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Cyclopedia of civil engineering; a general reference work on surveying, highway construction, railroad engineering, earthwork, steel construction, specifications, contracts, bridge engineering, masonry and reinforced concrete, municipal engineering, hydraulic engineering, river and harbor improvemen online

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on the rock. There were two of these piles maneuvered by hydraulic
power, enabling the vessel to advance a given distance after each
swing, and preventing her losing her position. Dredges somewhat
similar to the above are being used at present by the United States
Government on the Panama Canal construction.

ATLANTIC COAST HARBOR IMPROVEMENTS

Study of Sand=Movement Forces. The improvement of
harbors on that part of the Atlantic coast south of New England
involves almost entirely such protective or corrective works as
are necessary for a sandy coast line. In making a study of these
works, it is necessary to consider forces which cause the movement
of sand; for almost the whole object of river and harbor improve-
ments on a coast line of this character is the control of the move-
ments, or the retention in place, of large masses of sand that would
otherwise be prejudicially shifted by winds and waves or tidal
currents.

Wherever dry sands are found, they are always shifted by
prevailing winds; indeed sand is drifted about under the action of
winds precisely like snow. Captain Black, in his paper on the
improvement of harbors on the south Atlantic coast of the United
States, asserts that on the Florida coast sand moves like drifting snow
with a depth of from 4 inches to 8 inches, while a properly constructed
sand catch will form a dune 10 feet high in a single year. It is thus
evident that, inasmuch as the sand will be moved in the direction of
the wind, if that direction is across a channel the result will be

439



96 RIVER AND HARBOR IMPROVEMENT

deposition of sand in that channel, with a possible obstruction of the
latter so far as the passage of vessels of considerable size is concerned.
Similar results from the same wind forces are constantly produced on
those portions of the Pacific Coast which are sandy.

Waves and Currents. The most troublesome forces to control in
connection with the littoral movements of sand, are those of waves
and currents particularly tidal currents, which are largely shifted in*
direction by bars and shoals of their own creation, though the winds,
also, frequently create difficulty. The study of wave motion has been
largely developed by Scott-Russell, Hagen, the Weber Brothers,
Rankin, Froude, and others, but it is not necessary to consider in this
connection the analytic basis of such work. It is only requisite to
observe that wave motion is a combination of rotation and transla-
tion. Stevenson, in his w^ork on rivers and harbors, states that the
height of the highest point of a wave crest above the normal level of
the water in which it is produced is two-thirds of the wave height
from hollow to crest, and that the velocity of wave translation in
the Atlantic Ocean may reach as high as 115 feet per second, while
ordinary storm waves travel at a rate of 50 feet or 60 feet per
second.

It is a matter of common observation that waves break as they
roll in toward the shore in shallow water; and Stevenson states that
the mean depth at which they will commence to break is from once
to twice a wave height, but that they break earlier when the water
shoals rapidly than when the slope of the bottom is more gradual.
It is, of course, perfectly evident that the wind will have great influ-
ence upon the velocity of wave translation, as well as upon the shape
of the wave and the circumstances under which its crest will break.
A sufficiently high wind will cause almost any w r ave to break at its
crest, and will cause each molecule to describe a very different orbit
from that assumed in mathematical analysis.

It is evident that the crest of a wave will be at right angles to
the direction of, the wind, and that its motion will be along that
direction; but when adjacent to the shore line, if the wind is blowing
obliquely to it, one end of the crest will be retarded and deflected by
the shore, while the other will be in deep water. Consequently the
crest will swing around more or less toward the shore line, and pro-
duce complicated motions in the water.

440



RIVER AND HARBOR IMPROVEMENT 97

Back Flow. As a wave rolls inshore, the water is piled up on the
beach for a moment and then flows back; and inasmuch as successive
waves are rolling in along the water surface, its back flow will result
in an undercurrent along the bottom at a constantly decreasing rate
in receding from that part of the beach at the low-water line. The
motion of the waves will produce great disturbances of the bottom in
shallows; in fact it will cause the constant picking up of sand or
sediment which will be carried inshore by inshore winds and by the
motion of translation of the waves, until eventually the material is
piled upon the beach. The back flow or undercurrent on the bottom
will, on the other hand, tend to carry material deposited on the beach
out to deep water, but with very much less activity than the inshore
forces just mentioned. As a consequence, the beach constantly gains
material; and inasmuch as the back flow or undercurrent is relatively
weak, the finer portions only will be transported back toward deeper
water. This accounts for the clean and relatively granular character
of most sand beaches.

