Lec 20: Floods and Droughts & Lec 21: Reservoirs Flashcards
Why do Canadian hydrologist celebrate
New Year on November 1st?
The annual flow regime begins and ends on november 1st
What are some typical discharge indices?
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
What does the flow duration curve show?
the percentage of time for which flow is expected to equal or exceed a specified Q value
discharge vs percentage
What does a steep flow duration curve show?
high variability
What are hydrologic regimes?
They show monthly flow values (either average or normalized) for a region. This describes the characteristic hydrologic behavior of a basin
How are floods described?
by discharge volumes,
water levels, flood stages, or inundated areas
What are indicators of a flood?
- highest water level reached
- peak discharge reached
What units are used to express floods via discharge measurement?
maximum daily flow
absolute peak flow
What are the main causes of floods?
heavy rain
brief torrential rain
tropical cyclone
monsoonal rain
snowmelt
dam/levy break
ice jam/break-up
extra-tropical cyclone
tidal surge
avalanche related
What other factors influence flood occurrence?
antecedent soil moisture conditions
extent of rainfall in the watershed
relation between size of watershed and duration of storm
Why are floodplains important ecological systems?
- 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)
What is attenuation of flow
The attenuation of flow refers to any means by which peak
flows are reduced
How does attenuation occur naturally and through man-made methods?
naturally: occurs especially when a river has large flood plains and lakes
man-made: dams and flood-detention basins
How does forest cover affect flooding?
reduces flooding from small storms, but has little impact on the regulation of major floods
how have human activities affected flooding?
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
what is a drought
no one definition, but…
Condition of insufficient moisture caused typically by a deficit in
precipitation over some time period
define a famine
“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
What are the 5 types of droughts?
► 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
What are the 2 main causes of droughts?
► 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?
What is the relationship between forest change and droughts?
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.
Name some drought indices
Which characteristics do they take into account? (5)
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)
Define a dam
A dam is a barrier built across a body of water that controls the flow of water
► Most dams create reservoirs behind them
How has dam construction change since the 1950s?
it has increased sevenfold
What are the 5 positive uses of reservoirs?
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
What are the 4 main reservoir types?
- Municipal water supply
- Flood control
- Irrigation
- Hydropower
What are the main countries producing hydropower
Canada, USA, Russia, Brazil, China, Norway
What are the negative effects of dams/reservoirs?
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
How do dams create river fragmentation? What are the impacts of this?
they interrupt the connectivity of river systems
this impedes longitudinal connectivity and disrupts fish spawning
and migration
Who do dams impact flow regulation?
What are the impacts of this?
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)
What is a possible solution to the negative effects of dams?
Multi-level release dam (as opposed to simple spillway overflow)
- releases water with different temperatures and chemical characteristics, but more expensive to build
What are other impacts of dams? (5)
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
