HLTH 6200 Week 5 Flood Consequence Estimate Example

Consequences of a Major Mill River Flood, Estimated in Three Categories That Are Not Added Together

Student Name

Walden University

HLTH 6200: Risk Assessment, Preparedness, and Disaster Mitigation

Instructor Name

Month Day, Year

Consequences of a Major Mill River Flood, Estimated in Three Categories That Are Not Added Together

Scenario and Approach

This estimate describes the consequences of a major flood on the Mill River comparable to the 1996 flood of record, the reference scenario for the county's planning. All county figures here are invented for teaching. Consequences are estimated in three categories, life safety, property and continuity of services, and the categories are not converted into a common unit or added into a single score. Risk analysts have long described risk as a combination of what can happen, how likely it is and what the consequences would be (Kaplan & Garrick, 1981), and those consequences come in different kinds. Federal guidance for hazard assessment similarly asks planners to describe impacts in terms that matter to the community, including people affected and services disrupted, not only dollars (Federal Emergency Management Agency, 2018).

Life Safety

Mill River floods rise over one to two days, which gives warning time. Research on loss of life in floods finds that fatality rates depend heavily on water depth, the speed at which water rises, warning and whether people evacuate (Jonkman et al., 2008). Slow-rising riverine floods with warning tend to cause relatively few deaths, and those that occur often involve people driving or walking into moving water. The county's record fits that pattern: both deaths in the 1996 flood were drivers on flooded roads.

For the reference scenario, the estimate is zero to three deaths and ten to forty injuries requiring medical care, with most risk concentrated on flooded roads and among residents who stay in their homes. The 96-bed nursing home inside the 0.2% annual chance area is the largest single life safety concern: an evacuation that starts late, with frail residents and limited transport, could produce injuries or deaths that a timely evacuation would avoid. The 870 residents aged 65 or older and the 410 households without vehicles counted in the exposure summary are the groups most dependent on outside help to leave.

Property

Property loss was estimated by applying standard depth-damage relationships, which estimate the share of a building's value lost at each depth of water, to structures in the floodplain, using flood depths from the 1996 high-water marks. The result is an estimated $48 million to $70 million in residential building and contents damage, $15 million to $25 million in commercial damage and about $5 million in damage to public buildings, for a total of roughly $70 million to $100 million. Only about three in ten residential structures in the floodplain carry flood insurance (an illustrative figure for this composite county), so most residential losses would fall on owners, renters and disaster assistance programs. The range is wide because depth-damage relationships are averages and the 1996 marks do not cover every street.

Continuity of Services

The wastewater treatment plant, serving about 52,000 residents, would likely be out of service for one to two weeks if floodwaters reached its electrical equipment, as nearly happened in 1996. During that time, sewer backups and discharge restrictions would affect homes far outside the floodplain. Four miles of the state route that serves as the main ambulance route to the regional hospital would be closed for four to six days, adding about 18 minutes to transport from the east side of the county. Three substations feeding about 18,000 accounts could lose power for one to three days if flooded. Both schools in the floodplain would likely close for one to two weeks, and the nursing home's residents could be displaced for two to six weeks while the building is cleaned and inspected.

Summary Table

Table 1 summarizes the estimate.

Table 1

Estimated Consequences of a Major Mill River Flood, by Category (Composite County)

Why the Categories Are Not Combined

A single score would require converting deaths, dollars and days of lost service into a common unit, which means deciding how many dollars of property loss equal one death or one week without wastewater service. Those are value judgments a decision maker should make openly, not ones an estimate should hide. Keeping the categories apart also preserves information decision makers need when choosing among measures. Floodproofing the wastewater plant would sharply reduce the continuity consequence while barely touching property loss; an evacuation plan for the nursing home would reduce life safety risk while doing nothing for property; buying out the most flood-prone homes would reduce property loss and some life safety risk. A combined score would hide those differences and make the measures look more alike than they are.

Confidence and Limits

Confidence differs by category. The continuity estimates are the most reliable because they depend on which facilities lie below the flood level, which is known. The property estimate is moderately reliable, with a wide range from depth-damage averages. The life safety estimate is the least certain, since it depends on human behavior during the event, especially whether people drive through water and whether the nursing home evacuates in time. That uncertainty is a reason to treat life safety measures as high priority despite the small numbers involved.

The estimate should be revisited after any flood. Each event offers new high-water marks, new information on which roads people drove through and a test of the nursing home's evacuation timing. Updating the figures with that evidence, rather than leaving the 1996 flood as the only reference, would narrow the ranges over time.

References

Federal Emergency Management Agency. (2018). Threat and hazard identification and risk assessment (THIRA) and stakeholder preparedness review (SPR) guide: Comprehensive Preparedness Guide (CPG) 201 (3rd ed.).

Jonkman, S. N., Vrijling, J. K., & Vrouwenvelder, A. C. W. M. (2008). Methods for the estimation of loss of life due to floods: A literature review and a proposal for a new method. Natural Hazards, 46(3), 353-389. https://doi.org/10.1007/s11069-008-9227-5

Kaplan, S., & Garrick, B. J. (1981). On the quantitative definition of risk. Risk Analysis, 1(1), 11-27. https://doi.org/10.1111/j.1539-6924.1981.tb01350.x