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

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.