The herald., 1897-04-04

"
complete shadow decreasing as the dis
tance from the screen increases; outside of
this is q region of partial shadow of vary
ing intensity, the area of which increases
with the distance from the screen. The
same thing happens in the case Of q
breakwater; the waves are absolutely quiz
eted back of the breakwater for a width
which decreases with the distance from
the breakwater; they are partially quieteti
back of the breakwater for width which
increases with the distance from the
breakwater; and the amount of the quiet
ing beyond the length of the breakwater
is precisely equal to the amount of agita-
tion inside that length. The direction of the
shadow or protection is determined, not
by the direction of the breakwater but
by the direction of the swell. and will
therefore vary with the swell. Water be-
yond what is. strictly the protected area
may under ordinary storms be sufficiently
while portion of the water inside the
assumed protected area may be too rough
under exceptional conditions for anchor
age. There is no clearly defined line of
distinction, but general change, and
this will vary with many circumstances.
In general it may, be stated that the best
anchorage will be near to and immedi-
ately behind tho breakwater.
THE AREAS COMPARED.
For the purposes of comparison of
areas, there seems to be no more accurate
rule than to draw lines at right angles to
the breakwater at each end and assume
that the sea between these two lines is
protected. Under many conditions there
will be fairly good anchorage outside
give too great a measure; they are as
sumed, however, for comparison only, as
on the whole giving the best and fairest
average results.
The principal duty of breakwater is
usually TO provide safe and connections
anchorage. The protected area, though
comparatively still, feels to a limited ex-
tent the effects of an outside swell; it can
not have the absolute quiet which is found
in small nd-locked harbor, in an inte-
rior basin or in an artificial dock.
The practice of building unprotected
ocean piers is characteristic of the Cali
fornia coast. Where piers of this kind
are used there must be sufficient depth of
water to allow ships to rise and fall with
moderate swell, and in rough weather
the piers cannot be used. If piers are
built in the area protected by break
water, these difficulties will be
greatly reduced; ships can. lie
close to the piers instead OF
being breasted off; but it would
not generally be safe to allow them to
touch bottom. In completely enclosed
basin no swell whatever is felt and no
harm comes to ship if she occasionally
grounds, provided the bottom is soft; this
is sometimes done alongside the piers in
New York city. 1t would be fair to say
that at an ocean pier there should be at
least one fathom of water under ships
bottom at mean low tide. that at pier in
the area protected by breakwater there
should be half fathom, and that in a
landlocked basin only enough water to
float the vessel will be required. A COM
mon diurnal range of tides in both San
Pedro and Santa Monica bays is about six
feet, while the extreme of spring tides has
sometimes gone two feet below mean low
water. At this extreme low water there
would be four feet of water under a ship
lying at an ocean pier, one foot under a
ship lying at pier in a protected area.
while ship in an enclosed basin would
be aground. Under these conditions, in
order to accommodate vessels drawing 80
feet an ocean pier should be built in six
fathoms at mean low water, pier within
protected area in five and one-half faith
oms, and an interior basin should have a
little more than five fathoms The Port
Los Angeles pier which is built in from
five to five and one-half fathoms, would
not now at extreme low water afford
safe berth for a ship drawing 30 feet.
After this provisional consideration of
the requirements of the law and the pro-
lection which may be afforded by break
water, the consideration of the two re-
spective harbors may be taken up.