EESA05 Lecture 08 Notes.docx

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Environmental Science
Caroline Barakat

1 EESA05 Lecture 8 (Floods)  Flooding in the UK is common  The risk has increased due to the amount of new buildings on floodplains  Reduced economic year in 2011 (UK)  Because they depended on manufacturing (UK)  There have been flooding in Toronto (ex// Steeles Ave. W & Finch Ave. W.) and DVP was shut down due to the flood in Toronto, August 2005  Floods are powerful!  Hydrologic cycle: what it shows is the cyclical even  Water comes down through precipitation (P) and a lot of it get evaporate transpiration (ET) and the rest runs of (RO) and then it ends up in the lake or ocean or whatever, some go to the ground and it gets to the saturated sediment (groundwater = recharge)  Watershed = surface area of all the water that falls into that surface and drains into the place between mountains and coastal plain  At the top of the watershed we get river valley  And at the bottom of the watershed we get floodplain  Floodplain = the stretch of the river channel and all the sand and sediments fall into that area  Base level = lowest level the river can erode to and then it just goes into the ocean  Stream in balance with its surroundings is called a graded stream  Stream balances erosion, sediment, transport, and deposition  Erosion by streams has shaped the land surface worldwide  Ability to erode is based on river, velocity, and total discharge (total volume discharged by the river in a given time)  Stream systems everywhere tend to look the same….even on Mars…and it’s not unique to our planet  The overflow of water in stream bank, finer and lower stream load  Floodplain is fertile due to all the sand and sediments that fall in  The amount of sediment eroded and carried downstream increases with water velocity  Discharge = based on volume  Load = based on what goes  Suspended load makes up the largest amount of the load  Three loads: suspended, dissolved, and bed load  Confluence = when 2 separate bodies of water joins  Discharge will change if cross-sectional area changes  Sudden decreases in water velocity will decrease the carrying capacity of the water, resulting in deposition  Rivers deposit sediments in the shape of a fan when they slow  Deltas are when there’s a sudden change in the amount of deposits  The fan that forms in a lake or a sea is called a delta  Some features of deltas = multiple distributary channels  Rapid switching of flow from one channel to another  Shapes of delta depends on strength of river, tides, and currents  River channels and floodplains form as a result of the interaction of water flow and sediment transport 2 EESA05 Lecture 8 (Floods)  Main river patterns:  Braided rivers : need a good slope/gradient  Meandering rivers: snake their way around and not that steep  Composition of bed (gravel vs. sandy), type/amount of vegetation, humans, width and length, these all changes the way the river changes from erosion to deposition  Braided streams have multiple branching channels divided by many bars of sand or gravel; river is shallow; during high flows all of the bars will be covered by water; form where the gradient is steep and the river is carrying abundant coarse sediment; tectonic active and glacier areas  Meandering streams curve and ben that migrate across the floodplain; occurs where gradient is low; we don’t know why rivers meander; the river meanders then there’s a concave/outer-bend of the river called the cutbank; the river flows really fast near the cutbank; and it can even erode and then convex called riffle; meander scrolls have shrubby vegetation; pool is where you can swim; the river will get narrower after the riffle and then it'll get cut off (called the oxbow lake) and there will be a meander scar before it officially gets cut off  Hydraulic radius is the efficiency of the channel to move the water  For meandering streams water velocity on the outer bend is higher than on the inner bend so erosion will occur on the outer bend  The presence of pools and riffles created different environments  Depth is low in riffles, so speed is fast  Depth in pools is greater so the speed of slow is slow  A stream normally consists of one or more channels  Floodplain is where the water spreads out during floods  Since floodplains are fertile, we do agriculture on it  Water levels in the river channel may rise high enough to cover the floodplain  Common causes to increase in water levels = excess rainfall, snow melt, or excess water at the confluence of several streams  Stage of a stream = height of water in the stream  Flood stage = stage of a flood in the stream  Floods are characterized by flood discharge  To predict flooding of a specific stream, geologists make use of long-term records of previous floods on that stream  Recurrence interval (= flood frequency): the average length of time, in years, between floods of a particular size and the recurrence interval ® is calculated by the formula R = [(N+1)/M] where N is the number of years for which flood records are available for the stream and M is the rank of a particular size of flood during those N years  Recurrence interval is a probabilistic one  Probability = (1/R) where R is recurrence interval; this could be where r = 100 years but it’s possible to happen next year as r  P =T1 - P  Floods can be caused by local storms aka “flash floods”, rainfall lasting long periods of time, storm surges of tropical cyclones (ex// Sandy), formation of ice-jams in spring (ex// Red River flooding), natural dams made by landslides, failure of dams or levees 3 EESA05 Lecture 8 (Floods)  Flash floods happen when you get an intense local storm, they don’t last very long; typical in semi-arid areas with steep slopes and little vegetation; flash flood = toilet  On July 15, 2004, Peterborough caused by flash floods  Downstream floods last longer and are usually become natural disasters; require storms of long duration, and occur at the confluence of several flooding rivers; the size of floods increases as one moves downstream  Flooding will be progressively higher and of greater duration downstream  Two rivers that commonly downstream are the Red River (near Winnipeg) and the Mississippi River  Red river has a very low gradient and it flows to the north so that as snow melt in its upper reaches increases flow, its lower reaches may remain frozen; last major flood was in 1997…maybe have been a 200-500 year flood  Flooding in the Mississippi (largest river in N. America) was first noted the same year New Orleans was founded – 1717; and a response to this was to build levees which increases the risk of floods and increases the flow of water within this a
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