Table 8
Matagorda Jetties
Matagorda Peninsula, Texas
Date s
Construction and Rehabilitation History
part of the Matagorda Ship Channel project, twin rubble-mound
A
s
1966
jetties were constructed from Matagorda Peninsula into the Gulf of
Mexico (Figure 6 . The east and west jetties were 5,900 and 6,000 ft
)
long, respectively, spaced 2,000 ft apart, and terminated at approxi-
mately the -24 ft
contour. The jetties provide protection for
the 38- by 300-ft entrance channel. The design cross sections (Fig-
ure 10) consisted of a 16-ft crown width at
ft mlt and side slopes
of
and
on the trunk and head sections, respectively. The
long, followed by a 100-ft-long
head section on each jetty was 200
transition tying into the trunk section. The typical jetty cross
section was built on a bedding layer of
to 200-lb stone,
varying in thickness from 2 ft at the landward end to 5 ft at the
seaward end. To provide scour protection, the bedding layer extended
beyond the toe of the cover stone a minimum of 5
at the landward
ends to a maximum of 50 ft at the seaward ends. The core was built
using 200- to
stone (except the landward 600 to 700 ft of
the jetties which had a maximum stone size of 1,000 lb). To prevent
the flow of littoral drift through the jetty, a section of the core
side slopes) was
(extending below the width of the crown at
made impervious by adding
to
filler stone. A single layer
of cover stone was placed on the jetty trunk sections, and a double
layer of cover stone was placed on the head section side slopes. The
outermost two to three cover stones were placed horizontally on the
bedding layer, in effect buttressing the armor stone layer. The
rectangular granite cover stone varied in size from 2 to 4 tons at
the landward ends and 1 to 1 tons at the seaward ends. Cover stone
6
8
sizes were selected using Hudson's slope stability formula, design
and a +13
wave heights of 1 to 22.6
mlt surge level. Under
0
these conditions and no-damage criteria, calculated stone sizes
ranged up to 4 tons. The largest available stone was less than
8
20 tons. Thus, the design used a stability coefficient of 6.8,
allowing for a 10 to 20 percent damage level. The underlying founda-
tion materials are mostly fine sands with some shell to a depth of
about -60 ft mlt. During jetty construction the channel through the
peninsula had progressed to the point that only a narrow earthen plug
on the gulf side remained. On the night of 24 September 1963 the
earthen plug was breached by a high tide in the gulf, and the result-
ing tidal currents soon caused severe erosion and scour at the outer
ends of the partially completed jetties. The natural depths at the
outer ends of the jetties were about -15
mlt prior to the breach
but within a short period increased to about -40
mlt, during which
time jetty construction was interrupted. The remaining jetty
sections were completed under a new contract starting in 1964.
Construction of the jetties, a north dike, and channel revetments
totaled
using approximately 2,000,000 tons of stone.
(Continued)
34
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