SUSPENDED PARTICULATE MATTER MAPPING FROM MULTITEMPORAL SEAWIFS IMAGERY OVER THE SOUTHERN

NORTH SEA - SEBAB PROJECT"

B. NECHAD, D. VAN DEN EYNDE, M. FETTWEIS, F. FRANCKEN

Management Unit of North Sea Mathematical Model (MUMM) Gulledelle 100, 1200 Brussels, Belgium

E-mail: b.nechad@mumm. ac. be

Suspended particulate matter concentration (SPM) maps at water-surface were retrieved from 172 SeaWiFS images using the MUMM turbid water extension to the SEADAS4.4 software and a regionally calibrated hydro-optical model. Examination of surface-SPM maps and the depth-averaged SPM derived from a 2D-hydrodynamic and sediment transport model over the BCZ shows interesting similarities. The effect of the neap-spring tide cycle on the satellite-derived SPM distribution is compared to the significant impact of this factor on modeled SPM distributions. Classification of the Belgian waters is carried out using the time-series of SeaWiFS-derived SPM maps and is explained in the light of the SPM transport modelling knowledge. This is preliminary step in this project to provide boundary conditions and initial data of SPM distribution to the coupled model.

1. Introduction

The Sediment Balance of the Belgian Coastal Waters (SEBAB) project has the objectives to investigate the processes of suspended sediment transport along the Belgian-Dutch coastal zone and provide a support to the dredging and dumping activities. Such processes are analyzed and modeled using a 2D- hydrodynamic and sediment transport model and in situ measurement data and satellite imagery. In this study 172 SeaWiFS cloud-free images over the Belgian waters (see area of interest location in Figure 1) from 1997 to end 2003 feed the following image processing chain which yields SPM maps: a) SeaDAS atmospheric correction algorithm, extended to turbid waters [l], is applied to the top of atmosphere radiance measured at the sensor level, and produces the water-leaving reflectance, b) georeferencing and further image processing is carried out within ENVI software, c) a hydro-optical model designed for Belgian waters [2] converts water-leaving reflectance at band 765nm wavelength into suspended particulate matter concentration (SPM).

' This work is funded by the Maritime Access Division of the Waterways and Marine Affairs Administration, Ministry of the Flemish Community (AWZ).

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I"30'E 2"E 2'30'E 3"E 3'30'E

Figure 1: The Belgian coastal waters, area of interest is shown within the dashed rectangle.

2. The suspended sediment transport model

The The distribution of suspended matter is influenced by a range of interrelated processes such as meteorological and hydrodynamic conditions, turbulence, re- suspension and deposition, aggregation and flocculation, salinity, temperature, biological activity and -of course- the availability of fine-grained sediments or mud. The importance of each process is difficult to quantify, partly because of a lack of observations and partly because of its complex nature and the coupling existing between them. Therefore we have focus on the most important ones: the hydrodynamic conditions (tides, neap-spring cycle), the sedimentation-erosion processes of mud and the two major sources of this fine-grained sediment, i.e. the erosion of exposed Tertiary or Holocene clay layers and the import of SPM from the Strait of Dover.

In order to model these processes a coupled 2D hydrodynamic model, a wave model and a 2D sediment transport model has been used [2].A 2D horizontal model is appropriate for this area, because the water column is not stratified through the entire year. The high tidal amplitude (mean spring amplitude at Oostende is 4.6 m, mean neap amplitude is 3.0 m), the strong tidal currents (>1.2 m s-') and the low fresh water discharge of the Schelde result in generally well mixed water. The meteorological data for the hydrodynamic and wave models were provided by the meteorological forecasts from the United Kingdom Meteorological Office, Bracknell.

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Transport of SPM is determined by the settling of mud particles under the influence of gravity and by erosion and sedimentation due to the local current velocity. The sediment transport model solves the 2D depth-averaged advection- difhsion equation using the semi-Lagrangian Second Moment Method [3] for the advection of the material in suspension. The erosion and deposition are calculated using the formulations of [4] and [ S ] respectively. In the southern North Sea the sediment composition is not uniform: it varies from sand to mud (with different grain sizes, mineralogical compositions, organic contents etc.). The erosion formulae take the sediment composition of the bed into account. The model uses a value for the critical shear stress for erosion between 0.5 Pa (freshly deposited mud) and 0.8 Pa (after 48 hours). The shear strength of the parent bed was set to 2.0 Pa, which is a compromise between the erosion shear strength of Holocene mud layers found in the coastal zone near Zeebrugge, of the mainly sandy sediments outside the coastal area and of the results of calibration simulations based on seafloor erosion data. In sediments with lower mud content the erosion rate (0.12x10"kgm~2s") is multiplied by the mud fraction. In the model a constant fall velocity of 0.001 m s-' has been used. This relative high value implicitly accounts for flocculation processes.

