Transcript Document

4.
HYDROGEOGRAPHY
Key Concepts:
• Water Budgets
• Geography of water
• Global disposition
• Local watersheds
• Local hydrologic budget components
• Water Balance Prediction (genetic, generic)
• Management Practices (flood plain mapping), Best
Management Practices
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Water Balance Prediction; Genetic Approaches
• before runoff can occur, precipitation must satisfy the capacities of interception,
surface detention, soil moisture, evapotranspiration, interflow and ground water
• For any time period or event (storm or snowmelt episode) the absolute capacities and
antecedent moisture storage conditions must therefore be determined
• This necessitates ongoing monitoring of the cascade of stores, so that the available
water-holding capacity of each storage is known
• If stores are full prior to precipitation then more water will run off than if they had only
been half full
• Maintaining records of how much water-retaining storage capacity remains at any
point in time is a challenge as is the lag in the contribution of frozen precipitation to
stream flow
2
Water Balance Prediction; Genetic Approaches
• Rain falling on frozen ground may create immediate runoff, however if it freezes on
impact, there may be no runoff until it melts
• the hydrologic budget under these conditions is therefore very intimately tied to the
energy budget
• with complete knowledge, then the inputs, storages, transfers, transformations and
outputs would be additive (mass balance):
P = I + DS + SM + GW + IF + ET + S
• however, routine measurements are restricted to precipitation and runoff
•
given the difficulties of monitoring storages to produce meaningful records spread over
time and space, modelling has been undertaken to find readily available surrogate
measures
•
e.g., what are the shifts that take place over the course of a storm, as the disposition of
precipitation changes from primarily interception and soil-surface absorption, to
infiltration, interflow and runoff as capacities are reached?
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Water Balance Prediction; Genetic Approaches
• models must also contend with the differences between:
• light rain which may only generate runoff if prolonged, and
• heavy rain which will produce runoff, except if all storages have been emptied
• There are also differences among:
• initial spring melting episodes
• sudden and rapid melting, and
• melting accompanied by spring rain storms
• differences in land use, geology, soils, slopes and vegetative cover also significantly
affect responses to these runoff conditions, depending on their permeability etc.
• watersheds must therefore be modelled and calibrated separately
• water usage (agricultural, horticultural (golf), domestic, municipal or industrial
extraction) and land use changes are also factors that the models must accommodate
• only because the implications are so important, are efforts made to model so
thoroughly, but to be used, models must be simple
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Water Balance Prediction; Genetic Approaches
e.g., simplifying the water budget by aggregating all precipitation “depletions” together by:
• type of land cover
• steepness
• extent of impervious ground cover
• soil texture
• land use etc.
P
ET
DS
SM
S
High and
Low flows
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Water Balance Prediction; Genetic Approaches
The Rational Method: Q = 0.0028 C i A
• allows calculation of the peak flow rate (Q) on a channel
• by relying on Runoff Coefficients that have been developed to represent the expected
proportion (C) of rainfall (i = rainfall intensity, in mm/hr) that will run off from the area
(A, in m2) drained to a specified point,
• tables of C are published widely and customized to the local circumstances of specific
jurisdictions
• http://www.mto.gov.on.ca/english/engineering/drainage/section10.htm#flowrate
• primarily used as a design tool for the design of minor drainage systems such as storm
sewers and ditches
• mostly applied to an urban catchment (<50 Ha), but also rural watershed drainage areas
less than 100 ha
• where the variability in soil, vegetation or rainfall is minimal
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Water Balance Prediction; Genetic Approaches: The Rational Method
Land Use / Cover
Commercial
Residential
Industrial
C
0.50-0.90
0.50-0.75
0.25-0.75
From City of Brampton
http://www.city.brampton.on.ca/city_dept/works_trans/Development_Services/Subdivision_Construction/Subdesi
gnstandard.pdf
P
ET
DS
SM
S
High and
Low flows
GW
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Water Balance Prediction; Genetic Approaches: The Rational Method
• runoff has a genetic basis in the overall hydraulic conductivity of a basin
• assumption is made that the specific conditions of any area are the same as in other
areas with the same perviousness circumstances
• in practice, judgment is required in selecting from a range of precise coefficient values
to apply in a specific situation
• this may undermine confidence that the approach is reliable (consistently reproducible)
and accurate (close to reality).
