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

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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

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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

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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

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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

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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

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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

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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)

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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

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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

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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

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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

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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

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*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)

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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

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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

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Higher flood frequencies were found in mountainous areas (e.g. Cordillera) where several periods of flooding may result from snowmelt at different elevations

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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)

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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

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*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.