Chapter 13: Floods

June 2013 was considered the

• Flood of floods

• Three days of torrential rains

• Southern Alberta

• Canmore

Flood of floods: flooded _ then affected Calgary downstream as far as Medicine Hat

Bow River

• Back to normal in 2 weeks

Failure mechanisms: fill

Window/door breach

Failure mechanisms: collapse

Structural response

Failure mechanisms: float

Buoyant uplift

Rivers as response mechanism

• A treelike shaped drainage basin composed of four subbasins delineate by dotted lines

Rivers as response mechanisms —streams:

1) merge unto small subtributaries

2) larger subtributaries

3) tributaries

4) Finally unto main stream

Within a drainage basin, numerous factors interact to make streams seek equilibrium:

A state of balance where a change causes compensation actions

To grasp the fundamentals of how streams work, a few key variables must be understood:

1) discharge: the rate of water flow expressed as volume per unit of time

2) load: the amount of sediment waiting to be moved

3) gradient: the slope of the stream bottom

4) channel pattern: the sinuosity of the stream path

Too much discharge of equilibrium stream: the excess water erodes at the bottom, flattening the gradient and thus

Slowing water flow

The stream also responds by increasing the

Sinuosity of its channel pattern through meandering

A meandering stream cuts into its banks, thus using some of its

Excess energy to erode and transport sediment

In a meandering stream, the inside bank grows as a

Sediment and is deposited, while the outside bank is eroded and steep

Too much load: excess sediment is dropped on the stream bottom,

Increasing the gradient and thus speeding the water flow

Too much discharge: a braided stream develops as

Water flows through excess sediment within a fairly flat valley

Discharge equation

Q = V x A

• V = velocity of water

• A = cross-sectional area of a stream

A Hydrograph is

Q plotted against calendar time

Too much load: natural dam over time, a stream fills the lake with sediment then flows across the infilled lake, flow down the dam’s steep face is at a high velocity, causing

Erosion and transport of dam and lake sediment

Factors affecting infiltration

• increasing soil porosity

• Increasing vegetation cover

• Increasing duration of precipitation

Factors affecting runoff

• Increasing amount of precipitation

• Increasing soil saturation

• Increasing slope steepness

Hydrometeorological floods:

• Specific weather conditions

• Rate of precipitation exceeds the infiltration capacity of the ground

A local thundercloud

Flash flood in a few hours

A regional rainfall lasting days

Regional flooding lasting weeks

Storm surge

Coastal flooding

Winter breakup

Ice-jam flooding

Several flood fatalities in recent years in Canada have occurred when

Cars were swept off the road by flood waters

Do not _ through a flood

Drive

Flood water 0.6 meters deep both

Buoyantly lifts and laterally pushes a car