The simulation results show that the processes responsible for the high turbidity zone formation are the currents and the import of SPM through the Strait of Dover. Mainly because of the decreasing magnitude of residual transport and the shallowness of the area, the SPM is concentrated in the Belgian-Dutch coastal waters and forms a turbidity maximum in front of Zeebrugge (Figure 2). Mud is continuously deposited and re-suspended. Significant variations occur during tidal cycles and during neap-spring cycles. The mud which is deposited during a neap tide can sufficiently consolidate to resist (partly) to the high bottom stresses occurring during a spring tide. During spring tide the deposits are partly re-suspended again. This also explains - besides the fact that during spring tide more mud is imported in the area through the W-boundary- the higher SPM concentration during spring than neap.

Figure 2: Tide-averaged SPM concentration (mg 1-1) (a) spring tide of 21/03/1999 and (b) neap tide of 26/03/1999. Also indicated is the 10 m depth contour.

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3. SeaWiFS derived SPM maps

The SPM average map in Figure 3 shows the lowest concentrations around 20mg/l along the Belgian coast and 20km offshore. In average, SPM does not exceed 80mg/l which is frequently observed in the south west of Zeebrugge. However very high concentrations (over 200mg/l) occur in the 20km area along the coast where the maxima range from 40 to 250mg/l.

Z"30'E Z"45'E 3"E 3"15'E 3'30'E SPM (mg/l)

I I I I r Z"30'E Z"45'E 3"E 3"15'E 3"30'E

0

Z"3O'E 2'45'E 3"E 3"15'E 3"30'E SPM (mg'l)

0

Figure 3: Top: the average of 172 SeaWiFS derived SPM maps from 1997 to 2003. Bottom: the highest SPM concentrations observed during the same period.

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In the following section we examine the spatial correlation of SPM time-series to identify high and low sediment re-suspension areas in the Belgian coastal area, with regard to their time of occurrence; the links between spring-neap tide cycle and SPM distribution is studied. The effect of the modelled wind speed and SPM is also analysed.

3.1. SPM spatial correlation

Spatial correlation for the neap (resp. spring) tide cycle (Figure 4) gives the most extended high correlation (>0.7) areas as described here:

1. area A situated in front of Zeebrugge extending to the mouth of the Schelde estuary. This area has most of the time a high SPM concentration and can be described as a turbidity maximum zone.

2. area B in front of Oostende, which has high SPM less often than area A (only 10/62 images show high SPM in this area);

3. and area C located offshore where SPM concentrations remain generally low (<lo mg/l).

l"30'E 2"E Z"30'E 3"E 3"30'E l"30'E 2"E Z"30'E 3"E 3"30'E

Figure 4: Classification of the BCZ into area A,B and C (see text for more details) during neap tide (left) and the spring tide (right).

3.2. The neap-spring tidal cycle effect

As the main hydrodynamic factor -beside SPM availability- acting in the sediment transport model is the tidal cycle and the neap-spring tidal cycle, the 172 maps of SeaWiFS images are divide into a 96 images in spring cycle and 76 images during the neap tide. The distributions of SPM during these 2 periods are related to SPM classification discussed above.

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Spring-averaged SPM and neap-averaged SPM are shown in Figure 5 : the global SPM extension from the Schelde mouth to the south and the higher concentrations located near the coast. The main noticeable difference is that in the spring tide cycle the high SPM concentration area is extended more than in the neap tide about lOkm to the south and towards offshore waters. SPM standard deviation also follows this distribution through spring and neap tide cycle (Figure 6) . Note also that the couple of spring-neap tide depth-averaged SPM (Figure 2) is in good agreement with the couple of spring-neap at-surface SPM figures presented below.

3.3. The wind effect

In situ measurements of SPM and wind speed have shown significant correlation between these parameters when the wind direction is from south west. From the 172 SPM maps we select 16 (resp. 43) days at which the wind sector at location ( 5 1 .294N72.97E), situated in the area A, is south-west (resp. not south-west). The hydrodynamic parameters (the wind speed, the current velocity, the total water depth, the bottom stress and the significant wave height) were modeled using the 2D-hydrodynamic model, for the selected days and at the selected location. As the modeled hydrodynamic parameters have small spatial variability, the selected location modeled parameters are considered representative of the total area of interest. Correlation between the hydrodynamic parameters at this location and SPM at the whole area was computed and mapped (Figure 7)., The higher correlation (>0.6) occurs mainly during the neap tide in the area B to the south, relating low wind speed values to the generally low SPM values <30mg/l. During the spring tide the high correlations are compacted in a narrow band along the Belgian coasts, and turn rapidly to negative values ( ~ 0 . 4 ) towards the seashore; these negative correlation values cover area A and C.

The current velocity, the total water depth and the bottom stress have similar correlations to SPM (see Figure 7), whereas the significant wave height is more correlated to high SPM during spring (in area B and south of B) than during neap tide. For the days where the wind is not from the SW, very low correlations were found, except that 0.5 correlation with the significant wave height, during neap tide period in area B.