A current of water which is too weak to erode a bottom will
transport very considerable material in suspension or in semi-suspen-
sion if it can be once placed in that current. Various authorities
name currents from 6 inches to 15 inches per second as sufficient to
begin a scour upon a sandy bottom of various degrees of coarseness.
The liability to scour, however, of any sandy bottom, depends to a
considerable extent upon the degree of solidity with which the sand
is packed. Captain W. M. Black, in the paper already cited, states
that he has seen a bank of packed sand remain without erosion under
a current of 3 feet per second. With a relatively high current, con-
siderable material will, of course, be transported, while in the case
of a slower current the amount will be considerably less. Hence, if
the back flow or undercurrent on a beach is relatively strong, con-
siderable material will be carried into the water from the beach, and
the resulting bottom slope will be much more gradual than if the back
flow is weak and able to transport but little material, and that of the
finest.

If the shore is eroded by a littoral current, the sand which is
transported by the latter will be deposited with the decrease of
velocity which takes place as the current flows into deeper water, and
a spit or shoal point will thus be formed. Again, if these currents

441



98 RIVER AND HARBOR IMPROVEMENT

are formed and directed by the winds, this deposition of sand may
become extremely variable and form hooks or loops, thus producing
a shoal bottom of eccentric character; at the same time these sand
formations in the shape of bars or shoals may be spread across a river
entrance or harbor.

It is believed from many observations, that sand is much more
in motion or "alive" during flood tides than during the ebb, although
some perfectly reliable observations to the contrary have occasion-
ally been made. It is known that a new beach formed during a gale
will at first be comparatively soft, and eventually become very hard,
under the pounding action of breakers; in fact, some beaches become
almost as hard as asphalt pavements, so that they will ring under a
blow from the foot of a horse.

Wave Action on Jetties. The force of waves on the Atlantic
Coast of the United States is somewhat less than that of similar
action upon some foreign shores, although they are seen frequently
to exert destructive action upon protective works. The following
accounts are taken from the Report of the Chief of Engineers, U. S. A.
of 1890, on the Improvement of St. Augustine Harbor, as well as
from the paper of Captain Black already cited:

A wave may act on the jetty directly, either by a blow or a push, or a
blow and a push combined; and indirectly, by a pull, by compressing the air
in the voids of the masonry, by upward pressure due to the difference of head
produced on the two sides of the jetty, or by a combination of these actions.
The direct action measured on the dynamometers had effects equal to pres-
sures varying between 190 and 753 pounds per square foot. This action took
place when a wave broke directly on or in advance of the jetty; this also com-
pressed the air in the voids of the jetty Jets of water and sand were

sometimes projected up from the cracks in the jetty to some height. The

maximum height of any wave observed striking the work was 6 feet

Up to a height of about 2 feet above mean low water, riprap weighing 40 to
50 pounds was but little disturbed. Above this limit, to the height of 10 feet,
the highest point observed, riprap varying in weight from 40 to 200 pounds,
could not be held at any slope. An isolated piece of concrete weighing 350
pounds, and resting on its flat base 1.7 square feet in area, with its center of
gravity 7 feet above mean low water, was moved several feet by breakers whose
crests were about 1\ feet above mean low water. These breakers measured

3| feet from hollow to crest All that portion of a mound or wing

composed of riprap (varying in weight from 40 to 220 pounds) tightly chinked
with oyster shells, lying between 4 and 6 feet above mean low water, no matter
what side slopes the riprap was given, would "be carried away in a single tide

whenever breakers greater than 4 feet in height struck it fairly A

block of concrete weighing 527 pounds was elevated 1.3 feet by the action of

442



RIVER AND HARBOR IMPROVEMENT 99

a single breaker. During the same tide, it was moved 23 feet inshore. A
dynamometer within 8 feet of its original position recorded a maximum pressure
of 575 pounds per square foot during this tide. A piece of concrete weighing
200 pounds was lifted vertically to a higher level than that of the water surface,
by a wave which broke just in front of it. Another block of concrete weighing
1600 pounds was lifted from its bed vertically at least 14 inches, and then moved
several yards. Later, a concrete block 10 by 6 by 1\ feet and weighing dry
21,000 pounds, lying about at the mean low-water line of the beach, was lifted
vertically 3 inches, and there caught and held fast. The maximum wave height
and dynamometer readings during that gale were 5.5 feet and 633 pounds,
respectively.

