235
D.
DAM, PERMANENT. —See ‘ River Navigation.’
Dam, — a bank or obstruction built across a river or stream, for the purpose ofraising the level of the water on the upper side of it.
There are many objects for the attainment of which it may be necessary thus tocheck the course of a stream, and gain a head of water: it may be requisite to turnit for a time into another channel; to inundate the ground in front of part of amilitary position; to make a portion of the stream unfordable; to secure depthsufficient to enable vessels of a given draught of water to navigate the stream; or togain a power to be applied to mechanical purposes. The works for the attainmentof the first three of these objects belong more particularly to the class which Officersmay be called upon to execute in the course of a campaign; and the details heregiven refer entirely to works composed of such materials as may be expected to bewithin reach of an Officer on service.
Dams built for the purpose of inland navigation, or for that of securing a water-power, may be considered as having a more permanent character, and will be treatedof in the article on ‘ River Navigation.’
Choice of Site. The first consideration in forming a dam across a stream is the choice of a proper
site: this must of course be decided with reference to the objects to be attainedby the rise of water, but there are a few general rules to which attention shouldbe paid.
In streams liable to sudden floods it would be advisable to carry the dam acrossthe widest part of the stream, so as to allow ample space for the water to flow oyer,and thus to prevent any sudden and great rise above the dam; or it may even beadvisable to carry the dam in an oblique line across the stream.
In rivers where much drift timber is likely to be brought down, the dam should besituated below a bend in the stream where an eddy is formed, by means of which thecollection and removal of the timber will be facilitated.
The banks of the river or stream should be carefully examined with reference tothe quality of the soil of which they are composed, and their power of acting asabutments to the dam.
When the site of the dam has been decided upon with reference to the principalobjects which it is intended to answer, the necessary levels must be taken, and theheight of the structure determined: upon this will depend in a great measure (whenmaterials are plentiful) the plan to be adopted in forming the dam.
In shallow rivers, when the bottom is rock, a dam of the section shewn in fig. 1may be easily constructed, of 10 or 12 feet in height. The sill (a b) is bolted downto the rock with fox-wedge bolts. The standard (b c) is mortised into this sill, anda brace (a c) is framed into the two, making thus a strong vertical frame. When thedam is high, a second brace may be inserted, and the horizontal distances betweenthe frames diminished; but in general, 8 or 10 feet may be allowed as a fair distancebetween these frames from centre to centre. When the frames are securely fixed, afacing of logs, roughly squared on the upper and under sides, is laid in front ofthem, across the bed of the stream. These should be got as long as possible, and
* Chiefly by Capt. Sir Win. Denison, R. E. ? embodying some fragments by Capt. Bainbrigge, R.E.
Construction
™<*y sites.Plate I.
VOL. I.
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