Rivers and Flooding

what kind of disasters are the most common and affect most amount of people?

* meteorological

number of people affected by weather-related disasters

world - number of weather-related diseases reported per country (1995-2015)

what are some weather systems that cause flooding?

* tropical cyclones
* monsoon season
* sever isolated thunderstorms
* atmospheric rivers

what are the 2 types of flooding?

* high intensity & short duration
* lower intensity & long duration

what is high intensity & short duration?

* large amount of precipitation falling in a very short time period → flash flood

what is lower intensity & long duration flood?

* large amount of precipitation falls steadily over a longer time period → slow-onset flood
* low precipitation, high flooding

all about streams?

* flows to areas of low elevation → creates channels
* add to groundwater → river

what is a watershed?

* the land area providing water to a specific draining network

what is a draining network?

* an array of streams that drain into the same, larger (trunk) stream
* everything drains into trunk stream

what is the largest watershed in the world?

* amazon river watershed

watersheds of North America

what is a stream gradient?

* slope of the river channel; typically decreases downstream
* headwaters → mountainous areas
* end point of stream → ocean

what is base level?

* an elevation that a stream cannot erode past, controlled by the lvl of the body of water which the stream discharges into

what is bankfull?

* when the water lvl in a river is equal to the height of the banks
* flooding occurs when water rises over bankfull lvls
* rivers do not reach bankfull stage every yr
* on avg → they do not flood
* determining channel bankfull width and depth can approx the bankfull discharge and the capacity of the channel

what are floodplains?

* broad, flat area next to a stream that becomes either partly or comp. covered during a flood
* it is flat due to past floods (sediment deposition) → good for agriculture

what kind of loads do streams carry?

* dissolved
* suspended
* bed-load

what constitutes as suspended load?

* finer particles, such as clay, silt, and fine sand, carried in suspension

what is bedload?

* heavier sediment in a stream that is moved along the stream bed rather than in suspension

what do fast flowing stream carry?

* larger grain sizes (clasts) than a slower flowing stream
* even boulders

the volume of sediments carried in streams depend on?

* flow volume
* velocity;
* during flood, flow vol + velocity increases, so more sediment is transported

Natural Levees

* channel geometry is controlled by flow velocities and its associated sediment load carrying capacity
* as a stream spills over its floodplain, it moves from a deep channel with high velocity to a shallow, broad floodplain with a low velocity
* when velocity drops → causes sediments to also fall
* bigger material fall first → right next to channel
* small particles fall a bit farther
* this creates gradient → coarse to fine

what is an alluvial fan?

* a fan shaped deposit of sand and gravel at the mouth of a mountain canyon, where the stream gradient flattens at the main valley floor
* steep → high velocity
* lower → drop of velocity

what is a delta?

* accumulation of sediment deposited by a river at its entrance into a basin
* at the bottom, there is sediment deposition

what is a permanent stream?

* flows all yr, below the water table
* recharge
* in temperate climate

what is an ephemeral stream?

* flows only for part of the year; above the water table
* only flows when water flow is high (precipitation) will dry up

what is a meandering river?

* river with a single channel and high to moderate sinuosity

where do cutbanks form?

* on the outside of meander bends where the water is accelerated along the outside wall and erodes into the bank

where do point bars form?

* as sediment is deposited in the slower water on the inside of the meander bends

meandering river

example of meandering rivers

* Mississippi River and Floodplain
* streams change over time bc of flow velocity
* cutbanks are eroded, and pntbars are deposited
* always changing + moving

Clark Fork River

* gradually eroded the cutbank until it undercut the house that was originally built 10m back from the river
* issue → land erodes back

what is a braided river?

* a river characterized by multiple, frequent shifting channels
* common in regions where there is a strong seasonally and monthly variation in stream discharge
* during short periods of high discharge a braided river carries the coarsest sediment
* dev in regions where sediment is readily available

bedrock channels

* steep wall → reduce chance of flooding

what is discharge (m3/s)?

* measured vol of water flowing past a cross section of a river in a given amount of time
* D = A x v
* D = discharge (m3/s)
* A = cross-sectional area (m2) = width x depth (in m)
* V = velocity (m/s)

what is flood stage?

* the stage (water lvl) at which a stream rises above its banks in many places and submerges areas outside of the stream channel
* shape + depth is important
* in the top pic → no flooding cuz steep
* e.g., mountains
* bottom → not steep → high water lvl lead to flooding
* e.g., meandering bankfull, then over it → food

what is a hydrograph?

* length of lag time depends on:
* basin area and shape
* spacing of drainage channels
* vegetation cover
* soil permeability and land use
* time delay cuz water has to make way to river

more hydrographs

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what is a slow-onset flood?

* a flood that devs over a long period (days to weeks) and takes weeks to months to subside (seasonal or regional floods)

1931 Great China Floods

* flooding of the Yangtze and Yellow Rivers
* Flooding persisted for months, affecting crops and causing famine and disease
* Estimated up to 3.7 million people died and 50 million people displaced

Flooding in BC, November 2021

* Atmospheric river brought heavy rains to southern BC
* Caused mass wasting and severe flooding
* Disrupted major transportations routes
* 600,000 animals perished
* 5 deaths from a mudslide

what is a flash flood?

* a flood that devs within a short time frame (within 6 hrs) of intense rainfall, dam failure, or other cause
* most flash floods occur due to extreme rainfall where the ground cannot absorb the water
* e.g., 2015 flash flood in Joso, Japan

what are the dangers of flooding?

* floodwater residue contains garbage, sewage, and chemicals
* secondary disasters of cholera and dysentery are common
* submerged homes, businesses, fields
* transportation and communication networks fail
* deaths of animas, and people

what is recurrence interval?

* the avg time between floods of a crt discharge

what is annual exceedance probability?

* the likelihood that a flood of a given discharge or greater will happen in a given yr
* can occur at diff yrs

small floods happen often; large events are rare

what is an100-yr flood?

* a flood with a reoccurrence interval of 100 yrs

what is a 100-yr floodplain?

* the area likely to be flooded by a 100-yr flood

Flood-Hazard Map

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what features are used to determine flood lvls?

* paleoflood