IRM Guide for Local Jurisdictions
Step 3 – Give Context to the Risk
Example Planning Scenario
Dam Failure Scenario
Description of Event:
A heavy winter snowfall, rising temperatures, and a series of heavy rainfalls contribute to a major dam
failure. To avoid dam failure spill gates are opened and with the erosion of dirt levies the reservoir
ruptures and its contents are emptied over a few hours causing major flooding to downstream areas.
Introduction:
The risks associated with a dam failure are relatively high given the number of Class I (highest hazard
rating based on reservoir storage volume and potential for loss of life) within ABC County, the number of
Class I dams immediately to the north of ABC County, the age of some of the dams that were
constructed in the early- to mid-1900’s, and the size of the downstream population.
The 2011 Ohio Hazard Mitigation Plan states that the authoring agency “does not currently have a
means of determining downstream populations and/or potential monetary losses in case of (dam)
failure.” The Army Corps of Engineers publication: Estimating Life Loss for Dam Safety Risk Assessment:
A Review and New Approach, July 2002 acknowledges the significant uncertainties with forecasting loss
of life estimates associated with a dam’s failure because of the complexity of underlying events (e.g.
amount of rainfall, length of warning time, the time of day in regards to whether residents are at home
or at work).
Assumptions:
Assumptions developed in support of a “plausible, worst-case scenario,” include:

Dam failure occurred in evening hours when most residents were at home.

The highest impacts occurred in areas closest to the release; areas more distant to the release
were damaged to a lesser degree.

Residents in proximity to the release were assumed to have had less than 15 minutes warning
time, while those farthest downstream from the release were assumed to have at least 90
minutes.

Loss of life estimates were based on the population at risk within the inundation area.

Property Damage was based on structures found in the inundation area, the percent damaged,
and their assessed value.

The economic impact was based on the proportion of area’s Gross Domestic Product expected
to be lost as a result of dam failure.
1
IRM Guide for Local Jurisdictions
Step 3 – Give Context to the Risk
Probability of Event:
Quantifying the probability of dam failure for any one specific dam is difficult given the complexity of
underlying conditions that can lead to dam failure and the belief that many dam failures are the result of
a chain of events (Army Corp of Engineers). However from Jan. 1, 2005 through Jan. 1, 2009, state
dam safety programs reported 132 dam failures and 434 incidents (episodes that, without
intervention, would likely have resulted in dam failure).
This equates to approximately 140 dam
failures/incidents per year in the US, or an overall average of two to three failures/incidents per state
annually.
Potential Consequences:
In general, dam failure is expected to result in a loss of life, extensive property damage, and a
diminished local economy.
Roads, bridges, telephone lines, power lines, and the dam itself are likely to be damaged as they are
commonly found in most inundation areas. Damage, if any, to water and wastewater systems, electrical
substations, telecommunication towers, airports, railroads, specific businesses etc. depends on the
particular dam that fails and whether these critical infrastructures are located within the failed dam’s
inundation area.
The reservoirs from three dams in ABC County act as the drinking water source for about 65% of the
county. Five collector wells in the southern part of the county provide the other 35%. Despite the
inherent redundancy, a loss of any one dam could impact drinking water supplies and, on at least a
temporary basis, alternative drinking water sources may need to be identified.
The negative consequences associated with the dam failure and the underlying assumptions used in
quantifying the magnitude of the consequences are as follows:
Loss of Life: Loss of life from dam failures is directly proportional to the population at risk and the
percentage of population in their residence at the time of the flood and inversely proportional to the
depth and flow of the flood waters and the length of warning time.
The calculations for loss of life were based on the formula outlined in the 2011 Department of Homeland
Security (DHS) Dam Consequence Estimation - Loss of Life.
http://www.damsafety.org/media/Documents/Security/DamsSectorConsequenceEstimationLossofLife.pdf . Specifically with a no warning event, fatalities within the first 3 miles of the inundation
zone can be calculated at 0.25 times the population; 3-7 miles equates to 0.08 times the population; and
7-15 miles is 0.01 times the population. These calculations are based on the assumption that the closest
population will have the least amount of warning (likely less the 15 minutes). Those within the 3-7 miles
can be estimated to have between 15 and 90 minutes of warning time and the population beyond 7
miles of the dam failure would have greater than 90 minutes warning time.
2
IRM Guide for Local Jurisdictions
Step 3 – Give Context to the Risk
Injuries: The number injured is an indication of the stress placed on local health care systems. The types
of injuries that would be expected to occur include: hypothermia from exposure to weather conditions,
possible electric shock from downed utility lines, disease from drinking water contaminated with
sewage, and slip\fall accidents. In 2010, NOAA reported that in regards to flash floods, the ratio of
injuries to fatalities was approximately 3 to 1.
Property Damage: Property damage is assumed to be highest near the dam as the force of the flood
waters is expected to decrease with an increase in distance from the point of release from the ruptured
reservoir.
Damage estimates to residential, commercial, public, and agricultural buildings within the inundation
area were estimated based on their appraised structural value.
Damage to highways, secondary roads, railroads, and power lines were estimated based on their length
within the inundation area. A damage estimate was also assumed for critical infrastructures within the
inundation area, including structures such as airports, communication towers and lines, water and
sanitation treatment plants, electrical substations and the dam itself.
Local Economic Impact: Estimating the impact on the local economy often includes an estimation of:
property damage, lost jobs, lost wages, reduced profits by affected businesses (direct and indirect),
reduced tax revenues, etc. Much of this data was not readily available. Therefore, the local economic
impact was based, in large part, on an estimation of the flood’s impact on ABC County’s Gross Domestic
Product (GDP).
Drinking Water Supplies: An estimate of the temporary need for alternative sources of drinking water is
based on the assumption that the dam failed at one of the three primary reservoirs providing drinking
water to the county. The assumption is made that given the number of other drinking water sources
that one-fifth to one-fourth of the county’s residents may temporarily require an alternative source of
drinking water.
Geographical Area Impacted:
Based on an assumption that one of the Class I dams were to fail and release its entire stored volume of
water over a relative short period, the following data regarding the population at risk and assets within
the inundated area were provided using the county GIS data.

area inundated by flooding: X acres

number of households within the inundated area: X households

population residing within the inundated area: X population

number employed within inundated area: X employees

number of commercial parcels in the inundated area: X parcels

number of public parcels within the area: X parcels
3
IRM Guide for Local Jurisdictions
Step 3 – Give Context to the Risk

miles of surface streets within the area: X miles

miles of major highways in the area: X miles

miles of telephone lines within the area: X miles

miles of railroad tracks in the inundated area: X miles

miles of high voltage transmission lines within the area: X miles

high profile structures within the inundation area include: an airport, water and wastewater
treatment plants, and electrical substations.
Relevant Core Capabilities:
Fatality Management Services
Public Health and Medical Services
Mass Search & Rescue
Mass Care
Critical Transportation
Operational Communications
Public & Private Services and Resources
Environmental Response/
Health and Safety
On-Scene Security and Protection
Situational Assessment
Housing
Health and Social Services
Infrastructure Systems
Community Resilience
Economic Recovery
Threats and Hazard Identification
Risk and Disaster Resilience Assessment
Long Term Vulnerability Reduction
Planning
Operational Coordination
Public Information and Warning
4