1 Solid Weirs

1. SOLID WEIRS The simplest form of weir on an alluvial or erodible foundation is a dam of earthwork or rubble-stone faced with stone pitching, or masonry, or fascine mattresses weighted with stone, or with timber cribs filled with rubble. Many solid weirs were formerly constructed in North America of log-timber or stone-filled cribs. Large modern weirs on alluvial soil are now frequently built of concrete on piled foundations. When solid weirs are founded on rock, concrete or masonry construction is usually employed. It is frequently desirable to maintain a pool of water, by building a small dam below the weir against the down stream face of the weir to act as a water-cushion for the overflow.

Weirs founded on alluvial deposits are specially liable to under mining, and the consequent washing out of the foundation, and to the scouring action of the water down stream. Undermining is prevented by impeding the percolation of the water under the weir by means of a cut-off, either in the form of a deep curtain wall of masonry or concrete or of steel interlocked sheet-piling. Scour below the weir can be prevented by constructing a suitable apron of rubble-stone or cribwork of sufficient width. Frequently

cut-off walls are formed both under the crest of the weir and under the apron. In many Indian rivers weirs have been built with cut-off walls formed of brick or concrete wells. The weirs across the Damietta and Rosetta branches of the Nile below the Grand Barrage at the head of the delta (1899-1901) have cut off walls of cement-grouted rubble in a dredged trench.

Solid weirs, if properly constructed, possess the advantage of simplicity, strength and durability, and require no attendance.

They, however, block up the river channel to the extent of their height, and consequently raise the flood-level above them. This defect of solid weirs, when the riparian lands are liable to be injured by inundations, can be slightly mitigated by keeping down the crest of the weir a little below the required level, and then raising the water level at the low stage of the river by placing planks, called flash-boards, a few feet high along the top of the weir. The capacity of a solid weir is increased by building it obliquely across the river, e.g., some of the Severn weirs; or curved in plan, with the convex face up-stream.