impact of human interventions on estuarine dynamics regime shifts

Transcription

1 impact of human interventions on estuarine dynamics regime shifts Han Winterwerp Deltares and Delft University of Technology

2 concentration [mg/l] high and low water level [m] rationale for analysis 8 observations Ems River Papenburg 4 high water 2 low water C ~ 30 g/l distance from river mouth [km] near-surface sediment concentrations 1000 The Ems River, Germany Emden / / after February de Jonge, distance from Herbrum Hebrun [km]

3 high and low water levels [m rationale for analysis 8 observations Loire Rver Nantes 6 high water low water Nantes distance from river rmouth [km] The Loire River, France g/l

4 study approach & steps reduction of effective hydraulic drag with SPM (key issue) analytical solution of linearized water movement equations dimensionless parameter groups: estuarine convergence number L e dimensionless damping k i dimensionless roughness r * analysis of solutions w.r.t. human interventions (feed-back) analysis of historical data of four rivers calibration on analytical model to historic data (tuning r * ) snow-ball effect (positive feed-back) leads to regime shifts general perspective

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6 study approach & steps reduction of effective hydraulic drag with SPM analytical solution of linearized water movement equations dimensionless parameter groups: estuarine convergence number L e dimensionless damping k i dimensionless roughness r *

7 the configuration and our simplifications river s cross section with intertidal area b c Db h h u = 0 A a c h = constant river s plan view exponential trumpet shape ETM 1 balance between: river flow induced flushing & estuarine circulation ETM 2 balance between: river flow induced flushing & tidal asymmetry

8 present situation in Ems River (ETM2-conditions) no relation between SPM & salinity salinity suspended sediment ebb Q riv < 30 m 3 /s salinity suspended sediment tidal excursion flood Emden Herbrum (weir) Talke et al., 2009, data Aug 2, 2006

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12 study approach & steps reduction of effective hydraulic drag with SPM analytical solution of linearized water movement equations dimensionless parameter groups: estuarine convergence number L e dimensionless damping k i dimensionless roughness r * analysis of solutions w.r.t. human interventions (feed-back)

13 estuaries and navigation deepen for navigation natural estuaries are characterized by intertidal flats and multiple channel systems

14 tidal amplitude at 60 km [m] tidal amplification by ongoing deepening 1.5 tidal amplitude at 60 km relative to tidal amplitude at river mouth (= 1 m) sand bed, with intertidal area water depth [m]

15 navigation in estuaries yielded ongoing deepening and canalization narrow to keep depth Loire mouth

16 tidal amplitude at 60 km [m] tidal amplification by ongoing deepening and canalization 1.5 tidal amplitude at 60 km relative to tidal amplitude at river mouth (= 1 m) 1.0 sand bed, without intertidal area 0.5 sand bed, with intertidal area water depth [m]

17 tidal asymmetry by ongoing deepening and canalization seaward transport up-estuary transport tidal asymmetry at x = 25 km from the mouth pumping in mud sand bed, without intertidal area sand bed, with intertidal area water depth [m]

18 tidal pumping of mud by ongoing deepening & canalization seaward transport up-estuary transport tidal asymmetry at x = 25 km from the mouth muddy bed, without intertidal area sand bed, without intertidal area sand bed, with intertidal area water depth [m]

19 tidal amplitude at 60 km [m] tidal amplification by ongoing deepening and canalization 1.5 tidal amplitude at 60 km relative to tidal amplitude at river mouth (= 1 m) muddy bed, without intertidal area 1.0 sand bed, without intertidal area 0.5 sand bed, with intertidal area water depth [m]

20 study approach & steps reduction of effective hydraulic drag with SPM analytical solution of linearized water movement equations dimensionless parameter groups: estuarine convergence number L e dimensionless damping k i dimensionless roughness r * analysis of solutions w.r.t. human interventions (feed-back) analysis of historical data of four rivers: Ems (Germany) Loire (France) Elbe (Germany) Scheldt (Netherlands/Belgium) (Yangtze in progress)

