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Lecture

GOPH 375 Lecture Notes - Ice Dam, Saguenay Flood, 2013 Alberta Floods


Department
Geophysics
Course Code
GOPH 375
Professor
All

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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
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