Notes on irrigation, roads and buildings and on the water supply of towns

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1920 Excerpt: ... the foundations will be high, and the river will have established its own conditions of flow, and these the design of the work should maintain. The river banks upstream and downstream should be well defined and of a permanent character, so that the course of the river will not be likely to change. A favourable situation for the weir is below the confluence of a main tributary with the river, so that there may be backwater storage up the troughs and margins of each. A tributary flowing generally parallel and near to the river and meeting it below the weir is particularly dangerous, as it may lead to the main stream breaking into it and thus outflanking the work (Art. 182 (d) ). Another favourable site is one remote from the confluence of the first main tributary downstream with the river, so that the canal crossing of the former will be well above that of the level of the latter, and thus will not be interfered with by its floods. The finer the silt carried by the river, the slower will be the rate of its deposit in the backwater, as most of it will be carried over the weir by the current. Gravel and large sand are likely to be dropped in the backwater, and may thus very gently diminish its storage contents and interfere with the communication between the canal and the river. (d) Alignment of the Weir.–-When the bed is sandy, the alignment of the weir should be at right angles to the course of the river, so that it may discharge floods uniformly, and thus not create cross-currents parallel to the weir either upstream or downstream. Such a weir is, moreover, the most economical and practicable, as its rate of flood discharge per foot run will be at a maximum. When the bed is of hard, unerodible material, and especially if it is of rock, some consider an obl...