THE HYDROGRAPHICAL SURVEY 35 



In determining the discharge of a stream by means 

 of a weir great importance should be attached to the matter 

 of keeping the weir crest free from floating objects, which 

 by backing up the stream may invalidate the readings 

 even after the brush is removed. Weir frames are often 

 washed out by freshets. To prevent this the frame should 

 be firmly set in a trench excavated for the purpose with 

 riprap on the down-stream side to prevent scour. Should 

 the weir be found leaky or should leaks be apprehended, a 

 little clay placed on the bed of the stream against the 

 upper face will be found very satisfactory. 



Full hydrographical information is required by the land- 

 scape architect hi his preparation of plans for lakes and 

 water courses, and by the engineer in connection with 

 excavation and dredging operations and the design of 

 outlet sewers. 