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4. Discussion

1) Areas A and B are known to be a highly energetic hydrodynamic well-mixed zone, where the hydrodynamic conditions are such that SPM is concentrated (see section 2). It is remarkable that two zones can be statistically distinguished in the coastal zone the more that no closed 'physical' explanation can be found. We can however reach some arguments to explain some differences between both areas:

l "30'E 2"E 2"30'E 3"E 3"30'E l"30'E 2"E 2"30'E 3"E 3"30'E

L" z 9

z b 3 ?

m z

L" z

m z -

l"30'E 2"E 2"30'E 3"E 3"30'E l"30'E 2"E 2"30'E 3"E 3"30'E

Figure 5: Neap tide SPM average (left) and Spring tide SPM average (right).

l"30'E 2"E 2"30'E 3"E 3"30'E l"30'E 2"E 2'30'E 3"E 3"30'E

L" z 9

z b 3 - m z

L" z m z -

l"30'E 2"E 2"30'E 3"E 3"30'E l"30'E 2"E Z"30'E 3"E 3"30'E

Figure 6: Neap tide (resp. Spring tide) SPM standard deviation is shown on the left (resp. on the right).

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1. in area A 95% of the dredging and dumping occurs, which corresponds to about 10 million tons of dry matter (mainly mud) yearly. Nevertheless, model simulations have shown that dumping has only a local influence on the SPM concentration, and measurements and satellite imagery have indicated that an increase in SPM concentration in the surface layer could only be detected directly after the dumping operations [S].

2. The bottom near Zeebrugge (area A) consists of hard Holocene mud layers, which are difficult to erode. Soft mud is only found in the dredging areas, such as the navigation channels [2] . Close to the dumping site B&W Oostende, situated just to the northwest of Oostende, soft mud has been found on the bottom [ 8 ] . This could mean that in area A the mud stays during most of the tidal cycle in suspension whereas in area B the hydrodynamics are such that the particulate matter can be attached to the bottom and is, maybe only during extreme weather conditions (storms), brought into suspension.

South F st wind neap tide

No S' Spring tide

wind Neap tide

$ 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Figure 7: Going downward: correlation between SPM concentrations and the wind speed, the current velocity, the total water depth, the bottom stress and the significant wave height during spring (left) and neap (right) tide.

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2) All the hydrodynamic parameters show negative correlation with SPM maps in the area A. If SPM concentration remains weakly related to the hydrodynamic processes it indicates that another factor is more determinant in SPM balance in this region. The conventional understanding of the transport in this area may be inappropriate and the problem should be further investigated: the Schelde estuary may be the scene of more important SPM outflows from the river during the spring tide.

3) Through tide measurements [7] indicate that peak values in SPM concentration are coupled with peaks in current velocities, although often the peak SPM concentration occur after the peak current velocities. This phase lag is characteristic in tidal areas and can be explained by:

0 Deposition: after the bottom stress has decreased below a critical value the suspended particles are still transported over a certain distance before they can be attached to the bottom. Erosion: the critical erosion stress is greater than (or equal to) the one for deposition. Vertical mixing: the particles brought in suspension need a certain time before they are distributed over the water column.

0

The through tide measurements taken in the considered area show that a vertical SPM gradient can occur during part of the tidal cycle. The tidal cycle can generally be divided in a period with more or less constant SPM concentrations (2/3 of the tidal cycle) and a period with higher concentrations (1/3 of the cycle). During the longer period of the cycle the SPM concentration is well mixed through the water column and can be interpreted as a kind of background value, which can however reach values up to 50 mg/l in the high turbidity zone near Zeebrugge. This ‘background SPM concentration’ is varying with seasons, neap-spring cycles and meteorological effects. A satellite picture taken during this period can be a good representation of the total SPM concentration. During the other 1/3 of the cycle, and this during ebb as well as flood, a peak in SPM concentration occurs. The SPM concentration in the water column is stratified during these peak periods, the ratio between surface to bottom concentration can be smaller than 0.4. A satellite image taken during this period will underestimate the SPM concentration.

A long-term SPM and hydrodynamic parameters measurements in areas A and B may help correctly estimate the time needed for SPM deposition during a tide cycle, the time taken by the vertical mixing to homogenize the SPM vertical

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distribution and increase the chances for a satellite image to capture the surface SPM concentration that is better estimating the depth-average SPM. Long-time measurements at the sea floor will also provide a better understanding and quantification of the erosionhesuspension in both areas A and B (with a good knowledge of SPM types), so that a better modeling of these processes could be achieved.

Acknowledgments

This study was funded by the Maritime Access Division of the Waterways and Marine Affairs Administration, Ministry of the Flemish Community (AWZ). Support was also provided by the Belgian Office for Scientific, Technical and Cultural affairs (OSTC) within the framework of the STEREO programme BELCOLOUR project (SW00/03). The receiving station of Dundee University, Orbimage, the SeaWiFS and SeaDAS project teams, and the Distributed Active Archive Center at GSFC are acknowledged for providing, distributing and supporting SeaWiFS data.

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