reach-scale morphological changes of a braided river following a 15-year flood with multidate...

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

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

1) Study site

2) Data processing and error corrections

3) Morphological analysis

234 km²

Hydrometric station

Study reach

The Bès River (1/2)

5Reach-scale morphological changes of a braided river following a 15-year flood with multidate airborne LiDAR

• A very active braided river• Channel slope : 1,4%• Mean active channel width : 130 m• Mean discharge : 2,830 m3/s• Mean grain size (Surface D50) : 30 mm • Mediterranean stream flow regime

The Bès River (2/2)


1) Study site



• 2 airborne LiDAR surveys : 27/10/2008 and 19/04/2010

• A 15-year flood in december 2009 (instantaneous Qmax = 171 m3/s)


Studied data


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 mergingLiDAR 10 > LIDAR 08

After merging

Z10 – Z08


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 :

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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 2008DEM of difference

(DoD)

DoD production and uncertainty assessment (1/2)

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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


DoD production and uncertainty assessment (2/2)

dGPS - LiDAR

1) Study site




DEM of difference map


Volumetric changes

17%

0%

31%

5%0%

9%

37%

1%

Channel carving Channel deepening Bar sculptingBank erosion Channel plugging Channel fillingBar development Autres

Sediment budget (m3)

Bar sculpting -92754.

Bar development 107280.

Channel carving -50553

Bank erosion -14711

Channel deepening -990

Channel plugging 558

Channel filling 26110.

Autres 1519

-23540

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

0 1000 2000 3000 4000 5000 6000

-40

0-2

00

02

00

DISTANCE (m)

Se

dim

en

t b

ud

ge

t m3

Wid

th d

iffe

ren

ce

s (

m)

17

Upstream Downstream

0 1000 2000 3000 4000 5000 6000

-40

0-2

00

02

00

DISTANCE (m)

Se

dim

en

t b

ud

ge

t m

3

Wid

th d

iffe

ren

ce

s (

m)

Fill

Scour

Widening

Narrowing

Active width

Sediment budget

Active width Sediment budget


(m²)

Longitudinal variation of sediment budget (2/2)

• 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


Long profile responses of alpine braided rivers in SE France

Liébault, F. , Lallias-Tacon, S., Cassel, M., Talaska, N.

Liébault et al., 2012 (RRA)

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


Long profilesStudy 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


Degradation: 56% Aggradation: 20%Stability: 24%

Degradation Aggradation

Long profile evolution (1/2)


Degradation

Aggradation

Long profile evolution (2/2)


Morphological signatures

DegradationAggradation

Thank you for your attention

[email protected]

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reach-scale morphological changes of a braided river following a 15-year flood with multidate...

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