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Hack’s Law formula
A ∝ Lh
What is the h in Hack’s Law?
1.6
What does Hack’s Law mean?
Hack’s Law relates river-channel length, L, to drainage area, A
Colluvial valleys
Found in headwaters of drainage networks, where hillslope processes deliver sediment to valley bottoms by soil creep, rare debris flows and landslides also deliver sediment
Bedrock valleys
Narrow, steep valley bottoms with little sediment storage and a high capacity for sediment transport. Fluvial system moves sediment
Alluvial valleys
low gradient, filled with sediment, streams unable to scour bedrock, floodplains and terraces common
Estuarine valleys
Low-gradient valleys, filled with fine-grained sediment, wide, heavily vegetated, interface between marine and terrestrial realsm
Cascade channel
mountainous headwater, disorganized bed material, cobbles and boulders, large clasts protrude through flow
step pool channels
longitudinal steps formed by large clasts, steps separate pools containing finer material
plane-bed channels
characterized by long stretches of relatively featureless bed, composed of cobbles or gravel, large woody debris may force formation of pools and bars
pool-riffle channels
undulating beds with lateral bed-form oscillations that define a sequence of bars, pools, and riffles. often gravel-bedded, typically of lowland valleys
dune-ripple channels
sand-bedded, bedforms vary with increasing flow depth and velocity, from lower-regime plane beds to ripples, sand waves, dunes, upper-regime plane bed, and antidunes
How does sediment supply change from up to downstream?
increases
How does channel slope change from up to downstream?
Decreases
How does channel roughness change from up to downstream?
Decreases
What is the order of channel types from up to downstream?
Hillslope, hollow, colluvial, cascade, step-pool, plane-bed, pool-riffle, dune-riffle
How does stream flow change from up to down stream?
diffusion dominated to debris flow dominated to fluvial dominated
Is upstream supply limited or transport limited?
supply limited
Is downstream supply limited or transport limited?
transport limited
Sinuosity formula
Lc/LV
What is Lc in sinuosity formula?
Channel length
What is LV in sinuosity formula?
Valley length
What is the sinuosity of a straight channel
< 1.3
What is the sinuosityo f a meandering channel
sinuosity > 1.5
patterns of multi-thread channels
braided or anastomosing
braided channels
exhibit multiple unvegetated, frequently shifting channels that converge and diverge within a larger channelway
anastomosing channels
divind into multiple channels that flow around vegetated islands
slope vs drainage area
geometry of a meander
how do discharge and slope influence channel patterns?
slope and discharge influence likelihood of forming braided, meandering, or straight channels. sediment supply and discharge variability also play a role in determining channel form
what happens to floodplains dominated by overbank deposition?
develop natural levees, floodplain is lower than levees, leading to back swamps and yazoo channels
what is a yazoo channel
tributary stream that runs parallel to, and within the floodplain of a larger river
what happens to floodplains formed by lateral channel migration
bedload material builds point bars and digs out cutbanks
point bar
bedload material accumulates on the inside bend of streams and rivers
cutbank
the outside bank of a curve in a river, which is continually undergoing erosion
avulsion
floodplains formed around individual log jams are composed of patchwork of mid-channel islands that may stand above floodplain
decanting floodwaters onto floodplain
channel form based on pattern and sediment load
discharge formula
Q = WDV (discharge = channel width * channel depth * mean velocity)
channel width formula
W = aQb
channel depth formula
D = cQf
mean velocity formula
V = kQm
what is the sum of b+f+m?
1
what is the product of ack?
1
discharge is proportional to
drainage area
what is omega
<= 1
how does bed material change with slope?
particle size decreases
downstream hydraulic geometry chart
what happens to wetted width as discharge increases?
increases
what happens to flow depth as discharge increases and channel fills
increases
what happens to downstream flow velocity as discharge increases
increases
what is “at-a-station” relationship
A channel’s at-a-station hydraulic geometry describes how its width (W), depth (D), and velocity (U) vary as discharge (Q) rises and falls at a particular location or channel cross section over the course of many different flow events
braided rivers are characterized by
wide, shallow channels. Caused, primarily by weak banks, channels cannot incise very deeply, increase in discharge channel banks erode and the channel becomes wider. shear stress at base of bed cannot get very high (flow cannot get very deep).
