Upper Drac River restoration project

Transcription

1 Upper Drac River restoration project Restoration of a braided river bed incised in clay substratum through sediment reloading and bed widening SHF Conference, Nantes, 6 & 7 october 2014 Small scale morphological evolution - SESSION 6 : Braided streams F.Laval, E.Guilmin

2 Introduction Intervention contents : 1. Initial situation, diagnostic and causes 2. Project objectives and aborted solutions 3. Project engineering 4. Restoration works BURGEAP s role : Contracting authority : CLEDA (SAGE Drac amont, River Contract) Technical assistant : ETRM (Vincent Koulinski) BURGEAP (2012) : pre-faisability study BURGEAP ( ) : project design and works supervision ( winter)

3 1- Initial situation, diagnostic and causes What differences between these 2 rivers? Which one is in a good shape? Degraded reach (linear and incised pattern) Non-influenced reach (braided pattern)

4 1- Initial situation, diagnostic and causes Degraded reach (3.6 km) 2 to 4 m incision 10 m headward erosion

5 1- Initial situation, diagnostic and causes Historical pressions on Upper Drac river Extractions ( ) - 3 to 4 m incision St-Bonnet-en- Champsaur 3 long-profile stabilization weirs Champsaur lake (1990) 40 years of pression on sediment continuity Major floods (1990, 2001, 2006, 2008, 2011) Reference upstream reach : Chabottes plain (different geomorphological context ) m3/y Study reach : 3.6 km Gravel mining ( ) > m3/y

6 1- Initial situation, diagnostic and causes A major factor : soft clay substratum under bed materials Alluvial material thickness : 1 to 3 m Incision between 2005 and 2012 : 2 to 4 m in clayey substratum Alluvial materials Clay Bed incision in clay bedrock and chenalization

7 1- Initial situation, diagnostic and causes Impacts of incision were multiple Fluvial type consequence : Fluvial pattern tended from braided to recti-linear Active bed width reduced to m to 200 m 30 to 50 m

8 1- Initial situation, diagnostic and causes Ecological consequences : Drop of groundwater table Slow death for 60 ha of alluvial forest Alluvial forest has been deconnected Groundwater drop caused forest drought

9 Surface (m²) 1- Initial situation, diagnostic and causes Ecological consequences : Deconnection of tributaries Poor fish habitats and spawning (homogeneous and clogging) Drying out of tributary connected to groundwater («adoux») Colmatage (%) High clogging of fish habitats Adoux drying out Deconnected tributary after bed incision

10 1- Initial situation, diagnostic and causes Hydraulic consequences : bankfull discharge increased up to Q10 (200 m 3 /s) the river was more and more powerfull inside its bankfull bed Pour Q10 Pour Q10 Pour Q2 Pour Q1 Water level downstream STEP of Laye Q bf Q10 for most reaches Bankfull discharge increasing

11 1- Initial situation, diagnostic and causes Geological consequences : Bank and hillslope landslides Clay/silt material supply, useless for sediment reload and fish habitats Suspension material supply

12 1- Initial situation, diagnostic and causes Above all, potential damages for water and land uses : Lost of the river s social role (no fishing, no trail, no view on the river, etc.) Risks for security (lake dike : 5 m high / 8 m above Drac bed) Landslide threat : national road (RN85), isolated buildings, networks Headward erosion threat (3 to 5 km upstream : bridge, villages) Irreversible trends which needed urgent intervention 3 m incised bed along lake dike Eroded foot of dike and bank protection

13 2- Project objectives First principles : Stop upstream gravel mining (2012), Reload the bed with gravel/pebble sediments, above clay substratum

14 2- Project objectives Technical principles : Reload from old terraces and external supply, with the same or coarser granulometry ; Slope : 1,0% (continuous slope) ; Long profile level : close to 1913 long profile (downstream weir +1,6 m) ; Bed rewidening (> 80 m) ; Aborted solutions : Series of groins : Would not have been efficient Series of cross-section weirs : Would have deprived downstream reaches which were already fragile ; Would have been very expensive (150 to 250 m large needed to avoid by-pass) Move back of the Champsaur lake dike : Would have been very expensive (1km-long and 5m-high dike) ; Better solution : rewidening on the opposite bank (old alluvial terrace with supply in materials)

15 3- Project engineering Examples of cross-sections Profile #12 Forest clearing Forest maintaining Retalutage Argiles Excavation Elevation STEP

16 3- Project engineering Examples of cross-sections Profile #4 Pépinière Excavation Protection Lake Protection Elevation Clay lake fields

17 3- Project engineering Cross-section pattern V-profile with 1% side slope (0.40 m between thalweg and borders) and with a thalweg small sinuosity (1.1) The braided type modeling is left to the river (first flood is expected) 80 m 80 to 250 m

18 3- Project engineering Granulometry Reference granulometry (Chabottes plain) : 4 to 6 cm for Dm Coarser sediments in the excavation zones : 4 to 8 cm for Dm, blocks (5 to 25 %), sand (30%) Sand is kept as the sediment matrix

19 3- Project engineering Overview plan Excavation zones Elevation zones Kept alluvial forest

20 4- Works, results and survey Main figures : Main works : Total earthmoving : m 3, including m 3 external coarse supply Active bed surface restored from 14 to 40 ha (width : 38 to 110 m on average) Other works : Downstream weir (Déchèterie) : +1,65 m enhancement ; Bank protections were rebuilt for the lake dike Ecological engineering : wetlands, corridors, adoux, etc. Budget : 3,6 M Works conditions Period from nov 2013 to april months long, from November to April (up to m 3 /day) Adaptations during the works to balance pebble/gravel volumes Bad hydrological conditions because of high discharge Lost of m 3 within the derivation channel Morphological et ecological survey for 10 years.

21 4- Works, results and survey Pictures of the works Downstream weir, with fish and kayak passes

22 4- Works, results and survey Pictures of the works Earth-moving works

23 4- Works, results and survey Pictures of the works Before and after the works

24 4- Works, results and survey Pictures of the works Before and after the works

25 4- Works, results and survey Pictures of the works Waiting for the first flood

26 Conclusion Conclusion: It is probably the first sediment reload done at this scale It was motivated by irreversible conditions, special conditions (soft clay substratum) and high risks on water and land uses It was the only solution to reach good ecological status (SDAGE, DCE) the contracting authority s willpower was major in this case (only 2 years between decision and works) Main incertainty about permanent equilibrium : the wider river bed could cause sediment deposit et could slow down sediment transport to downstream reaches In mountain rivers, there is a real interest in doing works in winter, even if fish spawning is impacted for 1 ou 2 years. We expect much from the morphological et ecological survey (granulometry, hydrobiology, topography). To be followed

27 Thank you for your attention BGP280/1