1、Chapter 7CHANNEL ALIGNMENT7.1 GENERALTo minimize initial and maintenance dredging, the alignment of a nav- igation channel is usually designed to follow the course of the deeper channel in a river or estuary. Note, however, that there often are separate ood and ebb channels that may not be continuou

2、s. Although bends in the alignment should be minimized as much as possible, straight reaches with small curves may be needed to follow the natural deep water chan- nels in the estuary. Straight reaches between bends should be at least ve times the length of the longest vessel. Where gradual bends ar

3、e not prac- tical, cutoffs should be considered. Structures such as groins, revetments, and wave absorbers might be required to maintain acceptable channel alignment, channel dimensions, and wave conditions. Channel alignment that cuts across sandbars or mud bars shou1d be avoided unless train- ing

4、structures to control the movement and deposition of sediment are provided.7.2 VARIABLE ALIGNMENTThe strength of the hydraulic forces in the estuary will usually dic- tate channel alignment. Estuaries with high tide ranges and, therefore, strong ebb currents or large river discharges, normally will

5、have navi- gation channels with changing alignments. An example is Grays Harbor, Washington, which has high tides and a large tide prism, as shown in Figure 7-1.These systems may have channels that migrate, such as Grays Harbor, where the channel has migrated to a position adjacent to the South Jett

6、y (Figure 7-2). Normally, the ship channel alignment is adjusted over time to follow the natural channel migration.5758SHIP CHANNEL DESIGN AND OPERATIONNotes:Mean tide range 9.00 feet Maximum tide range 17.5 feet ship channel 26 miles longFIGURE 7-1. Variable Alignment Ship Channels (Adapted from AS

7、CEManual No 80).7.3 STRAIGHT ALIGNMENTStraight ship channels are possible when the hydraulic dynamics of the system are small. Projects in the Gulf of Mexico or South Atlantic coast have small tide ranges and, therefore, small tide prisms. The Gulf Coast not only has low tide ranges (1 to 3 ft) but

8、also has a tide cy- cle that is diurnal (i.e., one high and one low tide in a lunar day 24.84 hours). The East Coast has semidiurnal tides, which produce two nearly equal tide cycles in a lunar day. West Coat tides are mixed with two un- equal tide cycles a day. Therefore, channel tidal velocities a

9、re determined by the size of the tide prism and the number of tide cycles a day. Many of the Great Lakes ship channels have straight alignment because of the absence of tidal effects and because river discharges often are small. Fig- ure 7-3 shows an example of a straight ship-channel project in Mob

10、ile, Alabama.CHANNEL ALIGNMENT59FIGURE 7-2. Grays Harbor Channel Migration (Committee on Tidal Hydraulics, Corps of Engineers, 1995).7.4 PIANC METHODThe concept design guidance given in PIANC (1997) page 14follows.“Channel alignment should be assessed with regard to:the shortest channel length;condi

11、tions/basins, and so on, at either end of the channel;the need to avoid obstacles or areas of accretion, which are dif- cult or expensive to remove or require excessive (and, hence, costly) maintenance dredging;prevailing winds, currents, and waves; avoiding bends close to portentrances;the edge of

12、the channel should be such that ships passing along it do not cause disturbance or damage.60SHIP CHANNEL DESIGN AND OPERATIONFIGURE 7-3. Straight Ship Channel (From Lieutenant (jg) B. McCartney).Straight channel legs are preferable to curved ones. The designer should strive for an alignment consisti

13、ng of a series of straight legs connected by smooth bends, and not abrupt angles. Individual legs may have different widths and depths and should be navigated at different speeds.It is preferable to have the prevailing currents aligned with the channel to minimize cross currents and to avoid large g

14、radients inCHANNEL ALIGNMENTcross current along the channel. This also applies to wind and waves, although these may come from any direction. Usually, the prevailing wind and wave direction are considered in design, with a judgment on whether possible downtimes as a result of strong winds or high wa

15、ves from other directions are acceptable.Finally, it is advisable (and important in the case of channels navi- gated by ships carrying dangerous goods) that the channel be aligned to prevent the ship heading directly at the quay or jetty during its ap- proach. Any channel whose direction is perpendicular to the berthing face should be aligned to one side of the quay or jetty, so that a ship must turn (or be swung) to