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A floodplain is the flat land immediately

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A floodplain is the flat land immediately – PowerPoint PPT presentation

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Title: A floodplain is the flat land immediately


1
A floodplain is the flat land immediately
surrounding a stream channel and innundated at
times of high flow.
2
Aggradation occurs when deposition is greater
than erosion.
Aggradation of the Rivière des Ha! Ha! in Quebec
3
Incision occurs when erosion is greater than
deposition.
Broadstreet Hollow Stream, NY
4
  • Floodplain sediments derive from deposition of
    overbank suspended sediment or by deposition of
    bedload as the channel migrates across its
    valley

5
Levees
  • The boundary between channel and floodplain may
    be the site of a natural levee (a broad, low
    ridge of alluvium built along the side of a
    channel by debris-laden floodwater)

Levees form when debris-laden floodwater
overflows the channel and slows as it moves onto
the floodplain.
6
Flooding Sedimentation
7
Levee Deposits
Coarser sediment
Flood stage
Finer sediment
Finer sediment
The area adjacent to and outside of the channel
serves as an overflow area for excess water and
sediment
8
meander review!
  • higher velocities on outside of bends lead to
    cutbank erosion
  • lower velocities on insides of bends lead to
    point bar deposition

9
Point Bar Deposits
10
meandering
oxbows (sloughs)
  • meander belts

songhua r., china
11
Meandering stream flowing from top of screen to
bottom
12
Maximum deposition
Maximum erosion
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Meander scars (scroll bars)
Oxbow Lake
cutoff
23
The river cuts downward to form a V-shaped
valley.
The river starts to meander
24
River cuts from side to side eating into the
valley walls
Floodplain starts to form
25
Floodplain continues forming, increased sinuosity
26
Floodplain
Alluvial deposits
27
Floodplain landforms
  • oxbow
  • splay
  • scrolls
  • leveee
  • backswamp

28
Oxbow lake near the Chippewa River, Eau Claire,
Wisconsin
29
Splay
  • A deposit of coarse material resulting from a
    levee breach during a flood.

30
Meander scroll
  • A meander scroll consists of long, curving,
    parallel ridges (scroll bars) that are deposited
    during point bar growth

songhua r., china
31
Side looking radar (SLAR) image of floodplain of
an Amazon River tributary in 1971/2 flow is
toward lower right.
32
floodplain landforms
  • levee coarse sediment deposited near the channel
    during overbank flow
  • backswamp low-lying area separated from channel
    by natural levee

33
Alluvial Fans
  • when streams encounter a sudden change in
    gradient and confinement, e.g. leaving mountains,
    they often deposit alluvial fans
  • why? transport capacity suddenly decreases

34
  • Alluvial fans tend to be coarse-grained at their
    head. At their edges, however, they can be
    relatively fine-grained.

35
alluvial fans
  • bajada
  • coalescing alluvial fans
  • death valley

36
river terraces
Many stream valleys contain one or more
relatively flat alluvial terraces that lie above
the floodplain.
37
River Terraces
  • streams may create depositional landforms
    (especially floodplains) and then start to incise
  • terraces are abandoned floodplains

38
River Terraces
39
River Terraces
  • Changes in whether a stream is eroding or
    depositing in a given location can be caused by
    several different phenomena eg
  • tectonic uplift
  • change in base level downstream
  • change in climate modifying systems discharge
  • which brings us to river response

40
River Response
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42
river response case studies
  • skokomish river, WA
  • mount pinatubo, philipines
  • river restoration engineered log jams (ELJ)

Skokomish River, Washington Response to
intensive upland forestry in steep
landslide-prone terrain
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skokomish river - response
  • increased sediment input from landsliding in
    headwater channels
  • aggradation of channel bed in main stem
  • increased flooding due to decreased amount of
    water channel could hold
  • NOT by in creasing peak or mean flows

climate
catchment vegetation
riparian veg.
sediment wood inputs
fast
hydrology
flow obstructions
channel morphology
rate of change
geology
gradient
valley form
slow
49
river response case 2
Mount Pinatubo, Philipines Response to massive
volcanic disturbance
6 km3 of pyroclastic materials erupted and
subsequently deposited on flanks of
volcano Abundant fine-grained sediment available
for transport.
50
Porac River before eruption
51
Pasig-Potrero River after eruption
52
Antidunes
53
High-Flow Roll Waves Pasig-Potrero River
54
High-Flow Roll Waves ODonnell River
55
erosion of lahar depoisits
56
eruption consequences
  • large amount of new sediment available for
    transport
  • ashfall deposits
  • lahar terraces
  • the river bed
  • radical geomorphic changes
  • decreased grainsize in rivers (cobble/boulder ?
    sand/pebble)
  • switch to braided channels (single channel ?
    multi-thread)

57
river response
  • exponential decay of post-eruption sediment yield

58
river response
  • Pinatubo eruption effects were generally natural
    phenomena
  • BUT, Skokomish changes were not
  • what can be done about it?
  • river restoration
  • the science of properly restoring (as much as
    possible) rivers to their natural state
  • the hardest part
  • what was the natural state?
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