Colliery Dam (Nanaimo BC) Risk Assessment by Dr. Bill Roberds

21 Jan 2014 1.E-08

1.E-07

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1.E+00

1 10 100 1000 10000

An

nu

al P

rob

abili

ty o

f Ex

cee

dan

ce

Number of Fatalities

unacceptable bound

acceptable bound

Societal Safety Criteria (ref. CDA, 2013)

unacceptable

acceptable

examples of a complementary cumulative probabilitydistribution for a facility (considering all scenarios)

Develop Colliery Dams (Nanaimo BC) Plan

13 Dec 2013 Meeting Objectives - identify optimal dam rehab option plan Criteria - including (but not limited to) safety and

financial performance

Process - conduct risk assessment to appropriately evaluate potential performance (rather than worst-case scenario) of any plan, per recent dam safety guidelines

Risk assessment performance model translates inputs outputs

inherent uncertainties in inputs and in model result in uncertainties in outputs

quantify uncertainty in terms of probability

assess probability objectively or subjectively 21 Jan 2014 2

People/property/critical functions/etc. might exist within each element, but especially “downstream”

Outflow from Lower Dam can cause inundation downstream that might affect those people/property/critical functions/etc. outflow from Lower Dam consequences (safety and financial) of Lower Dam

outflow

Colliery Dam System: Elements

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Middle Reservoir

Lower Reservoir

Middle Res Watershed

Lower Res Watershed

Middle Dam

Lower Dam

Precipitation Event

Down- stream

Seismic Event

Lower Dam Release

Lower Dam outflow model

Lower Dam reservoir level Lower Dam reservoir inflow

Lower Dam res. watershed Precipitation Runoff

Middle Dam release Lower Dam storage capacity /

spillway release / overtopping release Undamaged Damaged

Breach by overtopping Seismic collapse Other (e.g., piping)

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Middle Dam outflow model

Middle Dam reservoir level Middle Dam reservoir inflow

Middle Dam res. Watershed Precipitation Runoff

Middle Dam storage capacity /

spillway release / overtopping release Undamaged Damaged

Breach by overtopping Seismic collapse Other (e.g., piping)

Safety / financial consequences

Inundation from Lower Dam release

Consequences (fatalities/damage) of inundation

Downstream Consequences

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# Peo. by Type Exp. in Area

# Peo. by Type Norm. in Area

#Fatalities (by Type, by Area)

P[Ind Fatal For Peo. Type

In Area]

Fatality Func- tions (by Type)

Depth&Velo- city in Area

Warning/Evac/ Prep in Area

Ind. Structure Values in Area

Structures by Type in Area

Timing of Failure

Damage (by Type, by Area)

p[% Damage for Struct. Type

in Area]

Damage Func- tions (by Type)

Depth&Velocity in each Area

by Downstream

Areas

Lower Dam Release

Downstream Hydro. Char.

Risk Inputs (1 of 10)

Precipitation/Hydrology/Reservoir Inflow Exceedance Frequency – Magnitude (flow rate/duration) for inflow

Middle Dam reservoir inflow Lower Dam reservoir inflow (excl Middle Dam reservoir inflow)

Status: We have precipitation frequency and hydrographs from previous studies, but will refine hydrographs (e.g., considering Nanaimo Parkway culvert) and subjectively assess uncertainties.

Reservoir Storage/Hydraulics/Outflow Storage(m3)—depth (m) – spillway (+ leakage) release (flow

rate/duration) – overtopping release (flow rate/duration) relationships Middle Dam Lower Dam

Status: We have these relationships (rating curves) from previous studies, but will refine them (e.g., considering Nanaimo Parkway culvert and spillway hydraulics) and subjectively assess uncertainties.

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Hypothetical Example Inputs

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1

10

100

1000

10000

1 10 100 1000 10000

Ave

rage

re

turn

pe

rio

d (

year

s)

Peak flood flow (m3/s)

Middle Dam flood flow

Lower Dam peak flood flow (excl Middle Dam)

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

0 2000 4000 6000 8000 10000

Pe

ak O

vert

op

pin

g Fl

ow

(m

3/s

)

Peak Flood Flow (m3/s)

Middle Dam

Lower Dam

Risk Inputs (2 of 10)

Seismic Load Exceedance Frequency – Magnitude (pga) relationship Status: We have this relationship from previous studies, but need to

develop site-specific seismic inputs and subjectively assess uncertainties. Dam “Failure”

Dam failure – seismic (pga) relationship Middle Dam Lower Dam

Status: We have “performance” of each dam for several pga values from previous studies. However, we will collect additional geotechnical data from the ongoing investigation (geophysics & drilling), which will be used to develop parameters for re-analysis. We need performance at several pga’s for each dam (also considering previous results) in order to subjectively develop the complete relationship (by interpolation/extrapolation), and subjective assessments of: a) the uncertainty in modeled performance; and b) the probability of failure - performance relationship and the uncertainty in that relationship. Note: not differentiating degree of dam failure.

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Hypothetical Example Inputs

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1

10

100

1000

10000

0.01 0.1 1

Ave

rage

re

turn

pe

rio

d (

year

s)

Peak seismic event (pga)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 1 2 3 4 5

Pro

bab

ility

of

Failu

re

Peak Ground Acceleration (g)

Middle Dam

Lower Dam

Risk Inputs (3 of 10)

Dam “Failure” (cont.) Dam failure/breach – overtopping (flow rate/duration) relationship

Middle Dam Lower Dam

Status: We do not have any overtopping “breach” analyses for either dam from previous studies. We need breach analyses at several overtopping values for each dam in order to subjectively develop the complete relationship (by interpolation/extrapolation), and subjective assessment of the uncertainty in that relationship.

