Question 1

What is the inflow design flood?

Accepted Answer

The Inflow Design Flood (IDF) is the flood event the spillway and freeboard must safely pass without overtopping the dam. It's the design event for the spillway, separately from the design event for the dam itself (which is set by hazard class and load combinations). For high-hazard dams, the IDF is typically the Probable Maximum Flood (PMF). For low-hazard dams, it's a return-period flood like the 100-yr.

Question 2

What is the PMF?

Accepted Answer

The Probable Maximum Flood is the runoff resulting from the Probable Maximum Precipitation (PMP) — the theoretically greatest depth of precipitation possible at a location for a given duration. PMP is published by NOAA HMR (Hydrometeorological Reports) for the eastern US and various state-specific manuals for the West. PMF computation requires routing PMP through a unit hydrograph or HEC-HMS model with no infiltration losses (or near-zero AMC III losses).

Question 3

What does '½ PMF' mean?

Accepted Answer

½ PMF means a flood with peak inflow and total volume equal to 50% of the PMF — typically achieved by scaling the PMP rainfall depth by 0.5 and re-running HEC-HMS. ¾ PMF is 75% scaling. These fractional PMFs are widely used for dams that don't quite warrant full PMF but where 100-yr is too small. FERC and many state programs recognize fractional PMFs as valid design events.

Question 4

How does dam size affect IDF?

Accepted Answer

USACE size categories (small, intermediate, large) consider both height and storage. Small: H < 40 ft AND storage < 1,000 ac-ft. Intermediate: 40-100 ft height OR 1,000-50,000 ac-ft storage. Large: H ≥ 100 ft OR storage ≥ 50,000 ac-ft. Larger dams have greater consequence-of-failure even at the same hazard class, so the IDF is escalated.

Question 5

Can I use a return-period storm instead of PMF for HH dams?

Accepted Answer

Most state programs accept a 1,000-yr or 10,000-yr ARI storm as an alternative to fractional PMF for high-hazard dams. The choice depends on data availability — frequency analysis becomes uncertain at long return periods, while PMF is non-statistical (an upper-bound estimate). FEMA P-94 prefers PMF-based design for HH dams; ASDSO is moving toward risk-informed approaches (incremental risk).  This calculator gives the deterministic-IDF answer; for risk-informed analysis see USACE EM 1110-2-1420.

Question 6

What about earthquake design flood?

Accepted Answer

Earthquake design uses a separate framework — Maximum Credible Earthquake (MCE) for HH dams, Operating Basis Earthquake (OBE) for service. Per USACE EC 1110-2-6053, the MCE is roughly equivalent to a 10,000-yr return-period seismic event. IDF and earthquake design typically aren't combined (unless required by state for HH dams in seismic zones).

Hazard	Size	FEMA P-94	FERC	USACE	NRCS TR-60
High	Large	PMF	PMF	PMF	—
	Intermediate	½ PMF – PMF	½ PMF – PMF	½ PMF – PMF	—
	Small	100-yr – ½ PMF	½ PMF – PMF	50% PMF	PMF
Significant	Large	100-yr – ½ PMF	500-yr – ½ PMF	100-yr – ½ PMF	—
	Intermediate	100-yr – ½ PMF	500-yr – ½ PMF	100-yr – ½ PMF	—
	Small	100-yr – ½ PMF	100-yr – ½ PMF	50- to 100-yr	100-yr
Low	Large	50-yr – 100-yr	100-yr	100-yr	—
	Intermediate	50-yr – 100-yr	100-yr	50- to 100-yr	—
	Small	25-yr – 100-yr	50-yr	25- to 50-yr	25- to 100-yr

Inflow Design Flood (IDF) Selector

USACE size categories (EC 1110-8-2)

Full IDF guidance matrix

Worked example

Example — small significant-hazard farm pond expansion

State-specific notes

Related tools

Category	Height (ft)	Storage (ac-ft)
Small	< 40	50 – 1,000
Intermediate	40 – 100	1,000 – 50,000
Large	≥ 100	≥ 50,000