Name Learning Unit 7 Flood Review Questions This assignment is designed to assess your understanding of Unit 6 and includes some of the Questions for Review 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 Floods 2 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 channel 7 How do the characteristics of upstream and downstream floods differ Upstream Flash Floods Caused by locally intense rainfall covering only one or two tributaries over a short period of time Steep Gradient High and Rapid runoff Narrow 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 Floods Caused 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 8 Why are upstream floods referred to as flash floods What effect can flooding an increase in discharge have on stream flow in an upstream channel 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 9 What changes imposed on a stream would likely lead to higher level floods with the same rainfall pattern Increased urbanization forest fire clear cut logging overgrazing 10 With urbanization of the drainage area what changes can be expected in the height or extent of the 100 year floodplain Such changes virtually always lead to a greater height for the 100 year flood level or a broader area covered by the flood 11 What information is represented by a hydrograph The relationship between precipitation and the effect it has on a streams discharge is often shown on a hydrograph 12 How would a hydrograph for a stream change if major urban growth were to occur 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 13 People should not build homes on floodplains because of the danger of flooding What are better uses for floodplains Agriculture parks playing fields golf courses 14 If floodplains are such poor places to live why are so many towns located on floodplains When people first settled in an area they chose to live next to streams for water transportation and fertile soils for growing crops As populations grew they grew in the same places 15 What do we mean when we refer to a flood as the 10 year flood A 10 year flood means that a given level magnitude of flooding is likely to reoccur once every 10 years There is a 10 chance per year that a 10 year flood will occur 1 year 10 years 10 16 What is the simple formula for calculating the recurrence interval T for a certain size flood on a stream in 1999 if there are sixty nine years of record Give a numerical example based on the largest flood RI 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 17 If a 100 year floodplain was flooded in 1999 and there is a slightly larger flood 4 years later in 2003 what if any change is there in the recurrence interval for the 1999 flood Be numerically specific Referring to the flood in question 16 Recurrence interval for 2003 flood T2003 69 4 1 74 37 year flood 18 If a 100 year floodplain was flooded in 2003 in how many years should we expect the next 100 year flood Could be anytime next year 10 years 100 years 200 years 100 years is a long term average not a specific interval 19 Why is the specified 100 year floodplain size or 100 year flood height not a very reliable indicator for future large floods 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
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