Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries Report and Symposium Proceedings (1987) / Chapter Skim
Currently Skimming:
Session E: New Facility Design Considerations
Session E: New Facility Design Considerations
Pages 277-337
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 280... ...
CHANNEL GEOMETRY/DREDGING Because of the potential magnitude and extent of its impact on the flow regime in an estuary, dredging is ~ major factor in the changes in sedimentation rates due to port development. Increases in sedimentation are primarily associated with changes in flow velocity and direction and saline intrusion induced by the introduction of navigation channels, turning basins, and berth facilities.
|
From page 283... ...
Of course, increases in channel depth and/or dimension tends to reduce flow velocities and increase the resultant settling of fine materials. From the perspective of port operations, large-radius turns are desirable, but the introduction of these types of channels may result in a tendency for the channel to meander.
|
From page 284... ...
The effect of relatively small changes in current magnitude and patterns in an estuary can have a marked impact on sedimentation rates. Notwithstanding the large- scale changes from dredging of turning basins and channels, the introduction of piers, berths, and other port structures can induce localized changes that may be equally damaging to the maintenance of the facility.
|
From page 286... ...
The practicality is, however, somewhat contingent on the consistency of sediment flow since the equipment is immobile and significant oversizing may be required to deal with seasonal fluctations. Sediment supply can also be reduced through the use of bank stabilization of navigation channels as well as upstream segments subject to natural erosion.
|
From page 287... ...
The coarse-grained fraction was concentrated in the outer areas of the port, so that a significant saving in maintenance dredging could be anticipated with the diversion of the river. Economically, however, maintenance dredging was a more practical alternative.
|
From page 290... ...
Such systems are often permanently in transition due to seasonal variations in water flow and sediment supply. Once artificial channels are dredged in such a system, nature will try to revert to the original natural state.
|
From page 293... ...
The breaking waves raise the mean water level shoreward of the breaker zone, thus a hydraulic gradient is created setting up a a s:i~n;Fi^~+ ;_=, _
|
From page 296... ...
. FIGURE 5 Areas of largest natural depth.
|
From page 297... ...
History shows many examples of projects which failed or had a much reduced financial return due to major miscalculations of sedimentation rates. A few such cases are illustrated in this paper.
|
From page 298... ...
298 A number of different sedimentation problems are shown with possible remedies. It is stressed that proper understanding of the physical processes causing the sedimentation in each case is an absolute requirement for proper calculations.
|
From page 300... ...
While operability and safety issues are typically recognized and resolved, the practical advancement of sediment control technology must also address the economic criteria against which new proposals for facility design will be evaluated. For a ship, the key economic factor is allowable vessel draft, which determines how much cargo can be carried.
|
From page 302... ...
, LEVEL UNCERTAINTY SEDIMENT DEPOSIT BETWEEN DREDGINGS (ACCEPTED) -~-~ DREDGING TOLERANCE FIGURE 2 Factors used to determine net underkeel clearance: deterministic approach.
|
From page 303... ...
for tankers operating in virtually any hard-bottomed port area, including SELECTED __ ~ TIDAL LEVEL EXPECTED LEVEL _ DURING POTAGE EXPECTED BED LEVEL DURING TRANSIT MEAN / CHANNEL DREDGED J l LEVEL - SELECTED TIDAL LEVEL TIDAL CHANGE DURING TRANSIT ~ MANEUVERING WATER LEVEL UNCERTAINTY a2 STATIC DRAFT ~ STATIC DRAFT (IN SEA WATER)
|
From page 304... ...
. Figure 4 is an example input data sheet for a calculation of required underkeel clearance for Exxon' s Fawley Refinery terminal, near Southampton in the south of England.
|
From page 305... ...
Cove Perio~l/Rol1 Period~ ~ . U;nrc Poriod/~$ech PeriodJet ~ FIGURE 4 Underkeel clearance input data sheet.
|
From page 317... ...
Adequate channel depth is the first requirement of safe navigation in a waterway. Channel depths substantially greater than the loaded static drafts of the vessels using the waterway are required in order to ensure safe and economical navigation.
|
From page 318... ...
When the bottom of the channel is hard this clearance should be increased to 3 ft. Since the static draft of the vessel may be different in entrance channels and interior channels due to the salinity of the water, and wave action may cause the vessel to roll and pitch more in entrance channels, a greater depth may be required for the entrance channel than the interior channel for the same design vessel.
|
From page 320... ...
Channel widths in entrances will have to be judiciously selected based on an analysis and evaluation of conditions at each proj ect . In order to minimize initial and maintenance dredging, the alignment of the navigation channel should follow the course of the deeper channel in a river or estuary as much as is practical.
|
From page 321... ...
located at Kings Point, New York. Information pertaining to the facility can be found in various publications and papers, including proceedings of periodical symposia held at the CAORF facility and in minutes of deep-draft navigation channel design courses held at the Waterways Experiment Station (U.S.
|
From page 322... ...
Jetties located at the entrance to a bay or river also serve to protect the entrance channel from wave action and cross currents. In most cases two jetties are needed to keep littoral drift from entering the channel.
|
From page 324... ...
In recent years, the use of relatively small quantities of sediment tracer material in fixed-bed models has generally been accepted as a reliable and relatively inexpensive method of studying sediment transport due to wave and tidal action. Results of these tests are usually qualitative rather than quantitative.
|
From page 327... ...
M;_~ Waterways Experiment Station.
|
From page 328... ...
Jenkins Center for Coastal Studies Scripps Institution of Oceanography Proper siting of a harbor is by far the most important consideration in determining its overall utility, cost of construction, and annual maintenance costs. Experience shows that annual maintenance cost for sediment removal is usually underestimated and frequently, with time, becomes the highest harbor cost.
|
From page 329... ...
. Examples are San Prancisco, Mare Island, Kings Bay and most harbors on the east and gulf coasts of the United States.
|
From page 330... ...
and eventual deposition of muds occurs when silt-laden fresh water from a river contacts salt water. The river water is usually lighter and rides of ~ ~ the salt water, their boundary forming a missing shear- layer where the dispersed suspended load of the river combines with salt water to form sediment aggregates termed 'iflocs." This floe-generating frontogenesis zone may extend a number of miles seaward of the river mouth where the freshwater plume spreads over salt water (Gross et al., 1965; Garvine, 1975; Trefry et al., 1985~.
|
From page 331... ...
. COMPOSITION AND PHYSICAL PROPERTIES OF MUDS The composition of the muds from the Sacramento-San Joaquin River systems are shown in the x-ray spectogram of mud from Mare Island Harbor in San Francisco Bay (Figure 2)
|
From page 335... ...
The lag occurs because high discharges displace the saline wedge seaward and cause the load to be deposited farther downstream. High spring tides are important in re-establishing saline wedges that then transport floes upstream.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.