ERDC/CHL CHETN-III-64
June 2002 (revised)
Damage Development on Stone Armored
Breakwaters and Revetments
by Jeffery A. Melby
method to calculate damage progression on a rubble-mound breakwater, revetment, or jetty trunk
armor layer. The methods apply to uniform-sized armor stone (0.75W50 ≤ W50 ≤ 1.25W50, W50
= median weight of armor stone) as well as riprap (0.125W50 ≤ W50 ≤ 4W50) exposed to depth-
limited wave conditions.
The equations discussed herein are primarily intended to be used as part of a life-cycle analysis,
to predict the damage for a series of storms throughout the lifetime of the structure. This life-
cycle analysis including damage prediction allows engineers to balance initial cost with expected
maintenance costs in order to reduce the overall cost of the structure. The equations are intended
to provide a tool for accurate damage estimates in order to reduce the possibility of unexpected
maintenance costs.
INTRODUCTION: Rubble-mound breakwater, revetment, and jetty projects require accurate
damage prediction as part of life-cycle analyses. But few studies have been conducted to
determine damage progression on stone armor layers for variable wave conditions over the life of
a structure. Previous armor stability lab studies were intended to determine damage for the peak
of a design storm. As such, most previous laboratory studies were begun with an undamaged
structure and damage measured for a single design wave condition (e.g., Hudson 1959; Van der
Meer 1988). The empirical equations derived from these studies were valid for determining
initial damage but not for damage progression through several storm events. Damage actually
occurs as a result of a sequence of storms of varying severity and with varying water levels. This
CHETN provides equations that allow the prediction of rubble-mound deterioration with time.
These relations are supplemented by predictive equations for the uncertainty or variability of
damage for more accurate estimation of reliability or, conversely, probability of failure.
Within this technical note, damage is defined in terms of the average normalized cross-sectional
eroded area of armor on the slope. Damage is defined up to the point that the underlayer is
exposed through a hole the size of a nominal armor stone diameter Dn50 = (M50/ρa)1/3, where M50
is the median mass of armor stone and ρa is the armor stone density. The condition where the
underlayer is exposed defines failure of the armor layer because rapid destruction of the structure
often occurs after this point. The damaged profile is described in terms of the engineering
parameters maximum eroded depth, minimum remaining cover depth, and maximum cross-shore
length of the eroded region. Relations for these profile descriptors are given in terms of the mean
damage. Further, relations describing the alongshore variability of damage and the profile
descriptors are provided to support reliability or uncertainty analyses. These relations apply for
single storms and for storm sequences given depth-limited normally incident waves. The
relations and supporting studies are described in a series of publications on damage (Melby
1999; Melby and Kobayashi 1998a, 1998b, 1999).
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