Journées d'Echanges ScienTifiques (JEST), 2015
On the sediment mobility and recent morphological changes in a mesotidal system: the Arcachon lagoon J. Arriagada Gonzalez1-4, A. Sottolichio1, V. Hanquiez1, F. Bertrand2, F. Ganthy3 1 Université
de Bordeaux, UMR EPOC, Pessac, France Université Paris IV, UMR PRODIG, France 3 IFREMER, LER Arcachon, France 4 Universidad de Chile, Departamento de Geografía, Chile 2
INTRODUCTION -
MATERIALS & METHODS
The Arcachon lagoon is a mesotidal embayment in the south coast of the bay of Biscay (Fig. 1). Its total surface is about 174 km² at high tide, where 65% is formed of tidal flats.
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Bathymetric charts: they were obtained on 1992, 2001, 2005 and 2012 (Fig. 3). For the 2005 situation, only the area scanned at low tide, i.e. the eastern lagoon, is suitable for the analysis.
(a)
(b)
Photo location
Source: Ifremer, 1993
Source: SIBA, 2001
Fig. 1: Location maps of the study area
Source: OCA, 2012
Source: OCA, 2005
Fig. 3: Bathymetric maps of the Arcachon lagoon
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Bathymetric changes were analyzed by L‘Yavanc (1995) for a period of 126 years (1864 – 1993). The most recent observations made by end-users of the lagoon, suggest relative infilling of the channels (Allard et al., 2009) and relative increase of turbidity in the inner waters.
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This suggests that a mobile stock of surficial fine sediment is available in the lagoon, which is able to be exchanged between the tidal flats and the channels, depending on the hydrodynamic conditions, reinforced by the recent loss of vegetation. For example, Fig. 1a and 1b shows a significant accretion of mud occurred in areas occupied previously by sand or by seagrass.
The aim of this study is to estimate the stock of mobile surficial sediment available in the the different compartments of the lagoon, transported under natural and anthropogenic forcings. This study is mainly based on information provided by bathymetric and topographic available data.
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Estimation of changes on bathymetry. The DMS were built with ArcGis 10.2 software, for the periods 2001-1993 (following L’Yavanc limits and our limits based on morphological criteria), 2005-2001, 2012-2005 and 2012-1993 (Fig. 4)
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The results for the period 2001/1993 are compared with the limits proposed by L'Yavanc (1995), then bathymetric changes are discussed with the new limits based on morphological criteria. These latter limits are used for the analysis of later periods.
Fig. 4: Differential bathymetric at different periods
RESULTS •
(1)
a) Situation between 2001 - 1993
(7)
Variation Erosion (-) Accretion (+) / (m)
(3)
(6)
(b)
(4) (5)
(a) (a)
Andernos east Arcachon north Claouey north Delta Eyre west Mapouchet Lanton west
Andernos west Arcachon south Claouey south Delta Eyre east Lanton east Île aux Oiseaux
6
b) Situation between 2012 - 2005
1
5 0 4
3
2
1
0 -20 -19 -18 -17 -16 -15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0
1
2
3
4
5
Variation Erosion (-) Accretion (+) / (m)
(2)
(8)
Sedimentary budget related to the depth and general morphology:
Fig. 6 shows the relationship between bathymetry of the oldest year (x axis) and the variation relative to the bathymetry with the most recent year (y axis). Fig. 6a shows the bathymetric variation of the years 2001/1993, while the Fig. 6b shows the bathymetric variation of the years 2012/2005.
0
2
3
4
5
6
7
8
9
10
Bathymetry (m) -1
-2
-3
-1 Bathymetry (m) -2
1
Andernos east Arcachon north Claouey north Delta Eyre west Mapouchet Lanton west
Andernos west Arcachon south Claouey south Delta Eyre east Lanton east
-4
Fig. 6: Variation accretion/erosion as a function of class of depth at each morphological zone
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In general, the accretion rate decreases as the depth decreases; and conversely, the rate of erosion increases as the depth decreases. Accretion occurs mainly in the channels, and decreases regularly towards the shallower areas; erosion occurs in the zones upper to 1 m.
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Although the general trend is the decline of the curves, the variation is less pronounced in the lower depths (< 0 m) at Île aux Oiseaux, Mapouchet, Claouey south, Delta Eyre west and Arcachon north areas (Fig. 6a). When comparing the situation of the years 2001/1993 and 2012/2005 (Fig. 6b) the same trend is observed in almost all areas, except at Claouey north, east and west Delta Eyre Lanton.
