1. 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 Séminaire GESTRANS Grenoble, 22 Nov. 2012

2. Introduction Characterize morphological changes of a braided river following a 15 yr flood from multidate airborne LiDAR - Assess errors - Calculate sediment budget - Explore spatial pattern Objectives of this study - 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) LiDAR Reach-scale morphological changes of a braided river following a 15-year flood with multidate airborne LiDAR 2

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

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

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

6. A very active braided river Channel slope : 1,4% Mean active channel width : 130 m Mean discharge : 2,830 m 3 /s Mean grain size (Surface D 50 ) : 30 mm Mediterranean stream flow regime The Bès River (2/2) 6 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) Morphological analysis

8. 2 airborne LiDAR surveys : 27/10/2008 and 19/04/2010 A 15-year flood in december 2009 (instantaneous Qmax = 171 m 3 /s) 8 Reach-scale morphological changes of a braided river following a 15-year flood with multidate airborne LiDAR Studied data

9. 9 Reach-scale morphological changes of a braided river following a 15-year flood with multidate airborne LiDAR Elevation difference of points situated on the road Vegetation points Ground points Pre-processing 1)Vegetation removal (Axellson ‘s filter of Terrascan + manual control) 2)Ground point cloud merging (Polyworks) Before merging LiDAR 10 > LIDAR 08 After merging Z10 – Z08

10. 10 Reach-scale morphological changes of a braided river following a 15-year flood with multidate airborne LiDAR TIN + Linear interpolation Ground points DEM of 0.25 m² resolution 1- Field sample of water depths for discharge equivalent to discharge during both 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) DEM production Water surfaces Laser not water penetrating Flow depths estimated by field investigation and statistical approach :

11. 11 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 Uncertainty assessment Reach-scale morphological changes of a braided river following a 15-year flood with multidate airborne LiDAR +/- error uncertainty DEM of 2010 – DEM of 2008 DEM of difference (DoD) DoD production and uncertainty assessment (1/2)

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

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

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

15. 15 Reach-scale morphological changes of a braided river following a 15-year flood with multidate airborne LiDAR Volumetric changes Sediment budget (m3) Bar sculpting-92754. Bar development107280. Channel carving-50553 Bank erosion-14711 Channel deepening-990 Channel plugging558 Channel filling26110. Autres1519 -23540

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

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

18. Control/correct LiDAR data before DoD calculation Provide a quantified spatial pattern of erosion and deposition patches highlighting geomorphic processes Improve longitudinal variation understanding Conclusion and perspectives 18 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. Long profile responses of alpine braided rivers in SE France 20 (1)Study of the long profile evolution of braided rivers during the last 100 years (2)Analysis of the spatial variability of braided channel responses in SE France (3)Characterization of morphological signatures of aggrading and degrading braided channels Objectives

21. Long profile responses of alpine braided rivers in SE France 21 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 Data Aigues River

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

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

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

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