Rivers and Flooding

meteorological

tropical cyclones

monsoon season

sever isolated thunderstorms

atmospheric rivers

high intensity & short duration

lower intensity & long duration

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

large amount of precipitation falls steadily over a longer time period → slow-onset flood

• low precipitation, high flooding

flows to areas of low elevation → creates channels

add to groundwater → river

the land area providing water to a specific draining network

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

• everything drains into trunk stream

amazon river watershed

slope of the river channel; typically decreases downstream

headwaters → mountainous areas

end point of stream → ocean

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

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

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

dissolved

suspended

bed-load

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

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

larger grain sizes (clasts) than a slower flowing stream

• even boulders

flow volume

velocity;

• during flood, flow vol + velocity increases, so more sediment is transported

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

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

accumulation of sediment deposited by a river at its entrance into a basin

• at the bottom, there is sediment deposition

flows all yr, below the water table

recharge

in temperate climate

flows only for part of the year; above the water table

only flows when water flow is high (precipitation) will dry up

river with a single channel and high to moderate sinuosity

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

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

Mississippi River and Floodplain

streams change over time bc of flow velocity

cutbanks are eroded, and pntbars are deposited

always changing + moving

gradually eroded the cutbank until it undercut the house that was originally built 10m back from the river

issue → land erodes back

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

steep wall → reduce chance of flooding

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)

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

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

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

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

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

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

the avg time between floods of a crt discharge

the likelihood that a flood of a given discharge or greater will happen in a given yr

can occur at diff yrs

a flood with a reoccurrence interval of 100 yrs

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

paleoflood