94 DIVISION OF WATER RESOURCES 



In 1932, flow recorders operated with 30-inch floats were installed 

 at the two controls operated in 1931 and bedrock exposures were 

 selected for the locations of the upper and branch controls. Low con- 

 crete dams were built on these sites and the flow passed through flumes 

 in the dams, water stage recorders being set for recording the gage 

 heights. 



Plate XXI is a view of the middle control showing the 3-inch 

 Parshall measuring flume and the flow recorder. 



Flume for Winter Measurements 



In order to measure small summer floAvs accurately and also to 

 obtain a record of large winter flows, a combination flume, such as is 

 shown in Plate XXII, was found desirable. This combination flume 

 consists of two Parshall measuring flumes, one large and one small, so 

 arranged that both large and small flows pass through the converging 

 section of the large flume, but the small flows are by-passed from the 

 dip in the large flume into a basin above the small flume and thence 

 through this latter flume, while the greater part of the large flows 

 continues on through the larger flume. 



A record of the larger discharges is obtained by a recorder operated 

 by a float in a still well connected to a larger flume, and a record of 

 the smaller discharges is obtained by a flow recorder operated by a 

 float in a still well connected to the smaller flume. Some overlap is 

 j)rovided so there is a small range during which a record may be 

 obtained from both flumes. Two separate recorders may be used, or 

 one duiDlex recorder is sufficient if it is desired to record gage heights 

 only. 



Plate XXII shows a combination of 3-inch and 2-foot Parshall 

 measuring flumes, providing a range in discharge up to 23 second- 

 feet. The sizes of both flumes may vary according to the accuracy 

 and range desired. It should be recognized, however, that there are 

 certain limitations on the accuracy of measurements over very wide 

 variations in flows, and the selection of sizes should depend on whether 

 the greatest accuracy is desired at very high, medium, or low stages. 



The application of Parshall measuring flumes to measurements in 

 mountain canyons involves problems not ordinarily met with in valley 

 areas. The stream gradient is steep, often 10 per cent or more, and the 

 water tends to cut a narrow channel and travel at a relatively high 

 velocity and carry a large bed load as well as a considerable amount 

 of suspended material. 



A large flume placed, for example, directly in a stream channel 

 where the grade is as high as 10 per cent will have a rating curve quite 

 different from the standard calibration. It may pass as much as 30 

 per cent less water than that given for the lowest gage heights in the 

 standard tables. The reason for this is that a small stream of Avater 

 entering the center of a wide flume at high velocity tends to proceed 

 through the center of the flume without changing its cross section 

 greatl}', leaving dead water along each side of the wide flume. . At high 

 stages, the flume may pass more water than is indicated by the gage 

 height in the standard tables, because of the high velocity of approach 



