The importance of sediment supply data to modelling river morphodynamics

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Gabriella Barker
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elevation (m NAP) Pannerden channel Astrid Blom Delft

1 The importance of sediment supply data to modelling river morphodynamics Wilbers & Ten Brinke (2003) bed elevation (m NAP) Pannerden channel Astrid Blom Delft University of Technology Netherlands Waal branch Dutch Upper Rhine

2 Gorinchem Zaltbommel Tiel Nijmegen Pannerdense Kop Lobith bed elevation [m] Dutch reach of the Rhine River Bovenrijn and Waal branches flow m 2010 Source: Rijkswaterstaat river km -3.3 < < -1.7 cm/a

3 Content 1. Long-term bed degradation 2. Potential causes of bed degradation 3. The equilibrium river profile The equilibrium river profile under steady flow The effect of variable flow The effect of tributaries The effect of mixed sediment 4. Conclusions

4 Culemborg Wageningen Arnhem Pannerdense Kop Lobith bed elevation [m] Dutch reach of the Rhine River Pannerden channel, Nederrijn, Lek flow m 2010 Source: Rijkswaterstaat river km -3.2 < < -1.6 cm/a

5 Zwolle Deventer Zutphen Westervoort bed elevation [m] Dutch reach of the Rhine River IJssel branch flow 1m Source: Rijkswaterstaat river km -2.6 < < -1.3 cm/a

6 water discharge at Lobith (m 3 /s) Does the degradation rate decrease? We may observe a decrease in the degradation rate over the past years. This may be due to a lack of peak flows over this period: year

7 German reach of the Rhine River flow Frings et al. (Catena, 2014)

8 bed elevation [cm] relative to 1888 Elbe River flow river km Source: PG Erosionsstrecke Elbe (2009)

9 Problems related to bed degradation and also: destabilization of structures; flood water level increases; flood risk downstream of bifurcations may increase. Frings et al. (Geomorphology, 2014)

10 Potential causes of long-term bed degradation a) construction of levees narrowing degradation b) construction of groynes narrowing degradation c) bend cut-offs shortening degradation d) extensive dredging reduction of sediment supply to downstream reaches degradation e) construction of dams idem degradation f) coarsening of the load coarsening of sediment supply to downstream reaches degradation + (later) aggradation g) coarse augmentation measures idem degradation + (later) aggradation h) sea level change? aggradation (migrating upstream) i) temporal change of the PDF of water discharge? very difficult to say

11 Potential causes of long-term bed degradation a) construction of levees narrowing degradation b) construction of groynes narrowing degradation c) bend cut-offs shortening degradation d) extensive dredging reduction of sediment supply to downstream reaches degradation e) construction of dams idem degradation f) coarsening of the load coarsening of sediment supply to downstream reaches degradation + (later) aggradation g) coarse augmentation measures idem degradation + (later) aggradation h) sea level rise aggradation (migrating upstream) i) temporal change of the very difficult to say PDF of water discharge?

12 Potential causes of long-term bed degradation a) construction of levees narrowing degradation b) construction of groynes narrowing degradation c) bend cut-offs shortening degradation d) extensive dredging reduction of sediment supply to downstream reaches degradation e) construction of dams idem degradation f) coarsening of the load coarsening of sediment supply to downstream reaches degradation + (later) aggradation g) coarse augmentation measures idem degradation + (later) aggradation h) sea level rise aggradation (migrating upstream) i) temporal change of the very difficult to say PDF of water discharge? Frings et al. (2009)

13 Coarse sediment nourishment Source: MSc project Bart Berkhout (ongoing) flow bed elevation high time (hh:mm) This preceding degradational wave was first observed by Ribberink (1987). x coordinate (m) low

14 time COARSE NOURISHMENT bed elevation high UNISIZE EXPERIMENT low x coordinate (m) x coordinate (m) Source: MSc project Bart Berkhout (ongoing)

15 A certain river reach in equilibrium

16 Local constriction (due to levees or groynes)

17 Local constriction (due to levees or groynes) M1 backwater M2 backwater INITIAL OR SHORT-TERM RESPONSE H n H n H n

18 Local constriction (due to levees or groynes) LONG-TERM RESPONSE OR EQUILIBRIUM RIVER PROFILE

19 Local constriction (due to levees or groynes) Source: Erik Mosselman

20 Content 1. Long-term bed degradation 2. Potential causes of bed degradation 3. The equilibrium river profile The equilibrium river profile under steady flow The effect of variable flow The effect of tributaries The effect of mixed sediment 4. Conclusions

the river tends to approach after human intervention 3.

