monitoring of a sediment replenishment operation · 18/08/16 26/11/16 06/03/17 14/06/17 22/09/17...
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4 > 8 juinRecherches et actions au service des fleuves et grandes rivières
Integrative sciences and sustainable development of rivers LYON 2018
Monitoring of a sediment
replenishment operation:
The case of the Saint-Sauveur dam in the
Buëch River (Southern Alps. France)
G. Brousse ; M. Bertrand ; G. Arnaud-Fassetta ; F. Liébault ;
L. Borgniet ; G. Melun ; R. Loire ; J.R. Malavoi ; G. Fantino
I.S.RIVERS 2018
• Research context
• Why restore - How to restore ?
• Monitoring approaches
• Hydromorphological evolution of the restored
reach
• Feedback and prospects
Outline
I.S.RIVERS 2018
Geology dominated by sedimentary rocks with black marls
A hot spot of the Alpine biodiversity with a mosaic of ecological habitats
Multiple economic uses (irrigation, hydro-electricity. Hunting, fishing, tourism …)
Research context
SMIGIBA©
I.S.RIVERS 2018
Human alterations of the physical fluvial
corridor :
• Gravel mining
> 3 Mm3 in the upper catchment
> 6 Mm3 in the lower catchment
• Flood protection and hydropower
equipments
River regulation (before 20th century)
Saint-Sauveur dam (1991 and 1992) => dredging
of 600,000 m3 of sediment
• Bedload has been strongly impacted
Why restore - How to restore ?
Rapid channel responses :
• Alluvial fan is aggrading upstream the reservoir
• Narrowing and degradation of the active
channel with downstream propagation
• Marly bedrock outcrops are observed along the
degraded reach
• A shift from a braided to a wandering pattern
I.S.RIVERS 2018
THE REPLENISHMENT OPERATION
• September 2016: 1 million euros
• Dredging the alluvial fan of the Saint-
Sauveur reservoir
• Clearing alluvial bar
• Sediment replenishment downstream
of the dam (44,000 m3)
• 2,500 m3 by day (7 tip-truck)
• Limiting flood risk
• Stop incision
Why restore - How to restore ?
NEW RULES OF DAM EXPLOITATION
1997 : transparency for Q>80 m3/s
(without forecast)
2015 : transparency for Q>60 m3/s
(according to forecast on 24h)
Frequency and magnitude of
transparency are higher in order to
optimize sediment continuity
Right berm central berm left berm
« Point bar stockpile method » (Ock et al.. 2013)
I.S.RIVERS 2018 Monitoring device
Database
Topography
Field surveys
Hydrology
Time lapse
• Grain-size sampling
• Marl outcrops
• Flood marks
• Bedload tracing with
active UHF RFID tags
• Sequential LiDAR
• Drone survey and SfM
• 2 cameras
• 1 photo / hour
• 6 months
OBJECTIVES :
CAPTURE THE GEOMORPHOLOGICAL AND BIOLOGICAL RESPONSES OF THE DEGRADED REACH TO THE ARTIFICIAL GRAVEL RECHARGE
• Floodgates hydrology
• Overflow wear hydrology
I.S.RIVERS 2018
0
50
100
150
200
250
300
350
400
18/08/16 26/11/16 06/03/17 14/06/17 22/09/17 31/12/17 10/04/18 19/07/18
Q (
in m
3/s
)
Time
Monitoring device
Q10
Q2
REPETITIVE DATA ACQUISITION
5 Airborne LiDAR
1 SfM
3 bedload tracing inventory
3 field surveys
Hydraulic transparency
LiDAR
Sediment tracking
Field survey
SFM
RESULTS
I.S.RIVERS 2018Hydromorphological evolution
CHARACTERIZATION – Just after restoration (September 2016)
I.S.RIVERS 2018Hydromorphological evolution
CHARACTERIZATION – During flood (November 2016)
Berms erosion
Hydraulic transparency
I.S.RIVERS 2018Hydromorphological evolution
CHARACTERIZATION – Just after flood (December 2016)
I.S.RIVERS 2018Hydromorphological evolution
THE REPLENISHMENT SITE – Post flood (December 2016)
622
623
624
625
626
627
628
629
0 20 40 60 80 100
Altitu
de
(m
)
Distance (m)
1
1
2
3
2
3
SC
SC
Very high erosion on the central and on the right berm during flood
➔ 20,000 m3 (46% of initial berms)
Net sediment budget ➔ -11,000 m3
Post floodPost restoration
-14 000
-12 000
-10 000
-8 000
-6 000
-4 000
-2 000
0
2 000
LEFT CENTRAL RIGHT
MC
MC
Δz (m)
1
2
Vo
lum
e (i
n m
3)
Alt
itu
de
(in
m)
I.S.RIVERS 2018Hydromorphological evolution
THE REPLENISHMENT SITE – Flood modelization
0; 0.5
0.5; 5
5; 10
10; 20
20; 30
30; 50
50; 70
70; 90
>90
• 2D Telemac model of the Q10 flood
(Durand-Gasselin. 2017)
• Field calibration (grain size and flood
marks)
Shaping of the secondary channel?
