Question 1

What does 'high hazard' actually mean?

Accepted Answer

High hazard means a dam where failure or misoperation will probably cause loss of human life. It is NOT a measure of how likely the dam is to fail — that's reliability or condition rating. A well-maintained, low-risk dam can still be high-hazard if there are homes downstream of it. Hazard classification is about consequences, not probability of failure.

Question 2

Who assigns hazard classifications?

Accepted Answer

In the United States, each state's dam-safety program assigns and reclassifies hazard categories. The state typically requires the dam owner to submit a breach inundation map, supported by hydraulic modeling, to demonstrate the proposed classification. Federal agencies (USACE, BOR, FERC, NRCS) classify their own dams. FEMA P-94 provides the framework but is not regulatory.

Question 3

Does 'probable' mean a 50% likelihood?

Accepted Answer

FEMA P-94 doesn't define a numerical threshold. Practice varies by state. North Carolina, Texas, and California all interpret 'probable' as 'reasonably likely given the inundation footprint' — typically meaning even one occupied dwelling in the inundation zone triggers high hazard. Some states require population-at-risk modeling (LIFESim, HEC-FIA) for borderline cases.

Question 4

What inundation depth or velocity is dangerous?

Accepted Answer

USACE / FEMA use a depth × velocity threshold of about 10 ft²/s for an adult to remain standing, and 7 ft²/s for a child. Below those, the floodwater is considered unlikely to cause loss of life unless other risks (vehicles, debris) are present. Most state programs default to 'any depth in an inhabited area = potential loss of life' for screening, since the 10 ft²/s threshold underestimates risk for vulnerable populations and at night.

Question 5

How do I tell if a structure is in the inundation zone?

Accepted Answer

Run a breach analysis (HEC-RAS 2D, FLO-2D, DSS-WISE Lite, or for screening, HEC-LifeSim or USGS DamWatch). The inundation footprint is the maximum extent of flooding from a hypothetical breach. Compare to downstream parcel and population data (county GIS, US Census blocks). Any inhabited residence, school, hospital, business, or critical facility within the footprint triggers significant or high hazard.

Question 6

What about hazard classification of a sediment lagoon or wastewater pond?

Accepted Answer

Industrial impoundments — sediment ponds, sludge lagoons, ash basins, tailings dams — are evaluated using the same FEMA P-94 framework but with content-specific consequence considerations. NPDES discharge violations and chemical contamination from a breach may push classification higher than just hydraulic-impact alone. CCR (coal combustion residual) impoundments are classified per 40 CFR 257 with parallel low/significant/high categories.

Hazard	Loss of life	Economic / environmental / lifeline
High (HH)	Probable. ≥ 1 expected fatality	Yes — typically extensive
Significant (SH)	No probable loss of life	Yes — appreciable damage to public/private infrastructure or environmental damage
Low (LH)	No probable loss of life	No — minimal damage, mostly to dam owner's property

Hazard class	Dam size	Recommended IDF
High	Large (H ≥ 100 ft or storage ≥ 50,000 ac-ft)	PMF
High	Intermediate	½ PMF — PMF
High	Small (H < 40 ft or < 1,000 ac-ft)	100-yr — ½ PMF
Significant	All sizes	100-yr — ½ PMF
Low	Large or intermediate	50-yr — 100-yr
Low	Small	25-yr — 100-yr

Dam Hazard Classification

Downstream consequences of a hypothetical breach

FEMA P-94 hazard classification matrix

Inflow design flood guidance by hazard class

Worked example

Example — small farm-pond dam below new subdivision

State-level overlays

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