geol FLoods (ch6)
geol FLoods (ch6) GEOL 110
Long Beach State
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This 6 page Class Notes was uploaded by Elizabeth Rubio on Monday March 7, 2016. The Class Notes belongs to GEOL 110 at California State University Long Beach taught by Ewa Burchard in Spring 2016. Since its upload, it has received 6 views. For similar materials see Natural Disasters in Geology at California State University Long Beach.
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Date Created: 03/07/16
GEOL 110 3716 Chapter 6 Con. Intro to Rivers •Streams and rivers are part of the hydrologic cycle –Evaporation of water from Earth’s surface –Water returns to ocean underground or across the land •Streams merge into tributaries and then into rivers –Streams only differ from rivers in size –Local usage varies –Geologists use “stream” for any body of water that flows through a channel •Drainage basin –Area drained by a single stream or river –Also called watershed, river basin, or catchment –Surface drainage is referred to as runoff Gradient –Slope of the land over which the river flows –Steepest at higher elevations –Levels off as river approaches its base –Shown on longitudinal profile •Crosssectional profile –Steepersided and deeper valley near headwaters –Wide floodplain usually present at base level –Differences due to higher flow velocity at higher elevations Earth Materials Transported by Rivers •Rivers transport materials along with water •Total load consists of: –Bed load •Materials that roll, slide, bounce along bottom –Suspended load •Silt and clay particles that are carried in the water –Dissolved load •Materials carried as chemical solution River Velocity, Discharge, Erosion and Sediment Deposition Rivers Primary erosion. Transportation in the rock cycle Velocity controlled •Discharge –Water volume flowing through a cross section per unit time –Constant along river if no additions or deletions –Changes in area lead to changes in velocity •Crosssectional area decreases, velocity increases •Crosssectional area increases, velocity decreases •Stream flow widens and slows from high to low gradient –Forms at base an alluvial fan or delta Channel patterns and Floodplain Formation •Streams and rivers develop distinct channel patterns •Meandering Pattern Most common are meandering and braided –Curving channel bends called meanders •Migrate back and forth across the floodplain •Velocity greater on outside of curves causing erosion (cutbank) •Rivers slow on inside of curves causing deposition (point bars) –Floodplains are created during overbank flows –During avulsion streams shift position –Contain pools and riffles (floodplain) (meandering river) Channel Patterns and Floodplain Formation, cont. •Braided Pattern –Tend to be wide and shallow –Contain sand and gravel bars that divide and unite a single channel Channel Patterns and floodplain formation cont. •River Systems –Drainage basin = sediment and water it transports –Consists of three distinct zones •Zone 1: zone of water and sediment production (zone of production) –Generally in upper parts where there is steeper topography and more precipitation –Water velocity is fast, downcutting and erosion occur •Zone 2: zone where water and sediment are conveyed (zone of transport) –Broad valley and floodplain –Sediment frequently deposited in river bars or on floodplain –Braided or meandering or combination of both •Zone 3: zone where water velocity slows near base level and sediment is deposited (zone of deposition) –Alluvial fan –Lake –Delta (most common) Flooding = Natural process of overbank flow •Related to: –Amount and distribution of precipitation in drainage basin –Rate at which the precipitation soaks into earth –How quickly surface runoff reaches river –Amount of moisture in the soil INTRODUCTIONS •Flood characterizations –Flood discharge – discharge of the stream at the point where water overflows the channel banks –Stage – height of water in the river •Flood stage — Elevation of water surface that is likely to cause damage to property •Hydrograph –Graph of stream discharge or water depth over time Magnitude and Frequency of Floods •Flooding intimately related to amount and intensity of precipitation and runoff –Catastrophic floods produced by infrequent, large, intense storms –Smaller floods may be produced by less intense storms that occur more frequently •Recurrence interval –Average time between flood events of a certain size –Example: 10year flood •Can be expected about every ten years •Probability of this size of flood in any given year is 1/10
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