Dam failure/breach – other causes (e.g., piping) relationship Middle Dam Lower Dam

Status: We do not have any other failure analyses for either dam from previous studies nor reliable models to do such analyses. We need subjective assessment of probability of dam failure by other causes (not seismic or overtopping, e.g., piping).

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Hypothetical Example Inputs

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0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 2000 4000 6000 8000 10000

Pro

bab

ility

of

Ove

rto

pp

ing

Failu

re

Peak Overtopping Flow (m3/s)

Middle Dam

Lower Dam

Risk Inputs (4 of 10)

Lower Dam Release Magnitude (flow rate/duration) for no Lower Dam failure in

combination with No Middle Dam failure Middle Dam overtopping failure Middle Dam seismic failure Middle Dam failure by other causes (e.g., piping)

Status: We have the magnitude of releases for each dam in the absence of dam failure from previous studies, but will confirm and need subjective assessments of the uncertainties in those releases (done elsewhere).

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Risk Inputs (5 of 10)

Lower Dam Release (cont.) Magnitude (flow rate/duration) for Lower Dam overtopping failure in

combination with No Middle Dam failure Middle Dam overtopping failure Middle Dam seismic failure Middle Dam failure by other causes (e.g., piping)

Status: We do not have any overtopping “breach” analyses to determine the magnitude of release for either dam if breached, from previous studies. We need breach analyses at several overtopping values for each dam (done elsewhere) in order to subjectively develop the complete relationship (by interpolation/extrapolation) of dam release magnitude to overtopping value, and subjective assessment of the uncertainty in that relationship, for each dam.

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Risk Inputs (6 of 10)

Lower Dam Release (cont.) Magnitude (flow rate/duration) for Lower Dam seismic failure in

combination with No Middle Dam failure Middle Dam overtopping failure NA Middle Dam seismic failure Middle Dam failure by other causes (e.g., piping) NA

Status: We do not have any analyses to determine the magnitude of release for either dam if it fails due to a seismic event, from previous studies. We need to subjectively assess dam release magnitude if the dam fails due to a seismic event and the uncertainty in that release, for each dam.

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Risk Inputs (7 of 10)

Lower Dam Release (cont.) Magnitude (flow rate/duration) for Lower Dam failure by other causes

(e.g., piping) in combination with No Middle Dam failure Middle Dam overtopping failure NA Middle Dam seismic failure NA Middle Dam failure by other causes (e.g., piping) NA

Status: We do not have any analyses to determine the magnitude of release for either dam if it fails due to some other cause besides overtopping or seismic event (e.g., piping), from previous studies. We need to subjectively assess dam release magnitude if the dam fails due to some other cause besides overtopping or seismic event (e.g., piping), and the uncertainty in that release, for each dam.

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Risk Inputs (8 of 10)

Downstream Inundation Downstream inundation (depth/velocity/location) – Lower Dam

release (flow rate/duration) relationship Status: We can get downstream inundation values at relevant locations for

several Lower Dam releases from previous studies, but we need additional analyses for other Lower Dam releases in order to subjectively develop the complete relationship (by interpolation/extrapolation), and subjective assessment of the uncertainty in that relationship.

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Hypothetical Example Inputs

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0

1

2

3

4

5

6

7

8

9

10

0 2000 4000 6000 8000 10000

Inu

nd

atio

n (

de

pth

an

d v

elo

vcit

y)

Lower Dam Release (m3/s)

Location XXX

Location YYY

Risk Inputs (9 of 10)

Downstream consequences Downstream consequences – downstream inundation

(depth/velocity/location) - warning (failure mode related) relationship Properties/facilities

Downstream properties and facilities (type, location, value, and occupant type/number)

Damage (% of value) – inundation (depth/velocity) – warning relationship by type

Individual casualty (occupant probability) – damage (% of value) – warning relationships by type

Population (outside of property/facilities) Downstream populations (type, location and number) Individual casualty (by type) – inundation (depth/velocity) – warning

by type

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Risk Inputs (10 of 10)

Downstream consequences (cont.) Status: We can get relatively current inventory (type, location, value,

number) of downstream property/facilities and populations from previous studies, but we need to confirm and project into future when a failure might occur, and subjectively assess uncertainties in those values. We have the damage – inundation and individual casualty – inundation relationships from previous studies, but still need to subjectively assess effectiveness of warning and individual casualty – damage relationship, and the uncertainty in all those relationships.

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Hypothetical Example Inputs

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0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 2 4 6 8 10 12

% o

f va

lue

Inundation (depth and velocity)

Structure Type AA

Structure Type BB

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 2 4 6 8 10 12

Pro

b o

f in

div

id f

atal

ity

Inundation (depth and velocity)

Person Type aa

Person Type bb

Risk Model

Algorithms (outputs from inputs in chains) implemented in MS Excel with @Risk (commercial add-in) to do probabilistic analysis: Inputs expressed probabilistically (representing their uncertainties) Outputs calculated probabilistically (representing their uncertainties)

via Monte Carlo simulation (many possible sets of input values are generated, each with known probability, from which outputs with known probability are generated)

Simulation Sequence: Maximum precipitation and seismic events Dam(s) failure mode (each with particular lower dam release, timing and

warning/no warning) Downstream inundation and downstream population/property Downstream damage and casualties

Status: In development

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Colliery Dam Risk Assessment

Thank you! Questions?

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