Within the scope of the long term vision of the Schelde Estuary and the Development Outline 2010 low dynamic intertidal and shallow water areas are considered to be of high ecological value. Little is known about the occurrence of such valuable  reas in the Schelde Estuary, nor are the physical and morphological processes which determine the occurrence of these areas fully understood. Based on the results and recommendations of a 2008-2009 research project in the Westerschelde, a new research project was defined to determine relationships between abiotic (bedforms, hydrodynamics, sediment) and biotic (benthos) characteristics in the Zeeschelde (Belgium) and to set up a classification of deep and undeep subtidal areas. A first phase in the habitat classification comprises of the analysis of bed form occurrence and the setup of a bed form classification based on multibeam echo sounding data. Results show within the different sub areas a variation in bed forms going from hard bed layers to no bed forms to ripples (H ~ 0,1 to 0,3 m | L ~ 10 m) and smaller dunes (H ~ 0,5 to 1 m | L ~ 15 to 25 m). On-going research investigates relationships between the bed form characteristics and flow and sediment characteristics. Preliminary results

De Sutter, R.

Maximova, T.

Plancke, Y.

Van Braeckel, A.

Vos, G.R.

Open Marine Archive

Marine and River Dune Dynamics – MARID IV – 15 & 16 April 2013 - Bruges, Belgium 
 
315 
A first phase in the habitat classification for the Zeeschelde: 
Bed form classification. 
G.R. Vos(1,2), T. Maximova(1), Y.M.G. Plancke(1), A. Van Braeckel(3), R. De Sutter(2) 
 
1. Flanders Hydraulics Research, Antwerpen, Belgium - yves.plancke@mow.vlaanderen.be 
2. Antea Group, Antwerpen, Belgium – gwendy.vos@anteagroup.com 
3. Instituut voor Natuur- en Bosonderzoek, Brussel, Belgium – alexander.vanbraekel@inbo.be 
 
 
Abstract 
Within the scope of the long term vision of the Schelde Estuary and the Development Outline 2010 low dynamic 
intertidal and shallow water areas are considered to be of high ecological value. Little is known about the 
occurrence of such valuable areas in the Schelde Estuary, nor are the physical and morphological processes which 
determine the occurrence of these areas fully understood. Based on the results and recommendations of a 2008-
2009 research project in the Westerschelde, a new research project was defined to determine relationships between 
abiotic (bedforms, hydrodynamics, sediment) and biotic (benthos) characteristics in the Zeeschelde (Belgium) and 
to set up a classification of deep and undeep subtidal areas. A first phase in the habitat classification comprises of 
the analysis of bed form occurrence and the setup of a bed form classification based on multibeam echo sounding 
data. Results show within the different sub areas a variation in bed forms going from hard bed layers to no bed 
forms to ripples (H ~ 0,1 to 0,3 m | L ~ 10 m) and smaller dunes (H ~ 0,5 to 1  m | L ~ 15 to 25 m). On-going 
research investigates relationships between the bed form characteristics and flow and sediment characteristics. 
Preliminary results show weak relationships between flow velocity and bed form size, and between depth and bed 
form size. 
 
1 INTRODUCTION 
Within the scope of the long term vision of the 
Schelde Estuary and the Development Outline 
2010, low dynamic intertidal and shallow water 
areas are considered to be of high ecological value. 
These areas are in fact linked to eutrophic foraging 
zones for birds and young fish and to refugees for 
tidal migrators as shrimps. However, this 
assumption is based on experiences in other areas 
and there is no evidence that all parts of the 
shallow water area are equally valuable. Little is 
known about the occurrence of such valuable areas 
in the Schelde estuary, nor are the physical and 
morphological processes which determine the 
occurrence of these areas fully understood. 
Therefore, a research project was defined in 2008-
2009 to investigate the relation between on one 
hand the bed forms, hydrodynamics and sediment 
properties and on the other hand the ecological 
value of the shallow water areas (Plancke, Y. 2009). 
This  ‘Walsoorden’ research project was conducted 
by NIOZ-CEME in collaboration with Flanders 
Hydraulics and IMARES, focused on the area near 
the Walsoorden Sandbar in the Westerschelde (the 
Netherlands). This study resulted in a criterion 
based on abiotic parameters (depth and duration 
during a full tidal cycle in which flow velocity 
exceeds 65 cm/s) that was able to predict habitats 
with a high ecological value. Where this criterion 
was found for only one specific area of the Schelde 
Estuary, a validation is necessary. Therefore 2 new 
studies were started: one to validate the criterium 
in the Westerschelde, a second one to validate it in 
the Zeeschelde. This paper describes the results of 
the abiotic analysis in the Zeeschelde, the Belgian 
upstream part of the Schelde Estuary. In a first 
phase a bed form classification was made for 4 
specific subareas (spread over the full salinity 
gradient), based on multibeam echo sounding data. 
In a second phase the hydrodynamics were 
analyzed, while in the third phase it was 
investigated whether relations existed between bed 
form and hydrodynamic characteristics. 
 
