no longer confined by a bank on each side, becomes dispersed, and
owing to the reduction of its scouring force, is no longer able at a
moderate distance from the shore
effectually to resist the action of
the waves and littoral currents
tending to form a continuous beach
in front of the outlet. Hence a
bar is produced which diminishes
the available depth in the approach
channel. By carrying out
a solid jetty over the bar, however,
on each side of the outlet, the tidal
currents are concentrated in the
channel across the bar, and lower it
by scour. Thus the available depth
of the approach channels to Venice
through the Malamocco and Lido
outlets from the Venetian lagoon
have been deepened several feet
over their bars by jetties of rubble
stone surmounted by a small superstructure
(fig. 3), carried out across
the foreshore into deep water on
both sides of the channel. Other examples are provided by the long
jetties extended into the sea in front of the entrance to Charleston
harbour, formerly constructed of fascines, weighted with stone and
logs, but subsequently of rubble stone, and by the two converging
rubble jetties carried out from each shore of Dublin bay for deepening the approach to Dublin harbour.
 |
| Fig. 3.—Lido Outlet Jetty, Venice. |
 |
| Fig. 4.—Mississippi South Pass Outlet Jetty. |
Jetties at the Outlet of Tideless Rivers.—Jetties have been constructed on each side of the outlet of some of the rivers flowing into the Baltic, with the objects of prolonging the scour of the river and protecting the channel from being shoaled by the littoral drift along the shore. The most interesting application of parallel jetties is in lowering the bar in front of one of the mouths of a deltaic river flowing into a tideless sea, by extending the scour of the river out to the bar by a virtual prolongation of its banks. Jetties prolonging the Sulina branch of the Danube into the Black Sea, and the south pass of the Mississippi into the Gulf of Mexico (fig. 4), formed of rubble stone and concrete blocks, and fascine mattresses weighted with stone and surmounted with large concrete blocks respectively, have enabled the discharge of these rivers to scour away the bars obstructing the access to them; and they have also carried the sediment-bearing waters sufficiently far out to come under the influence of littoral currents, which, by conveying away some of the sediment, postpone the eventual formation of a fresh bar farther out (see River Engineering).
 |
| Fig. 5.—River Maas Outlet, North Jetty. |
 |
| Fig. 6.—River Nervion Outlet, Western Jetty. |
Jetties at the Mouth of Tidal Rivers.—Where a river is narrow near its mouth, and its discharge is generally feeble, the sea is liable on an exposed coast, when the tidal range is small, to block up its outlet during severe storms. The river is thus forced to seek another exit at a weak spot of the beach, which along a low coast may be at some distance off; and this new outlet in its turn may be blocked up, so that the river from time to time shifts the position of its mouth. This inconvenient cycle of changes may be stopped by fixing the outlet of the river at a suitable site, by carrying a jetty on each side of this outlet across the beach, thereby concentrating its discharge in a definite channel and protecting the mouth from being blocked up by littoral drift. This system was long ago applied to the shifting outlet of the river Yare to the south of Yarmouth, and has also been successfully employed for fixing the wandering mouth of the Adur near Shoreham, and of the Adour flowing into the Bay of Biscay below Bayonne. When a new channel was cut across the Hook of Holland to provide a straighter and deeper outlet channel for the river Maas, forming the approach channel to Rotterdam, low, broad, parallel jetties, composed of fascine mattresses weighted with stone (fig. 5), were carried across the foreshore into the sea on either side of the new mouth of the river, to protect the jetty channel from littoral drift, and cause the discharge of the river to maintain it out to deep water (see River Engineering). The channel, also, beyond the outlet of the river Nervion into the Bay of Biscay has been regulated by jetties; and by extending the south-west jetty out for nearly half a mile with a curve concave towards the channel the outlet has not only been protected to some extent from the easterly drift, but the bar in front has been lowered by the scour produced by the discharge of the river following the concave bend of the south-west jetty. As the outer portion of this jetty was exposed to westerly storms from the Bay of Biscay before the outer harbour was constructed, it has been given the form and strength of a breakwater situated in shallow water (fig. 6). (L. F. V.-H.)
JEVER, a town of Germany, in the grand-duchy of Oldenburg,
13 m. by rail N.W. of Wilhelmshaven, and connected with the
North Sea by a navigable canal. Pop. (1901), 5486. The chief
industries are weaving, spinning, dyeing, brewing and milling;
there is also a trade in horses and cattle. The fathers (Die
Getreuen) of the town used to send an annual birthday present
of 101 plovers’ eggs to Bismarck, with a dedication in verse.
The castle of Jever was built by Prince Edo Wiemken (d. 1410), the ruler of Jeverland, a populous district which in 1575 came under the rule of the dukes of Oldenburg. In 1603 it passed to the house of Anhalt and was later the property of the empress Catherine II. of Russia, a member of this family. In 1814 it came again into the possession of Oldenburg.
See D. Hohnholz, Aus Jevers Vorgangenheit (Jever, 1886); Hagena, Jeverland bis zum Jahr 1500 (Oldenburg, 1902); and F. W. Riemann, Geschichte des Jeverlandes (Jever, 1896).
JEVEROS (Jeberos, Jibaros, Jivaros or Givaros), a tribe of
South American Indians on the upper Marañon, Peru, where they wander in the forests. The tribe has many branches and there are frequent tribal wars, but they have always united against a common enemy. Juan de Velasco declares them to be faithful, noble and amiable. They are brave and warlike, and
