Oct 7 Lecture: Streams and Flooding Flashcards
a flood occurs when ___ exceeds ___
input
output
T/F
velocity, height, and discharge of stream decrease during a flood
false
they all increase
stage=
the elevation of the water
- flood stage= stream exceeds the bank height
(2m stage= when water is 2m above banks)
crest=
maximum stage is reached (most amount of water and damage)
upstream flood=
occurs in a small, localized part of a basin (where the river is confined- water can enter fast and leave fast)
downstream flood=
occurs in a larger, lower part of a drainage basin (usually stays for longer)
flash flood=
a type of upstream flood characterized by a rapid rise of stream stage
T/F
upstream and downstream floods are directly associated with the up/downstream locations of a stream
false
floods affecting small, localized areas are usually called:
- they generally result from:
upstream floods
locally intense rainstorms
dam bursts
eg flash floods
floods affecting large river systems are called:
- they generally result from:
downstream floods
heavy rains or snow melts
tend to be much longer in duration
naturally, flooding will occur ___ before the excess water reaches ____, and the ____ discharge will peak before the ___ discharge
upstream
downstream
upstream
downstream
describe the hydrograph of an upstream flood
why is it shaped like this?
there is a sharp peak in the hydrograph (the discharge rises most steeply). After the flood, the discharge drops rapidly
b/c the drainage basin is smaller (surface runoff doesn’t need to travel far to enter the stream)
describe the hydrograph of a downstream flood
why is it shaped like this?
a flood is represented by broad, low peaks on a hydrograph.
the drainage basin is larger (smaller changes in stream discharge)
what 4 things influence the rate of surface runoff?
- ground cover- different rates of infiltration occur with soils, rocks, pavement, etc
- topography- steepness of terrain governs how much is infiltrated
- presence of vegetation: plants may provide a physical barrier (decrease water velocity, increase soil infiltration)
- climate- determines level of rain/ frequency of storms. Soil may be frozen during spring runoff
a flood frequency curve is a useful tool to evaluate:
plot ___ as a function of ___ ____
frequency of flood events
discharge as a function of recurrence interval
what is the formula for the flood frequency curve
R= (N+1)/M R= recurrence interval N= number of years M= ranking of annual maxima
___ reduces a stream’s capacity to rapidly carry water away
silt
How do the following increase the risk of flood after building on a floodplain?
- Asphalt
- buildings
- storm drains
- vegetation loss
- Asphalt: reduce infiltration
- buildings: replace water volume, raises stream height
- storm drains: rapid delivery of storm water to stream= rapid increase in stream height
- vegetation loss: soil exposed, can cause streams to silt up
The warmer the climate, the ___ the hydrological cycle
faster
= more frequent floods/ extreme events, sea level rise
How does channelization reduce flood hazard?
straightening stream channel will speed up the flow of water and reduce flood damage locally
BUT
this increases erosion and cause higher flood stages downstream
How do levees reduce flood hazard?
allow water to reach higher stage w/o spilling onto the floodplain
However
when levees contain floodwater, they increase stream velocity and may affect flooding downstream
- if they fail= more significant damage!
T/F
restrictive zoning, retention ponds, and diversion channels are all ineffective flood hazard reduction strategies
false
they are flood hazard reduction strategies
but
they are usually quite effective