2 BED FORM ANALYSIS 
2.1 Study area and used data 
Within the Zeeschelde detailed research has been 
conducted in 4 subareas. These subareas were 
Marine and River Dune Dynamics – MARID IV – 15 & 16 April 2013 - Bruges, Belgium 
 
316 
selected based on the salinity zones in the 
Zeeschelde and the occurrence of a vast undeep 
water area. The selected subareas are: 
• Mesohaline: Schaar van Ouden Doel and 
Galgenschoor area 
• Oligohaline: Notelaer and Ballooi area 
• Freshwater with long residence time: 
Branst area 
• Freshwater with short residence time : 
Appels area 
 
Figure 1. Location of analyzed subareas in the Schelde 
estuary 
 
The bathymetry of  the mesohaline area in the 
Beneden-Zeeschelde is based on multibeam echo 
sounding measurements from 2011, the bathymety 
for the more upstream areas in the Boven-
Zeeschelde is composed of data from 2009, 2010 
and 2011. For the intertidal area LIDAR data from 
2011 is used. 
 
2.2 Methodology 
The analysis was executed using the following 
steps: 
1 Visual classification in zones, starting from the 
shaded view image of the area 
2 Selection of zones without bed forms 
3 Definition of several longitudinal transects 
(along the direction of the flow) within each 
zone with bed forms 
4 Analysis of average bed form characteristics in 
each transect  such as length, height, 
asymmetry and steepness of the bed forms 
5 Classification of the sub area in a limited 
amount of bed form classes 
 
2.2.1 Visual classification in sub areas 
A so-called ‘shaded view’ map was used as a base 
for the visual classification in sub areas. This 
computer-generated map shows a simulated cast 
shadow of sun upon a raised bathymetry. The 
angle of the sun on the bathymetry has been 
chosen to discern the bed forms optimally. The 
optimal angle was found to be parallel to the 
direction of the flow, because the direction of bed 
forms is expected to be perpendicular to the 
direction of the flow.  
Based upon this ‘shaded view’ images, different 
sub areas were delimited (see zoom detail in 
Figure 3). Boundaries were defined visually in 
those places where differences in bed forms 
appeared to occur. 
 
2.2.2 Selection of zones without bed forms 
While determining the zones without bed forms, a 
distinction was made between flat zones and zones 
with irregular bed forms (caused by geological 
hard layers or human interference such as sediment 
disposal or extraction). This distinction is not 
always clear on the basis of topo-bathymetric 
maps. Especially the recognition of hard layers, 
which can have both irregular and a flat 
appearance, is often difficult. The in situ sediment 
sampling campaign that has been conducted in the 
scope of this research, will make this distinction 
more clear. 
 
2.2.3 Definition of longitudinal transects 
Within every subarea some longitudinal transects 
were defined, assuming that these sections are 
representative for the whole subarea. If possible, 
the length of the transects was chosen long enough 
to ensure a sufficient number of bed forms within 
one transect, in order to be able to conduct a 
representative analysis. The depth values of these 
Marine and River Dune Dynamics – MARID IV – 15 & 16 April 2013 - Bruges, Belgium 
 
317 
sections were exported from a 1m*1m raster 
covering the study area using GIS-software. 
 
