CHAPTER 36
PLANNING AND DESIGN OF A Lo\~-lvEIR SECTION JETTY AT
~ffiSONBORO INLET, NORTH CAROLINA Nels C. Hagnuson
Assistant Chief, Engineering Division IYilmington District, Corps of Engineers
ABSTRACT
A berm and dune project for shore and hurricane protection was recently completed at IYrightsville Beach, North Carolina. A navigation project provides for a channel 14 feet deep and 400 feet wide through Hasonboro Inlet which forms the southern boundary of I~rightsville Beach. It has been found infeasible to maintain the channel by dredging alone.
Ivork has been started on a 3,400-foot-long jetty on the north shore of Masonboro Inlet. The shoreward 1,700 feet will be constructed of con-crete sheet-piling and the contract calls for a rubble-mound structure for t~le oceanward 1,700 feet. The concrete sheet-piling section of the project will include a 1,000-foot weir section with an elevation of 2 feet above mean low water, which is about midtide. A deposition basin will be provided between the jetty and the navigation channel.
The paper will discuss erosio~ rates at IYrightsville Beach since the 1930's; shoaling in the navigation channel, describe briefly the berm and dune proj ect at Ivrightsville Beach and describe in some detail the jetty under construction, including the factors that led to the selection of a "low-weir" structure. Some of the elements that entered in the structural design will be mentioned, but complete design data will not be included since common procedures were followed and no unusual design problems were encountered.
SCOPE
I~ile this paper deals primarily with the Hasonboro Inlet jetty, both shore and hurricane protection and navigation are involved in the planning of the jetty and projects for those purposes are also discussed. Because of the broad area covered, detailed information on design is not included.
DESCRIPTION
Ivrightsville Beach. The town of IYrightsville Beach is primarily a summer resort with a permanent population of about 1,000. Up to 25,000 people visit the beach on summer weekends or holidays. It is an active recreational boating center, and about 35 commercial fishing vessels are based nearby. Hany other commercial fishing vessels use the inlet when project depths are available. The area under study and environs are shown on figures 1, 2, and 6.
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The town area on the ocean front about 10 miles east of Hilrnington, North Carolina, is about 3 miles long, from a few hundred feet to one-quarter mile wide and contains about 300 acres. The general elevation is
about 10 feet above mean low water along the ridge of the land. A shallow
sound, 1-1/4 miles wide, separates Wrightsville Beach from the mainland to
the west. Shallow sounds and marine marshes separate the barrier beaches and the mainland along the entire coast of North Carolina, except in the areas where the larger sounds are located. Until this year (1965), Moore
Inlet at the northern end of Wrightsville Beach has been open on every inspection since 1738, although it migrated about 1-1/4 miles. It was sometimes known as Barren Inlet. In recent years it has been shallow; it was closed in August-September 1965 as a part of the shore- and hurricane-protection project described below.
Masonboro Inlet. Masonboro Inlet, 25 miles above Cape Fear, sepa-rates Masonboro Beach, which is undeveloped, from Hrightsville Beach. The direction of the coast is south 410 west. The nearest inlet to the south
is Carolina Beach Inlet which was artificially opened by local interests by dredging and dynamiting in 1952. It connects a sound to the Atlantic Ocean, 10 miles south-southwest of Masonboro Inlet.
Masonboro Inlet has also been open on every inspection since 1738.
Bet1~een 1857 and 1887 the inlet migrated southward 3,800 feet, but it had
returned to within 700 feet of its 1857 position by 1932. The north shore which forms the southern boundary of Wrightsville Beach has been rela-tively stable since 1928, whereas, the south shore has alternately
elon-gated northward and been cut through leaving middle ground shoals in the
inlet. The north shore has been stabilized by groins, two of which were constructed by the Corps of Engineers in 1947. At that time sand ,/as deposited on the point of beach.
The width of the gorge has varied from about 1,000 to 2,000 feet; at present it is about 1,200 feet wide at mean low water.
