flushing flows in the lower ebro: monitoring and modelling · lower ebro: monitoring and modelling...
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Madrid Meeting, November 29th 2011
FLUSHING FLOWS IN THE LOWER EBRO:
MONITORING AND MODELLINGAlvaro Tena, Leszek Ksiazek, Damià Vericat and Ramon J. Batalla
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The lower Ebro River is experiencing a series of geomorphic and ecological alterations caused by human impacts.
Water and sediment transfer in the lower Ebro River are altered by dams. The Mequinenza-Ribarroja-Flix dam-complex retain, on average, up to 90% of the suspended load (Vericat and Batalla, 2006).
Downstream from this dam-complex, frequent floods (e.g. Q2 to Q25) have been reduced by around 25%(Batalla et al., 2004).
Persistent low flows, excess nutrients (nitrogen, phosphorus) and high light availability (due to the lack of suspended sediment) have been discussed as the likely causes for the recent uncontrolled growth of macrophytes.
This massive development of macrophytes generates a series of ecological and socio-economic problems (Palau et al., 2004).
INTRODUCTION
Logo grupoPotamogeton pectinatus
Ceratophyllum demersum Potamogeton friesii Rupr.
Myriophyllum verticillatum L.
Black Fly
Photo: Cristina Buendia www.heraldo.es
Photo: Cristina Buendia www.barranquismo.org
Clogging of water intakes of Flix Hydro-Electric Power Plant, AscóNuclear Power Plant, and irrigation stations.
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How to face this problem????Controlled water releases, have been implemented (i.e. designed, monitored, and modelled) in the lower Ebro River since 2002, with the objective of removing the excess of macrophytes and keeping sedimentary activity in the channel.Here we present an integrated monitoring design to assess the effectiveness of flushing flows.This design is based in the following scheme:
Monitoring and samplingFlushing flow designModelling adverse geomorphic effectsDesign validationFlushing flow monitoringFlushing flow evaluation
(A) MONITORING AND SAMPLING-Sediment Transport, Hydraulics and Morphosedimentary Dynamics-
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Bed Entrainment
Topography
(A)MONITORING AND SAMPLING-Sediment Transport, Hydraulics and Morphosedimentray Dynamics-
Particle Sizes
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((
(((
Embarcación con GPS y ecosonda
Sediment Transport
Bed Incision
Boat with GPS and ecosound
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Bathymetry
(B) FLUSHING FLOW DESIGN -Based on a specific objective and limitations-
Sediment Transport
(A) MONITORING AND SAMPLING-Sediment Transport, Hydraulics and Morphosedimentary Dynamics-Particle Sizes
Bed Entrainment
Bed Incision
Topography
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(B) FLUSHING FLOW DESIGN -Based on a specific objective and limitations-
vτc = g · ρs’ · τc* · Di
(after Shields, 1936)
τ = d · g · ρ · s
Gravitational forces
Lift Forces
Drag Forces
vτ
τc
τ > τc‐Di → Di initial entrainment
The design of flushing flows was based on mobilizing an active layer, equal the maximum root depth of the macrophytes. This was based on the Shields entrainment function(1936).
Mac
roph
yte
Rem
oval
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Resulting peak flows were equivalent to the 1.5- to 2-year flood of the river’s post-dam flow series (i.e. ca 1350 m3s-1).
