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Dam Hazard
Consequences
Assessment
ASFPM
May 2011
James Demby – FEMA
Sam Crampton - RAMPP
Mary Shaw - RAMPP
Brief History of the Project
 Project Goal: To develop a process that could be recommended to
communities to execute an economic, social and environmental
consequences of dam failure
• This was presented at 2010 ASFPM
 Created initial draft guidance document (summer-fall 2010)
• Relied heavily on existing procedures recommended for examining the vulnerability
to hazards
 Conducted initial pilot study (Winter 2010 - Spring 2011)
• For NRCS Flood Control Dam Y-15, Gwinnett County, GA
 Currently revising guidance document
• Utilizing a more qualitative approach and lessons learned from pilot study
 Currently preparing for a second pilot study
• Lake Barcroft Dam, Fairfax County, VA
 Will use to further refine procedures in draft document
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Initial Draft Guidance Document
 Prepared Initial Draft Guidance Document
 Used a How-To guide approach with worksheets...
 Heavily based on existing FEMA guidance about evaluating the consequences or
potential losses due to hazards
 Problem
 The other documents develop an understanding of probability leading to an analysis
of risk
 Risk is a function of probability and loss
 With dam failure
 Probability of collapse may be very low
 Probability of non-failure incident / malfunction may be higher
 Generally, probability of a particular type of dam failure is unknown
 Assessment of consequences, not risk since we are no looking at the probability of the
dam failing
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Dam Hazard Consequences Assessment
NRCS Dam Y-15 Pilot Study #1
 NRCS Dam Y-15 (Georgia)
• 41’ high earthen flood control dam
• Rehabilitated in 2007-2008 to create
RCC Spillway for 6-hour PMF
• Experienced a ~1/4 PMF in
September 2009
• High detail GIS data available from
County (LiDAR, Building footprints,
tax data etc.)
 Recreated September 2009 Event for
Original Spillway Configuration
(Assuming Dam would have Breached)
• Assumed breach at peak elevation
• Routed breach downstream until
incremental depth < 1.5 feet
• Applied, tested and refined the
methodologies of draft guidance
document
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Identifying Inundated Areas
 Used HEC-RAS Model and Identified the:
• Inundation extent
• Arrival time
• Inundation depths
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Identifying Assets and Population
at Risk (PAR)
Residential Structures
Elementary School
• Planning and development
• Online research/county
department provided
occupancy per structure
coordination to determine
student and staff numbers
• 2.8 per structure average (SFH)
Temporal Population Adjustment
Non Residential Structures
12am-7am
7am-5pm
5pm-12am
• Assumed 1.4 persons per
parking Space
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Residential
90%
15%
75%
Non-Residential
10%
85%
25%
HAZUS Economic Loss Assessment
User Defined Facilities Method (UDF)
General Building Stock Method (GBS)
 More time consuming, more
 Less time consuming, simplified,
detailed approach
less detailed approach
 User defined assets, values and
 HAZUS uses census data and
building types
assumes average distribution
 $1.9 million of building related loss
Model
GBS
UDF
% Error
$
$
Building
52,906,000
1,081,477
4,792
$
$
 $97 million of building related losses
Total Losses
Content
43,018,000
665,722
6,362
$
$
Inventory
1,110,000
120,214
823
~50× difference between UDF and GBS Method
 Gwinnett County largely developed post-FIRM
 Does good floodplain management explain difference?
