Channel Pattern Channel Pattern, Meanders, and Confluences Outline Description of channel pattern Alternate bars Channel pattern continua and evolution Controls of channel pattern Description of Channel Pattern Channel Types Defined by the nature of channel splitting around braid bars or islands, and the sinuosity of channel segments Single channels of varying sinuosity Straight Meandering Multiple channels of varying sinuosity Braided Anastomosed Since patterns can be flow dependent, should be described at some intermediate flow stage (Thorne et al., 1997) Bridge (003) 1. Bed evolves towards a statistically constant geometry composed of alternate bars. Channel responds to alternate bars by inducing bank erosion and channel widening 3. Bed adjusts to sediment fluxes and thalweg wanders Alternate Bars For single - row alternate bars and Fr 0.8; Fr u 5 wd C f H 1.45 d 1.51C w d for 6 w d 40 b C f gds u u u f f * gd (Ikeda, 1984) 1
Alternate Bars Alternate Bars Alternate bars in the Naka River, an artificially straightened river in Japan. Image courtesy S. Ikeda. Alternate bars in a flume in Tsukuba University, Japan: flow turned low. Image courtesy H. Ikeda. Alternate bars in the Rhine River between Switzerland and Lichtenstein. Image courtesy M. Jaeggi. Tokachi River, Japan Alternate Bars Channel Response to Alternate Bars As water level drops, highest parts of the bars become emergent Bar tail, riffle, and head become recognizable Cause localized flow diversions, and localized erosion and deposition Deposition on convex banks inside of river bend point bars Deposition in mid-channel braid bars (Unit bars, P. Ashmore) (Schumm and Khan, 197) Double-row alternate bars Single-row alternate bars (Braid bars, Saksatchewan River, CAN) (Point bar, Madison River, MT) (Bar tail, River Feshie, Scotland)
(Braid bars, P. Ashmore) Double-row alternate bars Channel Pattern Defined by the nature of channel splitting around braid bars or islands and the sinuosity of channel segments (Braid bars, Sunwapta River, CAN) Single channels Straight Meandering Multiple channels Braided Anastomosed (Braid bar, Sagavanirktok River, AK) Sinuosity Channel Description channel thalweg length Sn valley length Controls on Channel Pattern Braiding index mean number of active channels or braid bars BI cross sectional transect Conceptual (Knighton, 1998) Controls on Channel Pattern Controls on Channel Pattern S Q -b Braided Braided Meandering S Q -b D c 3
Controls on Channel Pattern Braided D 50 b Controls on Channel Pattern Parker (1976) using channel stability theory S d ; meandering stable Fr w S d ; braiding stable Fr w Meandering Braided Meandering Channel Pattern Conclusions Alternate bars play an important role in the evolution of straight channels Channel pattern defined by channel splitting around bars and the sinuosity of channel segments Discharge, slope, sediment load, etc., all interact with river flow to modify channel pattern River Meanders Primary flow and transport characteristics within a meander bend Controls on meander wavelength Meander migration and shift Red River, near Aubigny, southern Manitoba Turkey River, Clayton County, IA 4
Plan Pecatonica River near Mineral Point, WI Cross section Primary Flow Characteristics Flow-transverse component of water surface slope towards the inner bank Secondary flow pattern (spiral or helical flow) due to (a) curvature-induced centrifugal forces and (b) pressure gradient of the transverse sloping water surface Effects proportional to velocity and inversely proportional to radius of curvature Flow field is stage-dependent Secondary flow pattern in a meander bed Point bar (inner bank) Outer bank (Hey and Rainbard, 1996) Balance between centrifugal force and pressure gradient Mean Flow Velocity mu r F c Centrifugal force u r h g n 5
Water Surface Topography Flow Depth Relative Bedload Transport Rate Bedload Grain Size Maximum shear stress Flow field Controls on Meander Wavelength W 1.34W r c 4.59r c 0.98 Secondary flow Flow structure (Knighton, 1998) Combining: r c ~ to 3W (Julien, 00) 6
Controls on Meander Wavelength Meander Movement Q bb ; b~0.5 (Julien, 00) (Knighton, 1998) River Meanders Conclusions Spiral or helical flow in meander bend is a balance of centrifugal and pressure gradient forces All flow, bed, and sediment parameters then are modified by this flow pattern Meander wavelength can be determined from width and discharge relations River Confluences Outline Flow and sediment transport characteristics at river confluences Braid bar development Sacramento and Feather Rivers Ohio River and Mississippi River Entrance Mixing Minnesota River (lower branch) entering the Mississippi River 7
Flow Processes Flow and Sediment Transport Processes (Robert, 003) (Robert, 003) Primary Flow Characteristics Entrance zones Equivalent to riffle cross-over Inherited helical flow pattern from upstream Confluence mixing zone Super-elevation and two circulation cells Shear layer and zones of flow separation Sediment transport becomes spatially varied Localized erosion in scour hole ~4X average depth of incoming channels Localized deposition as side bars and downstream Braid Bar Development Confluence-Diffluence Couplet (Ashmore, 1993) Braid Bar Development River Confluences Conclusions Confluences also have generalized flow patterns All flow, bed, and sediment parameters then are modified by these flow patterns Confluence-Diffluence couplet critical in braided streams (Ashmore, 1993) 8