the
estimation of near – bottom sediment suspensions, their thickness and
concentration related parameters.
Investigations
on near – bottom sediment dynamics are especially important in tidal flats,
coastal areas, navigation channels and ports. These areas are mostly tidal
dominated, that means an additional temporal fluctuation of flow and sediment
dynamics exist. High effort of measurements has to be done to get a synoptic
view of the temporal and spatial variations of the flow in the investigation
area. Using ADCP’s for flow measurements it is common to use the
“transect” – method. The ADCP is deployed from a moving boat. The boat
moves along the transects and the flow velocity vector will be measured depth
– resolved. So the flow will be measured with a high data density on this
transects but between the transects remain more or less great areas where the
flow will not be measured. On the other hand, there is also a problem to get a
single flow measurement on the whole investigation area at a fixed time (see
Fig. 2). Evaluation of the measured data will be complicated, especially if the
time dependence of the flow is not strong correlated to the tidal curve. This,
for an example, holds for vortices, which are often present in harbour basins
and near coastal and port structures. A synoptic evaluation of the flow at the
whole investigation area at distinct times can only made by a spatial and
temporal interpolation of the temporal and spatial unbalanced measuring data.
This necessary interpolation has to be done on the basis of hydrodynamic rules. From ADCP measurements the 3 – D – flow – vector on the transects and corresponding time are known. Using continuity, momentum and energy equations, neglecting small scale viscous effects and confining on layer-integrated velocities (2 – D – approximation) it is possible to avoid the explicit appearance of the pressure by introducing the vorticity and stream function as dependent variables. Transport equation for the vorticity
and the Poisson equation for the stream function
has to be solved to evaluate the 2 – D – approximation of the flow in the whole investigation area. Stream function and vorticity can be calculated on the transects as a boundary condition for the numerical scheme. Due to repeated measurements on the same transects also the time dependence on these transects can be evaluated. So the two – dimensional approximation of the flow at the whole investigation area at any focussing time can be calculated (see Fig. 3).
This method is implemented in a software LOG_aFlow®. Using this hydrodynamic interpolation the two dimensional flow at every location at any time at the investigation area can be calculated from the temporal and spatial unbalanced measuring data.
The flow field,
calculated with LOG_aFlow®, can be superimposed with additional
physical scalar parameters like density, concentration of ingredients or other.
Also flow – related parameters like approximated values for bottom stress can
be calculated. By taking these advantages of hydrodynamic interpolation the
foundation of an information system for sediment dynamics has established.
Near – bottom
sediment dynamics will be calculated from (a) fluid velocities at near bottom
fluid layers and (b) thickness of suspension layers and their concentration –
related acoustic parameters detected by the DSLP® – method, both
combined with LOG_aFlow® to get temporal and spatial insight on the
phenomena of near bottom sediment dynamics.
The information
system for sediment dynamics was evaluated by an evaluation of ADCP- and DSLP®-
measurements in a harbour basin at Hamburg Port (Germany). The ADCP –
measurements were made on 5 transects over one tidal cycle. From these
measurements the temporal evolving depth – integrated flow at the
investigation area (harbour basin in the foreground of Fig. 3) was calculated by
LOG_aFlow®. Using a rough assumption on the relation flow – bottom
stress a normalised spatial distribution of erosion and sedimentation
probability at any time can be calculated. Summing up this probability it is
obvious to mark areas with high resulting erosion-/sedimentation- probability
over the whole tidal cycle (see Fig. 4). The common way to get a correlation
between flow and sedimentation probability is to use the back scatter strength
of the ADCP – signals and relate them to particle concentrations by probes.
This was also done here and a resulting particle concentration over the whole
tidal cycle was estimated (see Fig. 5a). On the other hand the thickness of the
high concentrated near – bottom suspension layer (see Fig. 5b) was detected by
the DSLP® – method. Both (thickness of the suspension layer and
resulting particle concentration) were considered as ‘indication for the
concentration of particles potentially available for sedimentation‘. So
correlation of this particle concentration with the calculated resulting flow
– induced erosion-/sedimentation probability lead to a real sedimentation
probability for both cases (see Fig. 6a, b). Physical relevance of these results
can be qualified by a comparison with the thickness of the weak consolidated
sediments, means consolidated at the last times (Fig. 7). These comparison
shows, that there is only a weak correlation between the real sedimentation
probability by particle concentration from back – scatter signal (comparison
Fig. 6a and 7) and a strong correlation between the real sedimentation
probability and the thickness of the high – concentrated near – bottom
suspension layer detected with the DSLP® – method (comparison Fig.
