Floods/Rivers Flashcards
2 Types of Rivers
Meadering
Braided
Braided River Features
- Number of channels:more than 1
- Size of channel:smaller
- Shape of Channelhigher w/d ratio
- Discharge:variable
- Sediment Load:poorly sorted coarse or large moving as bedload
- Gradient:Steep
Meandering River Features
- Number of channels:1
- Size of channel:larger
- Shape of ChannelLower w/d ratio
- Discharge:consistent and stable (uniform)
- Sediment loadwell sorted, fine or small moving in suspension
- Gradient
shallower
Braided Rivers are ____ in the drainage basin
higher up
Meandering River are ____ in the drainage basin
lower down
Flooding in a braided system (see photo)
The excess water of a flood will enter the channel very soon after the rainfall event.
Discharge will increase quickly and reach a maximum value quickly
Just as quickly, discharge will drop as this excess water passes through the river.
Potentially a lot of sediment will be moved.
Braided rivers usually lack a flood plain so water and sediment continue to move during a flood.
Flooding in a meandering river (see photo)
Excess water from precipitation will enter the system more slowly.
This means that discharge will increase starting later after precipitation, reach a lower maximum value over a longer period of time, and then decrease slow over time.
In a meandering river, less sediment may be removed from the system. More might be deposited or simply moved within the system.
Braided River flood plain
No flood plain
Meandering flood plain
Flood plain
Which type of river flood is more catastrophic?
Both rivers are capable of creating a catastrophic flooding event.
While a flood in a brained river may be more violent (faster water), it will be shorter in duration.
A flood in a meandering system may be less violent (slower water) but last longer. Also, a flood in a braided system will arrive sooner, whereas in a meandering system it may arrive later after the precipitation event.
Urban flooding (see graph)
An urban setting is less permeable:
- There are many hard surfaces that prevent water from infiltrating into the ground.
- There is also artificial drainage (i.e. sewers) that will channel water quickly into a river.
These both contribute to sudden increases and decreases in discharge.
Rural flooding (see graph)
In a rural setting, there are a lot of permeable surfaces (fields, forests etc. ) that allow water to infiltrate into the ground.
There is also little or no artificial drainage that will move quickly.
There is also lots of vegetation that can intercept and absorb water, slowing water movement over the surface.
?
Often in a single drainage system the maximum discharge (Q) will travel downstream through the system.
This maximum Q will occur up stream at an earlier date and later downstream.
This means a flood can travel downstream.
Unique River Conditions
In Canada, many rivers flow north.
As they flow north the landscape changes; permafrost is encountered.
Permafrost makes the ground impermeable.
Precipitation is therefore quickly channeled into the river adding to a faster increase in Q.
Also, if there is flooding, this excess water will remain on the surface, making the flood worse.
?
During the winter, most rivers in Canada are frozen over. They behave like a pipe in that the water moving in them is confined to a fixed channel site.
In spring, North flowing rivers in Canada thaw first in the south. Here the extra water from snow melt eventually flows into a channel to the North that is still sealed with ice. Here water velocity increases.
At the same time, water pressure under the ice increases eventually causing the ice to break, sometimes suddenly and violently.
