BACKWATER CURVES PAST ISLANDS 



109 



Backwater curves past islands. It is often necessary to compute a 

 backwater curve for a stream which is divided into two channels by an 

 island. The division of flow between the two channels is unknown. 

 To solve this problem, first estimate the division of flow. Then com- 

 pute backwater curves past each side of the island to the point where the 

 streams unite again, one set of curves with a smaller proportion of the 

 flow going on one side of the island than was estimated, and the other 

 set with a greater proportion. The elevations of the water surface at 



Given Total 

 '/Discharge 



Discharge 

 Fig. 906. 



the junction point are plotted against discharge, as shown on Fig. 906, 

 and lines (A and B) drawn representing the relation for the two sides. 

 A third line C is drawn adding the abscissas of the lines A and B, thus 

 giving the relation between total discharge and elevation .at the junction 

 point. The ordinate on this line opposite the point whose abscissa is 

 the total discharge of the stream is the required elevation of the junction 

 of the two channels. If extrapolation is necessary, or if extreme pre- 

 cision is desired, it may be advisable to compute a third set of backwater 

 curves, using the proportions indicated. 



This method of computing the flow and the backwater curves past 

 islands is applicable only when there is tranquil flow throughout. If 

 the velocities at the point where the flow divides are above the critical, 

 the proportion of flow going to each side of the island will be dependent 

 upon flow conditions upstream from the point of division. 