Coast from North Carolina to Florida. The coast from Cape
Hatteras to the southern extremity of Florida is low and sandy,
broken by many openings into rivers, bays, and inlets. The highest
point in the entire line is Mount Cornelia, at the mouth of St. John's
River, and it is only 60 feet high. There is a general motion toward
the south, of the sand on this entire stretch of coast, in consequence
of the direction of the prevailing winds, which is in the main southerly.

The slope of the country bordering the coast is invariably very
slight; consequently all the rivers emptying into the ocean through
it are tidal streams, the effect of the tides being felt for a considerable
number of miles back from their mouths. As a result, the material
brought down by the different streams does not reach the ocean, but
is deposited in the basins immediately back of their mouths, except
in a very few cases where small amounts may reach the ocean. The
slope of the beach along low-water line is also very slight, being but
1 :45 off Cape Hatteras, and as low as 1 : 1000 at Sapelo Inlet just south
of Savannah, from which point it increases southward until it reaches
1 : 55 at Cape Canaveral, about midway down the coast of Florida.
The decrease in bottom slope in the vicinity of Florida is largely due
to the bay-like shape of the shore between Cape Hatteras and Cape
Florida. As the tides move into this "Southern Bay", as it is called,
they become concentrated in their effects, and rise in height as the
point of the bay in the vicinity of Savannah is reached. The material
moved by the tides is therefore concentrated in the same vicinity,
and a steeper slope of bottom results.

In the vicinity of Cape Hatteras, the prevailing winds are from
the north and northeast, and the same is true for the vicinity of
Charleston, while on the coast of Florida the reverse condition takes
place. Nevertheless, in all cases, the heaviest storms and highest

443



100 RIVER AND HARBOR IMPROVEMENT

winds are from a northerly direction, which produce at all points
along the coast the southerly movement of the coast sand, which has
already been mentioned. It is important to regard this fact in the
designing and construction of correction works at the mouths of the
more important rivers; and it will be noticed in many cases that this
tendency has deflected the mouths of many of the rivers in a south-
erly direction.

Summary of Conditions. The general conditions prevailing on
the portion of the coast under discussion may be summarized thus:
(1) shores are low, fringed by a beach of light, shifting sand, the fore-
shore having a very gentle slope, giving comparative protection from
wave action; (2) heaviest and most prolonged gales are from the
northeast, with occasional severe gales from the southeast; (3) mean
tidal range is from 1.5 feet to 7 feet, with two tides daily; (4) there are
no continuous, strong littoral currents, and no strong tidal currents
excepting in the immediate vicinity of the openings on the coastline;
(5) coast storm drift of sand is large in volume, with a resultant
movement to the south; (6) this drift forms fan-shaped shoals across
all openings on the coastline; (7) there is little or no fresh- water
sediment brought to the coastline; and (8) channels across the shoals
are generally shifting in character; while the navigable channels
are maintained by the ebb outflow from the interior tidal basins,
reinforced by the fresh-water discharge from the interior.

Improvement at Cape Fear River Mouth. The city of Wil-
mington, North Carolina, is situated nearly 28 miles from the mouth
of Cape Fear River; and for about one-half the distance, the latter
is roughly parallel to the sea beach, from which it is separated in
the main by a narrow stretch or tongue of sand, in some places but
a few hundred feet wide, but, near its mouth, by the accretion of
sediment called "Smith's Island", which is 2 miles wide and inter-
sected by numerous threads of water. It is probable that in the early
age of the coast, Cape Fear River emptied into the ocean at a point
nearly opposite Wilmington, and that the southern motion of the
coast sand gradually deflected its mouth by slow stages to its present
position.

