RUBBLE-MOUND BREAKWATERS 647
ans they suggest mean values 0.004 as not uncomfortable but wet,
between 0.004 and 0.03 l/s/m as uncomfortable but not dangerous, and
0.03 l/s/m as dangerous.
15.6 Rubble-mound breakwaters
15.6.1 GeneralMost breakwaters are constructed for the protection of harbours. In some
cases they may become part of a jetty or support a roadway. A rubble-
mound breakwater and a vertical breakwater built on rubble-mound
(Quinn, 1972) are shown in Fig. 15.9. Rubble-mound breakwaters usually
consist of a core of small quarry-run rock, protected by one or more inter-
mediate layers or underlayers separating the core from the cover layers of
large armour units. Failure of the rubble-mound breakwaters may be due
to removal of or damage to armour units, overtopping causing scour, toe
erosion, loss of core material, or foundation problems (Institution of Civil
Engineers, 1983). The armour units of the rubble-mound breakwaters are
Fig. 15.9 Typical breakwaters