1: Universiteit Utrecht Faculty of Geosciences Evolution of a new tidal river bifurcation: Dept. Physical Geography [email protected] numerical modelling of an avulsion after a catastrophic storm surge 2: TNO Built Environment 1 2 1 and Geosciences / Geological Maarten Kleinhans , Henk Weerts , Kim Cohen Survey of The Netherlands Innundation was caused by a combination of storm surges, river floods and poor dike maintenance. Here we address: • How does an avulsion splay develop? • Is there simultaneous erosion upstream in the feeding channel? Biesbosch area, The Netherlands Reconstructed splay development Modelled splay development: (Zonneveld 1960) No tides, shallow basin, EH Tides, shallow basin, EH cum. erosion/sedimentation (m) cum. erosion/sedimentation (m) 07-Apr-1456 04:40:00 07-Apr-1456 04:40:00 12000 12000 old bifurcate silts up old bifurcate silts up Rhine 11000 11000 to avulsion locations Niie 10000 10000 eu → → w (R e M ) ) ot a m m tte a ( ( erd s 1421-1424: 9000 9000 a e e m) t t ) a a polder n n • 2 storm surges i i d d r 8000 r 8000 reclaimed o o • 2 river floods o o c c • poor dike maintenance by 1461 y 7000 y 7000 splay ¡ splay inundation from 2 sides 6000 1456 6000 1456 5000 5000 1.2 1.4 1.6 1.8 2 2.2 2.4 1.2 1.4 1.6 1.8 2 2.2 2.4 → 4 → 4 x coordinate (m) x 10 x coordinate (m) x 10 Haringvliet -4 -3 -2 -1 0 1 2 3 4 -4 -3 -2 -1 0 1 2 3 4 Haringvliet cum. erosion/sedimentation (m) cum. erosion/sedimentation (m) estuary 12-Oct-1461 02:00:00 12-Oct-1461 02:00:00 12000 12000 11000 11000 10000 10000 → → ) ) m m ( ( 9000 9000 e e t t a a n n i i d 8000 d 8000 r r o o o o c c beacons mark old y 7000 y 7000 Meuse channel ridge 6000 1461 6000 1461 5000 5000 1.2 1.4 1.6 1.8 2 2.2 2.4 1.2 1.4 1.6 1.8 2 2.2 2.4 Hypothesis (Kriele et al. 1998): x coordinate (m) → 4 x coordinate (m) → 4 x 10 x 10 -4 -3 -2 -1 0 1 2 3 4 -4 -3 -2 -1 0 1 2 3 4 cum. erosion/sedimentation (m) cum. erosion/sedimentation (m) 1. Antecedent 30-Aug-1473 08:40:00 30-Aug-1473 08:40:00 river 12000 12000 sea Channel prior to 1421 AD 11000 11000 10000 10000 → → ) ) m m ( ( 9000 9000 e e t t a a n n 2. Avulsion, new tidal basin i i d d r 8000 avulsions and r 8000 o o o o avulsions and c c y bifurcations in delta y avulsion 7000 7000 bifurcations in delta ingression splay progradation 6000 6000 continues 1473 1473 St Elisabeth storm surges 5000 5000 1.2 1.4 1.6 1.8 2 2.2 2.4 1.2 1.4 1.6 1.8 2 2.2 2.4 → 4 → 4 and river floods 1421-24 AD x coordinate (m) x 10 x coordinate (m) x 10 -4 -3 -2 -1 0 1 2 3 4 -4 -3 -2 -1 0 1 2 3 4 3. Splay/delta, upstream erosion cum. erosion/sedimentation (m) cum. erosion/sedimentation (m) 10-Jul-1497 15:20:00 10-Jul-1497 15:20:00 erosion? 