FLUSHING FLOW DESIGN
(B) FLUSHING FLOW DESIGN -Based on a specific objective and limitations-
(C) MODELLING ADVERSE GEOMORPHIC EFFECTS-Based on Monitoring and Sampling-
vτ > τc-Di → Di will be entrained
τc = g · ρs’ · τc* · Di (after Shields, 1936)
τ = d · g · ρ · s
v
Submerged Weight
Lift Forces
Tractive Forces
Sediment Transport
(A) MONITORING AND SAMPLING-Sediment Transport, Hydraulics and Morphosedimentary Dynamics-Particle Sizes
Bed Entrainment
Bed Incision
Topography
Mac
roph
yte
Rem
oval
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Hyd
raul
ic a
nd S
edim
ent T
rans
port
Mod
ellin
g Bed Elevation Channel Roughness
(C) MODELLING ADVERSE GEOMORPHIC EFFECTS-Based on Monitoring and Sampling-
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Hydraulic modellingVelocity Discharge
Shear stress
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Sediment transport modellingSuspended sediment concentration
Bed load transport rate
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Modelling geomorphic effectsBed changes
(B) FLUSHING FLOW DESIGN -Based on a specific objective and limitations-
(C) MODELLING ADVERSE GEOMORPHIC EFFECTS-Based on Monitoring and Sampling-
(D) DESIGN VALIDATION-Based on adverse effects, flooding limitations and water availability-
vτ > τc-Di → Di will be entrained
τc = g · ρs’ · τc* · Di (after Shields, 1936)
τ = d · g · ρ · s
v
Submerged Weight
Lift Forces
Tractive Forces
Sediment Transport
(A) MONITORING AND SAMPLING-Sediment Transport, Hydraulics and Morphosedimentary Dynamics-Particle Sizes
Bed Entrainment
Bed Incision
TopographyM
acro
phyt
e R
emov
al
Hyd
raul
ic a
nd
Sedi
men
t Tra
nspo
rt
Mod
ellin
g
Bed Elevation Channel Roughness Bed Changes
(D) DESIGN VALIDATION-Based on adverse effects, flooding limitations and water availability-
(E) FLUSHING FLOW MONITORING -Flushing Flow effects in Monitoring Sections-
a) Macrophyte density and topographyb) Flow hydraulicsc) Sediment transport
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a) Macrophyte density
Macrophyte densities were estimated in experimental sections before and after the FF by means of the backscatter of a sonar.
Depth
Distanceicc.cat
Macrófitos estables: 8,1 %Macrófitos incipientes: 3,3%Total macrófitos: 11,4 %
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A board-mounted ADCP was used to measure discharge and hydraulics in a monitoring section during the entire FF
b) Flow hydraulics
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c) sediment transport
Water samples were obtained at the same sections in order to calibrate the turbidity records, allowing us to obtain a continuous sedigraphs.
Punctual suspended sediment samples(Calibration of the turbidity record)
Continuous turbidity record
Turbidity has been measured in continuous at different sections by means of turbidity probes.
(B) FLUSHING FLOW DESIGN -Based on a specific objective and limitations-
(C) MODELLING ADVERSE GEOMORPHIC EFFECTS-Based on Monitoring and Sampling-
(D) DESIGN VALIDATION-Based on adverse effects, flooding limitations and water availability-
(E) FLUSHING FLOW MONITORING -Flushing Flow effects in Monitoring Sections-
(F) FLUSHING FLOW EVALUATION -Based on flushing flow monitoring and post-sampling-
vτ > τc-Di → Di will be entrained
τc = g · ρs’ · τc* · Di (after Shields, 1936)
τ = d · g · ρ · s
v
Submerged Weight
Lift Forces
Tractive Forces
Sediment Transport
(A) MONITORING AND SAMPLING-Sediment Transport, Hydraulics and Morphosedimentary Dynamics-Particle Sizes
Bed Entrainment
Bed Incision
TopographyM
acro
phyt
e R
emov
al
Hyd
raul
ic a
nd
Sedi
men
t Tra
nspo
rt
Mod
ellin
g
Channel Hydraulics, Bed Topography and Bed Mobility
Sediment Transport
Bed Elevation Channel Roughness Bed Changes
Macrophyte density control
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(F) FLUSHING FLOW EVALUATION -Based on flushing flow monitoring and post-sampling-
0 10 20 30 40 50 60 70 80 90-4,0
-3,5
-3,0
-2,5
-2,0
-1,5
-1,0
Distance (m)
Wat
er d
epth
(m
)
Bed Changes
ArmouringMacrophyte Removal
RB LB
RB LB
0 10 m
0 10 m
a)
b)
Incision
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0
5
10
15
20
25
30
35
40
08/07/2009 24/04/2010 03/06/2010 18/10/2010 05 y 22 /11/2010
Macrófitos %
t1
t2
t3
t4
t5
FFFF
MACROPHYTES COVER EVOLUTION IN THE LOWER EBRO (2009-2010)M
acro
phyt
es c
over
(%)
(B) FLUSHING FLOW DESIGN -Based on a specific objective and limitations-
(C) MODELLING ADVERSE GEOMORPHIC EFFECTS-Based on Monitoring and Sampling-
(D) DESIGN VALIDATION-Based on adverse effects, flooding limitations and water availability-
(E) FLUSHING FLOW MONITORING -Flushing Flow effects in Monitoring Sections-
(F) FLUSHING FLOW EVALUATION -Based on flushing flow monitoring and post-sampling-
vτ > τc-Di → Di will be entrained
τc = g · ρs’ · τc* · Di (after Shields, 1936)
τ = d · g · ρ · s
v
Submerged Weight
Lift Forces
Tractive Forces
Sediment Transport
(A) MONITORING AND SAMPLING-Sediment Transport, Hydraulics and Morphosedimentary Dynamics-Particle Sizes
Bed Entrainment
Bed Incision
Topography
Mac
roph
yte
Rem
oval
Hyd
raul
ic a
nd
Sedi
men
t Tra
nspo
rt
Mod
ellin
g
Channel Hydraulics, Bed Topography and Bed Mobility
Sediment Transport
Bed Elevation Channel Roughness Bed Changes
Bed Changes, Entrainment, Armouring
Macrophyte Removal
Macrophyte density control
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FLUSHING FLOWS IN THE LOWER EBRO
Flushing flows do not exhibit severe geomorphic impacts after evaluation.
