Sediment management plans in French mountainous catchments with illustrations from emblematic projects Dr. Benoit Terrier

Transcription

1 Sediment management plans in French mountainous catchments with illustrations from emblematic projects Dr. Benoit Terrier Agence de l Eau Rhône Méditerranée et Corse, FRANCE

2 Presentation layout 1. The context surrounding sediment management in France and in the Rhone catchment in particular 2. Three examples of (not always quite emblematic) projects in France in terms of sediment management 3. Short statements on: i. Policy framework concerning sediment management in France ii. Sediment monitoring (enough, gaps?) iii. Gaps in sediment management that require further research

3 Sediment management in the Rhone catchment On the Rhone and mediterranean catchment: 2/3 rd of rivers have hydromorphological alterations 60% among them have an altered hydrological regime 70% among them have their biological and sediment transport continuity disrupted by weirs or dams 70% have an altered morphology (reshaping of main channel etc ) => Many of the catchments for which sediment management represent the greatest challenge are located in mountainous regions Catchments identified with issues linked to sediment

4 Measures prescribed to reach good ecological status Several measures have been prescribed to tackle issues related to sediment in order to reach good ecological status (Rhone catchment) Manage sediment transport and the longitudinal profile Improve the management of floods (frequency, duration and peak flow) in favour of morphogenic floods (measure 3C06) Remove or adapt weirs or dams that alter sediment transport continuity (list all structures, lower the structures 3C07) Take action to improve the management of structures altering sediment transport (3C09) Limit or stop negative sediment fluxes (e.g. prevent sand fluxes3c37) Set up a sediment replenishment plan (3C32) => 5 measures that require studies to define the right actions to be applied

5 French Policy framework concerning sediment management Legal requirement to publish a list of rivers for which it is necessary to reestablish sufficient sediment transport and fish migration Two lists of rivers identified: List I: rivers in very good ecological status (preserve) List II: rivers for which work is required It is a legal requirement to intervene before 2017 on identified structures (dams and weirs mainly) which are located on rivers in list II. To encourage action, around 780 structures were identified and divided into 2 categories on the Rhone catchment. Actions are partly subsidised for owners (from 50% to 80%) until Category I: needs to dealt with in 2012 Category II: at least some studies for 2012 and dealt with in 2017

6 Sediment management in the Rhone catchment Main types of restoration projects related to sediment: Dam/weir removal, with clear targets and deadlines on identified structures (many examples) Sediment replenishing (from lateral sources, taking sediment u/s of a dam and re-injecting it further d/s) few cases Setting up of a low bottom weirs to stabilise the longitudinal profile, reshaping an altered main channel to a more natural state Setting back flood defences (few cases) / reconnecting oxbows Organise flushing of dams / gate operations (few cases) etc

7 Sediment management in the Rhone catchment Three restoration projects related to sediment 1. River Ain 2. River Durance 3. River Fontaulière

8 River Ain project (1/6) Issues: 5 large upstream dams for hydroelectricity, gravel extractions and bank protection => incised river bed, erratic lateral erosion But: the river Ain has potential to be dynamic downstream of the dams The project: bring back to dynamism the river (mainly by lateral recharge), promote sediment transport where possible (1.7M ) Steering group

9 Managing the erodible corridor (2/6)

10 Analysis of granulometry (3/6) Pont d Ain Priay Gévrieux Mollon Armoured area Armouring happening Charnoz Coarse sediment St Maurice de Gourdans

11 Sediment budget evaluation (4/6) Pont d Ain Incoming from river Albarine Channel incision Channel deposition Priay Incoming from upstream Outgoing into the river Rhone Gévrieux Mollon Storage in river banks Storage in floodplain Significant sediment sources Sediment sources Storage area Charnoz St Maurice de Gourdans Significant storage area

12 Sediment management scenarios

13 Sediment refilling and oxbow restoration (6/6)

14 River Durance project (1/6) Dam flushing to mitigate silting of the river bed ex: Flushing operations at the Saulce dam Reason for flushing operations: Between Serre-Ponçon and Sisteron, generalised silting of the river bed due to reduced flows and fine sediment inputs from tributaries and catchments =>Ecological value of the reach is very much decreased The river Durance in 2007 during a flushing operation

15 River Durance project (2/6)

16 River Durance project (3/6) Experimental approach: =>Finding the right flow rate and the right duration of the event In 18th Sept 2007: Q=80m 3 /s for 10 hours, increased by steps Monitoring with initial state and measures soon after the event (5 sites, 3 cross-sections per site): Topography, river patterns (pools and riffles sequences), granulometry, measure of siltation Biological Index Fish Monitoring the following months to evaluate the time taken to come back to the initial state

17 River Durance project (4/6) Conclusions for 2007: 80m 3 /s is enough to clean superficial silting but maybe too high as it leads to local overbank flow Substantial gain in terms of de-silting No adverse effects found on invertebrates and fish A few months after the operation, siltation goes back to the initial state (length of time dependant on hydrology) Before and during the dam flushing Silt in an oxbow

18 River Durance project (5/6) In 2009: 3 monitoring sites instead of 5, 60m3/s instead of 80m3/s Results 60m 3 /s high enough flow rate to clean superficial silting However, the relative gain in terms of de-silting is lower compared to 2007 (but initial state had higher levels of siltation) The gain is more visible in areas of higher velocities but is it good enough to significantly improve overall ecological conditions?

