What is the 1.5 foot rule for dam hazard classification in North Carolina?

Under 15A NCAC 02K .0105, damage to a downstream structure is minor only if the structure is in a backwater area outside the direct path of the breach flood wave and experiences no more than 1.5 feet of flood rise above the lower of the lowest adjacent outside grade or the lowest floor elevation. All other damage potential is considered serious, which makes the dam Class C high hazard.

Is flood rise measured above the floor or the ground?

Above whichever is lower. The rule states that the lower of the lowest ground elevation adjacent to the outside foundation walls and the lowest floor elevation governs. For slab-on-grade or crawlspace dwellings on sloping ground the adjacent grade is usually lower, and using the floor instead understates the rise.

What is the difference between Class B and Class C dams in North Carolina?

Class B is intermediate hazard, where failure causes only minor damage as defined by the 1.5 foot flood rise and backwater tests. Class C is high hazard, where failure will likely cause loss of life or serious damage. Both require an emergency action plan, but Class C requires the probable maximum flood as the inflow design flood.

Dam safety worked examples

Class B or Class C? Working the 1.5-Foot Flood Rise Test

By Michael Flynn, PE · water resources & dam safety engineer

The difference between an intermediate-hazard and a high-hazard dam in North Carolina comes down to a single measured quantity, and the rule leaves almost no judgment in it. This is that determination worked against surveyed elevations — including the part practitioners get wrong most often, which is which elevation the 1.5 feet is measured above.

The situation

Dam	22-ft earthfill, 145 ac-ft, recreation impoundment
Breach model	Froehlich 2008 / 1995 — B̄ = 59.4 ft, t_f = 0.39 hr, Q_p = 7,150 cfs
Structure of concern	One occupied single-family dwelling, ~2,400 ft downstream
Position	On a terrace off the main valley, outside the direct breach path
Lowest adjacent grade (surveyed)	812.4 ft
Lowest floor elevation (surveyed)	813.1 ft
Modeled peak breach WSE at the structure	813.6 ft

Step 1 · What the rule actually says

15A NCAC 02K .0105(a)(2)

Class B covers dams whose failure "may damage highways or secondary railroads, cause interruption of use or service of public utilities, cause minor damage to isolated homes, or cause minor damage to commercial and industrial buildings." Everything turns on minor, which the rule then defines with no wiggle room. Damage is minor only when the structures are:

"located in back water areas not subjected to the direct path of the breach flood wave"; and
experience "no more than 1.5 feet of flood rise due to breaching above the lowest ground elevation adjacent to the outside foundation walls or no more than 1.5 feet of flood rise due to breaching above the lowest floor elevation of the structure, the lower of the two elevations governing."

And then it closes: "All other damage potential will be considered serious."

Step 2 · Pick the governing elevation

The lower of the two — this is the common error

Lowest adjacent grade outside foundation walls = 812.4 ft
Lowest floor elevation = 813.1 ft

Governing elevation = min(812.4, 813.1) = 812.4 ft (adjacent grade)

Do not default to the finished floor. The rule says the lower of the two governs, and for a slab-on-grade or crawlspace dwelling on sloping ground the adjacent grade is almost always lower. Measuring rise above the floor instead of the grade understates it — here by 0.7 ft, which is nearly half the entire 1.5-ft allowance. That single substitution has turned Class C dams into Class B on paper.

Step 3 · Compute the rise

Peak breach WSE minus the governing elevation

Flood rise = 813.6 − 812.4 = 1.2 ft

1.2 ft ≤ 1.5 ft → damage is minor on the rise test
Structure is outside the direct breach path → backwater test also satisfied

Both conditions are met, so this structure supports a Class B classification — assuming nothing else downstream fails either test, and subject to the Director's approval under .0105(b).

Step 4 · How little it takes to flip

The same dam at a slightly higher stage

Re-run the breach with a modestly larger volume, a lower breach invert, or simply a less favourable roughness assumption downstream, and the peak WSE at the structure comes in at 814.1 ft instead:

Flood rise = 814.1 − 812.4 = 1.7 ft

1.7 ft > 1.5 ft → damage is serious
→ Class C, high hazard

Modeled WSE	Rise above 812.4	Test	Class	Consequences
813.6 ft	1.2 ft	Minor	B	EAP required
814.1 ft	1.7 ft	Serious	C	EAP required + PMF spillway

Half a foot of modeled stage — well inside the uncertainty of any breach regression — moves the dam between classes. In North Carolina both classes require an emergency action plan, so the EAP obligation does not turn on this. What does turn on it is the inflow design flood, which jumps from a 100-year-to-½-PMF range at Class B to the full PMF at Class C.

Step 5 · The tests you can still fail

Passing 1.5 ft is necessary, not sufficient

Direct path overrides the number. A structure in the direct path of the breach wave is serious damage regardless of depth. Half a foot of fast-moving water in the flow path is not "minor" under this rule.
Future development, .0105(c). Probable downstream development must be considered. Vacant land zoned residential does not stay vacant, and a classification that ignores that is one rezoning away from being wrong.
Cascade, .0105(e). If failure of this dam would likely fail a dam below it, the lower dam's consequences determine this dam's classification.
Roads only, .0105(f). Where a bridge or roadway is the only damageable property, you must still weigh possible loss of life, indirect economic impact from loss of service, and direct damage cost.

Tools used

Hazard Classification

Screen the consequence criteria

Dam Breach Peak Discharge

The hydrograph behind the stage

NC Class A/B/C Rule →

Full 15A NCAC 02K .0105 text

The rest of this series

These four examples follow one structure through the whole decision chain, because in practice that is how the questions actually arrive:

Breach parameters for a 22-ft earthfill dam — produces the failure hydrograph everything else depends on.
Class B or Class C? The 1.5-foot flood rise test — turns that hydrograph into a hazard classification.
Does an 18-ft farm pond dam need a permit? — the classification decides whether the size exemption survives.
Spillway design flood for a Class C dam — and the classification sets the storm the spillway must pass.

Scope. This page is general information about North Carolina dam safety regulation, current as of 2026-08-28. It is not an engineering opinion about any specific structure and it is not legal advice. Requirements change when the General Assembly amends the Dam Safety Law or the Environmental Management Commission amends the rules — always confirm against the current text of the statute and 15A NCAC 02K, and contact the NCDEQ Dam Safety Program for a determination that binds.