2.2.4 Analysis of the transects to obtain bed 
form characteristics  
For every longitudinal transect following 
characteristics were deduced: 
• Length of the individual bed forms 
• Height of the individual bed forms 
• Asymmetry of the individual bed forms 
• Average steepness of the bed forms per 
transect 
 
 
Figure 2. Methodology for determining characteristic 
parameters, above: step 1 – 4, below: step 5 – 7 
 
This analysis was executed using a self-developed 
Matlab routine, based on the methodology used in 
the Bed form Tracking Tool (van der Mark et al. 
2007). This routine consists out of the following 
steps to define the characteristic parameters of the 
bed forms in a certain transect (Figure 1) : 
1 Choosing a period for a floating average to 
remove trends (large scale depth variation) 
from the section without losing the individual 
bed forms. 
2 Detrending of the section by subtracting the 
floating average from the original data. 
3 Determining the intersections of the detrended 
signal and the zero-line. 
4 Determining the crests and troughs, based upon 
the assumption that between two intersections 
with the zero-line, a crest or a trough can be 
found. 
5 Determining the length of every individual bed 
form, defined as the distance between 2 
successive crests. 
6 Determining the height of every individual bed 
form, defined as the difference between the 
height of a crest and the following trough. 
7 Determining the asymmetry of every individual 
bed form, defined as the ratio between the 
inclination length (Ldstr) and the declination 
length (Lustr) of the bed form (from trough to 
trough). 
8 Determining the average steepness per transect, 
defined as the ratio between the average height 
and the average length of the bed forms (from 
trough to trough) in that section. 
Next, the characteristics of the individual bed 
forms were averaged per transect. The resulting 
average values for length, height and asymmetry 
were filtered to minimize outlier-effects: all values 
outside the interval [0,25*average: 1,75*average] 
were discarded and a new average was calculated 
using the remaining values. 
The transects were always defined from down-
estuary to up-estuary side. Thus, an asymmetry 
value greater than 1 implies a bed form where the 
seaward side is longer than the landward side. This 
indicates flood dominance. A value smaller than 1 
indicates ebb dominance. 
 
2.2.5 Classification of the study area in a 
limited amount of bed form classes 
In order to group the different sub areas in a 
feasible amount of classes, the averaged 
characteristics were compared and classified 
according to the classification used in the 2008-
2009 Walsoorden research project, which makes it 
easier to compare results of the Zeeschelde 
research project with those of the Westerschelde 
research project. 
 
 
Marine and River Dune Dynamics – MARID IV – 15 & 16 April 2013 - Bruges, Belgium 
 
318 
Class Length Height 
1 < 10 m < 5 cm 
2 ~ 10 m 15 – 30 cm 
3 10 – 15 m 30 – 50 cm 
4 15 – 25 m 50 – 100 cm 
5 15 – 30 m 100 – 150 cm 
6 > 30 m > 150 cm 
7 > 30 m < 100 cm 
Table 1 :Overview of length/height classes 
 
Class Asymmetry 
1 < 0,90 (ebb dominance) 
2 0,90 – 1,10 (no dominance) 
3 1,10 – 1,50 (flood dominance) 
4 > 1,50 (strong flood dominance) 
Table 2 :Overview of asymmetry classes 
 
Table 1 shows the classification based on length 
and height of the bed forms, table 2 shows an 
overview of the asymmetry classes. 
 
2.3 Results 
Based on the mean length, height and asymmetry 
of the bed forms per transect, each of the zones is 
attributed to one of the bed form and asymmetry 
classes. A class 0 is also defined, holding the zones 
with irregular bed-surface. Where small bed forms 
are superposed on larger ones (e.g. dunes of class 5 
with superposition of ripples of class 2), a 
combined class number is attributed (e.g. class 
5,2). In figure 3 an example of the length/height 
classification for the subarea Notelaer/Ballooi can 
be found, figure 4 shows the length/height 
classification for the upstream subarea Appels. 
 
