ENGINEERING CONSTRUCTION AND RAINFALL. XLIX. 



culvert will not discharge the required amount, the consequence 

 of which may be that the water will rise on the bank and cause 

 a washout, endangering life and property. 



In the case of M, however, if the assumed velocity be too low 

 and it rises above 6 feet per second his 10 square feet of section 

 only proves more than sufficient for the requirements, moreover 

 the smaller culvert is more liable to become obstructed and 

 rendered useless than the larger one. 



Engineer N may, however, say if for any reason the velocity 

 falls below 18 feet per second, the result will be that the water 

 will rise over the mouth of the culvert, until the head so acquired 

 furnishes the required velocity. But by what means can the 

 increase in velocity be calculated 1 Such an outlet cannot 

 be considered as an orifice, an adjutage, or a pipe under 

 pressure. No experiments, within the writer's knowledge, have 

 ever been made to determine velocity of efflux in such a case, the 

 reason being, no doubt, that such investigation not being con- 

 formable would be unnecessary to good practice. The permitting 

 of the flood waters to rise to formation level has been advocated 

 by some, but what guarantee is there that they will not rise 

 higher and cause a washout ? 



The attaining of a velocity greater than 4J miles per hour, 

 through a culvert of this class, by artificial means, such as by 

 surcharge at the outlet is undesirable. If a greater velocity can 

 be attained by a natural inclination of the bed of the culvert, by 

 all means let it be availed of, taking care, however, of two 

 things — first, that the outlet is left naturally clear, so that the 

 effluent water can get away from the culvert at least as quickly 

 as it passes through it ; and secondly, that the cross-section ot 

 the opening be not made so small as to render it liable to be 

 choked up, as the latter would inevitably lead to surcharge, and 

 possible wash away of the bank. 



Now, as regards this velocity of 18 feet per second, supposing 

 such could be attained by providing sufficient fall in the culvert 

 with a free get-away at the lower end, as a general rule, in such 

 structures, is it needed? The negative answer may be given, 



