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Lecture

Chapter 13.1.docx

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Department
Geophysics
Course
GOPH 375
Professor
All Professors
Semester
Winter

Description
Floods  Big floods are rarer than little floods  Hydrograph  Base flow is the part of the river that flows all the time o Ground water flows in winter  Stream floors have a steeper bottom/slope (higher gradient) near the stream source, and a flatter bottom (lower gradient) near the stream mouth  Base level - level below which a stream cannot erode  Discharge – the rate of water flow expressed as volume per unit of time  Load – available sediment amount waiting to be moved  Gradient – the slope of the stream bottom  Channel pattern – the sinuosity (curves) of the stream path  Graded stream theory: Streams maintain equilibrium by changing the gradient of the sea bottom o Too much discharge: rapid water flow erodes the stream bottom and increases the load, reduces the vertical drop, and creates more curves to use up extra water energy o Too much load: built up sediment on the stream bottom increases the gradient, thus the water flow, stream becomes straighter (braided stream?) o Lakes are temporary features streams are trying to eliminate o Typical stream has a braided stream upstream (too much load), and meanders downstream (too much discharge)  Rainfall infiltration into ground is most common in porous/dry soil that’s flat with vegetation. Runoff occurs on already saturated ground with steep slopes  Flood plains: floors of streams during floods; are at a high risk of flooding again. Short streams have a higher chance of flooding.  Flow duration curve plots discharge (vertical axis) vs. accumulated frequency % (horizontal) o Curve is not connected to vertical axis because you can’t ever say the river will never get higher than a particular point – can’t ever find top height o Flood frequency curve – historical floods plotted; the larger then flood, the rarer it is. Curve should influence building construction.  Recurrence interval how often, on average, a flood will reach a certain discharge o Calculated by T= (N+1)/m  T = answer is in years, how often that discharge level will occur  N = number of years of record  M = magnitude (rank in series from largest to smallest) o 100 year flood (recurrence interval of 100) means there’s a 1% chance of that discharge level happening each year o Cumulative probability – the longer the wait, the more likely a flood will occur; but the probability of flood each year is the same regardless of when the last flood occurred  Red River Floodway: in Manitoba, regular flooding because it’s relatively young so it hasn’t created a deep valley yet, the clay underlying the river impedes infiltration into the ground, the lake is shallow, melting winter snow o 1950 blizzard (winds + precipitation) combined with melting snow caused dikes protecting Winnipeg to collapse, mass evacuation o Floodway built in response to flood, trench diverting excess water o 1997 floods caused by larger than average drainage basin and melting of frozen water saturated in the soil. Floods caused fires, floodway protected Winnipeg Flood Styles:  Hydrometeorlogical floods occur in relation to specific weather conditions (thunderstorm, cyclone, snow melt). Most floods are most common in Canada o Rainfall floods: when precipitation rate exceeds infiltration capacity water becomes runoff. Flash floods. Meter deep water can wash away cars. Common in the Amazon/Brazil and other tropical areas  1972 Rapid Creek Flood, North Dakota. Dam collapsed, 23
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