Question 1

Why use multiple breach equations?

Accepted Answer

Each predictor was fit to a different (overlapping) historical-failure database, so they spread by a factor of 2–3× on the same input. USACE / FERC practice is to compute several predictors and use engineering judgment to pick a representative value or run the inundation analysis with the higher Q for conservatism. Reporting all four lets reviewers see the spread.

Question 2

What is V_w in the Froehlich equations?

Accepted Answer

V_w is the volume of water above the breach invert at the moment of breach formation — typically the storage at full pool above the breach bottom. h_w is the head on the breach (depth of water above breach invert). For an overtopping failure, the breach invert is the dam crest minus the breach depth (h_b), and h_w = h_b at full failure. Use V_w corresponding to peak flood pool, not normal pool.

Question 3

How is breach height h_b different from dam height?

Accepted Answer

h_b is the vertical distance from the final breach invert up to the original dam crest (or top of breach). For most overtopping failures, h_b = full dam height because erosion cuts to the foundation. For piping failures, h_b is sometimes less than full height because the breach stops eroding once the embankment material above the piping path collapses. Use h_b ≈ dam height for screening unless better information exists.

Question 4

What's k₀ in Froehlich 2008?

Accepted Answer

k₀ is a failure-mode coefficient: 1.30 for overtopping (wider breaches, faster erosion through saturated material), 1.00 for piping (narrower breaches, internal erosion). Pick the failure mode you're analyzing — for hazard classification, run both and use the larger Q.

Question 5

Should I use Q_peak or a routed hydrograph?

Accepted Answer

For preliminary hazard classification screening, peak Q downstream from breach is typically estimated by attenuating Q_peak based on distance and channel storage (USACE simplified routing curves). For a final inundation analysis, use a 2-D hydraulic model like HEC-RAS 2D, FLO-2D, or DSS-WISE Lite with the breach hydrograph derived from breach-formation parameters and routed through the river system.

Question 6

What about concrete dam breach?

Accepted Answer

These regressions are calibrated for earthfill / rockfill embankments. Concrete arch and gravity dams typically fail catastrophically — the entire dam goes at once. For concrete failures, assume instantaneous full breach with B = full dam length and use the dam-break wave (Saint-Venant) directly. Q_peak ≈ (8/27)·B·√g·h^(3/2) for an idealized rectangular dam-break (Ritter solution).

Method	N (failures)	Soil type	Notes
Froehlich 1995	22	Earthfill mostly	Most-cited Q_peak predictor; conservative for small dams
Froehlich 2008	74	All embankment types	Updated dataset; gives B_avg and t_f, not Q_peak directly
MacDonald 1984	42	Earthfill	Tends to underpredict for large dams; based on (V_w·h_w) product
Walder & O'Connor 1997	18	Volcanic landslide dams	Best for overtopping with rapid breach
Xu & Zhang 2009	182	Earthfill, rockfill, w/ core	Adds soil-erodibility and core-type adjustments

Dam Breach Peak Discharge Calculator

Method comparison and validity ranges

Worked example

Example — small earthfill dam, overtopping failure

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Dam Breach Peak Discharge Calculator

Method comparison and validity ranges

Worked example

Example — small earthfill dam, overtopping failure

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