Connecting channels. Three major channels flow through the shallow sound which lies bet1>leen Hrightsville Beach and the mainland. These channels connect the gorge in Masonboro Inlet 1>lith the Atlar.tic
Intra-coastal Water1>lay which flows near to and parallels the mainland. Masonboro channel is a natural channel which joins the ~terway 2 miles southwest of
the gorge. Shinn Creek, also a natural channel, joins the water1>lay 1-1/4
miles west of the gorge. Banks Channel and Motte Channel are improved channels which are part of the existing navigation project described
be-low.
PERTINENT DATA TIDAL FLO\·j AND RANGE OF TIDE
Data on tidal flow. Pertinent data as indicated by column headings are tabulated below. The data for 1937 and 1938 are taken from a report on the North Carolina shoreline
(1)
and the remainder are from reports on file in the Wilmington District. The flO1>lS in cubic feet per second are the maximum rates for the 1937, 1938, and September 1958 measurements. For the remainder they are the flows obtained during the range of tideshown. The maximum velocities for the 1958 measurements are the average velocities in the section at the time of maximum flow. The velocities in 1937 and 1938 are taken from the referenced paper (1). The volume of flow and rates of flow are based on the same measurements for the 1937 and 1938 data but this was not always the case for the other data shOlm.
Tidal range. Tide tables (2) issued by the U. S. Coast and Geode~ic Survey show that the mean tidal range in the ocean at Masonboro Inlet 1S 3.8 feet and the spring range, 4.5 feet. A mean high water elevation of 4.2 feet above mean low water was used in the shore protection investiga-tions. From July to November 1957 recording gages were maintained inside and outside the inlet by the Wilmington District. The tidal range in the ocean was found to be 0.5 foot greater than in the sound near the inlet.
TABLE 1
Tidal flow and related data Maximum Cross velocities section Tidal in feet (sq. ft.) range Iper second Date at MLW Flood Ebb Flood Ebb June 25, 1937 10,000 2.9 3.0 2.6 4.1 June 17, 1938 4,450 2.9 2.7 2.0 3.6 July 15, 1947 2.6- 0.7-3.1 0.25 July 1947 October 1947 Nov. 7, 1947 2.5-2.0 Nov. 10, 1947 4.1-4.6 Nov. 1948 3.0- 2.1-3.0 2.1 Aug. 19, 1958 7,790 Sep. 11, 1958 4.0-* 1. 6i>:* 4.0 1.6 4.2 4.2
*Range during tidal cycle, 5.4 feet. **Range during tidal cycle, 4.4 feet.
Rate of flow < cubic feet per second) Inflow Outflow 37,000 50,000 17,400 21,500 38,900 29,300 46,600 46,300 33,500 36,200 53,500 40,500 Total flow (acre feet) Inflow Outflow 12,000 1 8 ,900 4,700 8,200 16,100 12,000 10,200 19,200 13,100 19,200 20,700 1-6,800
Offshore velocities at inlet. On August 18, 1948, a series of ve-locity observations were made on ebb tide at the gorge, 1,000 feet o cean-ward and 2,000 feet oceanward of the gorge. Maximum observed veloc:ities
were 5.65 feet per second, 4.40 feet per second, and 3.50 feet per s econd, respectively. Observations were taken simultaneously at each locat:ion •
Differences in tide levels in the sound and the ocean were about 1 jfoot when the velocities were at the maximum.
LOW-WEIR JETTY
LITTORAL DRIFT
Available data indicate that the direction of littoral drift along
Wrightsville Beach is variable, but that the net direction is toward the
south. However, no firm data on which to estimate the volume of drift
are available.
SHORELINE CHANGES
811
Profiles. Twelve profiles were taken at each of three locations on
Wrightsville Beach between 1930 and 1957. One set of profiles was taken
near each end of what was then an island and one set was taken about half
-way between them. They ran from the dune line to the -20 to the -30-foot
contours. Profiles from a total of 12 points instead of three were taken
in 1957 and again in 1964.
The changes in the shoreline at mean low water and mean high water
taken from the profiles are tabulated below. All changes are landward,
that is, they show erosion.
