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| Titel |
Sedimentation and erosion trends over 50 years in the macrotidal Gironde estuary |
| VerfasserIn |
Aldo Sottolichio, Vincent Hanquiez, Joselyn Arriagada, Isabel Jalon-Rojas |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250146410
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| Publikation (Nr.) |
 EGU/EGU2017-10434.pdf |
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| Zusammenfassung |
| Recent studies have emphasized on the drastic morphodynamic evolution of many european urbanized estuaries, which have become more turbid during the XXth century because of human-induced deepening and narrowing. For some systems, the availability of data has allowed detailed analysis of evolution, while for some other estuaries, knowledge remains limited. For the latter, it is difficult to elucidate any effect due to climate change, extreme events or human activities. Among them, the Gironde estuary is a macrotidal funnel-shape system, the largest estuary of Western Europe, and characterized by high levels of turbidity. Despite numerous investigations on sedimentary processes carried out in this estuary in the past, there is poor knowledge on the evolution of its morphology and tides over the XXth century. Recently, an investigation on tidal patterns has started in the Garonne tidal river, where it is known that gravel extraction during the sixties has deeply modified the mean depth of the channel. In the meantime, there is evidence of a long-term shift of the turbidity maximum in this area, mainly due to a reduction of river flow in the last 40 years. However, for the main estuarine portion, which represents 80% of the total surface, morphology evolution and associated physical processes remain under-investigated. In this study the morphological evolution of the Gironde estuary has been documented and investigated for the first time, based on some bathymetric and tidal data collected from the archives of the port of Bordeaux. Six bathymetric maps covering a period of 50 years, from the 1953 to year 2000 were compared, highlighting areas of accretion and erosion. Results show that the zone of maximum volume of deposited sediment has migrated continuously towards the upstream portion of the estuary, which is coherent with the intensification of the low river flow periods and the upstream shift of the turbidity maximum zone to the riverine sections. In the meantime, despite relative weak visible transformation of the general morphology of the estuary, cross section areas have experimented high variation rate (up to 40% in some locations) with contrasted patterns between the upstream and the downstream portion of the estuary. While in the period 1962-1970 the downstream estuary experiences accretion and the upstream experiences erosion, the period 1980-1994 shows opposite trend, with a shift around an area at approximately km 45 downtream from Bordeaux, which acts as a nodal zone, where no net trend is observed. On the basis of available data, little effect of human activity (mainly channel dredging and gravel extraction) and extreme events are observed, and the averaged position of the turbidity maximum seems to be the main driver of changes in sections. Changes on tidal range as a response of change on morphology are relatively weak, with little tidal amplification through 50 years between the mouth and Bordeaux. However differences on tidal asymmetry (rate of flood duration / ebb duration) seem more significant, and could explain the contrasted areas and the shift around km 45. |
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