Scroll bars (ridge and swale, meander scrolls)

Scroll bars (ridge and swale, meander
scrolls)

• Longitudinal bar
• Low ridge of sediment on inside of
meander bend during bankfull flow
• Flow recedes, vegetation

Or
• Flow separation over
point bar during
single flood—
deposition
• Eventually vegetated
• 30 yrs ()

Point bar and floodplain formation of the
meandering Beatton River, northeastern British
Columbia, Canada. (1979) G. Nanson,
Sedimentology, v.27, n 1., 3-29 (quoted)
• Initial point bar platform (mainly coarse sediment) formed adjacent
to the convex bank of a migrating meander bend.
• Is the base for a single scroll bar of fine traction and suspended
load. Scroll bar grows, eventually supporting vegetation and
becoming a floodplain ridge.
• Form with greatest size and frequency in rapidly migrating bends
• Shape of the meander bend appears to determine both the location
of the initial bar deposit, and its direction of growth up or
downstream.
• Initiation of a scroll bar appears to be due to excessive deposition of
suspended load in a zone of flow separation over a point bar
platform.
– critical flow condition for the initiation of a scroll bar does not occur with
the same recurrence interval on different shaped meander bends!
– average recurrence interval ~ 30 years
• Despite 2–3 m of overbank deposition, the amplitude of floodplain
ridges is maintained by secondary currents which sweep sediment
from the swales towards the ridge crests.

New evidence of scroll-bar formation on the
Beatton River (1981) G. Nanson, Sedimentology, v.
28, n. 6, 889-891
• “observations… suggest that an initial
scroll bar may form around a stranded
dead tree that acts as a sedimentation
nucleus on a point-bar platform”

Swales exploited during high flow
• Develop into chutes
• Counteract sediment redistribution from
ridges

Mississippi entered union 1817

Levees
• Generally hundreds of m wide
• Slope away from channel
• V decreases as waters overtop bank
• Coarse sediment deposited adjacent to channel
– 53 cm levee, 1 cm backswamp (MS) (depends on V
decrease)

Distal floodplains
• Subdued topography
– Oxbows
• Clay plug
– Splay deposits
• Can get coarse
sediments!
– Cohesive banks
• Complex mosaics!
– Lateral migration 0-2460
ft/yr (lots in the 10s to
100s)
muller.lbl.gov

http://www.geo.uu.nl/fg/berendsen/pictures/photography/alaska/Crevasse.jpg

http://www.geo.uu.nl/fg/palaeogeography/pictures/results_avulsion
s/01_Development_avulsion.png
http://www.geo.uu.nl/fg/palaeogeography/results/avulsions

Floodplain evolution/formation
• Sediment “conserved” during meander
migration
– Bank erosion = point bar accumulation
• Vertical accretion decreases w/time
– Takes flood w/longer recurrence interval to
overtop banks
• Lateral migration rates far exceed vertical
accumulation (overbank flow) rates
– “dominant process”

• Incision can outpace bank erosion

What’s a terrace
• Abandoned floodplain
– Recurrence interval of flooding
• Composed of
– Tread
–Scarp
• May be erosional or depositional features
– Erosional: bench, strath, rock-cut
• Control: tectonic
– Depositional: Fill
• Control: climatic

• Depositional: All treads in alluvium
– First incision, then aggradation, then incision
• Alluvium= *temporary* sediment storage
– Sed production> Sed transport

Erosional terraces
• …”One sometimes wonders if any aspect of
fluvial processes escaped the genius of
G.K. Gilbert”.

• Formed during lateral migration
• Depth of alluvial deposit < scour depth of
river
• *Usually* requires
–Time
– Steady base-level
– Constant climate

Desert processes

Ralph A. Bagnold (1896-1990)
• Officer, British Royal Army of Engineers
• Stationed in Cairo between world wars
• Founded the LRDG
• In spare time, wrote “The physics of blown
sand and desert dunes”

• Deserts in:
More than just sand…
– North America: 2% sand cover
– North Africa (Sahara) 10% sand cover
– Arabia: 50% sand cover

Wind!
• Unequal heating of surface
• Winds pick up during day to 10-20 mph
• Governed by same fluid flow principles as H 2 O
• Mostly turbulent; HOWEVER, usually laminar
layer at base (~1mm)
• Also layer where v=0
– 1/30 grain diameter for flat surface of same size
grains
• Relate erosional potential of wind to
– Velocity, particle size

Wind velocity
• Increases with height
•V z = 5.75 V * log (z/k)
•V z ; velocity at height z
•V * ; drag/friction velocity;
surface friction speed
• K ; surface roughness
constant (height of v=0
zone)
• Increasing V * increases
shear on particle
τ=ρ V *2 ρ=density air
• k varies dramatically
(ocean, 1mm to city, 5 m)

Role of turbulence
• Creates some upward velocity
• Generally Vup = 1/5 V
• If a particle’s settling velocity < 1/5 V,
transport downwind
• Turbulence typical
• Eddies in many directions
• When hot, convection drives upward
plumes

Entrainment
• Can determine threshold friction velocity
– Surface roughness
– Soil moisture
– Salt precipitation
– Resistant crusts
• Threshold value of friction velocity: (for air,
A =0.1)
V
∗τ
=
A
ρs
− ρa
ρ
a
gD
• Avg. desert sand, threshold = 16 km/h

Modes of transport
• Image courtesy of “Manurenet”
• Surface creep also known as “reptation”

Suspension
• Upward velocity (wind) exceeds downward
velocity (gravity vs. drag)
• Restricted to silt and clay

Saltation
• Larger particles, discontinuous “bouncing”
• Once motion begins, surface is
bombarded
– Particles moved at lower wind velocities
– “new” Impact threshold