  We revisit the classical but as yet unresolved problem of predicting the
strength of breaking 2-D and 3-D gravity water waves, as quantified by the
amount of wave energy dissipated per breaking event. Following Duncan (1983),
the wave energy dissipation rate per unit length of breaking crest may be
related to the fifth moment of the wave speed and the non-dimensional breaking
strength parameter $b$. We use a finite-volume Navier-Stokes solver with LES
resolution and volume-of-fluid surface reconstruction (Derakhti and Kirby
2014a, 2016) to simulate wave packet evolution, breaking onset and
post-breaking evolution for representative cases of wave packets with breaking
due to dispersive focusing and to modulational instability. The present study
uses these results to investigate the relationship between the breaking
strength parameter $b$ and the breaking onset parameter $B$ proposed recently
by Barthelemy et al. (2018). The latter, formed from the local energy flux
normalized by the local energy density and the local crest speed, simplifies,
on the wave surface, to the ratio of fluid speed to crest speed. Following a
wave crest, when $B$ exceeds a generic threshold value at the wave crest
(Barthelemy et al., 2018), breaking is imminent. We find a robust relationship
between the breaking strength parameter $b$ and the rate of change of breaking
onset parameter, $dB/dt$ at the wave crest, as it transitions through the
generic breaking onset threshold ($B\sim 0.85$), scaled by the local period of
the breaking wave. This result significantly refines previous efforts to
express $b$ in terms of a wave packet steepness parameter, which is both
difficult to define robustly and which does not provide a generically accurate
forecast of the energy dissipated by breaking.
