454 



ords of these weirs have been used for years in determining large 

 payments of money for water dehvered. 



There are two general types of weirs — sharp-crested and broad- 

 crested. 



The sharp-crested weirs are separated into three classes, the 

 most popular of which is the rectangular sharp-crested wpir with 

 end contractions. The formula (Francis') is simple and well 

 known, and sufficient experimental work has been done on this 

 type to develop satisfactory coefficients for velocities of approach, 

 high heads over the weir crests, etc. If the crest of the weir 

 is sharp, straight and clean and sufficiently high above the bot- 

 tom of the approach channel and the end contractions complete — 

 and if the head on the crest is measured well back from the curve 

 over the crest — the Francis formula will give excellent results. 



The rectangular sharp-crested weir without end contractions, 

 called a suppressed weir, also gives excellent results. 



The third class of sharp-crested weir is called the Cippoletti, or 

 trapezoidal, weir after its inventor. This weir is a sharp-crested 

 weir with end contractions which are not vertical, but slope back 

 from the end of the crest on a batter of about one-fourth hori- 

 zontally to one vertically. This slope has the effect of simplifying 

 the formula by doing away with the correction for end contrac- 

 tions. (See pages 165-166, Merriman's Hydraulics.) 



Broad-crested Weirs. 



Broad-crested weirs are rarely constructed solely for measure- 

 ment purposes. It has been found that this type of weir when 

 already existing as a diversion structure or for some other pur- 

 pose may be utilized to measure water. A series of measure- 

 ments by some other method is usually necessary to determine 

 the formula to be used. The width and shape of the crest deter- 

 mine the coefficient to be used. Sufficient experiments have been 

 made to determine within fairly accurate limits, formulas for 

 various types and shapes of crests, but usually a careful investi- 

 gation by an engineer is needed to determine the formula used in 

 any specific case. 



Venturi Flumes. 



The use of the basic principles of the Venturi meter for open 

 channel discharge measurements has recently been tested by the 

 U. S. Reclamation Service and the Department of Agriculture, 

 and experiments thus far completed seem to indicate that the 

 "Venturi flume," as the device has been named, will prove suc- 

 cessful. As a substitute for ditch weirs where lack of fall, sedi- 

 ment carrying water, etc., make weirs impracticable, this type of 

 measuring device promises excellent results. It is described in a 

 reprint from the Journal of Agriculture Research by Mr. V. M. 

 Cone, under date of March 13, 1916. 



SUMMARY. 



The various devices are generally best used under the follow- 

 ing conditions : 



