A generic quantitative damage description for rubble mound structures: Investigation of damage to roundheads by using a 3D high-resolution measurement technique in a physical model

A generic quantitative damage description for rubble mound structures: Investigation of damage to roundheads by using a 3D high-resolution measurement technique in a physical model

Disco, M.J.

Uijttewaal, W.S.J. (mentor)
Hofland, B. (mentor)
Verhagen, H.J. (mentor)
Berg, A. (mentor)

Civil Engineering and Geosciences

Hydraulic Engineering

2012-12-06

Breakwaters are used for reducing wave height in harbours or divert sediment. One of the types of breakwaters is the rubble mound breakwater. These structures are constructed with smaller quarry rock as core and larger quarry rock as outer layer. For economical reasons, the design allows some damage to the armour layer. The amount of damage is objectively and quantitative determined by damage parameters. There are several different damage parameters which describe all different unique characteristics of the erosion hole. For a straight slope with a constant cross section, extensive research is executed for reliable design values of some of these parameters. For a 3D geometry such as a roundhead such design values are not formulated. This thesis is about roundhead model tests, executed at Deltares research institute. Four tests with increasing wave height are executed to eight identical roundheads. The roundheads weren’t repaired in between test so progressive damage occurred. The damage was analysed with the use of digital stereo photography (DSP), this high-resolution measurement technique creates a computer model of the physical model with a resolution of one mm. By subtracting the computer models of before and after a test, the erosion and deposition is accurately represented. The damage level is quantitatively determined by comparing different damage parameters. The compared parameters consider all different aspects of the erosion, the number of displaced stones (damage percentage Nd), the number of displaced stones per stone width (Nod), eroded area (van der Meer damage parameter S), nominal erosion length (L) & the nominal erosion depth (E). Before the results are being analysed, the data gathered from the DSP must be processed to smooth the individual stones but keep the erosion profile intact. The optimal smoothing process is based on a convolution or moving average principle. The size of the optimal smooth factor is two times the median nominal stone size (Dn50). The results show three erosion holes at the roundhead, the first erosion hole between 0~10° from the incident wave direction, the second between 60~70° from the incident wave direction and the third between 105~115° from the incident wave direction. The most damage was observed at the first and third erosion hole. The damage in the third erosion hole was slightly higher and more variable between the eight realizations compared with the first erosion hole. By comparing the different damage parameters it appears that only the erosion depth E, is representing the realizations correctly; the highest values in the third erosion hole with the largest deviation. Therefore design values of this damage parameter are proposed for roundheads with a slope of 1:2 and an armour layer thickness of two times Dn50. Initial damage: E = 0.2 ~ 0.3 Intermediate damage: E = 0.5 ~ 0.6 Failure: E = 1.0 ~ 1.1 Collapse: E = 1.5 ~ 1.6 To come to a generic damage parameter further research is necessary to validate the proposed design values for roundheads with different configurations, such as different slopes and different radius.

breakwater
roundhead
damage
Digital stereo photography
erosion depth
rubble mound

http://resolver.tudelft.nl/uuid:cb6658a3-c9ca-4c8a-a443-2094ba99b174

Student theses

master thesis

(c) 2012 Disco, M.J.