Evolution of the Fluvial System - PowerPoint PPT Presentation

Loading...

PPT – Evolution of the Fluvial System PowerPoint presentation | free to download - id: 406b2b-YTBlN



Loading


The Adobe Flash plugin is needed to view this content

Get the plugin now

View by Category
About This Presentation
Title:

Evolution of the Fluvial System

Description:

Drainage Basin Evolution ... Stream channels only account for about 1-5% of the total basin area Divides Define the boundaries of adjacent drainage basins May ... – PowerPoint PPT presentation

Number of Views:80
Avg rating:3.0/5.0
Slides: 96
Provided by: Davi223
Learn more at: http://www.westga.edu
Category:

less

Write a Comment
User Comments (0)
Transcript and Presenter's Notes

Title: Evolution of the Fluvial System


1
Evolution of the Fluvial System
  • Geomorphology
  • GEOL 4003

2
Drainage Basin Evolution
  • Rain falls
  • Water infiltrates or flows overland under
    influence of gravity
  • Water collects in surface rills, gullies, and
    channel networks
  • All the water (and sediment) eventually leaves an
    area (the drainage basin) through a single channel

3
Rainfall-erosion facility (REF) at Colorado State
University
4
The Drainage Basin
  • The surface upstream and uphill from a channel
  • It sheds water and sediment into that channel
  • Also called a watershed
  • An open systemmatter and energy flow in and
    outwith more or less well-defined boundaries
  • Stream channels only account for about 1-5 of
    the total basin area

5
(No Transcript)
6
(No Transcript)
7
Any drainage basin can be subdivided, each
considered as its own drainage basin.
8
(No Transcript)
9
Divides
  • Define the boundaries of adjacent drainage basins
  • May be well-defined, sharp ridges
  • Often not obvious in low-relief areas
  • No scale implied
  • Divides within the larger drainage basin are
    called interfluves

10
Channel Networks
  • Remember Playfairs law
  • Rivers occupy valleys they cut
  • Valleys proportional to size of river
  • Tributaries join main stream at level of main
    stream
  • See quote, Bloom p. 232

11
Hortons Laws
  • Beginning of quantitative fluvial geomorphology
  • Established quantitative relationships between
    geomorphic processes and landforms
  • Morphometrythe measurement and mathematical
    analysis of various landform parameters

12
Who is Horton?
  • Robert E. Horton (1875-1945)
  • Considered the father of modern hydrology
  • Remember Hortonian overland flow
  • Became NY District Engineer of USGS in 1900
  • Developed ideas of rainfall/runoff relationships
  • AGU awards Horton medal each year in hydrology

13
Horton Analysis
  • Each stream is assigned to a particular order
  • Stream order indicates that particular streams
    relative importance in the overall network
    (within the drainage basin)

14
Hortons Order Numbers
  • 1st orderdoes not receive a tributary
  • 2nd orderreceives 1st order tributaries
  • 3rd orderreceives 1st and 2nd order tributaries
  • 4th orderetc.
  • 5th orderetc.
  • Any order stream can receive tributaries from any
    lower order

15
Horton Stream Order for drainage basin
morphometry study. The main deal with a Horton
Analysis is that the entire streams are the
ordered units. Well see what that means.
16
Note how the order number extends for the entire
stream segment in the Horton Analysis.
17
Strahler Method
  • He refined the Horton approach in 1952 so that
    stream segments are the ordered units
  • 1st orderno tributaries
  • Where segments of equal order join, the order
    number is increased
  • If a lower order stream enters a higher order
    stream, there is no change in order number

18
Strahler (1952) refined Hortons approach so that
stream segments are the ordered units.
19
(No Transcript)
20
Morphometric Relationships
  • Order number versus
  • Number of streams
  • Area drained
  • Length of streams
  • Stream gradient

21
(No Transcript)
22
Drainage Density (D)
  • Average length of streams per unit area
  • Reflects stream spacing
  • Interaction between geology and climate

23
Low Drainage Density
  • Resistant rock
  • or
  • high infiltration capacity
  • Result is widely spaced streams (low D)

24
High Drainage Density
  • Low-resistance rock
  • or
  • Low infiltration capacity
  • Runoff increases
  • More and closely-spaced streams (high D)

25
A standard flood magnitude- drainage density
relationship, regardless of the size of the
basin, discharge increases with increasing
drainage density. In other words, high D means
lots of runoff
26
Higher drainage densities (D) increase the
steepness of the hydrograph because water more
quickly runs off into the stream channel network.
27
Idealized Flood Hydrographs
28
Floodplain Morphology
  • Genetic Floodplainthe largely horizontally
    bedded alluvial landform adjacent to a channel,
    separated from the channel by banks, and build of
    sediment transported by the present flow-regime.
    (Bloom, p. 240)
  • Floodplains are primarily depositional features,
    though some sediments may be relict

29
(No Transcript)
30
In time a river can affect most of the floodplain
directly by meandering, and all of the floodplain
indirectly by flooding.
31
Interesting web site
  • Check out the Virtual River at
  • http//vcourseware3.calstatela.edu/VirtualRiver/Fl
    ooding/index.html

32
Floodplain Deposition
  • Lateral accretion
  • Point bars
  • Vertical accretion
  • Flood deposits

33
Lateral Accretion
  • Deposits of lateral accretion are typically
    coarser than overbank flood deposits that cause
    vertical accretion
  • Point bars are the most important component of
    lateral accretion
  • Cut bank is the opposite side of the channel from
    point bars

34
(No Transcript)
35
(No Transcript)
36
(No Transcript)
37
(No Transcript)
38
Meander scars below Memphis are evidence of
lateral accretion along the Mississippi
River. See Also Bloom, figs 11-10 and 11-11.
39
Vertical Accretion
  • When a river exceeds its banks, flow velocity of
    the overbank water may be very low
  • This leads to deposition by vertical accretion
  • Deposits are usually fine sand or mud

40
Floodplain Features
  • Natural levees
  • Crevasse-splays
  • Oxbow lakes
  • Neck cuttoff
  • Plugs
  • Yazoo tributaries

41
(No Transcript)
42
(No Transcript)
43
Note that because we have built so many levees
along so many rivers that we now use the term
natural levee.
44
Note the sediment types and grain sizes across
the floodplain.
45
(No Transcript)
46
Mississippi River flooding, with levees visible.
Photo by Duncan Heron.
47
Mississippi River flooding, with levees visible.
Photo by Duncan Heron.
48
Sometimes the levees (natural or engineered)
break with flood water and sediment spilling onto
the floodplain.
49
The levee breaks are called crevasses, and the
resulting deposit is called a crevasse-splay.
50
(No Transcript)
51
(No Transcript)
52
(No Transcript)
53
(No Transcript)
54
Formation of an oxbow lake.
55
A Yazoo tributary is a tributary on the
floodplain. Note location of backswamp.
56
Aggradation in Rivers
  • We normally think of rivers as eroding and
    transporting material out of a basin
  • But the normal floodplain is made of material in
    transit or in temporary storage
  • Aggradation can be caused by
  • a rising base level
  • climate change
  • tectonics

57
Huntingtons Principle
  • Relationship between climate and fluvial systems
  • Increased aridity in areas of marginal rainfall
    leads to a loss of vegetative cover and increased
    mass wasting
  • Stream channels become choked with excessive
    sediment load because of decreased discharge
  • Aggradation results
  • As a result, gradient steepens
  • Thus, greater sediment load can be carried

58
River Terraces
  • Two parts
  • Treadflat surface of old floodplain
  • Scarpsteep slope connecting tread to next lower
    surface
  • Alluvial terraces
  • Cut in old floodplain deposits
  • Bedrock terraces
  • May have thin cover of alluvium
  • Called straths

59
(No Transcript)
60
(No Transcript)
61
Alluvial Terraces
  • Most terraces are this type
  • Abandoned floodplains
  • Material deposited when river was at higher level

