|
| |
Most dredging done by hopper dredges is to maintain safe navigation. There are two main
contributing factors to a navigational channel becoming too shallow to be safely used.
They are silting and littoral drift.
Silt. Silt is defined by the Encyclopaedia Britannica as sediment particles ranging from
0.004 to 0.06 mm (0.00016 to 0.0024 inch) in diameter irrespective of mineral type.
Silt is easily transported by moving currents but settles
in still water. It constitutes about 60 percent of the material in the Mississippi River
delta. Sediments are seldom composed entirely of silt but rather are a mixture of clay,
silt, and sand.
Littoral Drift. The other source of deposited material likely to obstruct navigation is littoral
(coastal) drift, especially in areas where there is a sizable tidal range. The incoming
tide frequently brings suspended material, some proportion of which settles to the bottom
around the turn of the tide when the movement of water is at a minimum. In the absence of
any countervailing tendency, an accumulation takes place, which requires dredging.
It is a common story in history of a prosperous harbour
that is abandoned due to silting and littoral drift. A few examples are:
Ephesus, Turkey. Roman city and harbour that thrived from the first century A.D. The
site of this ancient city is located about 160 km North of Bodrum. Because of its
fantastic coastal position and excellent harbour and owing to financial preference of many
rulers through several culture epochs, Ephesus was for many centuries the most important
city of Minor Asia. The Greeks founded Ephesus already in the and very soon it
became the center of culture, trade and power. Many historians refer to Ephesus as the
starting point of western civilization and occidental thinking.
The shifting delta of the river Cayster caused regular silting problems in the harbor, and
major engineering efforts to combat this were documented in 60 A.D. and 129 A.D. Further
silting of the habor was documented in 431, 449, and 716 A.D., and major port areas of the
city of Ephesus were completely abandoned by the tenth century.
Today Ephesus is about 5 km far away from the sea.
Bruges, Belgium. In the 11th century Bruges
became an international commercial center. Access with the sea was important for the
export of locally produced high quality cloths that were sold all over Europe. Because of
it's harbour, allowing easy shipping, Bruges became quite influential. By 1350 the
population was approximately 45,000. Modern population is less than this. Prosperity
ended for Bruges because of the silting up of the coastal area, which is now a natural
park located near Knokke, about 12 miles from modern Bruges.
Pisa, Italy. Pisa's Golden Age, which lasted from the eleventh to the thirteenth
centuries, ocurred when the city was still a port, and one of the great maritime powers of
the Mediterranean. The city's political zenith come in the second half of the eleventh
century with a series of maritime victories over the Saracens, whom the Pisans drove out
from Corsica, Sardinia and the Balearic Islands. Decline in power set in with defeat at
sea by the Genoese in 1284 followed by the silting up of the harbour, which doomed Pisa as
a practical sea port.
The US congress recognizes the necessity of navigatable harbours and ports. With this
in view the Corps Engineers was given the duty.
|