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Séminaire GESTRANS Grenoble, 22 Nov. 2012

Séminaire GESTRANS Grenoble, 22 Nov. 2012. Reach-scale morphological changes of a braided river following a 15-year flood with multidate airborne LiDAR S. Lallias-Tacon (1,2) , F. Liébault (1) , H. Piégay (2)

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Séminaire GESTRANS Grenoble, 22 Nov. 2012

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  1. Séminaire GESTRANS Grenoble, 22 Nov. 2012 Reach-scale morphological changes of a braided river following a 15-year flood with multidate airborne LiDAR S. Lallias-Tacon(1,2), F. Liébault(1), H. Piégay(2) (1) Irstea-ETGR, Saint-Martin-d’Hères, France ; (2) CNRS-UMR 5600, Lyon, France

  2. Introduction • LiDAR • Recent development • Rapid acquisition of topographic data at high-precision and high-resolution • Interesting on braided rivers because rapid changes through time and minimised elevation errors (almost no vegetation, low flows) • Objectives of this study Characterize morphological changes of a braided river following a 15 yr flood from multidate airborne LiDAR - Assess errors - Calculate sediment budget - Explore spatial pattern Reach-scale morphological changes of a braided river following a 15-year flood with multidate airborne LiDAR

  3. 1) Study site 2) Data processing and error corrections 3) Morphologicalanalysis

  4. 1) Study site 2) Data processing and error corrections 3) Morphologicalanalysis

  5. The Bès River (1/2) 234 km² Hydrometric station Study reach Reach-scale morphological changes of a braided river following a 15-year flood with multidate airborne LiDAR

  6. The Bès River (2/2) • A very active braided river • Channel slope : 1,4% • Mean active channelwidth : 130 m • Meandischarge : 2,830 m3/s • Mean grain size (Surface D50) : 30 mm • Mediterraneanstream flow regime Reach-scale morphological changes of a braided river following a 15-year flood with multidate airborne LiDAR

  7. 1) Study site 2) Data processing and error corrections 3) Morphologicalanalysis

  8. Studied data • 2 airborne LiDAR surveys : 27/10/2008 and 19/04/2010 • A 15-year flood in december 2009 (instantaneousQmax = 171 m3/s) Reach-scale morphological changes of a braided river following a 15-year flood with multidate airborne LiDAR

  9. Pre-processing Vegetation removal (Axellson ‘s filter of Terrascan + manual control) Ground point cloud merging (Polyworks) Vegetation points Elevation difference of points situated on the road Ground points Z10 – Z08 Beforemerging LiDAR 10 > LIDAR 08 Aftermerging Reach-scale morphological changes of a braided river following a 15-year flood with multidate airborne LiDAR

  10. DEM production TIN + Linear interpolation DEM of 0.25 m² resolution Ground points • Water surfaces • Laser not water penetrating • Flow depths estimated by field investigation and statistical approach : 1- Field sample of water depths for dischargeequivalent to dischargeduringboth LiDAR surveys 2- Random water depths derived from normal distribution 3- Subtraction of random water depths at each DEM elevation for wet pixels (mapping with points density map) Reach-scale morphological changes of a braided river following a 15-year flood with multidate airborne LiDAR

  11. DoD production and uncertaintyassessment (1/2) +/- erroruncertainty DEM of difference (DoD) DEM of 2010 – DEM of 2008 Uncertainty assessment 1- Assessment of spatially distributed error in DoD with dGPS checkpoints on different types of terrains (exposed gravel bars, sparse and dense vegetated areas) 2- Critical threshold error with propagation of error into DoD Lane et al., 2003 Brasington et al, 2000, 2003 Reach-scale morphological changes of a braided river following a 15-year flood with multidate airborne LiDAR

  12. DoD production and uncertaintyassessment (2/2) Uncertainty assessment • Systematic bias between LiDAR data and dGPS control ground points • Systematicerror and standard deviationincreasewith surface complexity LiDAR ground points > dGPS control ground points Gravel bars dGPS - LiDAR Vegetated areas Road Alluvial forest Reach-scale morphological changes of a braided river following a 15-year flood with multidate airborne LiDAR

  13. 1) Study site 2) Data processing and error corrections 3) Morphologicalanalysis

  14. DEM of differencemap Reach-scale morphological changes of a braided river following a 15-year flood with multidate airborne LiDAR

  15. Volumetric changes Reach-scale morphological changes of a braided river following a 15-year flood with multidate airborne LiDAR

  16. Longitudinal variation of sediment budget (1/2) Fast Fourier Transform (FFT) Upstream Downstream => a period of 661m = 5 times the mean active channel width ? Reach-scale morphological changes of a braided river following a 15-year flood with multidate airborne LiDAR

  17. Longitudinal variation of sediment budget (2/2) Active width Sediment budget (m²) Downstream Upstream Fill Widening Active width Sediment budget Narrowing Scour Reach-scale morphological changes of a braided river following a 15-year flood with multidate airborne LiDAR

  18. Conclusion and perspectives • Control/correct LiDAR data before DoD calculation • Provide a quantified spatial pattern of erosion and deposition patcheshighlighting geomorphic processes • Improve longitudinal variation understanding Reach-scale morphological changes of a braided river following a 15-year flood with multidate airborne LiDAR

  19. Long profile responses of alpine braided rivers in SE France Liébault, F. , Lallias-Tacon, S., Cassel, M., Talaska, N. Liébault et al., 2012 (RRA)

  20. Objectives • Study of the long profile evolution of braided rivers during the last 100 years • Analysis of the spatial variability of braided channel responses in SE France • Characterization of morphological signatures of aggrading and degrading braided channels Long profile responses of alpine braided rivers in SE France

  21. Data Long profiles Study sites • 31 reaches in the French Alps and piedmont • 129 km or river length • ~20% of the SE France braided channel network Topographic surveys • spring and summer 2009 • Level, total station, dGPS Historical long profiles • surveyed between 1894 and 1931 • Cross-sections • Level, total station Aigues River Long profile responses of alpine braided rivers in SE France

  22. Long profile evolution (1/2) Degradation: 56% Aggradation: 20% Stability: 24% Aggradation Degradation Long profile responses of alpine braided rivers in SE France

  23. Long profile evolution (2/2) Aggradation Degradation Long profile responses of alpine braided rivers in SE France

  24. Morphological signatures Degradation Aggradation Long profile responses of alpine braided rivers in SE France

  25. Thank you for your attention sandrine.tacon@irstea.fr

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