19 River Durance project (6/6) In 2010: 60m 3 /s twice in the year Results Confirm that 60m 3 /s high enough flow rate to clean superficial silting Gain two levels in terms of degree of silting in the Cemagref protocole However, silting levels go back to initial state after about three months Ecological gain not obvious yet after 4 years What next: retest the other flow rates and several flushing events per year (while avoiding the february-may period due to fish reproduction)

20 River Fontauliere project (1/3) Dam with storage capacity of m 3 Original purpose of the dam: water supply during draughts to people Capacity reduced by 1/3rd due to sediment build up Objectives: 1. Remove 45000m 3 of material (done last summer) to regain storage capacity 2. Re-inject sediment downstream of the dam to heal the incised bed. Originally the dam owner (private owner) wanted to sell the dredged material to the construction industry but the law on aquatic habitat in France now forbids it Removal of material

21 River Fontauliere project (2/3) Removal of the dredged material in 2011

22 River Fontauliere project (3/3) Re-injecting sediment downstream of the dam: what is the best option to do it? Where? Which quantity and in how many times? There is little experience in France yet on that topic. A few projects are about to start in the next couple of years (River Tet, 50000m 3 of dredged material, River Drac, 15000m 3 of dredged material ) Experience so far was done on re-injecting small volumes of material to see what the rivers can do There is a great deal of fear from local inhabitant s (flood, environmental impacts ) => A great deal of communication is needed to launch these projects

23 Brief statements (1/5) Policy framework concerning sediment management in France The issue is clearly identified in France since the law imposes the re-establishment of sufficient sediment transport => we aim for an identified result But there are technical questions in terms of priority (identification of the most affected reaches at stakes), how to tackle the issue and the project evaluation (monitoring)

24 Brief statements (2/5) Sediment monitoring (enough, gaps?) 1/2 Clearly there are gaps in terms of sediment monitoring The Water catchment Agencies together with ONEMA launched a network of sites selected to monitor the impacts of river restorations monitor biological parameters (related to fish and macroinvertebrates), water quality and hydromorphological parameters an initial state and a monitoring for up to 6 years after restoration 12 sites were selected in the Rhone catchment, 8 of them are in mountainous catchments with sediment related issues In parallel, a protocol to ensure minimum monitoring was developed and sent to many local river organisations to promote monitoring for project evaluation

25 Brief statements (3/5) Sediment monitoring (enough, gaps?) 2/2 Partnering between researchers and Water catchment Agency Together with researchers from Cemagref, we set up 2 research programs : _to develop a protocol in terms of direct sediment transport management _to investigate the use of Lidar to monitor bar migrations, changes to river reaches, evaluate bars morphometry

26 Brief statements (4/5) Gaps in sediment management that require further research when do we need to intervene on a river to remove sediment built up when there is a risk of flooding / when should we let the river do the job / how to build relevant cost benefits analysis? how will the river respond to dam/weir removal (impacts on groundwater, river bed evolution etc ) how to design sediment reinjection projects? what can sediment transport modelling really bring from an operational viewpoint? When is it really worth doing?

27 Brief statements (5/5) Gaps in sediment management that require further research What do we do of the braided rivers left in the catchment? => Research project to identify the braided rivers and their trajectories in terms of fluvial style (stable or not)

28 Thank you for your attention Agence de l Eau Rhône Méditerranée et Corse

29 River Ouvèze project Project background: Deficit in terms of sediment supply due to u/s dams Former gravel extractions led to incised river bed There is no susbtrate left on the river bed as the water flows directly over bedrock The river bed had been reshaped for flood alleviation purpose The groundwater table has lowered Project objectives: To promote stabilisation of river bed and new susbrate To minimise flood risk impacts To allow for fish migration

30 River Ouvèze project Project description: Pilote reach of 1km where bed level is raised by 0.4m up to 1.4m where the effects of incision are worst. Set up 4 bottom weirs to stabilise the river bed, retain substrate while allowing for fish migration Reshape the river bed to give «more» natural channel dimensions (find former plane forms)

31 River Ouvèze project

32 River Isere project Background project: At first, a serious flooding issue The river had also gone through severe hydromorphological pressures (many gravel extraction sites) which led to some severe incision In France the link between flood issues and river restoration is far from obvious. Only in rare cases like on this project do we see opportunities in flood related projects to promote river restoration Our water Agency tries to encourage ecological inputs by incentive funding

33 River Isere project Project targeting 40km of the Isere river: Flood management plan to ensure 1/200 flood defence level (by making more space to water, setting back flood defences) Ensure river restoration and promote tributaries reconnection (restore lost tributaries and former gravel extraction sites ) Target sediment transport equilibrium (removal 8 weirs)

34 River Isere project Flood management plan to ensure 1/200 flood defence level (by making more space to water, setting back flood defences) Current areas at risk of flooding New controled flooded areas

35 River Isere project Ecological input to the project 300 ha of alluvial forests 8 former gravel extraction sites restored 3 valued natural areas 7 reconnexions piscicoles

36 River Isere project Total cost of actions: 95 M, from which the environmental parts represent 20M Subsidies from the Water catchment agency : 8.5M Project schedule: