Environmental Geology
Chapter 6
STREAMS & FLOODING
提
纲
INTRODUCTION
6.1 THE HYDROLOGIC CYCLE
6.2 STREAMS AND THEIR FEATURES
6.3 FLOODING
6.4 CONSEQUENCES OF DEVELOPMENT IN FLOODPLAINS
6.5 STRATEGIES FOR REDUCING FLOOD HAZARDS
6.6 LOOKING OUT FOR THE “FLASH FLOODS”
EXERCISES
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Introduction
Water is the single most important agent sculpturing
the earth’s surface. Mountains may be raised but they
are shaped primarily by water. Streams carve valleys,
level plains, and move tremendous, amounts of
sediment from place to place.
Floods are probably the most widely experienced
catastrophic geologic hazards. On the average, in the united states
alone, floods annually take over eighty-five lives and cause well over $1
billion in property damage. The 1993 flooding in the Mississippi river basin
took 48 lives, and caused an estimated $15-20 billion in damages.
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Most floods do not make national to those affected by
them. Some floods are the result of unusual events, such
as the collapse of a dam, but the vast majority are a
perfectly normal, and to some extent predictable, part of
the natural functioning of streams. Before discussing
flood hazards, therefore, we will examine how water
moves through the hydrologic cycle and also look at the
basic characteristics and behavior of streams.
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Key terms you should pay attention
Alluvial fan
Base level
Braided stream
Capacity
Channelization
Crest
Cut bank
Delta
Discharge
Down-steam flood
Drainage basin
Flood
Flood-frequency curve
Flood plain
Gradient
Hydrograph
Hydrologic cycle
Hydrosphere
infiltrates
Levees
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Load
Longitudinal profile
Meanders
Oxbows
Point bars
Recurrence interval
Retention pond
Stage
Stream
Upstream flood
Well sorted
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6.1 THE HYDROLOGIC CYCLE
The hydrosphere includes all the water at and near the surface
of the earth.
All of the water in the hydrosphere is caught up in the
hydrologic cycle, illustrated in figure 6.1. And the total amount
of water moving through the hydrologic cycle is more than 100
billion gallons per year.
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6.2 STREAMS AND THEIR FEATURES
 streams and their features
A stream is any body of flowing water confined within a
channel, regardless of size.
The region from which a stream draws water is its drainage
basin (figure 6.2)
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The size of a stream may
be described by its
discharge (figure 6.3) ,
the volume of water
flowing past a given point
(or ,more precisely,
through a given cross
section) in a specified
length of time. Discharge
is the product of channel
cross section (area) times
stream velocity.
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 Sediment transport
saltation (figure 6.4)
load
bed load
suspended load
dissolved load
capacity
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• Velocity, gradient, and base level
1.
2.
3.
Gradient
Base level
Longitudianl profile
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Note: The base level is the lowest elevation to which the stream can erode downward.
• Velocity and sediment sorting and deposition
Variations in a stream’s velocity its length are
reflected in the sediments deposited at different points.
The relationship between the velocity of water flow
and the size of particles moved accounts for one
characteristic of stream-deposited sediments: they are
commonly well sorted by size or density, with
materials deposited at a given point tending to be
similar in size or weight.
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If a stream is still carrying a substantial load as it reaches its mouth, and it
then flows into still waters, a large fan-shaped pile of sediment ,a delta, may be
built up (figure 6.6A). A similarly shaped feature, an alluvial fan, is formed when
a tributary stream flows from mountains into a plain (figure 6.6B).
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• Channel and floodplain evolution
Streams do not ordinarily flow in straight lines for
very long. Small irregularities in the channel cause local
fluctuations in velocity, which result in a little erosion
where the water flows strongly against the side of the
channel and some deposition of sediment where it slows
down a bit.
One of the classification of river is by its topography
and physiognomy: straight river, braided river or steam,
meandering river.
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meandering river
cut bank
point bar
oxbow
Floodplain
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meandering river
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braided river or steam
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6.3




FLOODING
When the stream in times of higher discharge, the stream may
overflow its banks, or flood.
Factors governing flood severity
Flood characteristic
Stream hydrographs
Flood-frequency curves
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 Factors governing flood severity
major factors: the quantity of water、the rate at which the water
enters the stream system;
others: soil type、topography、vegetation、the plant、
meteorologic fluctuations, etc.
 Flood characteristic
water level is higher than usual;
velocity and discharge increase;
the higher volume and velocity together produce the increased
force that gives floodwaters their destructive power (figure 6.12).
Note :
1、The elevation of the water surface at any point is termed the stage of the stream.
2、The stream is said to crest when the maximum stage is reached.
3、Floods that affect only small, localized areas (or streams draining small basins)
are sometimes called upstream floods.
4、Floods that affect large stream systems large drainage basins are called
downstream floods (figure6.13).
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 Stream hydrographs
Fluctuations in stream stage or discharge over time can be
plotted on a hydrograph (figure 6.14).
FIGURE 6.14 The summer 1993 flooding in the upper Mississippi basin was not only
unusual in its timing (compare with 1965 and 1973 patterns) but, in many places, broke floodstage and/or discharge records set in 1973. in a typical year, snowmelt in the north plus
spring rains lead to a rise in discharge in late spring. Summer is generally dry. Heavy 1993
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summer rains were exceptional and produced exceptional flooding.
 Flood-frequency curves
Another way of
looking at flooding is in
terms of the frequency of
flood events of differing
severity. Long-term
records make it possible
to construct a curve
showing discharge as a
function of recurrence
interval for a particular
stream or section of one.
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6.4 CONSEQUENCES OF DEVELOPMENT IN FLOODPLAINS
• Reasons for floodplain occupation
Why would anyone live in a floodplain?
One reason might be ignorance of the extent of the flood hazard.
In mountainous areas, floodplains may be the only flat land on
which to build, and construction is generally far easier and cheaper
on nearly level land than on steep slopes.
Farmers have settled in floodplains since ancient times because
flooding streams deposit fine sediment over the lands flooded,
replenishing nutrients in the soil and thus making the soil especially
fertile.
Obviously, the more people settle and build in floodplains, the
more damage flooding will do.
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• effects of development on flood hazards
Two factors affecting flood severity are the proportion and rate
of surface runoff. (figure 6.19).
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Buildings in a floodplain also can increase flood heights (figure 6.20).
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Measures taken
to drain water from
low areas can
likewise aggravate
flooding along a
stream. The same
is true of the use of
tile drainage
systems in
farmland (figure
6.21).
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6.5 STRATEGIES FOR
REDUCING FLOOD HAZARDS
 Restrictive Zoning and “Floodproofing”
 Retention Ponds, Diversion Channels (figure 6.23)
 Channelization
 Levees (figure 6.24) (figure 6.25)
 Flood-control dams and reservoirs
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Retention Pond
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Levee
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 Flood-control dams and reservoirs
Yet another approach to moderating stream flow to prevent
or minimize flooding is through the construction of floodcontrol dams at one or more points along the stream.
The practice also has the drawback, however.
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exercises
1.
2.
3.
4.
5.
6.
Define stream load, and explain what factors control it.
Why do stream sediments tend to be well sorted? (relate
sediment transport to variations in water velocity.)
Explain how enlargement and migration of meanders
contribute to floodplain development.
Discuss the relationship between flooding and (a)
precipitation, (b) soil characteristics, (c) vegetation, and
(d) season.
Outline two potential problems with flood-control dam.
List several appropriate land uses for floodplains to
minimize risks to lives and property.
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