STORAGE AND REGULATION OP WATER. XLV. 



per cent. 



of the 



rainfall. 



26 occasions the flow was less than 10' 



44 ,, ,, ,, between 10 and 20 



25 „ „ „ „ 20 and 30 



19 ,, ,, ,, above 30, 



Storage Dams. 



Storage clams are of two kinds, still water dams which 

 are designed to hold water up to a fixed level, and overshot 

 dams, which are designed to permit of flood waters passing 

 over their crests. In the former, the exact and definite 

 strains, at least from outside sources — for there are un- 

 known strains in the dam itself — can be calculated and 

 located, but in the latter, by reason of the varying strains 

 set up by the falling water, the profile cannot be drawn 

 according to any fixed rules. The truth of this is well 

 exemplified by the case of the Gin Gin Weir on the Mac- 

 quarie River above Warren, which, constructed with the 

 profile of a still water dam, failed as an overshot dam by 

 breaking across, at a depth of about 20 feet from its crest. 



Now, it is not the dams which stand, but those which 

 fail, that teach engineers the lessons they are most anxious 

 to learn, and happily failures have been rare, when however 

 they do occur, an inquiry into the cause of their downfall 

 would furnish valuable information. The failure in this 

 case may have been from an inherent weakness in the con- 

 crete, or from the design of the profile, or from the lifting 

 power of the water which might have penetrated its face. 

 I do not know, but any information on this important sub- 

 ject will I am sure be appreciated by our engineers. 



In the ordinary calculations connected with the design 

 of masonry dams, we assume in the first place that the 

 foundations shall be solid and homogeneous rock, and in 

 the second place that the dam when completed shall be an 

 absolutely rigid structure, we then proceed to carry . out 

 the design in connection with the following conditions : — 