• with more categories of land surface, greater accuracy in prediction is expected
• subtle increments the coefficient if higher-magnitude (less-frequent) events are the
basis for runoff prediction or, if snowmelt is involved
http://www.th.gov.bc.ca/bchighways/operations/hwyeng/bridge/hydrology/hydrol2.doc
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Water Balance Prediction; Generic Approach
• an alternative approach to prediction is to adopt a purely statistical approach:
•
the frequency of discharge values in the past is the basis for prediction of values in
the future
• rather than indicating the day-to-day expected discharges, this approach defines a
standard condition – the maximum runoff expected over a given time period – based
on the frequency of peak (annual) flows already experienced over the period of
monitoring
• by ranking these runoff events over the period of discharge monitoring, and
converting frequency into periodicity (called the "recurrence interval") for each
discharge magnitude, an expected flow is based on the probability of it happening
again
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Water Balance Prediction; Generic Approach
• This is normally used to interpolate the magnitudes of floods for major
recurrence periods floods, and to extrapolate for longer "design" periods
Data is almost always based on longitudinal study:
• observing floods over time for N years
• measuring peak discharge (Qmax) annually (the highest flood for each year)
• ranking these peak annual floods from the very highest ever (1st) to the lowest of
the annual maxima (Nth, indicating that every year its peak flow has been
exceeded)
• converting the frequency distribution to a probability-based return period
(recurrence interval, in years) calculated as:
• R=(N+1)/M
• the number of years expected between when a flood of a given magnitude is
expected to be matched or exceeded
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Water Balance Prediction; Generic Approach
Hypothetical Case (N=29) excerpt:
Year
Qmax
m3/sec
M (rank)
Recurrence
= (N+1)/M
Log10 of Qmax
…
301030
3
5.47
2001
207440
17
5.23
2002
202420
11
5.31
2003
318780
2
5.50
2004
116120
20
5.06
2005
257510
8
5.41
2006
374110
1
2007
201890
16
2008
105220
30
30/1= 30yr
5.57
5.30
30/30=1yr
4.98
…
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Water Balance Prediction; Generic Approach
Hypothetical Case (N=29) excerpt:
Year
Qmax
m3/sec
M (rank)
Recurrence
= (N+1)/M
Log10 of Qmax
…
301030
3
30/3= 10
5.47
2001
207440
17
1.7
5.23
2002
202420
11
2.7
5.31
2003
318780
2
15
5.50
2004
116120
20
1.5
5.06
2005
257510
8
3.75
5.41
2006
374110
1
30/1= 30yr
5.57
2007
201890
16
1.9
5.30
2008
105220
30
30/30=1yr
4.98
…
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Water Balance Prediction; Generic Approach
By plotting the recurrence
interval by actual peak annual
discharge, extrapolation to
standard longer-term return
periods (50-hr or 100-yr) can
be accomplished
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Water Balance Prediction; Generic Approach
• From the graph relating discharge to flood
elevations (Stage-discharge diagram) the
elevation of the flood line for the two
design events can be mapped and depicted
on cross-sections
• Below the 100-year flood line, sites are at
risk of flooding at least once in every 100
years. Only certain land uses and
construction methods are permitted to
expose the public to this hazard to human
life and/or property
P
ET
DS
SM
S
High and
Low flows
GW
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Water Balance Prediction; Generic Approach
However
• in calculating the recurrence interval, precision is limited by the number of years
of record, not the actual magnitude of peak annual discharge
• like all summary statistics, much is overlooked in distilling the data
• for instance, the peak flow may have been nearly reached several times in
some years and only once in others
• the peak value in one year may well have been exceeded several times in
other years
• basin conditions may well have changed over the period of monitoring
• the N-year set of observations may not be representative of what to continue
to expect from the discharge record
• a storm with a likelihood of once in 100 year may or may not have occurred
during the period of record
• conversely, back-to-back small storms may produce as much runoff as one
very large one
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Water Balance Prediction; Generic Approach
• climate shifts may occur due to:
• inherent variability or progressive change
• natural or human-induced changes of genetic water-balance components in the
drainage basin
• erosion
• sedimentation
• water-taking
• land cover changes
• agricultural expansion/contraction
• urbanization
• reforestation
• quarry operations, etc.
• calculation of recurrence is primarily used in prediction of flood peaks
• other water-balance concerns are overlooked:
• low flows
• groundwater recharge
• soil moisture demand etc.
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Water Balance Prediction; Generic Approach
• The strength of recurrence interval use is that it allows standards to be applied. The 2year, 50-year and 100-year or even longer-recurring discharges are widely used as
standards for engineering design of water-management facilities and imposition of land
use restraints.
