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Severity of Flooding Reflects Amount and Intensity of PPT and MELT Intensifying Conditions
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Lecture 8-Floods
Introduction General Arctic Drainage Systems Glacier Ice Jams Statistical Methods for Snowmelt Floods
Flood Flows
Apart from the rare effects of landslides or dam failures (ice dams, glaciers, man-made dams)-river floods are caused almost entirely by excessively heavy and/or prolonged rainfall or melt
…..large in quick flow into the stream channel
Severity of Flooding Reflects
Amount and Intensity of PPT and MELT
Intensifying Conditions
Catchment Conditions
Area Time of concentration Total volume of runoff generated
Snow distribution over the area Slope, Aspect, Altitude
Water movement depends on slope materials
Slope…… Q
Continued
Aspect and Altitude….affect type of ppt. and energy balance (evap.)
Channel and Channel Networks Are dynamic and variable Can change dramatically within a few
hours
Drainage Pattern
Dendritic pattern Lower catchment-# of major floods from a
# of major tributaries meet at the outlet Sharp, high magnitude flood peak at the
catchment outlet VS
Patterns which allow water from lower tributary to be passed through before upstream tributaries have arrived……”Muted Flood Response”
Depression Storage
Total area of inter-connected saturated areas……determines volume of quickflow
Channel Conditions
Flood peak will move faster in an unregulated steep channels and slower in well regulated, flat ones.
Channel roughness (bed, bank materials, vegetation growth)
Channel shape and storage properties can vary widely with changing flow and local conditions
Glaciers
Aperiodic variations of runoff from glaciers…..High Q: A) periods of very rapid melt over a week
or more, which permits high rates of baseflow and quickflow
B) extreme high, intensity rainfall, especially late in the afternoon where meltwater runoff is at a maximum
Continued
C) Sudden release of water (jökulhlaup) which has either been held in storage within the glacier, or as surface lakes on or adjacent to the ice, or has been dammed back by tributary valleys.
Ice Jams-Floods
Historical Flood Review (i.e Mackenzie, Liard) Past….floods due to ice-jam backwater
and not by high flows during the open water period
Current Flood Predictions….derived from records of high flow under open water conditions
Continued
Do Not Take Into Account Flow Due to Ice Jams
Need Year Round Flood Predictions-N. Rivers Return Periods-Ice Jam Floods Return Periods-Open Water
Continued
A few sites in a river are usually prone to major jam formation every year with little variation in jam locationVarious stages of jam
formation and break-up with Q
Continued
1) progressive accumulation of ice pieces in front of the solid ice cover where the jam thickness is usually governed by the criterion of frontal progression or packing
2) with volume of ice in the jam and flow, the jam in the second stage, will attain equil. thickness if the channel is regular-the ice thickness is more or less uniform and the slope of the water line through the jam is approx. parallel to the water slope
Continued
3) foot of the jam-the cover will be made of large ice pieces. water will be steeper in this location….local
head losses and the higher friction of bigger pieces * Location where the thrust of the jam is the highest
Continued
If the internal resistance of the accumulated ice pieces becomes smaller than the thrust with an in Q, the jam will shove under the existing solid cover, break it upwards in a continuous movement and the ice jam will move downwards.
*Release of Ice Jams….Release to a Surge….Triggers break-up of the solid ice cover
Use of Critical Jam Depth
The criterion for jam release can be defined when the hydrostatic water depth of the solid ice cover downstream from the jam Yi becomes < Yc at the foot of the jam.
Yi< Yc (subcritical jam flow) Yi = Yc (limit equil. conditions) Yi> Yc (no jam there yet, stable jam flow)
Continued
Powerful tool Study existence and stability of ice
jams…..stream reaches Identify max. Q that these jams can
withstand before their release.
Maximum Possible Upstream Stage vol. of ice in the jam, its thickness and
the ice roughness are at a maximum Max. Q that the jam can withstand is:
QMax= 0.054 CYi2 (max) B0.5
Qmax = max. Q for which Yi(max) =Yc
Continued
C= R 1/6/n R is the Hydraulic Radius = A/2B A is the Area of the Channel B is the Width of the Channel N is Manning coefficient
Continued
N = [ ni 1.5 + nb 1.5]/2
Where, ni, nb are Manning’s roughness coeff. under the ice and on the river bed
Yc= 1.67 A/B
Continued
Typical Jam Sites Sharp bends Local restrictions Shoals Junctions of tributaries
Statistical Analysis of Snowmelt Floods Many parts of Canada-prone to flooding during
the snowmelt period 2 basic approaches have been used to
determine the timing and size of floods 1) permits the short-term forecasting of impending
hazards so that the correct evasive or regulatory action may be taken…models require conitinually updated reports of basin conditions and anticipated snowmelt and precipitation
Continued
2) provides estimates of the probabilities of hazards suitable for the long-term planning of flood plain use, design of flood damage reduction structures and the provision of emergency services and insurance. Models require statistical analyses of the
long-term streamflow records
Definition of Floods
The definition of floods is of both practical and theoretical importance
In past, the largest daily Q in year (annual flood) has been selected for study….this type of analysis has practical limitations in areas where more than 1 flood occurs/year
Continued
*statistical techniques have been developed recently, to make a more general treatment of flood problems possible A more comprehensive defin. of floods,
which inlcudes all daily Q which exceed some pre-determined level of concern (the partial durations series)
Continued
Frequency counts of the # of floods a location receives per year
The time and duration of their occurrence
Maximum flow that the river attains during any such period
Continued
The probability distribution used to describe these variables have a theoretical foundation and their parameters can be estimated from a limited number of years of flow record
Results
Dates when floods occur were found to be normally distributed and often with a mean during spring and a fixed variance that describes the year-to-year variability in the timing of snowmelt.
Canada-systematic pattern to the timing and magnitude of snowmelt floods…..regional and national scale
Continued
Higher flood frequencies were found in mountainous areas (e.g. Cordillera) where several periods of flooding may result from snowmelt at different elevations
Continued
Lower flood frequencies….are found in larger basins with large lake storage (e.g. Tree) where the flood peaks are attenuated.
A similar combination of conditions explains the variability in mean flood duration…ranging from < 2 days (Nith River) to 53 days (Tree River)
Continued
Large Basins-….abundant lake storage…prolong the flood period
Rivers near the coast display higher specific runoffs (Q/drainage area) than basins in central Canada.
Rain Shadow Affects-Barriere and McClintock
Continued
*mean date of flooding Varies-latitude, altitude, coastal proximity BUT variability in flood dates is LOW…
reflects the consistency of the snowmelt process
Snowmelt floods Rain on snowmelt….dangerous Rain floods
NOW
Statistical studies of floods-linked with meteorological records….floods may be classified according to their generating process (snow, rain, rain-snow).
Shift to link ppt, basin morphology and resulting flood characteristics
Aim to determine flood frequency data for ungauged stations (remote areas of Canada)
Summary-Floods
General Intense melt
Arctic Snow dams increase Q for short duration
Glacier Similar-increase Q due to breakage of ice dam,
tunnel…..jolkulhaulp Ice Jams
Impt. in Canada Must look at backwater stage and peak floods in open water Impt. to monitor critical ice jam depths
Statistical Methods Time-duration analysis (short record) Further link with met. info and basin info.