Studied by 5 people
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
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
what features are used to determine flood lvls?
paleoflood
