Lec 20: Floods and Droughts & Lec 21: Reservoirs

Question 1

Why do Canadian hydrologist celebrate
New Year on November 1st?

Answer 1

The annual flow regime begins and ends on november 1st

Question 2

What are some typical discharge indices?

Answer 2

MQ = mean flow
HQ = absolute peak flow (one per year)
7-day minimum flow
Q90 = discharge that is exceeded at 90% of the time (shows low flows)
Q10 = discharge that is exceeded at 10% of the time (shows high flows)
Flow duration curve

Question 3

What does the flow duration curve show?

Answer 3

the percentage of time for which flow is expected to equal or exceed a specified Q value

discharge vs percentage

Question 4

What does a steep flow duration curve show?

Answer 4

high variability

Question 5

What are hydrologic regimes?

Answer 5

They show monthly flow values (either average or normalized) for a region. This describes the characteristic hydrologic behavior of a basin

Question 6

How are floods described?

Answer 6

by discharge volumes,
water levels, flood stages, or inundated areas

Question 7

What are indicators of a flood?

Answer 7

• highest water level reached
• peak discharge reached

Question 8

What units are used to express floods via discharge measurement?

Answer 8

maximum daily flow
absolute peak flow

Question 9

What are the main causes of floods?

Answer 9

heavy rain
brief torrential rain
tropical cyclone
monsoonal rain
snowmelt
dam/levy break
ice jam/break-up
extra-tropical cyclone
tidal surge
avalanche related

Question 10

What other factors influence flood occurrence?

Answer 10

antecedent soil moisture conditions
extent of rainfall in the watershed
relation between size of watershed and duration of storm

Question 11

Why are floodplains important ecological systems?

Answer 11

• help attenuating flood waves
• are used for agriculture (i.e.
  importance for food production
• Floods trigger ecological processes (e.g. fish
  migration)
• Floods shape the river channel by flushing out or delivering sediments (habitat creation)

Question 12

What is attenuation of flow

Answer 12

The attenuation of flow refers to any means by which peak
flows are reduced

Question 13

How does attenuation occur naturally and through man-made methods?

Answer 13

naturally: occurs especially when a river has large flood plains and lakes

man-made: dams and flood-detention basins

Question 14

How does forest cover affect flooding?

Answer 14

reduces flooding from small storms, but has little impact on the regulation of major floods

Question 15

how have human activities affected flooding?

Answer 15

three activities: population growth, increased urbanization, and paving of more land

4 impacts: less infiltration and flood attenuation, higher peak flow and increased frequency of floods

we are also settling onto floodplains more now

Question 16

what is a drought

Answer 16

no one definition, but…

Condition of insufficient moisture caused typically by a deficit in
precipitation over some time period

Question 17

define a famine

Answer 17

“Drought” event that reduces food availability and causes
widespread and substantially increased morbidity and mortality

droughts are not the only factors that can lead to famine

Question 18

What are the 5 types of droughts?

Answer 18

► Meteorogical / Climatological / Atmospheric drought
* prolonged period with less than average precipitation

► Agricultural drought
* deficit in soil moisture affecting crop production

► Streamflow / Hydrological drought
* deficit in river discharge or surface water storage (lakes, wetlands)

► Groundwater drought
* deficit in groundwater storage

► Socioeconomic / Operational drought
* conflict of water shortage and water management demand
* when physical water shortages begin to affect people

Question 19

What are the 2 main causes of droughts?

Answer 19

► Climatic factors
* Decreased precipitation due to certain large scale
circulation patterns or recurring phenomena (e.g., El Nino)
* Increased evaporation due to higher temperatures or
intensified solar radiation
* Climate change

► Poor land-use practices and water mismanagement
* Land degradation, desertification
* Deforestation?

Question 20

What is the relationship between forest change and droughts?

Answer 20

there is a variable relationship and no scientific consensus. However:

► Typically, young forests and plantations (often with nonnative and fast-growing, water-consuming tree species such
as eucalyptus) have been shown to reduce net and dry water
flow.

Question 21

Name some drought indices

Which characteristics do they take into account? (5)

Answer 21

Palmer Drought Index

Standardised Precipitation Evapotranspiration Index (SPEI)

Aridity Index: P/PET

They can take
various physical characteristics into account (precipitation, ET,
soil moisture, snow cover, streamflow)

Question 22

Define a dam

Answer 22

A dam is a barrier built across a body of water that controls the flow of water

► Most dams create reservoirs behind them

Question 23

How has dam construction change since the 1950s?

Answer 23

it has increased sevenfold

Question 24

What are the 5 positive uses of reservoirs?

Answer 24

 Irrigation (i.e. food)
30-40% of the 271 million hectares of
irrigated agricultural land worldwide rely on
dams

 Water for consumption
12% of large dams are designated for water
supply

 Hydroelectricity
Approx. 20% of total global electricity supply
from hydropower

 Flood control

 Fishing, navigation, recreation, other uses

Question 25

What are the 4 main reservoir types?

Answer 25

• Municipal water supply
• Flood control
• Irrigation
• Hydropower

Question 26

What are the main countries producing hydropower

Answer 26

Canada, USA, Russia, Brazil, China, Norway

Question 27

What are the negative effects of dams/reservoirs?

Answer 27

social impacts
- people displaced, provide a breeding ground for disease carrying insects and parasites

environmental impacts
- dams alter flow regulation and create river fragmentation
- sediment retention
- alter temperature and chemical composition of released water
- destruction of habitat and fisheries, both upstream and downstream

Question 28

How do dams create river fragmentation? What are the impacts of this?

Answer 28

they interrupt the connectivity of river systems

this impedes longitudinal connectivity and disrupts fish spawning
and migration

Question 29

Who do dams impact flow regulation?
What are the impacts of this?

Answer 29

Dams alter natural flow
characteristics due to water
storage and managed
release

► This affects longitudinal and
lateral connectivity (on
floodplains)

Hydropower generation can
lead to extreme flow
fluctuations (hydropeaking)

Question 30

What is a possible solution to the negative effects of dams?

Answer 30

Multi-level release dam (as opposed to simple spillway overflow)
- releases water with different temperatures and chemical characteristics, but more expensive to build

Question 31

What are other impacts of dams? (5)

Answer 31

impact land use and agriculture
- upstream: flood forested or arable land
- downstream: restrict water from attaining floodplains which were previously supported by the sediment input from river

reservoir seepage –> groundwater rise –> evaporation –> salt contamination

silt buildup can fill the expensive reservoir

loss of 5% of storage volume, since evaporation increases in a reservoir

have greenhouse gas emissions