• Unfortunately, the standard is unevenly applied: some jurisdictions find the “designstorm” criteria too restrictive for their liking and have arbitrarily established other lessrestrictive bases for flood management
• in Ontario, the "Hurricane Hazel events (1954),” the "Timmins event (1961)" and the
100-year storms are widely used as standards for designing flood-risk responses, as part
of provincial policies on hazard management
(http://www.mnr.gov.on.ca/MNR/pubs/nat_haz2.pdf)
• the County of Oxford, for example overlaps in its jurisdiction with four conservation
authorities, with the following “Design Storms”: Grand River Conservation AuthorityRegional Storm (Hurricane Hazel) Long Point Region Conservation Authority- 100 Year Event
Catfish Creek Conservation Authority- Regional Storm (Hurricane Hazel)
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Water Balance Prediction; Generic Approach
• This kind of regulation for the public good is regarded by some people as an imposition on
individual property rights
• Since emergency response and public safety are accepted as social responsibilities in most
societies, protection from flood risk has only occasionally been controversial.
• Failure to protect, as in the case of Hurricane Hazel is more of an issue, and society
continues to pay a very high price for flood damage and prediction/response
infrastructure: (http://sciencepolicy.colorado.edu/socasp/data/tables1.html,
http://www.nws.noaa.gov/oh/hic/flood_stats/index.htm,l
http://www.worldbank.org/dmf/operations.htm ,
http://www.disastercenter.com/flood.htm ,
http://iwin.nws.noaa.gov/iwin/us/flood.html ,
http://www.mscsmc.ec.gc.ca/airg/doc/hogg.html ).
Definition of flood hazard lands is therefore taken seriously since it does impose restrictions
on personal freedoms in regard to what activities are permitted on a parcel of land.
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Water Balance Prediction; Generic Approach
• Management decisions therefore have focused not only on the lands susceptible to
flooding, but on managing the water itself, with implications for low-flow conditions and
water quality. Finding the best management practice for a given situation is often the
issue
• Both the Rational Method and Recurrence Interval are widely used for flood prediction,
the former in designing engineering works and the latter for both engineering of
stormwater conveyance and for land use planning to identify and avoid hazard lands.
• Early engineering works were primarily structural solutions to high flow: designing and
building to “fix the problem” of flooding
• Regulations have been developed to ensure that bridges and culverts will accommodate
expected discharges:
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Water Balance Prediction; Generic Approach
Span < 6.0 m
Span > 6.0 m
50 yr
100 yr
10 yr
Regulatory
25 yr
50 yr
2 to 5 yr
Regulatory
10 yr
25 yr
2 yr
Regulatory
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Management Practices: Best Management Practices
Hazard lands are often held in public ownership as parks (passive-use open space) which
also serve as flood-conveyance and storage facilities:
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Stormwater management: Best management practices (BMP) for stormwater management
(SWM) may include the following:
Practice
disconnecting roof downspouts from roadside stormwater
conveyance systems
landscaping design gently grading the land into vegetated swales,
“dry well” of stones
small to medium-sized “extended dry detention” ponds, off-channel
retaining/restoring pervious vegetated green spaces
Impacts
detention, increase infiltration, evapotranspiration,
intercept, capture more surface detention, increase infiltration,
evapotranspiration
capture more surface detention, increase infiltration and
evapotranspiration
intercept, capture more surface detention, increase infiltration,
evapotranspiration
rooftop gardens intercept, enhanced surface detention,
evapotranspiration, peak flow suppression
collect runoff from impervious areas; surface detention, increase
infiltration, evapotranspiration
retaining roadside ditches rather than engineered curb and gutter
storm sewers
porous “exfiltration” storm sewers
“wet” stormwater retention ponds/detention ponds
capture more surface detention, increase infiltration,
evapotranspiration
groundwater recharge, water quality
enhance surface detention, retain/detain runoff, diminish peak flow,
enhance base flow, evapotranspiration
enhance surface detention, evapotranspiration, base flow
maintenance, peak flow supression
recharge of soil moisture and groundwater reduction of overland flow
reduced runoff
constructed wetlands
infiltrating “soakaway” rock pits / trenches “French drains”
programs to discourage excessive irrigation, lawn-watering, hosing
down sidewalks/driveways, car-washing
programs to limit impervious surfaces, encourage preservation
of open space
porous pavement
underground storage tank or “tunnel”
reduced runoff, enhanced base flow, enhanced infiltration
reduced runoff, enhanced base flow enhanced infiltration
temporary storage, redirected runoff
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Stormwater management
Practices: $
http://www.city.toronto.on.ca/sewers/storm.htm
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Stormwater management
Practices: $
http://www.toronto.ca/greenroofs/pdf/executivesummary.pdf
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