EESA05 - Environmental Hazards - Lec 8 : Floods (near-verbatim)

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Environmental Science
Mandy Meriano

EESA05 Lecture 8: Floods PY Date: Nov 6, 2012 Hurricane Sandy and east coast flooding; surge water that caused all that flooding in NY Risk of flooding is increasing A lot of flooding events during spring time  Has to do with hydrologic cycle 2011: Bangkok Thailand continuous flooding – killed 300 ppl  Reduced economic growth in terms of manufacturing; it’s more than flooding of homes/streets, can actually have national impact Slide 4: Streams and flooding  Flooding in the UK is common and the risk has increased in recent years due to the amount of new building on floodplains o Besides the hydrologic cycle, we’ve affected flooding by building o Natural and human causes of flooding  Debris from flooding – damage  If ever stuck in a flood and driving – can’t drive thru flood; get away quickly  Insurance – if live on a floodplain and floods associated with that area – home insurance a lot higher Slide 5: Flooding in TO  Steeles Ave. W  Finch Ave. W  In Toronto, 2005  Flood waters are powerful Slide 6: Hydrologic Cycle  P = ET + R + RO  Cyclic event: o Water precipitation (rain/snow) comes down o A lot of this water actually transpires – if we were to quantify the water at each stage – evaporation/transpiration = one of the largest quantities of water o Whatever doesn’t get evaporated/transpired by plants, some of it runs off = RO = precipitation that runs off surface and ends up in streams/tributaries of streams and ends up in lake/ocean o Some of the water gets infiltrated thru the ground (percolation) and then into water table where you have saturated sediments underneath where you get aquifer o So after precipitation, you get some that:  Transpires/evaporates  Run-off  Into groundwater aquifer system (re-charged) o So P = ET + R + RO  Precipitation = Evap/Transpire + Re-charge + Run-off 1 EESA05 Lecture 8: Floods PY Date: Nov 6, 2012  When you have a year with a lot of rain, get soils really saturated – the more saturated soils = more run-off you get because the water isn’t able to percolate down and become ground water o So can expect to have a flood in the spring of following year – what happened in Bangkok Slide 7: Life of a stream  Streams are part of the hydrologic cycle  Watershed = surface area where all the water that falls on that surface drains into a particular stream and its tributaries – dashed line defines a watershed o This is how you use watershed to quantify the hydrologic elements from b4 – although there’s some uncertainty associated with this  Can have a diff type of river valley where most of the cross-sectional area here is actually the river valley and up here is the bed where it’s not really extended  Or you can get river channel, where you have the channel itself and then have a large floodplain o Depends on where you are, kind of topography, sediments o Not all rivers are built equally  In headwaters (top of the watershed), get this type of valley formation – mostly at this part of the watershed, get river channel formation and at the bottom towards the mouth of the river, get this much larger floodplains  The channel has certain capacity in terms of how much water it can hold o If you have a lot of water, it overflows the banks and spreads out; when it spreads out, whatever sediment, sand silt found on the land beside, that becomes the floodplain  The lowest level that the river can erode to – usually the mouth of the river where the river empties into lake/ocean – this is called base level – lowest level that river can go to and it can go no lower  For stream to flow, need some sort of gradient – if it was flat, no flow o In the headwater (=upper region), the elevation is higher; base level is the lowest; there’s a gradient that allows water to drain away from the headwater down to mouth of river Slide 8: Streams  A stream in balance with its surroundings is called a graded stream o Such a stream balances:  Erosion  Sediment transport and  Deposition…along its length, and has a smooth concave – upward profile from beginning to end  Streams have been eroding surroundings for millions of years – most of the morphology seen actually created by river systems/streams 2 EESA05 Lecture 8: Floods PY Date: Nov 6, 2012 o The more they do this, they become in equilibrium with their enviro; steady state with which they are happy with how much load they have, how fast they flow – get used to their place – get into equilibrium – when they reach equilibrium = called graded stream where they are in balance with erosion, sediment transport and deposition  longitudinal view o plains at base level; higher elevation of upper catchment areas o downstream = further down in the catchment Slide 9: Streams and Erosion  Erosion by streams has shaped the land surface worldwide over geologic time – graded system – has been shaping earth for long time  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… o Even on Mars Slide 10: Streams and Erosion  Discharge is found to be the amount of water flowing thru a given cross-section in a unit