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$
$
Total
97,034,000
1,867,413
Assessing Social Consequences
Loss of Life
Population at Risk (PAR)
Brown and Graham Method was applied
When warning time is:
0 – 15 Arrival Zone = 168
15 – 90 min Arrival Zone = 251
<15 minutes:
Probable Loss of Life = 0.5(PAR)
> 90 min Arrival Zone = 55
15 to 90 minutes:
Probable Loss of Life = PAR0.6
Warning time defined as the prebreach warning, plus the arrival time
>90 minutes:
Probable Loss of Life = 0.0002(PAR)
Performed a sensitivity
analysis to determine value
of advanced warning
systems
Loss of life highly sensitive on the
warning time
Estimated number of probable fatalities (per floodwater arrival time)
< 15 min
84
15 min – 90 min
28
> 90 min
1
Total
113
60 min
32
1
0
33
180 min
1
0
0
1
Warning Scenario
0
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Assessing Social Consequences
Social Consequences Assessment
• Economic Resource
• Infrastructure, jobs, tourism
• Environmental Resources
Will a dam breach…
Category
• Pollution, hazards, wildlife
• Public Services
Economic Resources
• Courts, transit, water, sewer, power
• Public Health and Safety
• Care facilities, emergency services,
disease, sanitation
• Recreation and Leisure
• Historical/cultural sites, lakes, trails
• Social Cohesion
• Activities, religious facilities
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Applicable
(Y/N)
Disrupt major roadways?
Y
Disrupt rail lines?
N
Disrupt the local school system?
Y
Disrupt a higher education system?
N
Disrupt the local manufacturing
industry?
N
Disrupt agriculture locally?
N
Disrupt the local tourism industry?
N
Disrupt the provision of banking
services locally?
N
Cause employers to leave the area?
Y
Assessing Environmental
Consequences
Pollutants
• Commercial buildings and garages within inundation zone, potential
source of chemical pollutants
• No major pollutant threats (gas stations, chemical plants, wastewater
plants etc)
• Y-15 sediment storage capacity of 334-acre feet, 91-acre feet estimated
as of 2009.
• Extensive sediment deposit potential downstream, possible heavy
metals
• Higher turbidity
• High potential for stream bank erosion exposing sewer lines
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Next Draft (currently under
development)
 Places greater emphasis on type of Dam Failure Scenario
• Sudden Collapse
 Worst case
• Planned Collapse
 Time to evacuate the inundation area
• Incidents
 Inability to function as flood control
 Uncontrolled releases (human/mechanical errors)
 Focus of recommended process is for community with limited
resources, little capability with HAZUS
• Will assess and refine more simplified approaches to a consequence assessment
 Assumptions are Required by Local Community
• How long it would take to repair / rebuild
• Relationship between number of lives lost and number of injuries
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Recommended Process
Outline of tentatively recommended process:
 Gather data on dam
• High hazard / Large impoundment...
 Identify Assets & Characteristics of Assets
• Include structures, infrastructure, vacant structures, open space, agricultural land
• Mention importance of CIKR – allow local definition
• Measures gathered for each asset in inundation area
 Occupancy
 Replacement Value, content value
 Jobs
 Sales / Usage / Productivity
 Existence of alternative space / cost of alternative space
 1st Floor Elevation, type of construction
 Dependencies, interdependencies, function
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Recommended Process Continued
 Identify Potential Impact
• Collapsed, damaged, leaking tanks, business temporarily closed, permanently
closed...
• Loss of life, injury
 Specify Consequences or Implications of Impacts
• Economic consequence
 Cost of evacuate, rescue, repair, debris removal,
• Social consequence
 Loss of social organizations, anxiety...
• Environmental consequences
 Pollution
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Recommended Process Continued
 Assess the consequences
• Economic costs may be
 Covered by insurance or grants
 Paid out-of-pocket by homeowners, business owners, tax payers
• Social consequences may be
 Obstacles that can be overcome
 Positive
 Negative and enduring
• Environmental consequences may be
 Obstacles that can be overcome
 Positive
 Negative and enduring

Pollution
 Final step
• Consider path forward
 Further study to determine probability
 Plan to mitigation in very long-term through zoning...
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Dam Hazard Consequences
Assessment
 Lessons Learned from Initial Draft / Pilot Study
• Quantitative approach only misses too many potential consequences
 Anxiety
 Loss of confidence in government
 Disruption of social fabric
• HAZUS’s Limitations must be carefully evaluated
 Caution must be applied when using GBS method
 Model is expensive to implement / requires advanced user capabilities
• Path forward may be unsatisfactory
 Further study rather than project / mitigation
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Dam Hazard Consequences
Assessment
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