Chapter 10 Streams and Floods

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Chapter 10 Streams and Floods
GEOL 101 Introductory Geology
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Hydrologic cycle

• The hydrologic cycle is a summary of the
  circulation of Earths water supply
• Processes involved in the hydrologic cycle
• Precipitation
• Evaporation
• Infiltration
• Runoff
• Transpiration

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The Hydrologic Cycle
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Channel flow and sheet flow

• Longitudinal profile of a stream

Head area erosion predominant
Mouth area, deposition mainly
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Downcutting
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Drainage Pattern
Radial
Dendritic
Trellis
Rectangular
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Dendritic Drainage
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Factors affecting erosion and deposition

• Velocity
• Gradient rise over run
• Channel shape
• Channel roughness
• Discharge amount of water flow per unit time,
  as discharge increases, load increases.

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Velocity
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Velocity
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Channel shape and roughness
Semicircular Flow fast
Wide channel Flow slow
Rough bottom Flow slow
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Channel shape and roughness
Narrow channel Flow fast
Wide channel Flow slow
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Stream Valleys

• Most common landform on Earths surface
• Two general types of stream valleys
• Narrow valleys
• V-shaped
• Downcutting toward base level
• Features often include rapids and waterfalls
• Wide valleys
• Stream is near base level
• Downward erosion is less dominant
• Stream energy is directed from side to side
  forming a floodplain

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Stream Valleys

• Features of wide valleys often include
• Floodplains
• Erosional river erodes laterally
• Depositional fluctuation in conditions, base
  level
• Meanders sweeping bends in river channel
• Cut bank active zone of erosion
• Point bar zone of deposition
• Cutoffs shortened channel segment
• Oxbow lakes abandoned bend

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Stream Valleys

• Features of narrow valleys often include Rapids
  Waterfalls
• Both occur where stream profile drops rapidly,
• Rapids resistive bed acts as temporary base
  level upstream, downcutting downstream
• Waterfalls stream makes vertical drop
• Resistive rock underlain by erosive rock
• Water plunges and erodes the underlying rock
• Niagra Falls

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Changes from Upstream to Downstream

• Profile
• Cross-sectional view of a stream
• Viewed from the head (headwaters or source) to
  the mouth of a stream
• Profile is a smooth curve
• Gradient decreases downstream
• Factors that increase downstream
• Velocity
• Discharge
• Channel size
• Factors that decrease downstream
• Gradient
• Channel roughness

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Stream erosion

• Lifting loosely consolidated particles
• Abrasion particles scrape, rub, bump together
  and wear down
• Dissolution dissolve soluble rock by chemical
  reaction
• Stronger currents lift particles more effectively

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Active Stream Erosion
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Base level and graded streams

• Base level lowest point to which a stream can
  erode
• Two general types of base level
• Ultimate (sea level)
• Local or temporary
• Changing conditions causes readjustment of stream
  activities
• Raising base level causes deposition
• Lowering base level causes erosion

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Adjustment of Base Level to Changing Conditions
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Local Base Level (waterfall)
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Sediment Transport by Streams
Rolling Suspension dissolution
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Sediment Deposition by Streams

• Caused by a decrease in velocity
• Competence is reduced
• Sediment begins to drop out
• Stream sediments
• Generally well sorted
• Stream sediments are known as alluvium
• Channel deposits
• Bars
• Braided streams
• Deltas

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Sediment Deposition by Streams

• Floodplain deposits
• Natural levees form parallel to the stream
  channel by successive floods over many years
• Back swamps marsh
• Alluvial fans
• Develop where a high-gradient stream leaves a
  narrow valley
• Slopes outward in a broad arc
• Deltas
• Forms when a stream inters an ocean or lake
• Consists of three types of beds (Foreset, Topset,
  Bottomset)

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Deposition
Bar
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Braided stream
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Braided RiverResurrection River, AK
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Erosion and Deposition Along a Meandering Stream
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Meander Loop on the Colorado River
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Flood plains
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Stream Valley in Arid RegionOwens Valley, CA
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Creation of oxbow lake
Creation of oxbow lake
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Formation of a Delta
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Deltasouthern, AK
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Nile and Mississippi Deltas
Bird-Foot
Triangle shape
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Alluvial FanDeath Valley, CA
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Floods

• Floods are the most common and most destructive
  geologic hazard
• Causes of flooding
• Naturally occurring
• Human-induced factors
• Types of floods
• Regional floods
• Flash floods
• Ice-jam floods
• Dam failure

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FloodingSalt River, AZ
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Floods

• What is a 100-year flood?
• It is not a flood that occurs every 100 years
• Flood of a given size that has the probability of
  1 in 100 of occurring in that year
• Better term 1-in-100 chance flood
• Urban planning based on FEMA 100-yr flood maps

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Flood Control

• Engineering efforts
• Artificial levees
• Flood-control Dams
• Channelization
• Nonstructural approach through sound floodplain
  management
• Identify high risk areas
• Zoning regulations for development

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Flood-Control Dams

• Store water for slow release
• Lowers crest of flood, spread out over time
• Often have other functions
• agricultural irrigation water
• hydroelectric power
• recreation
• Reservoir covers previous land use fertile
  farmland, historic sites, scenic valleys
• Sediment deposition behind dam
• Impediment for fish migration

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Channelization

• Altering a stream channel to speed flow of water
  to prevent reaching flood height
• Clearing channel of debris
• Dredging to widen and deepen channel
• Lining channel with concrete
• Artificial cutoff straightening the channel
• shorter stream increases gradient and velocity of
  water flow
• larger discharge associated with flooding
  dispersed more quickly
• Army Corp of Engineers

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Artificial Levees

• Earthen mounds on river banks to increase the
  volume capacity of river
• Steeper slopes than natural levees
• Trap sediment that otherwise would have been
  deposited in floodplain
• River bed build up often requires raising the
  height of levee over time
• Many artificial levees not built to withstand
  extreme flooding
• Levee failure numberous on Miss. (1993)

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Erosional Floodplain
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Incised Meanders and Stream Terraces

• Incised meanders
• Meanders in steep, narrow valleys
• Caused by a drop in base level or uplift of the
  region
• Stream Terraces
• Remnants of a former floodplain
• River has adjusted to a relative drop in base
  level by downcutting
• Results in horizontal plane above current
  floodplain

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Stream terraces
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Incised MeandersDelores River, CO
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Incised MeandersColorado River, Canyonlands NP,
UT