Intense or prolonged rain, rapid runoff (as in short basins with no tributaries, that is, first-order streams)A drainage basin is the region drained by a stream or river. It is a cup of land bounded by areas of high relief. Precipitation that falls within the cup of land flows to the stream as runoff over the surface or infiltration and Base flow through the groundwater system. It may also be called a watershed, a river basin, or a catchment.Water seeping into the ground raises the water table, causing more rapid outflow from groundwater into streams down slope.Upstream Flash FloodsCaused by locally intense rainfall (covering only one or two tributaries) over a short period of time.Steep Gradient.High and Rapid runoffNarrow V-shaped channels. No floodplains or levees.Waters rise quickly.Short lag time and high peak discharge.Q = A X V. Increased discharge increases water velocity resulting in an increase in erosion.Unpredictable (because it is a function of weather conditions), high loss of life.Flood damage mainly from the force of the rapidly moving water and deposition of sediment.Floods rapidly, recedes quickly.Downstream unaffected.Covers a smaller area of land.Downstream Regional FloodsCaused by large amounts of rainfall over an extended period of time over a large portion of the drainage basin.Shallow gradient.High infiltration (Natural)Wide, deep U-shaped channels with floodplains and levees.Water rises slowly.Long lag time and broad discharge curve.Q = A x V. Waters rise as soil becomes saturated. Water spreads out onto the floodplain (large cross-sectional area).Predictable. Low loss of life.Flood damage mainly from extensive wetting and deposition of sediment. Great property damage because it covers a large area of land and flood waters remain high for long periods of time.Recedes slowly.Floods affect tributaries.Flash floods are caused by locally intense rainfall (covering only one or two tributaries) over a short period of time. Because of the steep gradient runoff is high and rapid flowing in narrow v-shaped channels. As a result flood waters rise quickly and unexpectedly. As discharge increases, velocity increases. The force of the water is capable of capable of transporting large debris. It can wash cars downstream, and damage roads and bridges and trigger landslides and debris flows.Increased urbanization, forest fire, clear-cut logging, overgrazingSuch changes virtually always lead to a greater height for the 100-year flood level (or a broader area covered by the flood).Paved streets, parking lots, buildings, and storm sewer systems decrease infiltration, carrying rainwater as runoff quickly to streams channels.After urban development infiltration decreases and runoff increases. As a result floods tend to peak earlier after a rainstorm, and have higher peak discharge.Agriculture, parks, playing fields, golf coursesRI = (n + 1)/m [n = number of years in flood record; m = rank of the flood in question]Example: T1999 = 69 + 1 = 70 = 70-year flood [69 years of record] 1 1 [1 refers to largest flood on record]Could be anytime—next year, 10 years, 100 years, 200 years [100 years is a long-term average, not a specific interval]It is based on a sample of floods that is much too short to be representative. Most areas in North America have a record of less than 200 years of floods.Any changes in the upstream drainage area will change the rate of runoff and therefore the magnitude of a flood; upstream drainage area characteristics commonly do change,All floods in the sample used to determine recurrence interval should have the same origin (the sample size is so small that this condition is virtually never met).Lowest floor of a house must be at least two feet above the 100-year flood level (even on compacted fill)Building or encroachment must not raise the level of a 100-year flood by more than one foot (½ foot in some areas).Disposal of waste, hazardous materials, or sewage drain fieldsFloodwater is confined between the levees, causing the flood flow to be deeper and faster.Flooding upstream and downstream of the levees is increased.Water overtopping of the levee,Erosion of the levee by the river currents,Slumping of banks of the levee into the river,Piping or seepage through the leveeChannelization is the deepening, straightening, widening, cleaning, or lining of existing streams. All are attempts to increase the amount of discharge the stream can accommodate.An event in which a stream leaves its regular channel, flows in a new channel on its floodplain, and never returns to its previous channel.Name: _______________________Learning Unit 7: Flood Review QuestionsThis assignment is designed to assess your understanding of Unit 6 and includes some of the Questions forReview at the end of Chapters 10, 15 and 14 from your text plus a few additional questions. Each question can be answered in one to two sentences. Please limit yourself to a maximum of three sentences. Access the assignment, complete it with ANSWERS IN A DIFFERENT COLOR FONT as a separate file, and send it back for evaluation and grading through the assignment tab by or before the due date. 1. Of all of the common natural hazards, which causes more than 80 percent of all deaths?Floods2. What aspects of weather cause a flood? (Be specific: not merely “more water.”)Intense or prolonged rain, rapid runoff (as in short basins with no tributaries, that is, first-order streams)3. How is a drainage basin defined?A drainage basin is the region drained by a stream or river. It is a cup of land bounded by areas of high relief. Precipitation that falls within the cup of land flows to the stream as runoff over the surface or infiltration and Base flow through the groundwater system. It may also be called a watershed, a river basin, or a catchment.4. Why does stream level rise within a few hours of a heavy rain, even though you see no water running off the slopes?Water seeping into the ground raises the water table, causing more rapid outflow from groundwater into streams down slope. 5. How would you calculate the discharge of a stream?Q = v A [Discharge (Q) = velocity (v) x cross-sectional area (A = average width x average depth)6. What is the definition of a flood? (Be specific: not merely more water)A large discharge (or flow) compared with normal for that channel7. How do the characteristics of upstream and downstream floods differ?Upstream Flash Floods• Caused by locally intense rainfall (covering only
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