Chapter 7: Rivers and Flooding
Chapter 7: Rivers and Flooding GEOL 1005
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This 3 page Class Notes was uploaded by Kerrigan Unter on Monday October 19, 2015. The Class Notes belongs to GEOL 1005 at George Washington University taught by Brown, C in Summer 2015. Since its upload, it has received 20 views. For similar materials see Environmental Geology in Geology at George Washington University.
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Date Created: 10/19/15
GEOL 1005 Chapter 7 Rivers and Flooding 7 River Basics Watershed an area where the surface runoff from precipitation onto the land ows together toward lower areas such as lakes or oceans Tributaries streams that ow into one another Trunk river where tributaries merge Divides boundaries between watersheds of all sizes are elevated areas Runoff overland ow that occurs if the ground is saturated with water Runoff is in uenced by people Some runoff stored in dams and ponds infiltrates the ground Removal of vegetation can increase runoff Urbanization decreases infiltration and increases runoff Discharge amount of water owing in a river at any given time Discharge m3s Area m2 X Velocity ms Channel the place where water is more or less continuously present and the main current ows Base level elevation at which a river cannot ow farther or erode deeper into the ground Longitudinal profile shows the elevation of its channel bottom from the headwater areas to its downstream termination Mouth place where a stream discharges into a large stream lake or the ocean Gradient loss of elevation along a river s channel measured in meters per kilometer or feet per mile Cutbank bare steep or overhanging bank Meanders sinuous curves bends loops or turns in the course of a stream formed as the stream shifts its course laterally as it ows across its oodplain Oxbow lake sedimentation along the sides of the shortcut channel may then seal off the former loop Load all material that the river carries Bedload larger sediment Suspended load finer particles carried along in the river water Dissolved load dissolved minerals and some organic materials leached from the soil and rock in the river s watershed Capacity at a specific velocity and discharge a stream has a maximum amount of bed and suspended load it can carry Braided channel places where many small channels interconnect between areas of sediment deposition Alluvial fan gently sloping deposit of and or gravel shaped like an open fan deposited by a stream where its gradient rapidly decreases as it approaches a local base level Delta deposit of sediment at the mouth of a stream when it enters a body of water Distributaries disperse ow of water rather than bring it together as tributaries do Floodplain at or nearly at lowland bordering a stream that may be covered by its waters at ood stages Reaches specific sections of a river 72 Floods Flooding occurs when the discharge of a river is so great that the water rises and overtops the river s natural or arti cial banks Levees natural or artificial river banks Flood crest the highest level above ood stage that a river achieves during a ood Intensity duration timing Failure of landslide dams ice dams and constructed dams Effects of cultivation loss of wetlands urbanization Flash oods commonly occur when a storm dumps a large amount of rain over a wasteland in a relatively short period of time tend to crest rapidly and recede rapidly Permeability the capacity of rock sediment or soil to allow water to pass through Riverine involve large volumes of water than ash oods because they develop because they develop in larger watersheds and draw on more widespread sources 73 Measuring and Forecasting Floods Hydrographs plots of river discharge over time Stage measured water level at any time Bankfull stage the level at which the channel is completely full Recurrence intervals the average time between past ood events of a similar size calculated from the discharge history of a river recorded on a hydrograph Recurrence Interval R1 T2 T1 l r Annual ood peak discharge each year Flood frequency curve a plot of annual ood discharges and their recurrence intervals Probability P 1 RI 74 Living with Floods Decrease discharge increase channel s crosssectional area to allow more discharge without ooding allow a river s ow velocity to increase in response to increased discharge Channel alteration oodcontrol dams diversion channels oodways detention ponds Structural approach Expensive Levee failure Restricting rivers Dams create local base levels Flood control Channel alteration Environmental consequences