2 1 3 4
(c)
(d)
5
6
12
11 8
(a.1) Sediment budget: 2001 / 1993 (L’Yavanc limits)
Sediment budget: 2001 / 1993
4
3
2
AREA
1
5
2
0
1
3
6
7 8
4
10⁶ m³
1
-1
-1
-2
-2
-3
4
0
7
5
3
8
9 10
-3
Sediment budget: 2005 / 2001
Accretion Erosion (10⁶ m³) (10⁶ m³) 3,25
1,80
E
6,91
0,78
6,13
11,00
0,60
D
7,54
7,75
0,22
E
Eyrac
8,10
17,50
9,40
E
4,77
4,58
0,19
D
Belisaire
8,40
24,60
16,20
E
5,51
3,61
1,90
D
Claquey
18,60
8,70
9,90
D
6,22
4,09
2,13
D
Ares
9,60
7,10
2,50
D
3,97
5,58
1,62
E
4
3
3
2
2
2 1
3
8
5 4
-2 -3 (c.1)
6
7
9 10 11
10⁶ m³
1
2 1
D
Sediment budget: 2012 / 1993
Sediment budget: 2012 / 2005
4
0
BILAN Cat (10⁶ m³)
1,45 11,60
3
-1
Bilan 2001 - 1993
BILAN Cat Acretion Erosion (10⁶ m³) (10⁶ m³) (10⁶ m³)
Eyre Teychan
4
1
10⁶ m³
2
11 12
6
5
6
8
0
1
-1 -2 -3 (d.1)
2
3
4
7
9 10 11
10⁶ m³
10⁶ m³
Bilan L'Yavanc 1864 - 1993
2
1
(e)
9
(b.2)
4
3
7
10
(b.1)
12
6
0
1
2
3 4 5
7 8
9 10 11
-1 -2 -3 (e.1)
Fig. 4: Sedimentary budget over the period : (a-a.1) 2001-1993, L’Yavanc limits; (b-b.1-b.2) 2001-1993, geomorphological criteria, (cc.1) 2005-2001; (d-d.1) 2012-2005; (e-e.1) 2012-1993
•
Bathymetric variations in the different periods analyzed.
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Compared with results from L'Yavanc (1995), sedimentary budget of the period 2001/1993 shows opposite trends (Fig. 4a and 4b.2), except at Claouey area.
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With the new limits based on morphological criteria, we can clearly distinguish a pattern that tends to accretion in nearshore and a tends to erosion in the inner part (Fig. 4b), where the sedimentary budget is -0,36 10⁶m³. This situation is reversed in the period 2005-2001 (Fig. 4c), with a sedimentary budget of 3,65 10⁶m³, while in the period 2012-2005 the pattern is less clear from a spatial point of view (Fig. 4d); however, the sedimentary budget is similar to the previous period: 2,57 10⁶m³. In these last two periods (2005-2001 and 2012-2005) we can see the trend of erosion of the delta of Eyre, as opposed to previous reporting period (2001-1993).
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Finally, the sediment budget between all the analyzed period (2012-1993) shows a general trend of accretion (with a sedimentary budget of 8,20 10⁶m³) especially in the southern part of lagoon; while Lanton West and Île aux Oseaux have a tendency to erosion (Fig. 4e).
CONCLUSIONS & PERSPECTIVES ¾ This analysis suggests efficient mobility of sediment in the Arcachon lagoon. The system experienced significant changes due to accretion and erosion in the recent period covering the last 20 years. Differential dynamics between the inner zone of the lagoon and the nearest shore areas are not obesrved in the zoning proposed by L'Yavanc (1995), but becomes visible when considering morphology-based zones. Thus, one can see a cyclical pattern of erosion / accretion in some areas at different periods analyzed (eg: in Andernos east, west Lanton and Eyre Delta areas). ¾ From the spatial point of view, comparing the periods 2005/2001 and 2012/2005 shows a shift in the sedimentary trend for almost all areas (except Lanton east, Mapouchet and Delta Eyre east areas). However, from the point of view of sediment volume the variation is not as significant, which could be explained by a mobilization of sediments from the inner part of the coast and vice versa in these periods analyzed. ¾ Many factors can explain the changes observed. Among them, wind waves, which can develop at high tide over the entire surface of the lagoon (eg: A model exercise performed by Parisot et al., 2008). This effect is maybe reinforced by the decrase of the vegetation coverage (Ganthy, 2011). These factor are under investigation.
REFERENCES
Allard J., Chaumillon E., Fénies H., 2009. A synthesis of morphological evolutions and Holocene stratigraphy of a wave-dominated estuary: The Arcachon lagoon, SW France. Continental Shelf Research (29): 957 – 969. Ganthy F., 2011. Rôle des herbiers de zostères (Zostera Noltii) sur la dynamique sédimentaire du Bassin d’Arcachon. Thèse Doctorale, Université Bordeaux 1. L’Yavanc J., 1995. Evolutions bathymétriques et morphologiques du Bassin d’Arcachon. Raport interne, Ifremer DEL / 95-12. Parisot J. P.., Diet-Davances J., Sottolichio A., Crosland E., Drillon Ch., Verney R., 2008. Modélisation des agitations dans le Bessin d’Arcachon. Xèmes Journées Nationales Génie Côtier – Génie Civil, Sophia Antipolis.