• A net limit between MC and SC
• Flow dividing
• Minimize erosion in SC
• Erosion of SC du to lateral and regressive erosion
• No linear erosion
• Too far of the flow axis
Bed shear stress (in N/m²)
I.S.RIVERS 2018Hydromorphological evolution
622
623
624
625
626
627
628
629
-10 10 30 50 70 90 110
Altitu
de
(i
n m
)
Distance (in m)
1
2
3
SC
Post floodPost restoration 1 year later
Very small evolution during last year But the main channel erode
the right bank
Net sediment budget ➔ -150 m3
0 30 60 90 12015m
THE REPLENISHMENT SITE – One year later (September 2017)
1
2
3
Δz (m) MC
I.S.RIVERS 2018
Aggradation max in the 2.5 km
downstream reach
Global net deposition
Hydromorphological evolution
THE DOWNSTREAM REACH – Longitudinal profile
-1
-0,5
0
0,5
1
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500
-1
-0,5
0
0,5
1
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500
Talweg
After flood After 1 year
1
2
Δ(m
)Δ
(m)
Active channel
Replenishment
zone
POST FLOOD N+1
Min -0.85 -0.67
Mean 0.12 -0.08
Max 0.76 0.18
SD 0.34 0.11
POST FLOOD N+1
Min -0.86 -0.14
Mean 0.06 0.002
Max 0.60 0.21
SD 0.22 0.04
I.S.RIVERS 2018Hydromorphological evolution
THE DOWNSTREAM REACH – Sediment budget
Hypothesis: Sediments from eroded berms
have been exported to the downstream reach
2,500 m
REPLENISHMENTZONE
DOWNSTREAM REACH
Berms erosion = 20,000 m3
INPUT STORAGE CHANGE OUTPUT
500 m
Bedload deposit = 9,000 m3
DEM differenceBedload
calculation
DAM
I = ΔS + O
46,000 m3 34,000 m3 12,000 m3
Bedload passing = 26,000 m3
35
,00
0 m
3
I.S.RIVERS 2018Hydromorphological evolution
THE DOWNSTREAM REACH – Travel distance from berms erosion
n = 148
Recovery rate = 68%
Mean travel distance =1,150
Max = 3,240 m
Only informations upstream the dam
0
5000
10000
15000
20000
25000
30000
35000
0 500 1000 1500 2000
Vo
lum
e (m
3)
Distance (m)
Cumulative sediment budget
Bermserosion
20,000 m3
Distance travel from berms erosion: [0; 1600] m
I.S.RIVERS 2018Feedback and prospects
In one year: Hydromorphological objectives are achieved
• Great erosion of initial berms ➔ sediment input to the downstream
reach
• Net aggradation on the 6 km downstream reach (main channel and
active channel)
• Change are more important in the upper part (2.5 km) o Aggradation is greater
o Main channel erosion
Real impact of hydraulic transparency which permits a net gain of the
sedimentary budget
I.S.RIVERS 2018Feedback and prospects
Exploitation feedback
Positive
• Monitoring show the efficiency of the replenishment operation
• Efficiency is link to ideal conditions (hydrology and exploitation)
• New knowledge about the area of influence of hydraulic transparency
Negative
• Shaping of a secondary channel is not a good solution in this site
• Left berm are now non erodable for frequent flood
Prospects 2018-2020
• Distinguish impact of frequent flood with or without hydraulic
transparency
• Monitoring of the channel degradation
• Monitoring of the left berm pushing in the active channel (september
2018)
I.S.RIVERS 2018
Thank you for your attention
UFR GEOGRAPHIE HISTOIRE ECONOMIE ET SOCIETES (GHES)
UMR 8586 PRODIG
Guillaume BrousseDoctorant en Géographie
Bâtiment Olympe de Gouges – Case courrier 70015 rue Thomas Mann – F – 75205 Paris Cedex 13+33 (0)6 77 18 97 98www.univ-paris-diderot.fr | www.prodig.cnrs.frguillaume.brousse@[email protected]