time  The amount of sediment eroded and carried downstream increases with water velocity  Basic anatomy of a stream: o There’s stream, stream bed and water in the stream; there’s also stream bank o When the water flows over this bank, usually deposits things; overflow of water in this area results in deposition of finer load than the stream load o Silt and sand get deposited by the river on these floodplains – fertile  The amount of sediment eroded downstream increases with water velocity o The faster it flows, the more it can carry with it downstream; the slower, the less it can carry and it can deposit – when it reaches a certain threshold in its velocity, that’s when it begins to deposit stuff o Velocity = 0 = deposit? Slide 11: Sediment Theory - Animation  How the stream actually gets to carry its load  What is the load? o Load of the river/stream = the stuff that it is carrying; it is more than just sand and silt  Stream rolls the bigger pieces along; these are closer to the stream bed – called bed load = pebbles/gravel  The finer load (10 of a mm-mm in size), the sand, silt and some organic particles – they actually are suspended in the current – suspended load  In addition to the suspended load and the bed load – the stuff that’s dissolved in the water – the ions = dissolved load 3 EESA05 Lecture 8: Floods PY Date: Nov 6, 2012 o Which makes up the largest portion of the total load? – the suspended load = almost 90% o Dissolved = 10% o Volume = discharge  You hardly see a clear river – usually cloudy, brown/green – the suspended load – really small Slide 12: Suspended and Bed Load  River system – see 2 diff loads  Brown  More clear  Confluence = where 2 parts of the river join  More suspended load in this part of the stream than up here (not that it doesn’t have suspended load, it’s just that the other region has more) Slide 13: Streams and Equilibrium  Discharge of stream will change if cross-sectional area changes o If it is wide, then hydraulic radius is larger – it allows the water to flow in much larger area; can actually take and carry a lot of sediments and with a system like this with a canyon’?? where you have much lower cross-sectional area – water is travelling fast and is therefore moving a lot of stuff o As soon as it gets out of the canyon, it spreads out because it has much larger cross- sectional area – velocity drops – starts to deposit its load – get alluvial fans right outside of the canyon o If you go hiking/camping in Grand Canyon and see a trickle of water – need to get out immediately of the channel because a wall of water will be coming soon after – a lot of ppl killed  This happens when river comes out from canyon and empties into lake; velocity drops – deposit sediment load and that’s where you get deltas  Sudden decreases in water velocity will decrease the carrying capacity of the water, resulting in deposition  River deposit sediments in the shape of a fan when they slow  Lakes or seas  Sudden slope change  Mississipi = classic delta formation o There’s a difference in terms of flood hazards when talking about valley floods and floods that happen downstream in the lower parts of the channel where the delta is  Why?  b/c delta = combo of channels and these channels are transient based on tide and how much water is flowing in – therefore makes it extremely dangerous b/c there’s no way of predicting how it’ll behave 4 EESA05 Lecture 8: Floods PY Date: Nov 6, 2012  in terms of flood hazards, there’s big difference btn floods in valley/plain enviro and floods in delta enviro Slide 14: Delta Depositions  The fan that forms in a lake or sea is called a delta  Some features – multiple distributary channels  Rapid switching of flow from one channel to another  Shape of delta depends on strength of river, tides and currents Slide 15: River Channel Systems  How do river channels evolve/develop? o You need gradient to start the river but why do we have rivers that look differ? o It’s due to combo of interaction between flow of the water, discharge of water, velocity of the water and the amount of sediment it can transport and the type of sediment it is actually flowing on  River channels and floodplains form as a result of the interaction of water flow and sediment transport  Main river patterns include: o Braided rivers  Usually found:  Flown over certain parts of the world, will see them below  Gravel bars separating  Very large  Need good strong gradient, need slope to be there  Usually developed in areas of coarser sediments o Meandering rivers  Snake their way around  They don’t have steep gradients that braided rivers need in terms of slope  4 reasons for the erosion and deposition of these river systems: o 1) a change in its width, depth or slope/gradient o 2) because of composition of its bed; whether it is gravel, more sandy etc; o 3) type or amount of vegetation in the area o 4) humans can change the dynamic; urbanization, deforest – makes a difference in the way river behaves in terms of erosion/deposition Slide 16: Braided streams  Photo of braided stream = copper river in Alaska o Almost random; these rivers = powerful – gravelly o When water level is