A Treatise On The Principles And Practice Of Dock Engineering

2^8 DOCK ENGINEERING. and branching into a series of outlets, discharging as low as possible, at convenient intervals along the channel frontage. During the lowest period of ebb-tide, water from the dock is allowed to run off through these culverts and the velocity, which it possesses in consequence of the head of water within the dock, enables it to stir up and remove the mud in front of the outlets. The quantity of water run off is controlled by a penstock, or paddle, near the entrance of the culvert, and, in addition to this, other pe stocks are often provided, one at each outlet in order to regulate the numher of exits, for it may often be desirable to concentrate the whole discharge at a few points in order to obtain the maximum effect. Where this system is adopted, it is very essential to provide a masonry or concrete apron in front of the wall, otherwise there will be a decided risk of the wall becoming undermined. For the same reason the discharge should be perfectly horizontal, as any downward inclination causes the water to act the part of an excavator. The ground in such cases is ploughed up, and the excavated material is deposited a short distance away as soon as the current slackens, in such a manner as to form a ridge, which, being out of range of the sluice, is very dangerous, and can only be removed by dredging. This tendency to excavate below the toe of a wall is one of the draw- backs of a mural sluice; another is that its effective action is restricted to a very small area immediately in front of the opening, so that it lowers the sides of the channel at the expense of the middle of the bed. A third objection lies in the fact that the formation of numerous outlets at the base of the wall weakens the wall at the locus of greatest intensity of pressure. A fourth objection is the very serious loss of head due to friction and bends, whereby the force of the discharge is materially diminished. Accordingly, it is not surprising that the alternative method of sluicing through apertures in the dock gates has been adopted in many cases. There is an absence of skin friction, there are no bends, and the only loss of head is that due to discharge through a thin orifice, which is much less than the loss due to friction in a long conduit. Furthermore, by this means a large body of water is discharged along the axis of the channel, the bed of which is thus kept clear without endangering the stability of the wing walls. On the other hand, the provision of sluice valves and gear adds considerably to the weight of the gates and entails greater strength in their structure. Velocity of Efflux from Sluices.—The velocity of efflux, from which the scouring effect of a sluicing current can be gauged, is calculated from formulæ based upon the following principles : — The theoretical velocity of a liquid issuing from an outlet under a given head or charge, considered without reference to friction, is the same as that acquired by a solid particle in falling freely from a height equal to the head—i.e., 2 9