Dam Hazard Consequences

AssessmentASFPM May 2011

James Demby – FEMASam Crampton - RAMPPMary Shaw - RAMPP

2

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Created initial draft guidance document (summer-fall 2010)• Relied heavily on existing procedures recommended for examining the vulnerability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

3

Initial Draft Guidance Document Prepared Initial Draft Guidance Document Used a How-To guide approach with worksheets...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

4

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• Applied, tested and refined the methodologies of draft guidance document

5

Identifying Inundated Areas

Used HEC-RAS Model and Identified the:• Inundation extent

• Arrival time

• Inundation depths

6

Identifying Assets and Population at Risk (PAR)

Non Residential Structures• Assumed 1.4 persons per

parking Space

Elementary School• Online research/county

coordination to determine student and staff numbers

Residential Structures• Planning and development

department provided occupancy per structure

• 2.8 per structure average (SFH)

Temporal Population Adjustment

Residential Non-Residential12am-7am 90% 10%7am-5pm 15% 85%5pm-12am 75% 25%

7

HAZUS Economic Loss AssessmentUser Defined Facilities Method (UDF)

More time consuming, more detailed approach

User defined assets, values and building types

$1.9 million of building related loss

General Building Stock Method (GBS)

Less time consuming, simplified, less detailed approach

HAZUS uses census data and assumes average distribution

$97 million of building related losses

~50× difference between UDF and GBS Method

Gwinnett County largely developed post-FIRM

Does good floodplain management explain difference?

Model Building Content Inventory TotalGBS 52,906,000$ 43,018,000$ 1,110,000$ 97,034,000$ UDF 1,081,477$ 665,722$ 120,214$ 1,867,413$ % Error 4,792 6,362 823

Total Losses

8

Assessing Social ConsequencesLoss of LifeBrown and Graham Method was appliedWhen warning time is:

<15 minutes: Probable Loss of Life = 0.5(PAR)

15 to 90 minutes: Probable Loss of Life = PAR0.6

>90 minutes:Probable Loss of Life = 0.0002(PAR)

Estimated number of probable fatalities (per floodwater arrival time)

Warning Scenario < 15 min 15 min – 90 min > 90 min Total

0 84 28 1 113

60 min 32 1 0 33

180 min 1 0 0 1

Population at Risk (PAR)

0 – 15 Arrival Zone = 168

15 – 90 min Arrival Zone = 251

> 90 min Arrival Zone = 55

Warning time defined as the pre-breach warning, plus the arrival time

Loss of life highly sensitive on the warning timePerformed a sensitivity

analysis to determine value of advanced warning systems

9

Assessing Social ConsequencesSocial Consequences Assessment• Economic Resource

• Infrastructure, jobs, tourism

• Environmental Resources

• Pollution, hazards, wildlife

• Public Services

• 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

10

Assessing Environmental ConsequencesPollutants• 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

11

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

12

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

13

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

14

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...

15

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

16

Dam Hazard Consequences Assessment