Rivers and Flooding

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what kind of disasters are the most common and affect most amount of people?

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1

what kind of disasters are the most common and affect most amount of people?

  • meteorological

<ul><li><p>meteorological</p></li></ul>
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2

number of people affected by weather-related disasters

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3

world - number of weather-related diseases reported per country (1995-2015)

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4

what are some weather systems that cause flooding?

  • tropical cyclones

  • monsoon season

  • sever isolated thunderstorms

  • atmospheric rivers

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5

what are the 2 types of flooding?

  • high intensity & short duration

  • lower intensity & long duration

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6

what is high intensity & short duration?

  • large amount of precipitation falling in a very short time period → flash flood

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7

what is lower intensity & long duration flood?

  • large amount of precipitation falls steadily over a longer time period → slow-onset flood

    • low precipitation, high flooding

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8

all about streams?

  • flows to areas of low elevation → creates channels

  • add to groundwater → river

<ul><li><p>flows to areas of low elevation → creates channels</p></li><li><p>add to groundwater → river</p></li></ul>
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9

what is a watershed?

  • the land area providing water to a specific draining network

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10

what is a draining network?

  • an array of streams that drain into the same, larger (trunk) stream

    • everything drains into trunk stream

<ul><li><p>an array of streams that drain into the same, larger (trunk) stream</p><ul><li><p>everything drains into trunk stream</p></li></ul></li></ul>
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11

what is the largest watershed in the world?

  • amazon river watershed

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12

watersheds of North America

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13

what is a stream gradient?

  • slope of the river channel; typically decreases downstream

  • headwaters → mountainous areas

  • end point of stream → ocean

<ul><li><p>slope of the river channel; typically decreases downstream</p></li><li><p>headwaters → mountainous areas</p></li><li><p>end point of stream → ocean</p></li></ul>
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14

what is base level?

  • an elevation that a stream cannot erode past, controlled by the lvl of the body of water which the stream discharges into

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15

what is bankfull?

  • when the water lvl in a river is equal to the height of the banks

    • flooding occurs when water rises over bankfull lvls

    • rivers do not reach bankfull stage every yr

      • on avg → they do not flood

    • determining channel bankfull width and depth can approx the bankfull discharge and the capacity of the channel

<ul><li><p>when the water lvl in a river is equal to the height of the banks</p><ul><li><p>flooding occurs when water rises over bankfull lvls</p></li><li><p>rivers do not reach bankfull stage every yr</p><ul><li><p>on avg → they do not flood</p></li></ul></li><li><p>determining channel bankfull width and depth can approx the bankfull discharge and the capacity of the channel</p></li></ul></li></ul>
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16

what are floodplains?

  • broad, flat area next to a stream that becomes either partly or comp. covered during a flood

  • it is flat due to past floods (sediment deposition) → good for agriculture

<ul><li><p>broad, flat area next to a stream that becomes either partly or comp. covered during a flood</p></li><li><p>it is flat due to past floods (sediment deposition) → good for agriculture</p></li></ul>
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17

what kind of loads do streams carry?

  • dissolved

  • suspended

  • bed-load

<ul><li><p>dissolved</p></li><li><p>suspended</p></li><li><p>bed-load</p></li></ul>
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18

what constitutes as suspended load?

  • finer particles, such as clay, silt, and fine sand, carried in suspension

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19

what is bedload?

  • heavier sediment in a stream that is moved along the stream bed rather than in suspension

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20

what do fast flowing stream carry?

  • larger grain sizes (clasts) than a slower flowing stream

    • even boulders

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21

the volume of sediments carried in streams depend on?

  • flow volume

  • velocity;

    • during flood, flow vol + velocity increases, so more sediment is transported

<ul><li><p>flow volume</p></li><li><p>velocity;</p><ul><li><p>during flood, flow vol + velocity increases, so more sediment is transported</p></li></ul></li></ul>
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22

Natural Levees

  • channel geometry is controlled by flow velocities and its associated sediment load carrying capacity

  • as a stream spills over its floodplain, it moves from a deep channel with high velocity to a shallow, broad floodplain with a low velocity

  • when velocity drops → causes sediments to also fall

    • bigger material fall first → right next to channel

    • small particles fall a bit farther

  • this creates gradient → coarse to fine

<ul><li><p>channel geometry is controlled by flow velocities and its associated sediment load carrying capacity</p></li><li><p>as a stream spills over its floodplain, it moves from a deep channel with high velocity to a shallow, broad floodplain with a low velocity</p></li><li><p>when velocity drops → causes sediments to also fall</p><ul><li><p>bigger material fall first → right next to channel</p></li><li><p>small particles fall a bit farther</p></li></ul></li><li><p>this creates gradient → coarse to fine</p></li></ul>
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23

what is an alluvial fan?

  • a fan shaped deposit of sand and gravel at the mouth of a mountain canyon, where the stream gradient flattens at the main valley floor

  • steep → high velocity

  • lower → drop of velocity

<ul><li><p>a fan shaped deposit of sand and gravel at the mouth of a mountain canyon, where the stream gradient flattens at the main valley floor</p></li><li><p>steep → high velocity</p></li><li><p>lower → drop of velocity</p></li></ul>
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24

what is a delta?

  • accumulation of sediment deposited by a river at its entrance into a basin

    • at the bottom, there is sediment deposition

<ul><li><p>accumulation of sediment deposited by a river at its entrance into a basin</p><ul><li><p>at the bottom, there is sediment deposition</p></li></ul></li></ul>
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25

what is a permanent stream?