high, most of these gravel bars get submerged – so you only see these gravel bars during lower water levels  Braided streams have multiple branching channels divided by many bars of sand or gravel 5 EESA05 Lecture 8: Floods PY Date: Nov 6, 2012  The river is shallow most often  During times of high flow, all of the bars will be covered by water  Braided rivers form where the gradient is steep and the river is carrying abundant coarse sediment  Find braided rivers where you have glaciers and when you have good steepness/gradient (ex: areas that are tectonically active – getting uplift – big change in slope – get braided river sys) Slide 17: Meandering streams  Are completely diff from braided rivers – these curve and bend – as they curve and bend – the gradient isn’t as steep and so the curving/bending allows the river to facilitate the movement downstream (it almost looks like it’s flowing on flat plain – but it’s not absolutely flat of course but it’s not as steep as for braided rivers)  We don’t exactly know why rivers meander – we know some of the characteristics/behavior of these meandering rivers but why? Not known  Rivers with gentle curves and bends that migrate across the floodplain are called meandering rivers  Occurs where gradient is low Slide 18: Meandering streams - Animation  Meandering river – cut-back o The river meanders and there’s this concave (outer bend) part of the river where you see cut-back o River cut-back – can be part of bluff o The river flowing really fast around the outer concave shaped bend and because it’s flowing faster here, it can actually erode the sediments here o As it gets into the convex part of the river – velocity decreases – starts 2 deposit slough – this is where you get point bars o Elevated ridges – called meander scrolls? – due to the way that the river has overflown it has migrated and moved down  Meander scrolls usually have shrubby vegetation  Are parallel to the convex (slower part of the stream) Slide 19: Meandering streams  Water velocity on the outer bend is higher than on the inner bend, so that erosion occurs on the outer bank and deposition occurs on the inner bank, resulting in slow migration of the meanders Slide 20: Meandering Streams  The presence of pools and riffles create diff environments characterized by diff water velocities  Depth is low in the riffles, so speed is fast o In the straight part of the channel where it becomes shallower and picks up speed = riffles 6 EESA05 Lecture 8: Floods PY Date: Nov 6, 2012 o  Depth in pools is greater, so speed of flow is slow o Calm, more glassy – slower, deep part  When a river meanders – moves back and forth o Meander loop getting bigger and bigger – the neck of it gets narrower and narrower – it eventually gets so narrow that the meandering loop gets cut off –meander cut-off – as time goes on, meander cut-off becomes oxbow lake by itself  Picture of meandering stream in Labrador o Going thru changes; base level is changing b/c sea level is dropping by 1-2mm/yr o System is trying 2 adjust to that new base level o There’s oxbow lake; there’s cut-back – which is deep b/c has been eroding – has larger hydraulic radius and therefore can actually become eroded enough to make it deeper and deeper o Along where you have deposition – reduction in stream velocity – deposition of seds  Animation of meandering system: o Direction of flow – have some kind of gradient; very shallow gradient o Stream goes back and forth – meander loops are developing o Loops get larger and larger and when the neck gets too narrow, it gets cut-off and eventually meander cut-off becomes oxbow lake o Outer concave part of the bend –erosion; inside convex part of the bend = deposition  Hydraulic radius = channel efficiency to move that water o If it’s efficient enough to move the water, it increases its velocity; can move faster when it has larger hydraulic radius and is eroding that part of the stream o The area where it deposits the sediments = smaller hydraulic radius – channel is not as efficient in moving the water downstream – so velocity drops and there is deposition – that’s why they meander – often efficiency of channel to allow this water to move  Photo: o Glassy and calm = deeper than the riffles o Riffles o Can tell a bit about how deep it is by looking at it Slide 21: Floodplains  A stream normally consists of one or more channels, where the water normally flows, and a floodplain, where the water spreads out during floods  Intensive human use of floodplains is the main environmental problem associated with streams  Floodplains develop because of over-bank deposition = deposition of sediments over the stream bank – where you get build-up of floodplains – floodplains continuously get built up because the river continuously overflow over the bank and deposits material  We tend to want 2 live near water but there’s a cost when build structures on floodplains  Photo: 7 EESA05 Lecture 8: Floods PY Date: Nov 6, 2012 o Extent of the flood – Seine river – flown over its bank and covered much larger area of flood plain around it  Lighter lines = how far it went and therefore that is how far it deposited  At some point