Notes for Exam 2

Notes for Exam 2 - 10.1.07 Floods and Flood Control Flood:...

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-110.1.07 Floods and Flood Control Flood: discharge from a stream that exceeds the capacity of its channel. Most common and most destructive geological hazard. Part of the natural behavior of streams Stage is the fundamental hydrologic measurement of a river Flash floods: occur very rapidly w/ very little warning Factors incl Rainfall intensity & duration, surface conditions, topography, human interference w/ stream systems (dam failures, levee failures) Causes of flooding: Resulting from naturally occurring and human0induced factors: Causes incl Heavy rains Rapid snow melt Dam failure Topography Surface conditions (Johnstown, Penn: 1889 Flash flood killed 2200 Built n flood plain poorly maintained dam broke sending 20 million tons of water into the town Wall of water up to 60 ft high, 40 mph Many bodies never identified or found 5 yrs before town fully recovered) Approaches to flood control Prevention Adjustment Artificial Levees - earthen mounds built on the banks of a river to increase the volume of water the channel can hold. Can protect against MOST floods (but not all) Problems: During high flow (flood stage) discharge increases, sediment is deposited, buildup at the bottom occurs, less water is needed to reach the top of the levee and it floods 2. Mmany artificial levees arent built to withstand high periods of extreme flooding Channelization - involves altering a stream channel to speed the flow of water to prevent it from reaching flood height. Dredge the channel to make it wider and deeper. Clear the channel of obstructions (trees, debris, etc) More radically: change te coure of the channel, using artificial cutoffs Shorten the stream, increase gradient, increase velocity get a larger discharge, rapid dispersing flooding. Ex. 1930s US Army Corp of Engrs. Created cutoffs on the Miss River, shortening it by about 150 miles. Red River of the North River flows North. Ice melt occurs I nthe South and it is carried north and creates a dam. Floods every year. 1997 - Fargo, ND floods, no deaths Floodway - diversion channel Built to transport floodwaters away from inhabited areas Fairly successful for example, Winnipeg, Canada
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Expensive - costs 63.2 million about 500 million in 1997 dollars Flood control dams - dams on streams built to store floodwater and let it out slowly. Lowers the flood crest by spreading itout over a longer period of time. Benefits: Irrigation Water source Hydroelectric power Recreation Costs and consequences: Reservoirs may cover fertile farmland, useful forests, historic sites, and senic valleys Dams trap sediment - changes base level of stream Large dams cause significant ecological damage to river environments “Temporary” solution - gradually fill w/ sediment Lake Allatoona: Provides most of the drinking water from Cobb, Cherokee, and Bartow County. Popular for summer recreation.
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Notes for Exam 2 - 10.1.07 Floods and Flood Control Flood:...

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