Effect of channel‐contraction works upon regimen of movable bed‐streams

Abstract

When water flows in an open channel having an erosible bed a condition of equilibrium is approached in which the water ceases to cause further erosion of its bed, although sedimentary material is continuously transported by the stream. Various control‐measures required to improve the conditions of a river‐system for use in navigation, irrigation, and power‐development set up a new regimen or condition of equilibrium of the system. Thus, for example, channel‐contraction works (structures used for narrowing the river so as to obtain a greater navigable depth) set up new flow characteristics causing changes in the erosive force of the stream and therefore changes in the slope of the channel. Likewise in irrigation‐systems, if the various branch‐channels are not properly designed with reference to slope, relation of breadth to depth, and water‐discharge, serious difficulties will arise in consequence of silting and erosion, because the net of irrigation‐canals seeks a condition of equilibrium defined solely by natural laws. Any attempt to prevent such a change to take place will ordinarily prove fruitless and extremely expensive because of the magnitude and persistence of the forces and energy involved.

Scour and fill in sand-bed streams

“…These amounts of moving sand are so small that they can usually be disregarded in studies of scour and fill. SOME PREVIOUS STATEMENTS OR USES OF THE PRINCIPLES Straub (1934) computed scour for constant flow at a laterally contracted cross section from his form of the Du Boys equation and from the principle of continuity of the bedload discharge. He concluded that observed amounts of scour at narrower-than-average cross sections of a stream probably were not representative of long reaches of a river.…”

Pier and Contraction Scour in Cohesive Soils

“…These equations involve one soil parameter: the mean grain size. The studies on cohesionless soils include those of Straub (1934), Laursen (1960Laursen ( , 1963, Komura (1966), Gill (1981), Lim (1998), Chang (1998), andSmith (1967).…”

The Impact of River Contractions on the Bed Morphology under Unsteady Flows

“…In a very early paper, Straub [1] highlights how river contractions frequently occur when structures are built in restricting the river to achieve greater navigable flow depths. Lim [2] emphasizes how examples of river contractions are the gradual closing of a river prior to the construction of a dam or the partial constriction of a river either to build a spur dike into the flowing water or a cofferdam to enclose the work area for a structure from one side of the riverbank.…”