Section III., 1904 [ 127 ] Tkans. R. S. C 



XII. — On the Backwater Produced by Weirs of Different Widths 

 By Henry T. Bovey, LL.D., RR.S. 



(Read June 22, 1904.) 



The jDresent paper proposes to give an account of certain experi- 

 ments made to determine the effect of obstructing a water-way by wing 

 dams of varying widths. In the carrying out of these experiments a 

 flume, 61 ins. in width, was used. The flume may be considered as a 

 longitudinal slice of a water-way and the dimensions are given in units, 

 which may be of any magnitude. Previous experiments, with models 

 designed for hydraulic investigations on rivers and estuaries, carried out 

 by many eminent hydraulic authorities, including Douglas, Unwin, 

 Osborne Eeynolds, Leader Williams, Deacon, Shelford, etc., indicate 

 that important and reliable results may be obtained by the use of such 

 models. 



In the first and second series of experiments, two weirs, of the full 

 width of the flume, namely, 61 units, were constructed, the one at the 

 point of discharge and a second higher weir at 25 units from this point, 

 the bottom of the flume being level. The portion of the flume between 

 the two weirs was therefore of the character of a pond and any increase 

 of depth at the discharge was necessarily accompanied by a correspond- 

 ing increase of depth at the upper weir. The results of these exper- 

 iments are shown on diagram I, from which the exact amount of back 

 water due to the introduction of wing dams of 2, 4, 7, 7.6, 8, 10, 12, 

 18 to 24 units wide, can be at once measured. 



For the third series of experiments, a false bottom, of approximately 

 the same form as that of the Eiver Ottawa between the Chaudière Falls 

 and the Little Chaudière Falls, was constructed of grooved and tongued 

 timbers, laid transversely, the clear width of the water-way being 

 53 units. Diagram II shows the results of this series of experiments. 

 The increments of depth, due to the introduction of wing dams 6, 7.6, 

 18, 18 and 24 units in width, -were detemiined at three points, namely, 

 at the point of discharge and at distances of 4.2 and 15.2 units from 

 this discharge. These increments are plotted on the diagram. Some 

 difficulty was experienced in making accurate readings, owing to the 

 •rippling of the water caused by the irregularities of the bottom of the 

 flume, but there were very definite indications that, for every given 

 depth at the point of discharge, there was a limiting position in the 

 bed of the water-way where back water would be produced by the in- 

 troduction, at the point of discharge, of any obstruction, however small. 



