ti> THE IMPROVEMENT OF RIVERS. 



named being also used on small rivers of quick rises and where the low- water discharge 

 is small. In this case the weir is usually closed with Chanoine wickets, although 

 needles are also used. For large rivers subject to high floods the Chanoine wicket 

 has been almost always adopted, and on the whole seems best adapted to such 

 streams. Bear-traps and drum-shutters have not been used except as weirs to regulate 

 the pools. 



The calculations for the foundation of a movable dam are based on the same general 

 principles as given for lock- walls, combined with the strains from the movable* parts. 

 Thus let A BCD (Fig. 130) represent a section of a foundation for a needle dam. 

 The forces acting per foot run are: 



P pressure of water on CD, CD in this instance being supposed vertical. 



V vertical water-pressure on the apron above the sill. 



N pressure from the needles against the sill. 



T thrust from down-stream leg of trestle. 



U = upward pull from up-stream leg of trestle. 



W ** weight of masonry. 



As mentioned in the chapter on locks, W must be calculated with loss of weight 

 by immersion or otherwise, according to the material of the natural foundation. The 

 pressure V will be a function of the difference between pools, if the lower pool is above 

 BF. In this case the lower pool will also cause a downward pressure on BF, which 

 may be combined with V. 



Resolve T, N, and U into vertical and horizontal components, assuming N to act 

 upon the same axis as T and U. We thus obtain a horizontal force Q, acting along 

 FB, a downward force R, acting at E, and an upward force S (the algebraic sum of 

 the vertical components of N and U), acting at F. 



Combining Q and P we obtain as their resultant the force L, and combining R, V, 

 and W, we obtain their resultant M. Lastly, combining 5 and M we' obtain the 

 force X. 



We thus have finally to deal with the two forces L and X, whose resultant must 

 lie within a certain zone of the base as indicated in the calculations for lock-walls. 



Similar methods can be applied for calculating the foundations for other types of 

 movable dams. 



It should be noted that the water-pressure P acts a little below the middle of CD, 

 as a graphic representation of its elements would be a quadrilateral. 



Elevation of Sills. The depth of the pass sill below water is decided by the mean 

 position of the river-bed and the navigable depth desired. Its top may be placed on 

 the line which will connect the crests of the nearest bars above and below, but must not 

 be higher, otherwise it will itself become an obstruction at the stage occurring just pre- 

 viously to that at which the dam is to be raised, when the lock is out of service, a 

 condition quite common during the winter and spring, when it is not always advisable 

 to raise the dam on account of ice or the approach of floods. 