There are historical reasons for believing that Cape Fear River
had a channel capable of passing vessels of 14-foot draft through
Bald Head channel to the city of Wilmington, up to 1761 ; but in that

444



RIVER AND HARBOR IMPROVEMENT 101

year a violent equinoctial storm of 4 days' duration caused the sea
to break through the narrow sand beach at what is now known as
New Inlet, some 8 miles up the river from the former entrance. This
new channel was reported at different times to have a depth of from
2J feet to 18 feet; but the effect of this shortened route to the sea
for the river water was a scouring from a certain depth of 6 feet at
low water in 1797 to 10 feet at low water in 1839, with a corresponding
shallowing by bar formation in both the Western and Bald Head
channels, although up to 1839 the latter was the main entrance to
the river.

From 1839 to 1872, that channel was discontinued, while New
Inlet and the Western channel were used as the main entrances to the
river. Inasmuch as New Inlet was vastly more exposed to. the
prevailing northeast winds than the natural or Bald Head channel,
making both the entrance for vessels and the preservation of correc-
tion works much more expensive at the former point, it was deter-
mined in 1872 to close New Inlet and reopen and maintain Bald
Head channel. In consequence of the decreased velocity in the old
channel, due to the efflux of water through New Inlet, there were
produced horseshoe shoals and a fan-like bar formation at the outer
extremity of Bald Head channel. The shoaling influence at different
points was also considerably complicated by the tidal impulse up the
river through Bald Head channel, not being felt at New Inlet until
considerably later than on the sea front.

New Inlet Dike. The New Inlet dike was constructed between
1874 and 1881. It was begun by constructing a crib work pier 500
feet long out from Federal Point for the usual pier purposes for such
work. The same scouring effect around the advancing end of the
cribwork was experienced which gave the same kind of trouble in
the construction of the Zekes Island dike, and resulted in deepening
the water from 6 feet to 12 feet at a point 200 feet from the shore end
of the pier. The pier was completed in the month of November,
and the end crib was sunk in 19 feet of water, where the original
depth was but 6 feet. The settlement of these cribs and the extreme
difficulty in building upon them fast enough to keep them above
low water, caused their abandonment for all subsequent work.
In 1875 it was determined therefore to close the remainder of the
New Inlet opening by first sinking an apron of log mattresses cov-

445



102 RIVER AND HARBOR IMPROVEMENT

ered with a layer of brush and loaded with stone, in which the logs
were laid normal to the axis of the jetty. The scour in front of the
advancing end of the apron did not exceed 3 feet, and there was
no settlement after the mattresses were placed in position.

The jetty was completed by dumping one-man riprap stone
upon the apron already in position. The slope of the finished work
on the sea side was about 1 : 2, and on the land side 1 : 1J. On the to;>
of this mass of riprap and above low water, there was carefully
laid a capping of undressed granite blocks weighing from J to:i
to 4 tons each, with a slope of 1 : 2 on the sea face, and a slope some-
what less on the land slide. While this system of construction was
an improvement upon the crib plan, it was found that the logs did
not close the bottom of the jetty in a perfectly water-tight manner;
hence material was gradually scoured out between the logs, causing
them to settle. No settlement, however, was found where the
original depth was greater than 13 feet below low water; but in some
places there was a scour of 8 feet to 12 feet along the edges of the
foundation, due to the flow of water over the top of the dike. In
consequence of the irregularity of the tide effects at this point,
the difference in head of water on the two sides of the dike was as
much as 2 feet at some stages of the tide. This difference in head,
combined with the flow over the top of the dike, caused the scour
between the logs.

The crest of the completed dike is 1 foot above high water of
ordinary spring tides, and the total length of the entire work is 4752
feet; it contains nearly 183,000 cubic yards of material; and it cost
complete about $2.75 per cubic yard, or $105 per linear foot. This
completed work joined the Zekes Island dam at its northernmost
point, and completely closed the opening between Federal Point
and the northern extremity of Smith's Island.

Dike at Bald Head Channel Mouth. The only remaining jetty
work for the improvement of this river mouth, is the dike across
the shoals at the mouth of Bald Head channel, which has a length
of 5200 feet, and was built between 1883 and 1888.