Near Masonboro Inlet
Near middle of island
TABLE 2
Change in shoreline
1930
-MLW 75 100
Near northern end of island 500
Average of 12 profiles
-Average per year
PROJECTS IN AREA
SHORE AND HURRICANE PROTECTION
Change 1957 MHW 100 175 500
-in feet 1957 - 1964 MLlof MHW 28 6 32 11 26 38 38 15 5.4 2.1Groins. In 1939 the town of Wrightsville Beach constructed a groin
system generally in conformance with plans of the Beach Erosion Board,
U. S. Army (now Coastal Engineering Research Center), except that a bulk
-head was not provided because of insufficient funds (3). The work con
-sisted of 16 creosoted pile and timber groins, each 325 feet long with an
average spacing of 800 feet and placement of about 700,000 cubic yards of
fill.
Artificial fill. Pertinent data on sand deposited on Wrightsville
Volume in cu. yds. 700,000 43,000 38,000 35,000 305,000 100,000 1,221,000 TABLE 3
Volume of sand placed on beach
Year 1939 Nov. 1947 Early 1955 Fall 1956 Feb.-Apr. 1957 April 1959 Location Entire Beach
3/4 mile above Masonboro Inlet Not known
Near middle of beach
Lower mile of beach
8,500 feet beginning 4,500 feet above Masonboro Inlet
Shore- and hurricane-protection. In 1962, Congress authorized a
shore- and hurricane-wave protection berm and dune project for
Hrights-ville Beach (4). A similar project ~TaS authorized for Carolina Beach and
vicinity, 15 miles to the south-southwest.
The dune was constructed with the shoreward side of its base
gener-ally near the building line, with a crown width of 25 feet at elevation 15 feet above mean low water, together with integral construction of a
beach berm ldth a crown width of 50 feet at elevation 12. The southern
end of the berm and dune, which is 14,000 feet long, is at Masonboro Inlet.
The lands ide slope of the dune is 1 vertical on 5 horizontal. The ocean
side of the dune and of the berm from the 12-foot elevation to the mean-high-water line at 4.2 feet was constructed with side slopes of 1 on 10.
Oceamlard of the mean-high-l"ater line, the slope was assumed at 1 on 20.
The dune and berm \o.ere constructed by hydraulic dredging. Shoreward of the mean-high-l{ater line the structure lms shaped by mechanical graders.
Oceanward of that point waves and currents formed the slopes. The shore
at mean high water has been moved oceanward a minimum of 150 feet. A typical cross section is shown on figure 3. In addition to constructing the berm and dune, l-loore Inlet 1{aS filled as part of the project. The in-place volume ydthin the dimensions and slopes described above was
estimated at 1,441,000 cubic yards for the berm and dune project. A total
of 2,655,000 cubic yards were deposited based on borrow area measurements indicating a retention rate of 54 percent. At Moore Inlet the in-place volume was estimated at 210,000 cubic yards, and 340,000 cubic yards were deposited indicating a retention rate of 62 percent. Some tolerance was
allowed and the finished section generally included more fill than the design section. Much of the material not retained within the berm limits was deposited in the offshore area and provides a source of sand for natural nourishment of the shore. A contract has been let for stabiliza -tion of the dune by planting American Beach grass during the fall and winter of 1965-66. Part of the dune at Carolina Beach was planted in beach grass during the spring of 1965 and it is growing well, and it
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Existing pr9ject. The existing navigation project provides for a channel 14 feet deep at mean low water and 400 feet wide across the ocean bar at Masonboro Inlet, thence 12 feet deep and 90 feet wide to the cnan
-nel of the Atlantic Intracoastal Haterway by the way of Banks and Motte channels, a turning basin inside the inlet, and a jetty on each side of the inlet extending to the 14-foot depth in the ocean. The jetties ~ere to be constructed only if it were found impracticable to maintain tbe . channel by dredging and if a study showed the jetties economically just~ fied. Dredging of Banks and Motte channels to project dimensions was t completed in April 1957. The spoil was placed on the southern 4,500 fe~ of vlrightsville Beach. On June 1, 1959, dredging of the Masonboro rp.l.e t Channel was completed by a pipeline dredge since depths were insuffic ieXl to float a hopper dredge. A total of 345,700 cubic yards of sand were d dredged at a cost of $372,900. By September, the channel had shoaled aXl 110,000 cubic yards of sand were removed at a cost of $72,800. In 4 ~y.
months after the maintenance dredging the channel had shoaled extensive Since then the channel has shifted frequently and the controlling deptD has been about 7 feet at mean low water.