Effectiveness of flushing flows (i.e. rate of macrophyte removal) attains 95%, but decreases substantially downstream.
Flushing flows are an important instrument of river management in rivers subject to regulation specially in large Mediterranean rivers such as the Ebro.
Flushing flows exhibited high transport capacity for suspended sediment.
Fine to medium gravels are mobilized but bedload rates are typically low given the short duration of the events
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o BATALLA, R.J., GOMEZ, C.M & KONDOLF, G.M. (2004): Reservoir-induced hydrological changes in the Ebro River basin (Northeastern Spain). Journal of Hydrology, 290, 1-2, 117-136.
o BATALLA, R.J., VERICAT, D. (2009): Hydrological and Sediment Transport Dynamics of Flushing Flows: Implications for Management in LargeMediterranean Rivers. River Research and Applications, 25, 297-314.
o PALAU A, BATALLA R, ROSICO E, MESEGUER A, VERICATO D. (2004). Management of water level and design of flushing floods for environmental river maintenance downstream of the Riba–roja reservoir (lower Ebro River, NE Spain). HYDRO 2004: A New Era for Hydropower. Porto, Portugal, 18–20 October 2004.
o TENA, A., KSIAZEK, L., VERICAT, D., BATALLA, R.J. (2011):Assessing the geomorphic effects of a flushing flow in a large regulated river . River Research and Applications (under review).
o VERICAT, D. & BATALLA, R.J. (2006): Sediment transport in a large impounded river: The lower Ebro, NE Iberian Peninsula. Geomorphology, 79, 72-92.
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- Elaboración de una metodología de base física para la preparación de crecidas generadoras aguasabajo de embalses: Aplicación al tramo inferior del río Ebro, CICYT (REN2001-0840-C02-01/HID)- Diseño y análisis de crecidas generadoras como estrategia de reequilibrio hidrológico y sedimentario del curso inferior del río Ebro, CICYT (CGL2005-06989-C02-02/HID)- Diseño y aplicacion de crecidas generadoras como estrategia de reequilibrio hidro-sedimentario en rios regulados, CICYT (CGL2006-11679-C02-01/HID)- Encomienda de gestión relacionada con el medio ambiente en el marco del Proyecto ‘ModeloConceptual para comprender el riesgo de los fangos tóxicos contenidos en el embalse de Flix. Propuestas para su estudio científico’, CSIC, 2006-2008- Desarrollo y experimentacion de un sistema de crecidas de mantenimiento en cascada con base en criterios fisicos y economicos para la mejora hidrosedimentaria del bajo Ebro y sus principales afluentes, CICYT (CGL2009-09770 (subprograma BTE)- Assessing and predicting effects on water quantity and quality in Iberian rivers caused by global change SCARCE, MICINN, Consolider Ingenio 2010 CSD2009-00065- Diseño y monitorización de una crecida generadora en el río Ebro aguas abajo del sistema de embalses Mequinenza-Ribarroja, Endesa Generacion SA, 2002-2011- Analisis de la dinámica de las poblaciones de macrófitos en el tramo bajo del río Ebro. URS-España & Confederacion Hidrográfica del Ebro, 2008-2010
ACKNOWLEDGEMENTS