In consequence of the experience with the log foundation of
the mattresses already described, it was determined to employ a
different plan for this dike of mattresses. The foundation consisted
of a brush mattress apron, part in sections and part continuous, in

446



RIVER AND HARBOR IMPROVEMENT 103

5 feet to 12 feet of water. This mattress was broad enough to extend
beyond the side slopes of the dike, and thus to act as an apron for
overflow. It consisted of three layers of brush, of which the lowest
was transverse to the axis of the jetty, the next longitudinal, and
the third transverse. These were compressed and held by fascine
copes, or binders, 5 inches to 8 inches in diameter, strongly bound
with spun yarn. The binders were parallel to the axes of the jetty,
and were spaced 3J feet or 4 feet apart in sets of two, one above and
one beneath the mattress. The brush was compressed between the
binders by a lever to the full strength of the 9- to 18-thread ratlin
which tied the upper and lower binders together. The average thick-
ness of the mattresses after compression was 9 inches. They are
described as strong and pliable.

A portion of the foundation was made of separate mattresses,
constructed on a tilting table, 90 feet by 30 feet, and towed to the
site of the work and sunk. A length of 5200 feet of the foundation
was made of continuous mattress work in two parts of 3100 feet and
2100 feet, on a scow 80 feet by 28 feet, provided with an inclined
table 60 feet by 36 feet. A section having been launched, its binders
were spliced to binders on the scow; another section was made, and
the scow run from beneath on the line of the axis as before. The
sinking of the mattress by stone followed a short distance behind
the scow. In the small mattresses made on the tilting table, it was
found that by practice an average of 25 feet in length of fascine
binders was made each hour by one man. After the fascines were
ready and the binders spliced, the mattresses were made and fully
bound at the rate of 7J square yards for each hour's labor.

The dike was finished in 1888 with a total amount of 76,000
cubic yards of stone used in the construction, and a crest 6 feet wide
at the level of high water of ordinary spring tides. The side slopes
are 1 : 1J. The crests and slopes to low water are capped with heavy
stone, hand-laid to a smooth face, with an average thickness of
from 9 inches to 12 inches. The average subsidence of the dam due
to settling in the mud, compression of mattresses, and consolidation
of riprap, was about 2 feet.

It was necessary to dredge channels both through Horseshoe
Shoal and the short shoal about 1| miles to the north of it, as well
as through the outer bar along Bald Head channel. The dredge

447



104 RIVER AND HARBOR IMPROVEMENT

used for this work was of the hydraulic type. She was a small
propeller of 145 tons' burden, and was able to work very satisfactorily
on the outer bar, except in stormy weather, when she was usually
operated on the shoals inside the river mouth. The excavation was
made with a 9-inch centrifugal pump driven by a double oscillating
engine with 10-inch cylinders and with two suction-pipes of wrought
iron, of 6f inches internal diameter. She was able to dredge in
a depth of water from 6 feet to 14 feet, and frequently raised her
bin capacity of 45 cubic yards in 25 minutes; and the average day's
work was 515 cubic yards.

Lesson of Experience. The experience gained in this work all
shows in a marked manner the inadvisability of attempting to force
a dike or a jetty of full depth across a channel with a sandy and
shifting bottom, in water in which there is an appreciable current.
An increasing depth of scour will invariably take place in front of
the advancing point of the work, and the difficulties caused both by
construction and maintenance will be very largely increased.

The proper plan to follow, then, is the one which has been almost
universally adopted by the later and similar works, that of first
sinking a pliable foundation which can be quickly put in place with-
out greatly disturbing the flow. In this manner a bottom or founda-
tion is secured for the superstructure which will not yield to destruc-
tive forces. It was found, however, that water would even find its
way under the Bald Head channel dike to some extent, and pro-
duce a slight settlement. Some engineers have recommended
that such foundations be formed of riprap which settles into the
soft or yielding bottom and forms a part of it; but, on the whole,
brush fascines have been found to be quite satisfactory.

The result of all this Cape Fear River improvement work has
been a channel with a low-water depth of 17 feet through the outer
bar in Bald Head channel, and 16 feet through Horseshoe Shoal,
the dredged channels having a width of 200 feet, with the depths
named.



448



REVIEW QUESTIONS



449



REVIEW QUESTIONS

ON THE SUBJECT OF

WATER-POWER DEVELOPMENT

PART I



1. Define the terms work, power, and energy. What are
their units of measurement?

2. Illustrate the relation between pressure-head, velocity-



Online LibraryAmerican Technical SocietyCyclopedia of civil engineering; a general reference work on surveying, highway construction, railroad engineering, earthwork, steel construction, specifications, contracts, bridge engineering, masonry and reinforced concrete, municipal engineering, hydraulic engineering, river and harbor improvemen → online text (page 30 of 32)