160 Bulletin 86 



be only one-half as great as for cement pipe lines of equal diameter. 

 For short pipes, such as culverts, the differences are not so great. 

 Fig. 36 is presented to show the relative capacities of 25-foot cul- 

 verts of the two kinds and of three sizes. The capacities as given 

 in Fig. 36 are computed according to the principles of hydraulics. 

 Actual experiment might vary them to a slight degree. Many 

 scores of small corrugated iron culverts in Arizona have become 

 clogged and filled with earth. 



The uncovering and washing out of corrugated metal culverts 

 has been very frequent in the past. Doubtless these misfortimes 

 have been due, in part, to the fact that the county officials who pur- 

 chased the culverts have not appreciated the low carrying capacity 

 of the corrugated culverts, and have purchased culverts too small 

 for the locations where they have been installed. 



4. Permanence: Both classes of pipe may be expected to have 

 long life. Good cement construction grows harder with age, and 

 the ingot-iron, also, has been proven to resist oxidation much more 

 than ordinary steel. 



Metal culverts may be subjected to two destructive influences: 

 the erosive action of water carrying sharp sand, and chemical ac- 

 tion. Galvanized metal flumes have been used extensively in the 

 U. S. Reclamation Service projects during the past ten years, and 

 experience indicates that unprotected galvanized flumes will have a 

 life of 10 or 12 years, except under the most trying conditions, i. e., 

 high velocity of water carrying sand and fine gravel, where the life 

 in one particular instance was only four seasons use*. Tests were 

 made on a flume of the Uncompahgre project using various pro- 

 tective coatings such as paints, elastic graphite, and tar compounds. 

 The conclusion reached after the coatings had been on one season 

 was that coal tar is the best and cheapest mixture available. The 

 erosive action of sand or grit, carried at velocities over 3 feet per 

 second, is quite pronounced, but road culverts in Arizona, as a rule, 

 do not carry water save for a few hours each year. In other states 

 both kinds of culverts have been destroyed by alkali, but no cases 

 of injury of this sort in Arizona are known. 



5. Cost: In Arizona the cost of ingot-iron culverts up to 30 inches 

 in size is just about double the cost of cement pipe culverts of the 

 same nominal diameter. On the basis of carrying capacities, the 

 ratio is about two and a half to one. The cement pipe culverts are 

 admittedly much more economical. 



•Reclamation Service Record, Vol. 7, No. 11, 1916, p. 519. 