62
Fluvial Terrace Morphology
63
Types of Terraces
  • Floodplain terracessmall, deposited or eroded
    during floods, not preserved for long
  • Unpaired terracesslow downcutting with
    meandering
  • Paired terracessignificant evidence of
    downcutting
  • Fluvial terraces are cut, not built, by rivers

64
Paired Terraces
  • Multiple terraces exist on each side of the
    valley
  • Pairs of terraces are at the same elevation
  • Suggests something affecting the entire system to
    start river downcutting

65
Unpaired Terraces
  • Can exist in alluvium or bedrock
  • Interpreted as normal stream lateral erosion
  • Downcutting with meandering of stream channel
    across the valley

66
Paired Terraces
Unpaired Terraces
67
Terraces are cut into previously existing
material. They are evidence that the river is
undergoing a downcutting phase.
68
Types of River Terraces
69
Rock Defended Terraces
70
Channel Modification
  • Channelization (engineering)
  • For safety but has unintended consequences
  • Interferes with natural habitats and ecosystems
  • Aesthetics
  • Groundwater problems
  • Pollution
  • May cause more severe flooding downstream

71
Natural river system, undisturbed
72
Channelized river
73
Development and Floods
  • Subsidence from groundwater removal (remember
    Baytown, TX)
  • May decrease cross-sectional area of river, so
    flooding occurs with less discharge
  • Runoff increases with more paving and with storm
    sewer drainage (less infiltration), called urban
    runoff

74
(No Transcript)
75
Development on a floodplain can alter
cross-sectional area of channel
76
Development impacts shown on flood
hydrographsnote decrease in lag time, increase
in peak discharge.
77
Structural Responses to Flood Hazards
  • Channelization
  • Dams, retention ponds, and reservoirs
  • Controlled release of flood water
  • Levees, dikes, flood walls
  • May worsen flooding upstream and downstream

78
Observing modern floods is a good way to learn
what areas may be flooded in future events. This
is the 1993 Mississippi River Flood near St.
Louis, before and after.
79
(No Transcript)
80
Lateral displacement of a stream from its main
channel into a new course across its floodplain
is called an avulsion. The result is abandonment
of part of the river. The Mississippi River is
trying to change its course to flow down the
Atchafalya River, abandoning the Mississippi
River course through New Orleans.
81
River control structures built to maintain course
of Mississippi River
82
(No Transcript)
83
One of the problems with levees is the false
sense of security they give. When the big flood
finally occurs, they are overwhelmed and fail.
84
Nonstructural Approaches
  • Reduction of Vulnerability
  • National Flood Insurance Act, late 1960s started
    ball rolling
  • Floodplain zoning
  • Specialized building codes
  • Open-space planning in flood zones
  • Floodplain buyout programs
  • FEMA plays big role

85
Flood Hazard Mapping
  • To depict the impact and areal extent of a flood
  • Actual floods
  • Modeling
  • Paleohydrology
  • FIRMsFlood Insurance Rate Maps

86
Flood Zones
  • A-zone 100-year flood
  • V-zone 100-year flood with waves
  • Old terms
  • B-zone 100-500-year flood
  • C-zone above 500-year flood
  • New term
  • X-zone outside 100-year flood zone

87
Coastal Flood Zones
88
FIRM for a coastal NC community
89
FIRM for a river system in Puerto Rico
90
Same area showing historical river flooding in
Puerto Rico.
91
(No Transcript)
92
(No Transcript)
93
(No Transcript)
94
Name that feature A. _______, B. _______, BS.
_______, C. _______, F. _______, L. _______, LA.
_______, N. _______, P. _______, S. _______, T.
_______, VA. _______
95
Aalluvial fan, Bbackland, BSbackswamp,
Ccolluvium, Fchannel fill, Llag
deposit, LAlateral accretion, Nnatural
levee, Ppoint bar, Ssplay, Ttransitory bar,
VAvertical accretion.
About PowerShow.com