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ERDC/CHL CHETN-III-64 December 2005 (revised)

Damage Development on Stone-Armored Breakwaters and Revetments
by Jeffrey A. Melby

PURPOSE: This Coastal and Hydraulics Engineering Technical Note (CHETN) provides a 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 depthlimited 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 lifecycle analysis including damage prediction allows engineers to balance initial costwith 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. However, few studies have beenconducted 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 (Hudson 1959; Van der Meer 1988). Theempirical 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 predictiveequations for the uncertainty or variability of damage for more accurate estimation of reliability or, conversely, probability of failure. Within this CHETN, 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 theeroded 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

ERDC/CHL CHETN-III-64 December2005 (revised)

relations and supporting studies are described in a series of publications on damage (Melby 1999; Melby and Kobayashi 1998a, 1998b, 1999). DAMAGE DESCRIPTION: The Shore protection manual (1984) provides damage as a function of the marginal wave height exceeding the zero damage wave height. The Shore protection manual damage D% was defined as the normalized eroded volume in theactive region, extending from the middle of the breakwater crest down to one wave height below the still-water level. This design information is based on laboratory tests limited to monochromatic nonbreaking waves impinging on long structure slopes. The background reports provide little insight and no data. Broderick and Ahrens (1982) provided a definition of damage that was not a function of...