Results from the three-dimensional stability tests indicated the most stable
plan was Plan 2B, which consisted entirely of 11-tonne Core-Locs, a constant
crest elevation of +8 m CD, and a vertical-face toe trench 1.5 m high.
Toe Trench Construction
For the conditions tested in the model, the breakwater was not damaged if a
stable toe trench, 1.5 m deep and near vertical, was installed. The model tests
were conducted on a fixed bottom, which in nature would be analogous to a
smooth rocky bottom in the prototype. An example of a prototype toe trench
constructed in a rocky bottom is shown in Figure 20. The seaward face of the
trench should be as near vertical as possible.
No model tests were conducted on stability with a movable bed; therefore, it
was not possible to quantify the effects of a sandy bottom on the stability of the
toe trench because the model floor was fixed. For structures placed in shallow
water the Shore Protection Manual (1984) recommends a toe protection scheme
similar to Figure 21, in which a wide trench is constructed and replaced with
armor. The Shore Protection Manual suggests constructing the trench horizon-
tally 2 times the water depth or 2 to 3 times the design wave height for the most
severe scour.
Figure 20. Example of toe trench constructed on rocky bottom
31
Chapter 5 Summary
Previous Page