JErTY
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-General. Hhen it was found impracticable to maintain a channe~ >-I~ out jetties, a study was made which found jetties economical. The be s ini tial project appeared to be the construction of a jetty on the nor-tO be side of Masonboro Inlet. If necessary, a jetty on the south side wi}..}.. provided later. During early stage of the design of the project, coP- ~
ferences were held which were attended by representatives of the Coa.s-t8- ng-Engineering Research Center, the South Atlantic Division, and the Ifi}..rn:i d ton District, Corps of Engineers. At these conferences, it was propos e red to change the design from the rubble-mound structure previously cons:i.de to a structure having a low-weir section to pass littoral drift, wh:ic:P
has been aptly described as a "sand spillway" (5). A deposition bas:i.X: J..~ed would be provided between the jetty and channel. Hhen the basin is :f:l- a ' the sand will be pumped across Masonboro Inlet to Masonboro Beach us:i.pg; red pipeline dredge. This type of structure was selected because it apJ7~a. to that the low-weir design would localize the sediment, make it feasibJL~ _
maintain the project by pipeline dredging, and supply material for n~~X ishment of Masonboro Beach.
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e-The elevation of the top of the weir and the length of weir ~s
quired were based on the judgment of the conferees. One requirement 'xl
that the weir be high enough to afford protection to a dredge workin~
:l-the channel or deposition basin. The Chief of Engineers approved t~~
7.
change in design. Pertinent information is shown on figures 4,
5,
a~DESIGN OF JE'lTY
Foundation conditions. Six wash borings were taken. Dense sand
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found at -11.4 feet near the shore and at -28.8 feet near the end of 400 jetty. Two split-spoon borings, one taken about 2,100 feet and one3
£ oot feet from shore, near the location of the jetty, showed sand to a 40LOW-WEIR JETTY
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COASTAL ENGINEERINGStructural design is in accordance with Technical Report Number 4 by the Beach Erosion Board (6), other Corps of Engineer manuals, and
engineering books. No unusual design problems were encountered and de
-tailed design data are not included herein. However, general informa
-tion on which the design was based and a general descrip-tion of the project as designed and under construction follows. A profile and typical sections of the project are shown in figure 5.
Concrete sheet pile section. For the first 1,700 feet, the water
along the site of the jetty under construction has a maximum depth of
3 feet at mean low water. The plans call for a jetty 3,400 feet long
extending to the -12-foot contour. The shoreward 1,700 feet are to be
constructed of concrete sheet piling and the oceanward 1,700 feet of
rock. An alternate bid was requested using sheet pi,ling for an
addi-tional 450 feet. However, longer and heavier piling' were required and
the bids were higher for sheet piling than they were for rock.
Work started on the concrete sheet piling part of the project iP
August 1965. The structure is tied into the berm, described above, at
12 feet above mean low water. The oceanward 1,000 feet of the weir bas
a top elevation of 2 feet above mean low water which is about midtid
e-The concrete piles are tongue and groove precast prestressed un:i t S _
3 feet wide, 16 inches thick, and 23 feet 6 inches long. The specif:iC8
tions require that the cable stress be maintained until the concrete
reaches a compressive strength of 5,000 pounds per square inch. They
also require that the piles not be driven until the concrete is 7 daYS
old. The specifications stated that water jets may be used to assist
in driving the piles to within 1 foot of the required depth. Howeve:lC':f
it was found that the sand was so dense that driving caused spalling 0
the pile. Therefore, the procedure was changed so that three of eactl ~es.
