182
CHAPTER 5. COASTAL STRUCTURE MODELS
with an example calculation using typical values for a 1:10 scale stability model where fresh water in the model represents a saltwater prototype. This particular example indicated that Hudson’s scaling called for
model armor units about 8% lighter than suggested by Sharp and Khader’s
(1984) method. This means that scaling by Hudson’s stability parameter
(Eqn. 5.25) will produce more conservative designs for stable rubble-mound
structures because the lighter model armor units are more easily moved by
waves.
Example 5.1. Scaling Armor Unit Weight
Part I: A jetty located in salt water is to be rehabilitated using 360-kN (36.1-longton) unreinforced dolos armor units made of concrete that has a specific weight of
30 kN/m3 (191 lbs/ft3). A 1:50 structure stability model will be used to test the
design. The model dolosse are to be cast using a special mixture of resin and brass
filings. In order to determine the correct proportions of brass filings and resin, it is
necessary to calculate the required specific weight of the model dolos material. The
model is to be run in fresh water. Assume -ym = 9.79 kN/m3 (62.3 lbs/ft3) for fresh
water, and yp — 10.05 kN/m3 (64.0 lbs/ft3) for salt water.
Solution. In a Froude-scaled stability model, the model armor units should conform
to the criterion given by Eqn. 5.15, if possible. Noting that 7 = pg, Eqn. 5.15 can
be rewritten as
(5.30)
m
and the model unit specific weight is found as
(7“)™ =
The dolos armor unit weight scale is determined from Eqn. 5.17, i.e.,
Nw, =
(Nl j3 =
30 kN/m3
29.22 kN/m3
= 128 337
and the weight of the model armor units will be
(W ) =
_ 360 kN
V a)m ~ Nw, ~ 128 337
2.8 N (0.63/6s)
Part II: Later, it is proposed to re-use the above-designed model dolos units for a
model study of a fresh-water breakwater that is proposed to be built using 120-kN
(12-long-ton) dolosse having the same geometric shape as the above dolosse, but
fabricated using concrete with specific weight of 28 kN/m3 (178 lbs/ft3). Determine
CHAPTER 5. COASTAL STRUCTURE MODELS
with an example calculation using typical values for a 1:10 scale stability model where fresh water in the model represents a saltwater prototype. This particular example indicated that Hudson’s scaling called for
model armor units about 8% lighter than suggested by Sharp and Khader’s
(1984) method. This means that scaling by Hudson’s stability parameter
(Eqn. 5.25) will produce more conservative designs for stable rubble-mound
structures because the lighter model armor units are more easily moved by
waves.
Example 5.1. Scaling Armor Unit Weight
Part I: A jetty located in salt water is to be rehabilitated using 360-kN (36.1-longton) unreinforced dolos armor units made of concrete that has a specific weight of
30 kN/m3 (191 lbs/ft3). A 1:50 structure stability model will be used to test the
design. The model dolosse are to be cast using a special mixture of resin and brass
filings. In order to determine the correct proportions of brass filings and resin, it is
necessary to calculate the required specific weight of the model dolos material. The
model is to be run in fresh water. Assume -ym = 9.79 kN/m3 (62.3 lbs/ft3) for fresh
water, and yp — 10.05 kN/m3 (64.0 lbs/ft3) for salt water.
Solution. In a Froude-scaled stability model, the model armor units should conform
to the criterion given by Eqn. 5.15, if possible. Noting that 7 = pg, Eqn. 5.15 can
be rewritten as
(5.30)
m
and the model unit specific weight is found as
(7“)™ =
The dolos armor unit weight scale is determined from Eqn. 5.17, i.e.,
Nw, =
(Nl j3 =
30 kN/m3
29.22 kN/m3
= 128 337
and the weight of the model armor units will be
(W ) =
_ 360 kN
V a)m ~ Nw, ~ 128 337
2.8 N (0.63/6s)
Part II: Later, it is proposed to re-use the above-designed model dolos units for a
model study of a fresh-water breakwater that is proposed to be built using 120-kN
(12-long-ton) dolosse having the same geometric shape as the above dolosse, but
fabricated using concrete with specific weight of 28 kN/m3 (178 lbs/ft3). Determine