This course: the morphodynamic system Research group River and delta morphodynamics River patterns GEO3-4305, 2014 Dr. Maarten Kleinhans [email protected] www.geo.uu.nl/fg/mkleinhans Introduction 1. Channel pattern classification • • • bars channels channel pattern prediction 2. What really determines bar pattern? 3. What really determines channel pattern? flow sediment transport morphology • • • • Introduction River flooding Hydraulic roughness and bedforms • • Sediment transport Mixture effects • • • • • Hydraulic geometry Bars, bends, islands Overbank sedimentation Channel patterns vegetation 1. Channel and bar patterns Many classifications and phenomenologies… A few theories… Meandering: initiating from alternating bars (vertical) initiating from bend instability (planform) BIG QUESTIONS What determines bar pattern? What determines channel pattern? Alternating bars braided river transitional meandering river W/h < 20 no bars 20 < W/h < 30 stable alternating bars W/h > 30 dynamic alternating bars 1 Andre Ermolaev 8/30 Channel pattern stability diagram 0.42 v,t 900D50 Kleinhans & van den Berg, cond. Acc ESPL After Nanson & Knighton (1996) 9/30 van den Berg 1995 The power of patterns Explanation?? Why valley stream power? Streampower? ω = τu = ρgQS/W Van den Berg (1995): potential specific valley-related streampower Why median grain size? where Braided separated from meandering by 12/30 2 The data Kleinhans & Van den Berg (2010, ESPL) • No human impact • No entrenchment or confinement • Data from literature • Google Earth • Mean annual Q indept. of morph • Valley gradient unpolluted by sinuosity 14/30 Why does it work?? Deliberate misprediction of channel width wide shallow river → narrower, deeper, higher ω → braided river Bank strength! → floodplain formation and destruction (see review paper Kleinhans 2010) Kleinhans & Van den Berg (2010, ESPL) 16/30 2. Explaining bar patterns Ships don’t like bars... Bar pattern interaction flow and sediment in a channel width-depth ratio! Channel pattern interaction flow (and bars) with banks out of the channel: bank erosion and floodplain formation 3 Bar theory (1) Forced bars and free bars (Struiksma et al. 1985) Forced bars: stationairy, initiated and forced by channel curvature flow and sediment interact qs ~ mun (m=constant) ■ n>3 for theoretical reasons ■ n=3 for Meyer-Peter & Mueller ■ n=5 for Engelund & Hansen Free bars: migrating, initiated by perturbation that grows (=unstable) slope effects on sediment: downslope easier ■ transverse slope in bends >> river gradient secondary flows in bends: upslope ■ sharper bends → stronger secondary flow Damping and exciting bars (1) Bar theory (2) flow needs length to adapt to bed (bars…) relaxation length λw: Bar types: forced bars: forced by flow curvature; static position free bars: spontaneously develop and migrate; initiated at perturbation (groyne, bend, tree…) imagine: momentum! sediment too! relaxation length λs: Analogy: spring-mass-‘damper’ damper can also excite imagine: (transverse) slope effect → bed cannot suddenly jump 22/51 Three regimes for free (alternating) bars Overdamped graphics: after Mosselman et al. 2006 Underdamped Excited 23/51 24/51 4 Bar theory (4) Bar theory (3) from theory: (just accept this… full derivation in Struiksma et al.!) bar damping length LD so, most important parameter (spring-damper!) is Interaction Parameter: λs/λw s 2 g W 2 f 2 w C h 2 IP depends mostly on W/h and a bit on friction and on the slope effect f(θ) bar length Lp Bend flow conservation of momentum AND logarithmical flow velocity profile helical (spiral) flow bed shear stress towards inner bend s: longitudinal coordinate n: transverse coordinate v: velocity (u for us) z: elevation, s: surface, b: bed h: depth R: bend radius Bend flow (2) Koen Blanckaert inner-bend bar main flow forced towards outer-bend transverse movement of momentum and net transverse flow velocity with: Think! infinitely long bend? Transverse slope effect Transverse bed slope effect Gravity acts on particles on transverse slope particles pulled towards outer-bank counteracts spiral flow Schuurman, Marra & Kleinhans 2013, JGR 5 How to reduce transverse slope? fixed layers longitudinal dams Underdamping overshoot! bendway weirs Struiksma et al (1985) bottom vanes Adaptation and overshoot (Delft3D) Examples: a bit overshoot and much excitation Damping and exciting bars (2) three regimes: profiles along outer-bend bank bit overshoot overdamped ■ just forced bars in reaction to curvature straight bend straight long bend straight underdamped ■ overshoot superimposed on forced bars unstable, exciting excitation ■ free bars grow spatially; ‘spread like a disease’ ■ damping length negative Wider channels: braid bars Narrow channel Wide channel: stability of higher wave modes mode m=2, etc.! 6 Is this getting chaotic? chaotic forcing violin: ■ horsetail on string river: ■ many perturbations ■ turbulence dominant modes violin: ■ length of the string, +overtones river: ■ width Andre Ermolaev Andre Ermolaev Andre Ermolaev 3. From bar pattern to channel pattern Width-depth ratio!! remember hydraulic geometry: width depends on bank strength (depth depends on width and roughness) ■ vegetation ■ cohesive sediment Andre Ermolaev 7 Yep! John Bridge Courtesy Jim Best Remember: bank strength!? Imagine a river in sand without mud or vegetation Bars and bank erosion pools between the bars: velocity high bank deeply undercut celerity and size of bars: θc,banks < θc,bed (slope of bank!) initially: θ >> θc so, erosion of bank toe and collapse of bank! eroded sediment deposited on bed shallowing continue until θ = θc in bankfull conditions ‘threshold channel’ wide river: fast, small bars → everywhere bank erosion → straight planform narrow river: slow, large bars → alternating bank erosion → meandering planform Alternate bars and bank erosion low flow conditions! Bridge (2003) Rivers and Floodplains 8 But, does it really work like this? Ongoing work! Wout van Dijk PhD finished Filip Schuurman PhD study ongoing work in Delft, Illinois and Japan (with us) bank sedimentation floodplain sedimentation Effect of mud in rivers Meander migration Wout van Dijk, Wietse van de Lageweg 9 1977 1989 Ganges River 1985 1997 Simulation bend instability Sun et al 2001 1999 10 Camporeale et al 2007 These models look better than they are! One pattern explanation interaction between pools (bars…) and banks Nevengeul van de Vecht, Junne, bij Dalfsen stronger banks → narrower channels → slower, alternating bars → meandering weaker banks → braiding Bank strength derived from self-formed floodplains vegetation So far state of the art… much work in progress! Major points 1. What determines bar pattern? 2. What determines channel pattern? Let’s overheat the brain... Example calculation for 18th century Rhine river upstream of Lobith: S_valley = 0.00016, S_channel = 0.00013 Qmaf = 5580 m3/s, Qbf = 3370 m3/s D50 = 2 mm, D90 = 8 mm Width = 520 m, depth = 5 m calculate: θ’, Fr, Re, λBW, ωpv, LD, Lp, λs, λw, IP 11
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