four piles are jetted in place and the fourth is driven the last 6 i~c
Rubble-mound section. A top elevation of 6 feet was selected fC>~
the rubble-mound structure to allow for some settlement and still ha~~ ~
a top elevation of 5 feet, a few tenths of a foot above high tide. ~ e
8k-head of the structure was designed to withstand a 12-foot high wave ':~ ~
ing upon it; along the trunk, a design wave of 8 feet was used. DeS3-&-t::h
waves were selected on the basis that they would be 0.78 times the deJ? a
-of the water at the part -of the structure under consideration. CompL1~
tions were made for several side slopes. Slopes of 2.5 horizontal o~ ~~
1.0 vertical for the head and 1.5 horizontal on 1 vertical for the t~
of the jetty were selected.
\
LOW-WEIR JETTY
DEPOSITION BASIN
Location and size. Upon completion of the jetty, a deposition basin will be dredged between the jetty and the navigation channel. Although
819
no firm decision on dimensions have been made, a basin about 600 feet wide and 1,500 feet long is under consideration. The general location is shown on figure 4.
Sand movement. No estimate has been made of the volume of littoral drift from either the north or the south. Data which indicate that the movement is relatively large are the rather severe erosion that has oc-curred since 1930 at Wrightsville Beach and the rapid shoaling of the navigation channel in 1957. Information tabulated above shows that in spite of the deposition of 1,221,000 cubic yards of sand between 1939 and 1964, a substantial landward movement of the shoreline occurred. The sand deposited averages 49,000 cubic yards a yearj between 1957 and 1964 the 405,000 cubic yards of sand deposited averaged 58,000 cubic yards a year. Based on the average loss of 2.1 feet of shore at the mean- high-water line a year between 1957 and 1964 (table 2) and the rule-of-thumb estimate that 1 square foot of change in beach surface area equals 1 cubic yard of beach material (6) the average erosion from the 3-mile shoreline at Wrightsville Beach averaged 33,000 cubic yards during the 7-year period between 1957 and 1964. The 1957 profiles were taken just before sand was deposited on the beach in 1957. Adding 33,000 cubic yards to the average volume deposited on the beach indicates an average net loss of 91,000 cubic yards a year. Shoaling in the navigation channel in 4 months to-taled 110,000 cubic yards. However, the source of the sand causing the shoaling is not known. No estimate has been made of how often the deposi -tion basin will have to be dredged.
POST-CONSTRUCTION INVESTIGATIONS
The Wilmington District, the South Atlantic Division and the Coastal. Engineering Research Center, Corps of Engineers, plan to obtain data on the berm and dune, jetty, deposition basin and navigation channel. In-cluded in the investigations are offshore profiles to the -30-foot con-tours on ~Trightsville Beach and Masonboro Beach which are to be made yearly beginning in October 1965j short profiles on Hrightsville Beach 1,000 feet apart to the -2-foot contour which are to be made every 4 months and hydrographic surveys of the inlet to be made every 6 months, starting in June 1965.
ACKNOWLEOOEMENT
lVhile the information contained in this paper is generally taken fr~JQn published reports of the Corps of Engineers, U. S. Army, some is from th~
files of the Hilmington District. The plan for the jetty was formulated and the jetty was designed by the staffs of the l1ilmington District, the South Atlantic Division and the Coastal Engineering Research Center with only nominal participation by the author. The permission of the Chief o~ Engineers to publish previously unpublished data is appreciated.
REFERENCES
(1) Beach Erosion Board (1947). North Carolina Shore line (H. Doc. 763,
80th Congo 2d sess.)
(2) Coast and Geodetic Survey (1965). Tide tables.
(3) Beach Erosion Board (1934) Wrightsville Beach, N. C. (H. Doc. 218, 73d Congo 2d sess.)
(4) Wilmington District, Corps of Engineers (1961) Wrightsville Beach, N. C. (H. Doq. 511, 87th Congo 2d sess.)
(5) Beach Erosion Board (1955) Bulletin, Vol. 9, No.2; Paper by Thelbert
K. Hodges, Jacksonville District, Corps of Engineers.
(6) Beach Erosion Board, Corps of Engineers (1961), Shore protection
