1 chapter 10 streams and floods geol 101 introductory geology
TRANSCRIPT
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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 Earth’s 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 flowLongitudinal profile of a stream
Head area erosion predominant
Mouth area, deposition mainly
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Downcutting
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Drainage Pattern
Dendritic Radial
TrellisRectangular
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Dendritic Drainage
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Factors affecting erosion and deposition
VelocityGradient – rise over runChannel shapeChannel roughnessDischarge – 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
SemicircularFlow fast
Wide channelFlow slow
Rough bottomFlow slow
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Channel shape and roughness
Narrow channelFlow fast
Wide channelFlow slow
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Stream ValleysMost common landform on Earth’s surfaceTwo 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
RollingSuspensiondissolution
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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 DeltasBird-FootTriangle shape
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Alluvial FanDeath Valley, CA
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FloodsFloods 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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FloodsWhat 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 ControlEngineering 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 DamsStore water for slow releaseLowers 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 valleysSediment deposition behind damImpediment for fish migration
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ChannelizationAltering 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 LeveesEarthen 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