  • flows all yr, below the water table

  • recharge

  • in temperate climate

<ul><li><p>flows all yr, below the water table</p></li><li><p>recharge</p></li><li><p>in temperate climate</p></li></ul>
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26

what is an ephemeral stream?

  • flows only for part of the year; above the water table

  • only flows when water flow is high (precipitation) will dry up

<ul><li><p>flows only for part of the year; above the water table</p></li><li><p>only flows when water flow is high (precipitation) will dry up</p></li></ul>
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27

what is a meandering river?

  • river with a single channel and high to moderate sinuosity

<ul><li><p>river with a single channel and high to moderate sinuosity</p></li></ul>
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28

where do cutbanks form?

  • on the outside of meander bends where the water is accelerated along the outside wall and erodes into the bank

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29

where do point bars form?

  • as sediment is deposited in the slower water on the inside of the meander bends

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30

meandering river

knowt flashcard image
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31

example of meandering rivers

  • Mississippi River and Floodplain

  • streams change over time bc of flow velocity

  • cutbanks are eroded, and pntbars are deposited

  • always changing + moving

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32

Clark Fork River

  • gradually eroded the cutbank until it undercut the house that was originally built 10m back from the river

  • issue → land erodes back

<ul><li><p>gradually eroded the cutbank until it undercut the house that was originally built 10m back from the river</p></li><li><p>issue → land erodes back</p></li></ul>
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33

what is a braided river?

  • a river characterized by multiple, frequent shifting channels

  • common in regions where there is a strong seasonally and monthly variation in stream discharge

  • during short periods of high discharge a braided river carries the coarsest sediment

  • dev in regions where sediment is readily available

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34

bedrock channels

  • steep wall → reduce chance of flooding

<ul><li><p>steep wall → reduce chance of flooding</p></li></ul>
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35

what is discharge (m3/s)?

  • measured vol of water flowing past a cross section of a river in a given amount of time

  • D = A x v

  • D = discharge (m3/s)

  • A = cross-sectional area (m2) = width x depth (in m)

  • V = velocity (m/s)

<ul><li><p>measured vol of water flowing past a cross section of a river in a given amount of time</p></li><li><p>D = A x v</p></li><li><p>D = discharge (m3/s)</p></li><li><p>A = cross-sectional area (m2) = width x depth (in m)</p></li><li><p>V = velocity (m/s)</p></li></ul>
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36

what is flood stage?

  • the stage (water lvl) at which a stream rises above its banks in many places and submerges areas outside of the stream channel

  • shape + depth is important

  • in the top pic → no flooding cuz steep

    • e.g., mountains

  • bottom → not steep → high water lvl lead to flooding

    • e.g., meandering bankfull, then over it → food

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37

what is a hydrograph?

  • length of lag time depends on:

    • basin area and shape

    • spacing of drainage channels

    • vegetation cover

    • soil permeability and land use

  • time delay cuz water has to make way to river

<ul><li><p>length of lag time depends on:</p><ul><li><p>basin area and shape</p></li><li><p>spacing of drainage channels</p></li><li><p>vegetation cover</p></li><li><p>soil permeability and land use</p></li></ul></li><li><p>time delay cuz water has to make way to river</p></li></ul>
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38

more hydrographs

<p></p>
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39

what is a slow-onset flood?

  • a flood that devs over a long period (days to weeks) and takes weeks to months to subside (seasonal or regional floods)

<ul><li><p>a flood that devs over a long period (days to weeks) and takes weeks to months to subside (seasonal or regional floods)</p></li></ul>
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40

1931 Great China Floods

  • flooding of the Yangtze and Yellow Rivers

  • Flooding persisted for months, affecting crops and causing famine and disease

  • Estimated up to 3.7 million people died and 50 million people displaced

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41

Flooding in BC, November 2021

  • Atmospheric river brought heavy rains to southern BC

  • Caused mass wasting and severe flooding

  • Disrupted major transportations routes

  • 600,000 animals perished

  • 5 deaths from a mudslide

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42

what is a flash flood?

  • a flood that devs within a short time frame (within 6 hrs) of intense rainfall, dam failure, or other cause

  • most flash floods occur due to extreme rainfall where the ground cannot absorb the water

  • e.g., 2015 flash flood in Joso, Japan

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43

what are the dangers of flooding?

  • floodwater residue contains garbage, sewage, and chemicals

  • secondary disasters of cholera and dysentery are common

  • submerged homes, businesses, fields

  • transportation and communication networks fail

  • deaths of animas, and people

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44

what is recurrence interval?

  • the avg time between floods of a crt discharge

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45

what is annual exceedance probability?

  • the likelihood that a flood of a given discharge or greater will happen in a given yr

  • can occur at diff yrs

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46

small floods happen often; large events are rare

knowt flashcard image
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47

what is an100-yr flood?

  • a flood with a reoccurrence interval of 100 yrs

<ul><li><p>a flood with a reoccurrence interval of 100 yrs</p></li></ul>
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48

what is a 100-yr floodplain?

  • the area likely to be flooded by a 100-yr flood

<ul><li><p>the area likely to be flooded by a 100-yr flood</p></li></ul>
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49

Flood-Hazard Map

<p></p>
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50

what features are used to determine flood lvls?

  • paleoflood

<ul><li><p>paleoflood</p></li></ul>
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