Question 1

What goes into a freeboard calculation?

Accepted Answer

Required freeboard = wind setup + wave runup + safety margin. Wind setup is the rise in water surface at the leeward shore from sustained wind blowing across the reservoir. Wave runup R is the vertical distance the highest waves climb up the embankment slope above the still-water level (which already includes setup). The safety margin (typically 1–3 ft) accounts for spray, gust effects, and uncertainty in the design wind.

Question 2

What design wind speed should I use?

Accepted Answer

USACE EM 1110-2-1420 recommends the 50-year, 1-hour overland wind speed at 30 ft above ground, blowing along the longest reservoir fetch toward the dam. For preliminary design use the ASCE 7 wind-zone map or NWS wind-rose data for the project location. For high-hazard dams, also check the 100-year wind. Typical design winds for inland US sites are 50–80 mph.

Question 3

What's a typical fetch and depth?

Accepted Answer

Fetch is the longest reservoir distance over which the wind blows toward the dam. Use the geometry of the full pool (or PMF pool for spillway-design freeboard). Average depth d_avg along the fetch enters the SPM shallow-water wave-height formulas. For deep reservoirs, depth becomes irrelevant and Hs depends only on wind and fetch.

Question 4

What is the Iribarren number?

Accepted Answer

ξ = tan(α) / √(Hs/L0), where α is slope angle, Hs is significant wave height, and L0 = g·T²/(2π) is deep-water wavelength. ξ < 0.5 = spilling breakers (lots of foam, low runup). 0.5 < ξ < 3.3 = plunging breakers (peak runup energy). ξ > 3.3 = surging/non-breaking (waves reflect off the slope). Most embankment dams operate in the plunging regime where R/Hs is roughly equal to ξ (Hunt 1959).

Question 5

How much does riprap reduce runup compared to smooth concrete?

Accepted Answer

USACE recommends γ_r = 0.55–0.60 for two-layer riprap on a granular filter, vs γ_r = 1.00 for smooth concrete. Roughly half the runup. Two-layer riprap is the typical upstream-slope protection for embankment dams. Concrete-block (e.g., Petraflex) and grass or asphalt slopes have intermediate γ_r values published in EM 1110-2-1420.

Question 6

What freeboard is required if I have an emergency spillway?

Accepted Answer

At maximum design pool (with the IDF passing the spillway), the dam crest must remain above the maximum reservoir surface plus wave runup with some margin. For high-hazard dams, FERC and most states require a minimum 3 ft freeboard above the maximum routed pool. For an emergency spillway, freeboard is measured from the auxiliary spillway crest, not the principal spillway.

Slope surface	γ_r
Smooth concrete, asphalt-paved	1.00
Single-layer cellular concrete blocks	0.65
Two-layer riprap on granular filter	0.55
Single-layer riprap, dumped	0.65
Grass-lined embankment	0.85
Asphalt or fabric mat	0.85

Wave Runup & Freeboard Calculator

Wind setup formulas

Wave height and period (SPM 1984)

Surface-roughness reduction γ_r

Worked example

Example — small high-hazard embankment dam

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