Channel Approaches

The upstream face of the weir plate should be smooth, and the plate should be strictly vertical. The crest should have a sharp, square upstream edge, a top width of He to % in, and a bevel on the downstream side, so that the nappe springs clear, making a line contact for all but the very lowest heads. If it does not spring clear, the flow cannot be considered as true weir flow, and the coefficients do not apply. If the crest extends downstream so that it supports the nappe, the weir is broad-crested. The velocity at any point in the nappe can be determined. The approach channel should be long enough so that normal velocity distribution exists, and the surface should be free of waves. It may sometimes be necessary to install baffles to ensure a quiet flow of approach. All deviations from proper weir construction affect the flow in the same way. That is, they increase it over the computed value. 

Weir with end contractions. When the length B of the crest of a rectangular weir is less than the width of the channel, there will be a lateral contraction of the nappe so that its width is less than B. It is believed that end contractions are a source of error, and so this type of weir is not considered so accurate as the preceding. Its chief virtue is that the approach channel need not be of uniform cross section or have smooth sides. 

This equation was obtained by fitting a curve to the plotted values of Cd for a great many experiments and is purely empirical. Capillarity is accounted for by the second term, while velocity of approach (assumed to be uniform) is responsible for the last term. Rehbock s formula has been found to be accurate within 0.5% for values of Z of 0.33 to 3.3 ft and for values of H of 0.08 to 2.0 ft with the ratio H/Z not greater than 1.0. It is even valid for greater ratios than 1.0 if the bottom of the discharge channel is lower than that of the approach channel, so that backwater does not affect the head. 

The approach channel, which streamlines the melt into the final die opening. 

Fig. 12.1 Schematic representation of a sheet die, including manifold, approach channel, and die lip regions. The restrictor bar is incorporated so that the die can be used with different polymers of varying viscosities.

The die design equation proposed by Pearson (60) utilizes a constant die lip opening, but an approach-channel-taper that varies with the die width. Thus, in the region between the manifold and the die lip opening both the pressure and flow fields are two-dimensional. This may affect the flow in the die lip region, since the fluid is viscoelastic with memory of this recent upstream flow experience. 

Turning to the cross-fed tubular dies, we note that, to develop die design expressions, we must model the two-dimensional flow in the z- and 0-directions. This is a task of considerable difficulty. Pearson (73) was the first to model the flow for narrow dies. The flow region was flattened, and the two-dimensional flow in rectangular coordinates between two plates was considered. The plate separation was allowed to vary in the approach channel so that the resulting output is constant. The final die lip opening is constant, formed by the concentric cylinders. 

It is clear from this analysis that the most favored approach channel for electrophilic agents is directed towards the carbonyl oxygen. The channel ending on the NH2 group is decidedly less favoured. As far as the specific reaction of protonation is concerned, this prediction is in complete accord with ab initio" SCF calculations17). After a long discussion on the interpretation of relevant experimental data, the experimenters seem to have reached the conclusion at O-protonation occurs first18). 

In pyrrole (XV), a wide approach channel for electrophilic reagents leads to positions 3 and 4 (see Figs. 33 and 34). This finding is in accordance with the experimental evidence24) that protonation in 3,4 is faster than in 2,5, though the 2-protonated salts are more stable. As has been repeated many times, the electrostatic approximation can give at most a picture of the first part of the reaction and it is not able to predict the energetically most stable final product. 

The shape of the electrostatic potentials was employed in Section IV to reveal the reactivity capabilities (position and depths of the approach channels for electrophilic reagents, values of the W(r) minima, etc.)... 

PIANC, Approach Channels A Guide for Design, Final Report of the Joint PIANC-lAPH Working Group 11-30 in cooperation with IMPA and lALA, Supplement to... 

Puertos Del Estado, Recommendations for Maritime Works (Spain) ROM 3.1-99 Designing Maritime Configuration of Ports, Approach Channels and Floatation Areas, CEDEX, Spain (1999). 

The International Navigation Association (PIANC ) addresses the issue of safety for navigation project design in their June, 1997 publication, "Approach Channels, A Guide for Design" (PIANC 1997). It provides the following in Section 2.3 ... 

A thorough analysis of ship accidents shows that only a small percentage of accidents and marine casualties in approach channels and ports are due to channel design, but it is essential, with future commercial, economic and environmental pressures placed on port operators, that this percentage remains low. 

Before considering the various stages in the design process, it is necessary to define some basic terms. Most important of these is the "approach channel."... 

An approach channel is defined as any stretch of waterway linking to berths of a port and the open sea. There are two main types ... 

All sizes of approach channel are considered in this report the problems of catering for small coasters in a small port may be as great as those for a large tanker at an oil terminal. 

In this report approach channel design is considered to be a two-stage process consisting of ... 

FIGURE 6-1. PIANC Approach Channel Design Procedure. Courtesy of International Navigation Association. 

International Navigation Association (PIANC). (1997). Joint PIANC-IAPH Report on Approach Channels—A Guide for Design (Volume 2), Marcom Working Group 30, International Navigation Association, Brussels. 

One of the most important conditions, having a significant impact on the feasibility of a land reclamation project, is the availabihty of sufficient suitable fill material within a reasonable haulage distance to the project location. In most cases special borrow areas are allocated for the supply of fill material, but fill may also have to be retrieved from capital or maintenance dredging. For instance in harbour projects, deepening of basins or approach channels are often combined with reclamation projects. 

The ground investigation indicates that the material dredged from the approach channel and berth pocket used for the core will be predominantly a uniform, fine grained sand. 

PIANC 2014. Harbour Approach Channels Design Guidelines. PIANC Report PIANC Secretariat General. Bruxelles. 

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