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Canada (162,429)
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Chapter 8

Chapter 8 - Flooding

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Rob Milne

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Risks and Disasters – Chapter 8  Red River Flood of 1997 o River flows through Winnipeg o Red River basin is large, heavy winter snowpacks accumulate in upper parts o 1997 snowpack particularly heavy and snow melted more rapidly than normal o River rose relentlessly, created a temporary lake o Three people drowned, $815 Million in damages o Many similar floodings o Large clay deposits reduces permeability of water into the ground o Flooding is a common hazard, and slow to realize urbanization in floodplains  Introduction to Rivers o Rivers and streams part of the hydrologic cycle o Much of the water that fills on land return to ocean via topographic determined surface flows  Also known as runoffs  Small streams, rivers o Each stream has a drainage basin o Gradient – slope of surface over which a river flows, drop in elevation over some horizontal distance  Greatest in its headwaters, decreases as it goes downstream, lowest at river mouth  Base level – at or near sea level o Steeper gradients facilitates erosion and downstream transfer of sediments o Earth material transported by rivers  River moves both water and material  Total load – bed load, suspended load, dissolved load  Bed load o Sand and gravel that slide, roll and bounce along river channel (<10%)  Suspended load o Silt and clay (~90%)  Dissolved load o Electrically charged molecules from chemical weathering of rock and sediment o River velocity, discharge, erosion and sediment deposition  Important agents of erosion and sediment deposition, major role in landscape sculpting  Discharge – volume of water that moves through cross-section of a river per unit of time  Measured by - flow velocity and cross-sectional area of water  Fan shaped body of sediment on land – alluvial fan  Triangular or irregular shaped water deposit - delta o Channel patterns and floodplain formation  Rivers and streams flow in different channel patterns  3 typical channel patterns  Braided – large number of intersecting channels  Anastomosing – two or more channels and intervening stable islands where sediment is stored temporarily  Meandering – single snake-like channel  Braided tend to be wide and shallow compared to meandering  Many rivers have meanders  No idea on why rivers meander  Water moves faster along outside of meander bend, forming cutbank  Inside of bend starts to form point bar (sand and gravel deposits o Meanders move laterally  Pools – deep areas produced by high flow scour  Riffles – shallow areas formed by eroded sediment from pools 1 Risks and Disasters – Chapter 8  Flooding o Overbank flow = flooding o Magnitude and duration determined by:  Amount  Distribution  Duration of precipitation  Rate of precipitation soaking into ground  Presence or absence of snowpack  Air temperature  Speed of run off  Moisture of soil before precipitation plays a large role o Floods begin when stream achieves bankfull discharge  Discharge at which water first flows out of the channel o Flood discharge – level of river surface at a point  Hydrograph – change in discharge, water depth or stage over time o Flood stage – used to indicate a level that’s likely to cause property damage o Reoccurance interval – average time between events of that magnitude o Upstream and downstream floods  Upstream floods  Occur in upper parts of drainage basins  Produced by intense rainfalls in a short duration  Flash floods – sudden with large discharge o most common in arid and semi-arid environments o most deaths occur in automobiles  Can be very damaging  Downstream floods  Can affect larger areas and much more deadly  Produced by long duration storms or rapid snowpack melting  Large, slow rise in discharge in particular location  Outburst floods o Peak discharges many times larger than normal o Can be deadly when people liv along flood path  Geographic regions at risk for flooding o Most widespread natural hazard o Can cause billions of dollars of property damage and large number of deaths o Developing countries suffer much greater losses o Primary damages  Loss of life  Injury  Damage to property and infrastructure  Remove or bury soil and vegetation o Secondary damages  Pollution  Hunger  Disease  Displacement of people  Losses of services and income  Effects of flooding and linkages between floods and other hazards o Several factors affect damage  Land use on floodplain 2 Risks and Disasters – Chapter 8  Extent, height and strength of dykes  Depth and velocity of floodwaters  Rate of rise and duration of flooding  Season of the year of flooding  Amount and type of sediment deposited by floodwaters  Effectiveness of flood forecasting, warning and evacuation o Damage far greater in areas with commercial and residential development o Flooding during growing season may damage/destroy crops o Floods can be primary effects of hurricanes and secondary effects of earthquakes/landslides o Floods can cause fires in populated areas  Electrical circuits and broken gas lines sparking fires  Natural service functions of flooding o Fertile land  Fine sand, silt, clay and organic matter deposited on the floodplain  Add nutrient-rich sediments, creating most fertile and productive agricultural areas in the world  Best crops grow after a flood o Aquatic ecosystems  Flush out stream channels and remove accumulated debris  Have positive effect on aquatic life  Carry nutrients downstream, increasing food supply o Sediment supply  Keeps surface of delta above sea level  Human interaction with flooding o Land use changes can increase or decrease sediment supply to a stream o Urbanization usually increases number of buildings and infrastructure  Increasing the risk of local flooding o Dyking of rivers  Increases because flood waters can no longer spread out o Land-use changes  Rivers are open systems that maintain a dynamic equilibrium  Sediment is supplied by tributaries, landslides, erosion of bank materials  River maintains the gradient and cross-sectional shape that provides flow velocity needed to move sediment load  Change in amount of water or sediment brings about changes in channel gradient or cross- sectional shape  Changing velocity of water o May increase or decrease amount of transported sediments  Land use changes that alter sediment or water supply may set into motion series of events that bring about a new dy
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