12000 12000 alternating bars alternating bars splay 11000 11000 Subsequent evolution 10000 10000 → → ) ) m m ( ( 9000 9000 e e tidal basin and upstream river t t a a n n i i d 8000 d 8000 r r o o o o c c y 7000 y 7000 6000 6000 Prelim sediment budget 1461-1650 splay prograd slows, 1497 1497 5000 5000 fluvial mud import continues 1.2 1.4 1.6 1.8 2 2.2 2.4 1.2 1.4 1.6 1.8 2 2.2 2.4 → 4 → 4 x coordinate (m) x 10 x coordinate (m) x 10 fluvial import Total imported+reworked -4 -3 -2 -1 0 1 2 3 4 -4 -3 -2 -1 0 1 2 3 4 sediment composition avulsion splay 1421-- in representative 1461 AD set of corings • Delta-like progradation; with bifurcations/avulsion mud iiniitiiall llevee 31% 4 and spllay 84 88 • Tides affect overall planform 18 sand buiilldiing; 1 69% nn mud iimport reclaimed i reclaimed i g 84 g 1461-1950 AD 188 by 1461 iinn 10 20 30 40 mud u u g uug d d 46% d d sand import ee sand import d sand e d e e e 7 d w 7 w 54% r e r 1461-1650 AD ee w rr MM A. no tides B. 2 m tides e ee e w w ) 6 M uu 6 iiee e N m w 1461 AD k u ( iie 1421- 5 5 N dike n o 1968 AD i 1968 AD t a 4 d 4 a r tidal reworking g o 3 r 3 of sand & mud p of sand & mud a extra for shallow: t l 2 deep 2 e north d shallow Setting: southwest 1 0.5 1 • Total imported and reworked sediment budget • Shallow lake (2-4 m) 0.5(time) south 6 3 650x10 m (30x10 km, 2m thick, 50/50 sand/mud) • Freshwater input 10 20 30 40 10 20 30 40 time (years) time (years) • Splay sediment: 280x106 m3 sand+mud in 200 yr • Tides? 2 m range in 19th/20th century • Fluvially imported sand: • Progradation depends on basin depth 180x106 m3 = ~900,000 m3/year Modelled adaptation of upstream • General declining progradation rate due to semi- • For comparison: modern upstream channel circle expansion in constant sediment feed transport rate = ~560,000 m3/year channel for 6 scenarios -0.3 -0.3 deep deep Conclusions -0.4 -0.4 ) Idealised 3D splay modelling ) m m ( -0.5 ( -0.5 l l • Model agrees with reconstructed e shallow e shallow v initial bed level (m) Model settings: v e e -0.6 -0.6 l l 3 d 01-Jan-1450 00:00:00 Q = 1600 m /s d e e 1/2 b 15000 C = 45 m /s b -0.7 -0.7 splay progradation rates and Initial idealised bathymetry no basin D50= 0.25 mm no basin S = 0.0001 -0.8 -0.8 sediment budget → old Rhine branch (Nieuwe Maas) ) -0.9 A. no tides -0.9 B. 2 m tides m 10000 dike ( levee 0 5 10 15 20 0 5 10 15 20 e breach • Sandy delta progradation faster t distance (km) a distance (km) n i Meuse ridge Rhine d r o 5000 (Merwede) with tides and in shallow basin o • No backward erosion in upstream channel c y estuary 10 20 30 40 0.06 0.06 • Entire sediment load 0 A. no tides B. 2 m tides 0 0.5 1 1.5 2 2.5 3 3.5 4 0.05 0.05 of upstream river captured in splay ) x coordinate (m) → 4 ) m m ( x 10 ( 0.04 0.04 n n o o i height above sea level (m) i t t • No erosion in upstream channel a a t t 0.03 0.03 n n e -7 -6 -5 -4 -3 -2 -1 0 1 e m m i i d d 0.02 0.02 e no upstream impact of avulsion! e s s deep • deep (4m) / shallow basin (2m) / no basin 0.01 shallow 0.01 • no tides / tides (M2, O1): about 2 m range no basin 10 20 30 40 10 20 30 40 0.06 • simple basin / antecedent relief time (yr) time (yr) Acknowledgements NWO grant ALW-VENI-863.04.016 to MK • Engelund-Hansen / Van Rijn • After year 5 constant sedimentation Janrik van den Berg, Aat Barendrecht.