The most downstream subareas Ouden Doel and 
Galgeschoor in the Beneden-Zeeschelde are 
strongly influenced by human interference 
(disposal/sand extraction). This means only few 
zones contain bed forms, which makes it difficult 
to draw general conclusions on the occurrence of 
bed forms. Most of the bed forms in this area are 
small ripples (class 2). 
In the other sub areas, in the upstream Boven-
Zeeschelde, bed form characteristics are more 
diverse. Next to zones with small ripples, sub areas 
Notelaer/Ballooi and Branst also contain dunes and 
dunes with superimposed ripples. In most 
upstream sub area Appels no dunes can be found, 
but ripples from class 3 are present, next to the 
small class 2 ripples. 
It was noticed that most of the classes with bed 
forms are located in the deeper, central parts of the 
river. 
 
The classification in asymmetry classes is executed 
using the classes in table 2. Class 0 is again used 
for zones with irregular bed surface, while a class 
5 was defined for zones where different transects 
are characterised by different asymmetry. 
 
Most areas are flood dominant, but within each sub 
area ebb dominant zones (or zones with no 
dominance) are found. No relationship can be 
found though between location of the zones and 
dominance class. 
 
3 HYDRODYNAMIC ANALYSIS 
In a next phase of the study the relation between 
the occurrence of bed forms and hydrodynamic 
parameters has been analysed.  
 
3.1 Used data 
A 2D validated numerical model is used as a base 
for the hydrodynamic analysis (Maximova, T. 2013). 
This model is a refined version of the NEVLA-
model (Maximova, T. 2009; Verheyen, B. 2012), 
which was too coarse to be able to predict the 
velocities in sufficient detail for this study in the 
upper part of the estuary. 
The downstream boundary of the model is located 
at Walsoorden in the Westerschelde; the upstream 
boundary is located at the tidal border. Measured 
water levels at Walsoorden (HMCZ database) are 
used as a downstream boundary condition. 
Measured discharges are used as an upstream 
boundary condition. The time step used for the 
model simulations is 3s. For the bathymetry, the 
multibeam and LIDAR data from the bed form 
analysis is used in the model. 
Every 10 minutes a ‘map file’ with flow fields was 
exported for the period from 26/06/2009 to 
27/06/2009 (spring tide conditions). 
 
3.2 Methodology 
Based upon the flow fields, following 
characteristic hydrodynamic parameters were 
deduced: 
• Average flood velocity 
• Average ebb velocity 
Marine and River Dune Dynamics – MARID IV – 15 & 16 April 2013 - Bruges, Belgium 
 
319 
• Ratio average flood velocity to average 
ebb velocity 
• Maximum flood velocity 
• Maximum ebb velocity 
• Duration that flow velocities exceed 65 
cm/s during one spring tidal cycle (figure 
5 shows the duration map for Appels). 
Similar to the bed form analysis, the hydrodynamic 
analysis was performed using 2 types of 
classification: first a classification based on the 
magnitude of average velocities, second a 
classification based on the ratio between average 
velocities. 
For the first classification, the used boundary 
values are related to characteristic velocity 
boundaries for e.g. initiation of sediment transport 
(Flanders Hydraulics Research, 2007) and low versus 
high dynamic conditions (Bouma et al., 2005). For 
every point in the flow field, ebb as well as flood 
velocity has been classified using these boundary 
values. Based on the combination of ebb and flood 
velocity the eventual classification is made (see 
Table 5). 
 
[cm/s] < 50 [50, 60] 
[60, 
80] 
[80, 
100] > 100 
< 50 1 2 3 4 5 
[50, 60] 6 7 8 9 10 
[60, 80] 11 12 13 14 15 
[80, 100] 16 17 18 19 20 
> 100 21 22 23 24 25 
Table 5: Classification based on hydrodynamic 
characteristics. 
 
3.3 Results 
The hydrodynamic analysis shows as can be 
expected an increase of velocities with increasing 
depth. The duration during which flow velocities 
are higher than 65 cm/s exceeds 240 minutes 
almost everywhere, except along the river banks 
and on the leeward side of the guiding wall. 
Flood/ebb ratio shows a similar pattern in all sub 
areas, namely an alternating pattern of ebb and 
flood dominated zones along each river bank. 
 
