Sediment management plans in French mountainous
catchments with illustrations from emblematic projects
Dr. Benoit Terrier
Agence de l’Eau Rhône Méditerranée et Corse, FRANCE
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
Sediment management in the Rhone catchment
On the Rhone and mediterranean
catchment:
2/3rd 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
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
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 2017.
⇒Category I: needs to dealt with in 2012
⇒Category II: at least some studies for 2012 and dealt with in 2017
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…
Sediment management in the Rhone catchment
1.
2.
3.
Three restoration
projects related to
sediment
River Ain
River Durance
River Fontaulière
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
Managing the erodible corridor (2/6)
Analysis of granulometry (3/6)
Pont d’Ain
Priay
Gévrieux
Mollon
Armoured area
Armouring happening
Coarse sediment
Charnoz
St Maurice
de Gourdans
Sediment budget evaluation (4/6)
Incoming from river
Albarine
Channel incision
Priay
Channel deposition
Outgoing into the
river Rhone
Incoming from
upstream
Pont d’Ain
Gévrieux
Mollon
Storage in river banks
Storage in floodplain
Significant sediment sources
Sediment sources
Storage area
Significant storage area
Charnoz
St Maurice
de Gourdans
Sediment management
scenarios
Sediment refilling and oxbow restoration (6/6)
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
River Durance project (2/6)
River Durance project (3/6)
Experimental approach:
=>Finding the right flow rate and the right duration of the
event
In 18th Sept 2007: Q=80m3/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
River Durance project (4/6)
Conclusions for 2007:
• 80m3/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
River Durance project (5/6)
In 2009:
3 monitoring sites instead of 5, 60m3/s instead of 80m3/s
Results
•60m3/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?
River Durance project (6/6)
In 2010:
60m3/s twice in the year
Results
•Confirm that 60m3/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)
River Fontauliere project (1/3)
Dam with storage capacity of 180 000m3
Original purpose of the dam: water supply during draughts to
160 000 people
Capacity reduced by 1/3rd due to sediment build up
Objectives:
1. Remove 45000m3 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
River Fontauliere project (2/3)
Removal of the dredged material in 2011
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, 50000m3 of dredged material, River Drac, 15000m3
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
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)
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
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
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?
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)
Thank you for your attention
[email protected]
Agence de l’Eau Rhône Méditerranée et Corse
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
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)
River Ouvèze project
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
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)
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
River Isere project
Ecological input to the project
300 ha of alluvial forests
3 valued natural areas
8 former gravel extraction sites
restored
7 reconnexions piscicoles
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: 2009 - 2015