Notes For Test Three

Notes For Test Three - Floods Flood any relatively high...

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Floods - Flood : any relatively high stream flow that overtops the natural or artificial banks of a river. - Causes: 1) Heavy, intense, or prolonged rainfall. 2) Runoff from a deep snow cover. 3) Over-saturated soil. 4) Frozen soil. 5) High river, stream, or reservoir levels. 6) Ice jams in rivers. 7) Urbanization. 8) Lots of buildings. 9) Lots of parking lots. 10) Deforestation. - Weather patterns have a strong influence of when & where floods happen. - West: storms that bring moisture inland from the ocean cause floods in winter & early spring. - Southwest: Thunderstorms cause flashfloods in the late summer & fall. - Southeast: Hurricanes cause floods during late summer & fall. - Northeast: Frontal storms cause floods during winter & spring. - Floodplain : the flat land on either side of the river; it is considered part of the river. - Zoning & ordinances can help prevent building in floodplains. The government restricts building in 100-year-flood zones. - Landowners are advised to purchase flood insurance. - Natural levee : a ridge on made of sediment that helps stop a river from flooding. - Characteristics: - Stream stage: the level of water in the river; elevation of the water surface of a stream. - Flood stage: stream stage exceeds banks. - Flood/river crest: maximum stage (height) reached. - Hydrorecord : the level of the river over time. - Types of floods: 1) Upstream flood: small; localized; small drainage areas; short duration; result from sudden, intense rainfall. 2) Downstream flood: large river systems; large drainage basins; long duration; result from prolonged rainfall & regional snowmelt. Ex/ Mississippi & Missouri floods of 1993. - Recurrence interval : the average amount of time between flood events of a give size. To find the annual probability= 1/type of flood . For example, a 100-
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year-flood would have a 1% annual probability of occurring. - Flash flood : upstream flood where floodwater rises rapidly in a confined area; urban areas & canyons are where they are most popular. - An area’s probability for flooding can be assessed by Rainfall- Runoff relationships. - Physical mechanisms of protection against floods: - Flood-control dams store storm runoff & reduce flooding downstream. - Damns change a stream’s base level & longitudinal profile. - Straighten & deepen channels (channelization). - Levees control spread of floodwaters & limit amount of land covered by floods. - Problems?—can be over-topped or eroded away by large floods. Discharge - Discharge : the volume of water passing a given point at a given time; the amount of water in the river Q (m 3 /s )= cross-sectional area (m 2 ) x velocity (m/s) - As the discharge increases, width depth, & velocity increases. - As the discharge increases, the gradient decreases.
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This note was uploaded on 07/05/2009 for the course GEOG 1024 taught by Professor Neile.johnson during the Spring '09 term at Virginia Tech.

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Notes For Test Three - Floods Flood any relatively high...

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