Managing sediment continuity at hydropower dams : conceptual and operational approach

Transcription

1 Workshop 2 : Planning, managing and restoring hydromorphology at river basin scale, within an ecosystem services framework Managing sediment continuity at hydropower dams : conceptual and operational approach MALAVOI Jean-René Rémi Loire EDF DPIH CIH

2 1. General comments about sediment transport and dams 2. How to broach the sediment continuity concept - Assessment of Sediment Continuity (SC) at the dam level (typology, dam diagnosis, sediment transport measurement) - Assessment of Sedimentary State (SS) downstream of the dam - Some results: SC et SS 3. Examples of operative measures

3 1. General comments about sediment transport and dams

4 The classic pattern : hungry water (Kondolf, 1997)

5 Downstream incision, loss or pavement of the alluvial bed cover -2.7m 60 km downstream

6 Fluvial metamorphosis (Isar at Ascholding). Dnstrm Sylvenstein Dam(1959) Caution intensive past gravel mining

7 2. How to broach the sediment continuity concept?

8 Legislative context The french law : L (2006) and the implementation circular (2013) The law : L defines A list (list 2) of streams, parts of streams or channels in which it is necessary to assure sufficient sediment transport and circulation of migratory fishes

9 Help to implementation : the circular (2013) The primary goal that is targeted by the article L , is to guarantee the survival of aquatic communities (fishes, invertebrates, vegetation), habitats of which are subordinate in the alluvial substratum. It is a question of insuring, on the average/long term, a surface, a thickness a gravel size distribution a layout (weak armouring) a frequency of movement of alluvial substratum, allowing for the survival of the species of the aquatic communities inside the considered reach

10 First level Typology of the «sediment continuity» class of the EDF dams The «transparency» of a dam to coarse sediments transport can be characterized by 3 metrics : Passing through volume (PV) / incoming volume (IV) Passing through gravel size (PG) / incoming gravel size (IG) Passing through frequency (PF) / natural frequency (NF)

11 sufficient sediment transport Dam totally transparent to coarse sediment transport Dam usually transparent to coarse sediment transport PV= to IV PG= to IG PF= to NF PV to IV and/or PG to IG and or PF to NF Class % Dam transparent to a part of coarse sediment transport Dam impermeable to coarse sediment transport but sand and finer sizes can pass through PV to << IV PG to << IG PF to << NF PV << to 0 IV PG << to 0 IG PF << to 0 NF 3 20 % Dam totally impermeable to sediment transport % Summary assessment for 960 EDF dams and weirs (currently being validated. )

12 Weir Examples of dams class 1 or 2 Mobile dam 2 Barrage mobile (prise d eau de montagne) Fixed dam with large low level gates 1 And frequent sluicing 2

13 Weir or mobile dam with long reservoir 4 Examples of dams class 4 or 5 Very large dam 5

14 Second level Meanwhile the typology and sediment transport measurement at the dam level, we make «sedimentary state diagnosis» based on the concepts developped by the Circular

15 Concepts and objective of the SS diagnosis Verify if, in the river reach concerned by the dam (that is downstream), the 5 descriptive parameters of the coarse alluvial substratum quoted in the Circular : surface, thickness gravel size layout frequency of movement are compatible with the survival of the aquatic community "targets" present (or in theory present) in this reach

16 We thus try to estimate if the potentially impacted reach presents a sufficient state" from the point of view of alluvial habitats

17 15 to 20 transects measurements along the river reach Bankfull bed Wetted bed On every measurement point (10 pts / transect) : - water depth - granulometric patch in a 0.5 m radius around the point (EVHA method) - coarse sediments thickness > or < 25 cm - benthic clogging class (protocol Irstea Archambaud)

18 Gravel Patches EVHA (Malavoi, 1989) PG CG Here : + Gros Dom1 Dom2 PG PF CG PF PF CG PG PG PG PF CG CG CG PF PF

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20 RésultsRésultats générauxdiagnosis to date for 60 sedimentary 40 based on typology 20 based on the description of «sedimentary state» 5 cases with sed state «insufficient» NB : many cases with no incoming sediments

21 * If sufficient state : no immediate action * in the other cases, IMPLEMENTATION OF OPERATIONAL MEASURES - improvement of sediment management (sluicing), - dredging => downstream sediment replenishment - improvement of bank erosion downstream - dam or weir removal

22 Most recommended method : the sluicing Concept : avoid/limit sedimentation => full drawdown and gates opening during floods

23 Alternative method for coarse sediments continuity : dredging => downstream replenishment Fontaulière à Pt de Veyrière Vieux Rhin à Kembs upstre Buech à St downst

24 Alternative method for coarse sediments continuity : improve downstream bank erosion (free space) Vieux Rhin à Kembs after bank protection removal

25 Most radical method : weir or dam removal Seuil de Trentepas sur la Bléone Barrage St E. du Vigan sur l Allier

26 Thank you for your attention! Sediment continuity is really cool for sleeping