Oct 7 Lecture: Streams and Flooding

Question 1

a flood occurs when ___ exceeds ___

Answer 1

input

output

Question 2

T/F

velocity, height, and discharge of stream decrease during a flood

Answer 2

false

they all increase

Question 3

stage=

Answer 3

the elevation of the water
- flood stage= stream exceeds the bank height
(2m stage= when water is 2m above banks)

Question 4

crest=

Answer 4

maximum stage is reached (most amount of water and damage)

Question 5

upstream flood=

Answer 5

occurs in a small, localized part of a basin (where the river is confined- water can enter fast and leave fast)

Question 6

downstream flood=

Answer 6

occurs in a larger, lower part of a drainage basin (usually stays for longer)

Question 7

flash flood=

Answer 7

a type of upstream flood characterized by a rapid rise of stream stage

Question 8

T/F

upstream and downstream floods are directly associated with the up/downstream locations of a stream

Answer 8

false

Question 9

floods affecting small, localized areas are usually called:

- they generally result from:

Answer 9

upstream floods

locally intense rainstorms
dam bursts
eg flash floods

Question 10

floods affecting large river systems are called:

- they generally result from:

Answer 10

downstream floods

heavy rains or snow melts
tend to be much longer in duration

Question 11

naturally, flooding will occur ___ before the excess water reaches ____, and the ____ discharge will peak before the ___ discharge

Answer 11

upstream
downstream
upstream
downstream

Question 12

describe the hydrograph of an upstream flood

why is it shaped like this?

Answer 12

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)

Question 13

describe the hydrograph of a downstream flood

why is it shaped like this?

Answer 13

a flood is represented by broad, low peaks on a hydrograph.

the drainage basin is larger (smaller changes in stream discharge)

Question 14

what 4 things influence the rate of surface runoff?

Answer 14

1. ground cover- different rates of infiltration occur with soils, rocks, pavement, etc
2. topography- steepness of terrain governs how much is infiltrated
3. presence of vegetation: plants may provide a physical barrier (decrease water velocity, increase soil infiltration)
4. climate- determines level of rain/ frequency of storms. Soil may be frozen during spring runoff

Question 15

a flood frequency curve is a useful tool to evaluate:

plot ___ as a function of ___ ____

Answer 15

frequency of flood events

discharge as a function of recurrence interval

Question 16

what is the formula for the flood frequency curve

Answer 16

R= (N+1)/M
R= recurrence interval 
N= number of years 
M= ranking of annual maxima

Question 17

___ reduces a stream’s capacity to rapidly carry water away

Answer 17

silt

Question 18

How do the following increase the risk of flood after building on a floodplain?

• Asphalt
• buildings
• storm drains
• vegetation loss

Answer 18

• 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

Question 19

The warmer the climate, the ___ the hydrological cycle

Answer 19

faster

= more frequent floods/ extreme events, sea level rise

Question 20

How does channelization reduce flood hazard?

Answer 20

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

Question 21

How do levees reduce flood hazard?

Answer 21

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!

Question 22

T/F

restrictive zoning, retention ponds, and diversion channels are all ineffective flood hazard reduction strategies

Answer 22

false

they are flood hazard reduction strategies
but
they are usually quite effective