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@univ-paris-diderot.frguillaume.brousse@gmail.com