21 evolution tidal range in 4 rivers Ems Loire Elbe Scheldt

22 relative damping ki [-] example of analysis (1) evolution dimensionless damping Emden - Leeroort Leeroort - Papenburg < > estuarine convergence number L e [-] every data point represents a river stretch and a certain time period

23 example of analysis (2) relative damping ki evolution dimensionless damping Emden - Leeroort Leeroort - Papenburg r * = 1.7 < r * = > 1995 r * = 7 r * = 2.5 r * = solution of analytical model after tuning r * assess C (eff. drag) estuarine convergence number L e every data point represents a river stretch and a certain time period

24 Chezy coefficient [m 1/2 /s] evolution of hydraulic drag in 4 rivers Ems evolution of hydraulic drag Loire Emden - Leeroort Leeroort - Papenburg year Elbe Scheldt

25 Chezy coefficient [m 1/2 /s] summary of drag reduction 125 effecitve hydraulic drag of various rivers hyper-turbid C = 18log(12h/k s ) "normal" characteristic water depth [m] 1: Elbe-outer; 2: Elbe-inner; 3: Ems/E-L; 4: Ems/L-P; 5: Loire/P-C; 6: Loire/C-laM; 7: Loire/LaM-N; 8: Scheldt/S-T; 9: Scheldt/T-StA; 10: Scheldt/StA-D; 11: Thames; 12: Severn-inner; 13: Severn-outer; 14: Western Scheldt; 15: Gironde-outer; 16: Gironde-inner; 17: Yangtze estuary; 18: Vilaine

26 tidal range [m] summary tidal amplification 6 summary tidal evolution Antwerp Bremen Hamburg Nantes Papenburg year

27 the snow-ball effect leading to a regime shift too big ships too much deepening reduced river flushing tidal amplification reduction in hydraulic drag increase tidal asymmetry pumping of mud this takes time (decades?)

28 general conclusions (1) creating extreme rivers by engineering works

29 general conclusions (2) necessary conditions for regime shift: large tidal range (macro-tidal) and flood-dominant conditions sufficient fine sediments (mud) little accommodation space (siltation areas) historic development of many European rivers: end 19 th century profound embanking & narrowing: loss of resilience loss of accommodation space 2 nd half 20 th century large deepening triggered snow-ball effect climate change: further decrease in resilience??? regime shift (symptoms) takes time (availability of sediment) new regime is very stable new physics dominate

30 tidal range [m] general conclusions (3) 6 summary tidal evolution 4 Antwerp 2 Bremen Hamburg Nantes 0 Papenburg year tidal amplification because of: 1. geometrical effects (deepening and narrowing) 2. loss of hydraulic drag (takes time decades?) 3. reflection against weir this effect increases with loss in drag

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32 effect suspended matter on hydraulic drag 30 C/g C0/g 0.5 decrease in effective drag 20 h = 1 m h = 5 m h = 10 m h = 20 m C SPM g 4h Ri b typical regime of rivers discussed here, increase in C by m 1/2 /s 10 Ri * = Richardson number Ri * b increase in SPM b = Rouse number C = Chézy coefficient semi-analytical approach by by Winterwerp et al., 2009

33 water movement in compound channel h Au c t x u h ru g 0 t x h c b Db 0 h Ac u 1 bc 1 h Ac u t bc Db x bc x h x bc Db u h ru g 0 t x h linear friction: r 8c U 3 D exponential convergence: b b x L c exp 0 b 0 exponentially converging channel

34 dimensionless parameter groups k k ik r b * 2 * e * 2kL r i b r gu h hc bc Db b L b c Lb 4Lb gh ga b c tot dimensionless imaginary wave number (= tidal damping/amplification) dimensionless effective hydraulic drag estuarine convergence number (topography & bathymetry)

35 the analytical solution the (complex, dimensionless) dispersion equation: 2 k k k 2ik L 1 ir 0 the real and imaginary (dimensionless) wave numbers: e * r 2 Le 1 Ler* 2 Le L 1 L r 2 L 1 2 i e e * e (proxi for) tidal asymmetry: c c HW LW k k r, LW r, HW +/- for incoming wave -/+ for reflecting wave