in time, a bit of this river was much higher – can see ancient floodplain up here  River has been changing – there’s a change in base level; things change and the river adjusts  Terraces  Highland creek – it wasn’t always where it is now – can see 3-4 floodplains around there o Floodplain gets built up over time o Floodplains are flat  Makes it easier for us to build; agriculture b/c fertile o Slide 22: Floodplains - Animation  Flooding and levee development  River went over its banks Slide 23: Flooding  Taken from newspaper: o Cross-sectional river system o When there’s flood, flood can go over (when water exceeds the cross-sectional capacity) the bank and deposits a floodplain o Smaller channels = more prone to flooding even with smaller events  Water levels in the river channel may rise high enough to cover the floodplain  Common causes are excess rainfall, snow melt or excess water at the confluence of several streams o River system like Mississippi – drains very large area – as it moves downstream – starts to collect more and more – additional tributaries that add to it o If you live further downstream – chances of you being affected by flood is greater than ppl living upstream – although not always o Because as you go downstream – river is collecting more and more water o So at the confluence – can get greater flooding  Floods are characterized by flood discharge and flood stage o stage = type of the water in the stream; so, stage of the stream at the flood event = flood stage Slide 24: Floodplains and Flooding  Recurrence interval of a flood of a given discharge is found from the historical record, plotting flood size against average interval between floods of that size 8 EESA05 Lecture 8: Floods PY Date: Nov 6, 2012 o Likelihood of a flood or a river discharge of a certain size o Useful estimate; can use the historical occurrences of these events to come up with an average time – esp when trying to determine land use or develop policies around these river systems o Graph: shows discharge = cubic meter per second; recurrence interval is calculated – dashed lines = projection o If plotted on log scale – becomes straight line o Equation below  These plots form straight lines if both axes of the graph are logarithmic  The ‘100-year’ flood is size of flood with an expected recurrence interval of 100 years Slide 25: Flood Measurement and Prediction  To predict flooding for a particular stream, geologists make use of long-term records of previous floods on that stream  The recurrence interval (aka flood frequency) is the average length of time, in years, between floods of a particular size. The recurrence interval (R) is calculated by this formula: o R = (N + 1)/M o N = # of years for which flood records are available for the stream o M = the rank of a particular size of flood during those N years = magnitude = assigned by humans  Largest magnitude in recorded history = #1  Accordingly, we assign M to all other events  In slide 24: we have 9 years of record – from 1995-2003 o Can calculate recurrence interval for largest M = 1 (largest event – 280 discharge)  recurrence interval for this type of flood with this discharge = 10 years o If talking about EQ, in Pacific Northwest, around subduction zone, in the past 35,000 years – have had 7 subduction zone EQs over these years – magnitude 7  Can do recurrence interval for more than just floods Slide 26: Flood Measurement and Prediction  Suppose, in a 140-yr record of floods for a particular stream, a flood 6m above normal was the 4 largest. The recurrence interval for a flood 6m high or larger would be: o R = (N + 1)M = (140 +1)/4 = 35 o For this stream, you could expect to have a 6m or larger flood once every 35 yrs on avg. o In this example, we would call a 6m or larger flood a ’35-yr flood’. People commonly refer to ’50-yr floods’ or ‘100-yr floods’. Again, it is important 2 remember that this wording reflects the average time interval between floods of a particular size. A ‘100- year flood’ could occur 2 years in row. o Can do some planning around these estimates but must be cautious o Probability = inverse of recurrence interval – can be very quick calculation o Recurrence interval of ten years; probability = inverse = 1/10 9 EESA05 Lecture 8: Floods PY Date: Nov 6, 2012 o In terms of the probability of a given event, whether or not that probability will be exceeded will be the probability of some event is  What is the probability of a certain event, the probability of it exceeding certain # of years?  R = years that we’re trying to figure out – what is the probability of this event exceeding  P = 1/recurrence interval; for this example, want to know with the recurrence interval of 100 years what is the probability that the event will occur in 2 yrs  So, P becomes P = 1 – (1/100)^2 – when you calculate this = 0.99 – 99%  so can see that although the recurrence interval might be 100 yrs but the probability that it can happen in the next 2 years is very high  Don’t need to remember this formula  So, there is solid math behind this, not just guessing Slide 27: Recurrence Interval of Floods 
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