EFC - FAO WP MMW - Working Group IIWorkshop n°3

Sediment and woody debris transport regulationin torrential catchments.

Case study : Torrent of “La Balme” (Savoy - France)

May 8th, 2017

PRESENTATION OF THE « TORRENT DE LA BALME »

• Description of the catchment : historical events, geology,

land use, stakes …

• Description of the works along the torrent : in the state forest, in the village,

• Hydraulic context : Type of floods, woody debris, analysis of the longitudinal profile,

• Hazard map,

EFC WP MMW - WGII - Workshop n°3

LOCATION

EFC WP MMW - WGII - Workshop n°3

DESCRIPTION OF THE CATCHMENT

EFC WP MMW - WGII - Workshop n°3

Surface :

6.64 km2

Altitudes :

max : 2 696 m

min : 340 m

Longest path :

4 860 m

Mean slope :

0,12 m/m

To fullfill the request of the Ministry of Agriculture for establishing « Watershed Risk Studies » :

• assess the degree of hazard control in state owned forests,

• fix the management measures over several years,

To help the municipality of Argentine deal with a repetition of events in the lower catchment (numerous rock falls since 2011, obstruction of the torrent’s channel in 2013),

As a result : a global study covering the whole catchment,

EFC WP MMW - WGII - Workshop n°3

Why carry out a study on the « torrent de la Balme » ?

EFC WP MMW - WGII - Workshop n°3

Municipality of Argentine

RTM Forest « La Lauzière »

Forest perimeters

EFC WP MMW - WGII - Workshop n°3

Torrentiel events

• XVIIIe century : March 1776

• XIXe century : 9 events that caused damage to the road and houses

(1810, 1811, 1812, 1846, 1859, 1860, 1893, 1899, 1900)

• Between 1900 and 1940 : 7 events

(1901, 1908, 1911, 1912, 1931, 1940)

Historical record

1912

EFC WP MMW - WGII - Workshop n°3

Torrentiel events

• Since 1941 : 5 events

(1977, 1987, 1995, 2005, 2007)

Landslides and rock falls

• 2011, 2013,

Historical record

EFC WP MMW - WGII - Workshop n°3

Torrent de la Balme, 2013

Workshop WPMMW - Woody Debris

Geology

Biotite Granites

Granitic inclusions

Amphibolites

Green schists

Gneissic granite

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Natural hazards

Potential Rockfalls

Active Rock falls

Potential Landslides

Active Lanslides

Historical floods

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Land use

Coniferous forest

Deciduous forests

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Stakes

Main road

Railway

Secondary RdARGENTINE

Water canalization

Workshop WPMMW - Woody Debris

Torrent equipment (in the state forest)

• Serie of 10 check dams created after 1941 (acquisition of the forest by Agriculture Ministry

Workshop WPMMW - Woody Debris

Torrent equipment (in the state forest)

• Periode of construction: 1957-2000

• Structures made of concreted riprap

• Dimensions: 10m < width < 20m - mean height :1-2m (max height=10m)

• Objectives: riverbed and banks stabilisations

Workshop WPMMW - Woody Debris

Torrent equipment (in the village)

bridges

Workshop WPMMW - Woody Debris

Torrent equipment (in the village)

Bank protectionAgainst torrent divagation

Workshop WPMMW - Woody Debris

Torrent equipment (in the village)

digue

• Old digues created to prevent overflow in the village and to contain debris flow deposit > Ineffective today

Workshop WPMMW - Woody Debris

Torrent equipment (in the village)

Chek dams

Banks protections

Workshop WPMMW - Woody Debris

Torrent equipment (in the village)

channel

Workshop WPMMW - Woody Debris

Type of floods

• A succession of period with no activity and period with activity

• Activity of the torrent is related to rock slide in the catchment

Return period 10 years Return period 100 years

8 m3/s 17m3/s

• Hydrology

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Woody debris

• 2 types of floods:

– Flood with bedload transport

– debris flow related to rock slide (1908-1931-1977…)

debris flow are the most important events

Workshop WPMMW - Woody Debris

Longitudinal profile

– Gradual reduction of the slope:

– No regulation of sediment transport

– Progressive decrease of transport capacity

Contribution of the « ravine » (2013)

low slope for debrisflow transition

No more transport capacity

Arg

enti

ne

bri

dge

Transport capacity: 20 000m3

Transport capacity: 2000 m3

Transport capacity: 150 m3

Workshop WPMMW - Woody Debris

Bedload transport during floods

Argentine bridge

• Flow modeling including bed evolution during the flood (EFL model – HEC RAS model)

– Important deposition but no overflow– The section of Argentine bridge is OK– BUT, woody debris are not taken into account

– Overflow in the lower part – risk for the railway

??

Workshop WPMMW - Woody Debris

Debris flow

• contribution of debris flow (quantification)

– Empirical formula (Rickenmann, d’Agostino…)

– ECSTREM method

– Historical approach

Scenario volume Return period

Rare 40 000 m3 100 years

Exceptional 90 000 m3 > 100 years

maximum 150 000 m3 ?

• Reference event: volume 40 000m3

70m3/s< flow rate < 130m3/s

Workshop WPMMW - Woody Debris

Woody Debris

Sources of WD:

• Forest cover 65% of the catchment

• Vegetation presence in the riverbed and banks

• Land slides contributions

Consequences of WD?

• Modification of flow direction

• Trapping under bridges

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Debris flow: flow condition

• Herschel-bulkley model

5m

Flo

w h

eig

ht

Chanel slope

Workshop WPMMW - Woody Debris

Debris flow: flow condition

• Herschel-bulkley model

Arg

enti

ne

bri

dge

Flow height2 - 4m

Flow height2,5 - 5m

Flow height3 - 4m

2,7m

Workshop WPMMW - Woody Debris

Hazard map

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Aim of the exercise

• Describe the best adapted methodology to study a torrential catchment such as the “torrent de la Balme”,

• Propose a project (or variations) to manage sediment and woody debris transport in this case :

o Location,

o Schematic design of works,

o Aim of each work

o Maintenance and management aspects,

• Discuss and share experiences around this case study