Geomorphological adjustements of the middle Garonne River downstram of Toulouse (South-West, France) since the 1950s under the effect of in-stream gravel mining

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(1)Geomorphological adjustements of the middle Garonne River downstram of Toulouse (South-West, France) since the 1950s under the effect of in-stream gravel mining Hugo Jantzi, Jean-Michel Carozza, J.L. Probst, Philippe Valette. To cite this version: Hugo Jantzi, Jean-Michel Carozza, J.L. Probst, Philippe Valette. Geomorphological adjustements of the middle Garonne River downstram of Toulouse (South-West, France) since the 1950s under the effect of in-stream gravel mining. 3ème conférence internationale sur les recherches et actions au service des fleuves et grandes rivières, Jun 2018, Lyon, France. �hal-01875582�. HAL Id: hal-01875582 https://hal-univ-tlse2.archives-ouvertes.fr/hal-01875582 Submitted on 17 Sep 2018. HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés..

(2) I.S.RIVERS LYON 2018. Geomorphological adjustments of the middle Garonne River downstream of Toulouse (South-West, France) since the 1950s under the effect of in-stream gravel mining Ajustements géomorphologiques de la moyenne Garonne en aval de Toulouse (sud-ouest, France) depuis les années 1950 sous l’effet des extractions de granulats Background  During the 20th century, significant geomorphological adjustments are observed on the Garonne river leading to channel narrowing (-60 m) and incision (-1,7 m).  It occurred especially from the 1950s as a consequence of intensive human interventions, such as dam construction, channel works and mainly by in-stream gravel mining with 20 M.m-3 extracted in 20 years.  Nowadays the river presents a significant sediment deficit resulting in bedrock outcrops over 51% of the total channel surface.. Study site  The study reach is located on the Garonne River, 10 km downstream of Toulouse at the town of Beauzelle.  Width: 130 m ; slope: 20% ; stream power: 400 W.m-2.. Location map of the middle Garonne and the study reach (red dot) (IGN).. Methods. Aerial image of the study reach, Beauzelle (Haute-Garonne) (IGN)..  Spatial analysis from aerial images:. Objectives  Identify the geomorphological changes occurring in the study reach under the effect of in-stream gravel mining during the 2nd half of the 20th century.. Aerial images. Georeferencing (1st order polynomial). Mapping riverbed units.  Quantify the riverbed degradation..  Topographic data analysis from cross profiles..  Highlight the transition processes between alluvial channel and bedrock channel..  Hydrological data analysis based on flood records.. Analyzing geometry (surface area). Changes in channel morphology 1958 50 m3.s-1. Undisturbed conditions Incision. 1976 49 m3.s-1. 1973 68 m3.s-1. Start of in-stream mining. 1980 60 m3.s-1. Knickpoint emergence. End of in-stream mining. Process of incision  In-stream gravel mining with 389 000 m3 extracted over the 1970s according to archive sources.  In response, the channel experiments:  1) bed channel load removal,  2) erosional regression estimated at ̴27 m.an-1 leading to a gas line rupture (1982) 300 m upstream of the extraction site.. Narrowing. 1997 54 m3.s-1. 1983 43 m3.s-1. 2010 61 m3.s-1. 2006 55 m3.s-1. Knickpoint of Beauzelle at low flow ̴45 m.s-1 (Smeag).. Quarry in floodplain Knickpoint. Narrowing. Channel limits. --- -- - =. Oxbow lake ( ̴1850s). Oxbow lake ( ̴1975). Intensity of processes. In-stream mining. 4000 3000 2000. Channel degradation and simplification.  Narrowing :  Incision :. 1960s-1990s -61 m / -1,3 m.an-1 -3,6 m / -11 cm.an-1. 116. 1970s -38 m / -4,7 m.an-1 -2,7 m / -14 cm.an-1. In-stream mining. 115 114 113.  Between 1958-2010 channel morphology experiments drastic changes with 41% increase in outcrops and 10%-15% decrease in gravel and vegetated bars.. 112 1945 1955 1965 1975 1985 1995 2005 2015. In-stream mining. 70 60 50 40 1945 1955 1965 1975 1985 1995 2005 2015. 117. Altitude (m).  Significant narrowing and incision is recorded between 1960s and 1990s with a maximum intensity over 1970s corresponding to the period of in-stream mining:. 1000 1945 1955 1965 1975 1985 1995 2005 2015. 60. Riverbed units area (%). In-stream mining. QImax (m3.s-1). Lateral stabilization. 80. Channel area (ha). 5000. 50. In-stream mining. AF ATN ATVM. AFI ATVN. 40 30 20 10 0 1945 1955 1965 1975 1985 1995 2005 2015. Riverbed units: EO. Emerged outcrop ; SO. Submerged outcrop ; GB. Gravel bar ; IVB. Incipient vegetated bar ; MVB. Mature vegetated bar.. Conclusion  This study highlights the role of in-stream mining as the dominant factor in channel degradation at the local scale.  In-stream mining and lack of sediment connectivity resulted in an incision greater than 4 m and a high sediment deficit.  Change of incision process from evacuation of alluvial cover by regressive erosion and gradual outcropping of the bedrock to erosion of this latter.  Appearance of a knickpoint of more than 3 ha with 3 m difference in height. Ql. Discharge ; Qs. Solid discharge ; W. width ; Z. depth ; Tr. Return period Hugo Jantzi1*, Jean-Michel Carozza2, Jean-Luc Probst3, Philippe Valette1 1 Université. Jean Jaurès, GEODE UMR 5602 CNRS, Toulouse, France 2 Université de La Rochelle, LIENSS UMR 7266 CNRS, La Rochelle, France 3 Université Paul Sabatier, ECOLAB UMR 5245 CNRS/INPT-ENSAT, Toulouse, France * Corresponding author : hugo.jantzi@univ-tlse2.fr / +33 (0)5 61 50 36 26 View publication stats.

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