Unit 7: streams and flood processes
the regulatory floodplain is based on areas flooded during a
100 year floos
What is avulsion?
A flooding stream permanently leaves its old channel to form a new floodplain channel.
Delta
A landform made of sediment that is deposited where a river flows into an ocean or lake
Why does a stream bottom erode more deeply when its water level rises in a flood?
Its water level rises, so water velocity increases and it can carry more sediment.
recurrence interval flood formula
T=(n+1)/m n=number of years in flood record m=rank of flood in question
Floodplain
The area subject to flooding by a stream overflowing its levees
How does urbanization affect flooding?
Urbanization causes increased runoff because less water infiltrates the ground.
when a flood is classified as a 50 year flood it is understood that
a flood of thag size occurs on the average of once every 50 years
any surface water flowing through a channel under the influence of gravity is
a stream
difference between alluvial fan and delta
alluvial fans are in land deltas are in water
If a 100-year floodplain was flooded in 2003, when should we expect the next 100-year flood?
anytime
lowest depth a stream can erode its channel
base level
flood
excessive discharge
characteristic of a downstream regional flood
extended rise of river elevation for weeks large storm systems or extended rainfall floodplains covered in flood waters
as a result of the great floods along the mississippi river in 1993 the FIA now has chosen the 500 year flood level as the regulatory floodplain? true or false?
false
what can occur in upstream prt of a drainage basin during brief heavy rainfall
flash floods
people die from what natural disaster the most?
flash floods and flooding
With urbanization of the upstream drainage area, what changes can be expected in the height of the 100-year flood?
greater height
is the 100 year floosplain size a reliable indicator for future floods?
no
hydrograph
plot of stream stage or discharge vs time
dams are designed to reduce flooding by
providing storage for flood water
upstream part of a drainage basin when heavy rainfall occurs there may be
rapid runoff flash floods
how woukd a hydrograph for a stream change if major urban growthh occured
shorter lag time and higher peak discharge
channelization of streams reduces flooding by
speeding the passage of floodwaters
stream velocity
the distance the water travels in a given amount of time
discharge of a stream is
velocity times cross sectional area volumw of water flowing pasr a given point in a certain amount of time product of velocity times its width times the average depth at a particular point
stream gradient
vertical drop in elevation of the stream channel divided by the horizontal distance it travels
what leads to the failure of a river levee?
water overtopping the levee erosion of levee slumping of banks of levee into the river seepage through levee
why does stream level rise without water running into it
water seeping into the ground raises the water table
characteristics of a downstream channel
wide floodplain natural levees shallow gradiant
do natural levees created during flooding prevent later floods?
yes
do flash floods cause deaths?
yes bc hard to predict