what are individual channels within a braided river like
active, changing form via sediment erosion and deposition
shear stress in braided streams
just above critical for coarser grain sizes, results in lots of local erosion and deposition with many bars and channels being formed
why are the banks of braided rivers weak?
high sedimentation rates, reduced vegetation rates, highly flashy hydrographs
why are braided streams steeper than meandering streams?
meandering streams are deep, so they don’t need to be steep to have high enough shear stress. braided rivers can’t be deep, so they become steep. t = rfghS
what are scroll bars
the old position of point bars are preserved topographically as a system of ridge and swales, can be seen across flood plains and in ancient sedimentary deposits
Qw/Qs what happens when Qw increases?
lots of water, not much sediment, slope goes down, deep and narrow
Qw/Qs what happens when Qs increases?
slope steepens, river widens and becomes braided
rise of vascular plants
devonian
radius of curvature of river
rcurvature = 1/curvature
how can we tell what direction a river is flowing?
where point bar is relative to apex of meander
how does hydrostatic pressure change at different points in the river?
gradient in thickness of water leads to gradient in pressure
change in hydrostatic pressure formula
△PH = (ρgh1-ρgh2)/x
gravitational versus centripetal force in rivers
gravitational force dominates at bed, centripetal force dominates at surface
what is the condition at bed at 0 velocity
no-slip condition
what is a jet in a delta
the spatial structure of the flow emanating from a river mouth
distributary mouth bar
deposition at the end of a river channel
how many people live in deltas worldwide
over 300 million, deltas are vital to agriculture and aquaculture
what causes land loss in deltas
sea level rise and coastal erosion due to global warming, sediment starvation due to reservoirs and control structures, and subsidence due to water and gas extraction
what are the three types of delta classifications?
fluvial dominated, tide dominated, and wave dominated
fluvial dominated deltas are characterized by
sediment input
tide dominated deltas are characterized by
tidal energy flux
wave dominated deltas are characterized by
wave energy flux
when was the last glacial maximum
19000-7000 years ago
what happened in relation to deltas during LGM
rapid sea level rise, deltas lost land
what happened after LGM 7000 years ago and what did it mean for deltas
SLRR dropped so deltas expanded and built land
what does cohesion do to deltas
controls shape, number of channels, and average bifurcation angle
what is bifurcation
the division of something into two branches or parts
what does increasing sediment cohesion do
increases variability in terrestrial delta-top area
river-dominated deltas
relatively unaltered channel-forms protrude into body of water, waves and tides do little to rework coastline (ex: mississippi river delta)
what happens when distributary channels avulse
entire delta lobe stops getting sediment and becomes abandoned, new lobe starts to form and propagate
what happens to stratigraphy when distributary channels avulse
coarsening and thickening upward sequence. The abandonment of the channel causes the sequence to be overlain (usually abruptly) by mudstones.
wave-dominated deltas
wave action reworks sediment delivered to the delta by longshore drift, smooth shorelines produced (Nile river delta)
tide-dominated deltas
shape is extensively reshaped by flood and ebb tidal currents, funnel-shaped mouth an linear island bars (ex: Ganges-Brahmaputra River)
topset of deltas
river-channel deposits
foresets to deltas
subaqueous front to delta
bottomset to deltas
gently inclined fine-grained sediments
macroscopic form of deltas
made up of topset, foreset, and bottomset
examples of sediment remobilization
slides, slumps, debris flows, faults, and diapirs
what is shelf bounded by
shoreline in shallow water, continental slope in deeper water
why does continental shelf depth vary
subsidence, uplift, glacial maxima
tempestite
deposits associated with large storm events
bioturbation
sediment mixing due to organisms
what is shallow water wave speed?
froude number
wave speed formula
Fr = sqrt(gD)
why do tsunamis get taller as they reach shoreline
waves decellerate as they reach the shore because depth decreases, so wave grows taller
why is storm energy reduced in deeper water
diameter that particles move through decays, path is always circular
deep water waves
short when d > lambda/2, At depth of lambda/2 waves are not feeling the bottom, uorbital = 0
Note
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