21 The equilibrium river profile 1. The undisturbed equilibrium river profile The equilibrium longitudinal profile the river tends to approach before human intervention 2. The engineered equilibrium river profile The equilibrium longitudinal profile the river tends to approach after human intervention 3. The restored equilibrium river profile The equilibrium longitudinal profile the river tends to approach after river restoration measures Viparelli et al. (2015)

22 Equilibrium river profile, for unisize sediment Conservation of sediment mass (Exner) Q cb b t x K where Q B U D Engelund & Hansen (1967) n Conservation of water mass BH Q w t x 0 Conservation of streamwise momentum 2 UH U H H gh gh CfU t x x x 2 In a steady state all t 0 Van Bendegom (1967), De Vries (1971, 1974), Jansen (1979)

23 Equilibrium river profile, for unisize sediment Flow velocity Flow depth U H e e QD BK 1/n Q Q K w w 11 / n BUe B QD 1/ n Slope VAN BENDEGOM- DE VRIES EQUATIONS S e 13 / n 3/ n CfB QD gq w K H w Van Bendegom (1967), De Vries (1971, 1974), Jansen (1979)

24 Morphodynamic steady state, for unisize sediment U H S e e e QD BK 1/n Q Q K w w 11 / n BUe B QD 13 / n 3/ n CfB QD gq w K 1/ n H w Frings et al. (Catena, 2014)

25 Equilibrium river profile, for unisize sediment Importance of the load! Flow velocity Flow depth U H e e QD BK 1/n Q Q K w w 11 / n BUe B QD 1/ n Slope VAN BENDEGOM- DE VRIES EQUATIONS S e 13 / n 3/ n CfB QD gq w K H w Van Bendegom (1967), De Vries (1971, 1974), Jansen (1979)

26 Local constriction (due to levees or groynes) LONG-TERM RESPONSE OR EQUILIBRIUM RIVER PROFILE

27 Content 1. Long-term bed degradation 2. Potential causes of bed degradation 3. The equilibrium river profile The equilibrium river profile under steady flow The effect of variable flow The effect of tributaries The effect of mixed sediment 4. Conclusions

28 water level at Lobith (m NAP) water discharge at Lobith (m 3 /s) Water discharge at Lobith Temporal variation of the flow Factor year Water level at Lobith Source: Rijkswaterstaat year

29 How to deal with temporal variations pq w Q w upstream from backwater effects De Vries (1974), Jansen (1979) S d dx

30 The effect of tributaries As they generally add more water than sediment (De Vries, 1974, Parker, 2004), tributaries induce a stepwise reduction in slope. tributary S e 13 / n 3/ n cfb Q gq w m tributary Importance of the load!

31 Yet, we see no stepwise reduction in slope. Typical river long profile bed elevation η slope S x S typically 0 x upward concave distance x Parker (2004, E-Book)

32 Content 1. Long-term bed degradation 2. Potential causes of bed degradation 3. The equilibrium river profile The equilibrium river profile under steady flow The effect of variable flow The effect of tributaries The effect of mixed sediment 4. Conclusions

33 Coarser unisize grains are harder to move bc D

34 Coarser unisize grains are harder to move bc larger exposure of coarse particles mixed sediment hiding of fines D flow Hiding effects After Wilcock & Crowe, 2003 Egiazaroff (1965)

35 Particle abrasion gravel sand silt

36 Abrasion acts as a trigger for profile concavity and so downstream fining

37 grain size (mm) Grain size (mm) Ongoing work: comparison between model results and field data Grain sizes IJssel (SOBEK-uniform sediment model) IJssel branch of the Dutch Rhine D 90 measured flow D D e αl d50 average, 1995 d90 average., Sternberg (1875) Uniform D50 Uniform D D 50 measured NEEDED: sand and gravel sediment supply data GSD bed surface distance (m) Distance from upstream boundary (m)

WL delft hydraulics. Voorspelinstrument duurzame vaarweg. Rijkswaterstaat RIZA. Prepared for:

WL delft hydraulics. Voorspelinstrument duurzame vaarweg. Rijkswaterstaat RIZA. Prepared for: Prepared for: Rijkswaterstaat RIZA Voorspelinstrument duurzame vaarweg Report April Q357.3 WL delft hydraulics Prepared for: Rijkswaterstaat RIZA Voorspelinstrument duurzame vaarweg Mohamed Yossef, Migena