4 ON-GOING RESEARCH 
Based on the bed form classification the Research 
Institute for Nature and Forest has taken ca. 200 
samples, stratified random over the different 
classes and subareas. At this moment the samples 
are being analysed on both grain size and the 
density and number of benthic species. These 
results will be used to investigate potential 
relations between abiotic characteristics presented 
in this paper, and biotic characteristics. 
 
5 CONCLUSIONS 
Based on the bed form classification and 
hydrodynamic classification a relationship between 
both aspects has been investigated. Therefore the 
hydrodynamic characteristics were attributed to the 
bed form classes by calculating the average value 
per bed form zone for each of the hydrodynamic 
parameters. Figure 6 shows an example of the 
classification, based on the ratio between average 
flood and ebb velocity. 
 
Additionally the relationship between the 
occurrence of certain types of bed forms and each 
of the hydrodynamic characteristics was examined 
Figure 7 shows a weak positive trend between 
average ebb velocity and length/height classes. 
 
 
Figure 7: Relationship between length/height class and 
average ebb velocity 
 
A clear relationship between hydrodynamic and 
bed form characteristics could not be found. A 
general increase of the size of the bed forms with 
increasing velocities can be determined, but the 
scatter is fairly high. However, the linear 
relationship between depth and bed form size 
explained 56% of the variation (R² = 0,568). 
 
6 ACKNOWLEDGMENT 
This study was performed within the scope of the 
long term vision of the Schelde estuary 
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320 
commissioned by the Flemish Maritime Division 
and the Dutch Rijkswaterstaat. 
 
7 REFERENCES 
Bouma H., de Jong D.J., Twisk F., Wolfstein K. 2005. 
Zoute wateren Ecotopenstelsel (ZES.1) – Voor het 
in kaart brengen van het potentiële voorkomen van 
levensgemeenschappen in zoute en brakke 
rijkswateren, juli 2005. 
Maximova, T., Ides, S., De Mulder, T., Mostaert, F. 
2009. Verbetering 2D randvoorwaardenmodel. 
Deelrapport 4: Extra aanpassingen Zeeschelde. WL 
Rapporten, 753_09. Flanders Hydraulics Research, 
Antwerp, Belgium 
Maximova, T., Vanlede, J., Plancke, Y., Verwaest, T., 
Mostaert, F. 2013. Habitatmapping ondiep water 
Zeeschelde: Deelrapport 2 - Numeriek 2D model. 
Version 1.2. WL Rapporten, 00_028. Flanders 
Hydraulics Research.  Antwerp, Belgium 
Plancke Y., Vos G., De Mulder, T., Mostaert, F. 2009 
Habitatmapping Westerschelde – Deelrapport 1: 
classificatie op basis van bodemvormen en 
hydrodynamica. WL Rapporten, 754-06. 
Waterbouwkundig Laboratorium, Antwerpen, 
Belgie. 
van der Mark C.F., Blom, A. 2007. A new and widely 
applicable tool for determining the geometric 
properties of bedforms. april 2007. 
Verheyen, B., Leyssen, G., Vanlede, J., Schramkowski, 
G., Mostaert, F. 2012. Verbetering 
randvoorwaardenmodel: Deelrapport 7: Afregeling 
van het 3D Scheldemodel. WL Rapporten, 753_09. 
Flanders Hydraulics Research & IMDC: Antwerp, 
Belgium  
Vos, G., Plancke, Y., Verwaest, T., Mostaert, F. (2013). 
Habitatmapping Zeeschelde: Deelrapport 3 – 
Relaties abiotiek. Versie 1_3. WL Rapporten, 
00_028. Waterbouwkundig Laboratorium: 
Antwerpen, België. 
Waterbouwkundig Laboratorium Borgerhout (WLB), 
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Figure 3. Notelaer and Ballooi: classification in length/height classes, based on shaded view map 
 
 
Marine and River Dune Dynamics – MARID IV – 15 & 16 April 2013 - Bruges, Belgium 
 
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Figure 4. Appels : classification in length/height classes 
 
 
 
Figure 5: Appels: duration that flow velocities exceed 65 cm/s during one spring tidal cycle 
 
 
Figure 6. Notelaer and Ballooi: classification in hydrodynamic classes based on flood/eb ratio 
 


A first phase in the habitat classification for the Zeeschelde: Bed form classification

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