6b and 7).
A remote sensing
technology for evaluation of near – bottom sediment dynamics combining DSLP®
-method, ADCP measurements and LOG_aFlow® -a hydrodynamic data
interpolation was established. Both measuring technologies were deployed often
successfully. With ADCP – measurements and LOG_aFlow® fluid
phenomena in tidal dominated areas were investigated. On the other side the DSLP®
– method was successfully deployed at bathymetric surveys in areas with
complex stratifications of suspensions and sediment layers. The DSLP®
– method detects the fluid – solid interface physically proven. Proof of the
combination of DSLP® - and LOG_aFlow® - methods for the
investigation of sedimentation processes were also successful. So this
‘information system for sediment dynamics’ should be an effective tool for
dredging management.
References
Eden, H., Vorrath, D., Müller, V.: “Neuartige Echolottechnologie in der Gewässervermessung zur hochauflösenden Detektion von Sedimentprofilen und deren Eigenschaften”, in: W. Frohwein, H. Schlemmer (Eds.): INTERGEO 1998, 82. Deutscher Geodätentag in Wiesbaden “Geodäsie vernetzt Europa”, DVW-Schriftenreihe Band 33, pp. 33-43.
Eden, H., Müller, V., Vorrath, D.: “DSLP®-Method – A New Surveying Technologie”, in: W. Augath (Ed.): “Gewässervermessung und Hydrographische Informationssysteme”, DVW-Schriftenreihe Band 37.
Seefeldt, D., Eden, H., Müller, V., Vorrath, D.: Measurements With The New Acoustic DSLP® – Method in Hamburg Port, Fifth Workshop on Dredging and Surveying, 22-23. April 1999, Rotterdam.
Eden, H., Müller, V., Vorrath, D.: “DSLP® – an innovative echo sounding technology”, Hansa, (2000)10, pp. 36-39.
Fig. 1 Detection problems of
conventional echo sounder – technologies exemplary represented in a comparison
of the local readings of
the
DSLP® – method (left) and a conventional dual – frequency echo
– sounder – system (right) at a fixed position in the port of
Emden
(Germany); acoustical classification with DSLP®: S1 – S4 = high
concentrated suspensions, S5 = weak consolidated
sediment, S6 – S7: consolidated sediment
Fig. 2 Example for a
‘moving boat’ ADCP – measurement on two transects (yellow and green) at a
harbour basin over a half tide cycle
(measuring times are the yellow and green sections on the tidal curve) showing
the reason for temporal and spatial unbalanced
measuring data
Fig. 3 Streaklines of the 2-D
flow field for a fixed time (yellow circle on the tidal curve), calculated from
temporal and spatial
unbalanced ADCP – data on 5 transects at the harbour basin (see Fig. 2) by
LOG_aFLOW®
Fig. 4 Spatial distribution
of temporal mean of sedimentation probability (colours related to probabilities
by the colour bar) calculated
from
the spatial flow velocity distributions over a tidal cycle (result of LOG_aFLOW®)
Fig. 5a Spatial distribution of temporal mean of particle concentration (colours related to concentrations [mg/l] by the colour bar) calculated from calibrated ADCP – back – scatter strength over a tidal cycle (result of LOG_aFLOW®)
Fig. 5b Spatial distribution of thickness of near – bottom high – concentrated suspension (colours related to thickness [m] by the colour bar); detected with the DSLP® – method
Fig. 6a Spatial distribution of temporal mean of sedimentation probability from particles potentially available for sedimentation (see Fig. 5a) (colours related to probabilities by the colour bar) (result of LOG_aFLOW®)
Fig. 6b Spatial distribution of temporal mean of sedimentation probability from particles potentially available for sedimentation (see Fig. 5b) (colours related to probabilities by the colour bar) (result of LOG_aFLOW®)
Fig. 7 Spatial distribution
of thickness of weak consolidated sediments (colours related to thickness [m] by
the colour bar); detected
with
the DSLP® – method