65 percent gravel. In our tests, the 

 combinations of sand and gravel used covered 

 the greater part of the range of water per- 

 meabilities. 



Seepage in the flume is measured by 

 dividing the rate of inflow or outflow (they 

 must be equal) by the cross sectional area 

 occupied by water in the flume. In labora- 

 tory tests it was found that a minimum of 4 

 hours is needed for complete adjustment to 

 occur after changing flow rate in the flume 

 and before running another test. In the 

 determination of the cross-sectioned area, 

 the height dimension is taken from the 

 height of water in the standpipe. The fol- 

 lowing equation is used for determination 

 of seepage in terms of feet per hour. 



D= V 

 A 



Where: D = total volume of water displaced 

 in the flume in cubic feet per 

 hour. 



A = cross-sectional area in square 

 feet at the standpipe as deter- 

 mined by multiplying the height 

 of water measured in the stand- 

 pipe by the width of the flume. 



V = rate of flow, seepage, at the 

 standpipe in feet per hour. 



A clarification of the terms "seepage" 

 rate or "velocity" in the gravel is appro- 

 priate. "Apparent" velocity is derived from 

 the formula shown above; "true" or "absolute" 

 velocity can be detected only at one speci- 

 fic point because of the complex pattern of 



Seepage. True or absolute velocity is best 

 interpreted by the use of the standpipe 

 System described and must be expressed in 

 terms of "apparent" velocity. The relation 

 between "apparent" velocity and "true" or 

 "absolute" velocity is illustrated in figure 

 4. The "true" or "absolute" velocity of 

 flow in the gravel may be several times 

 faster than "apparent" rate of flow, since 

 the path actually followed by the water be- 

 tween points A and B may be several times 

 longer than the linear distance between 

 these points. 



In the laboratory the attempt is made 

 to relate "true" or "absolute" velocity as 

 interpreted by the standpipe with "apparent" 

 velocity as measured in the flume by the 



D 

 .A 



formula, -=^ = V. ActuEilly, units of dis- 



placement are counted by changes in resist- 

 ance of a salt solution that has been intro- 

 duced into the chamber of the standpipe. 

 The end objective sought in the calibration 

 process in the laboratory then is to corre- 

 late changes in resistance with measured 

 apparent velocity. 



To accomplish this, two all-important 

 factors must be considered: (1) the volume 

 of water to be displaced, cuid (2) the con- 

 version of the dilution of a Salt solution 

 into units of displacement as recorded by 

 resistance readings on a conductivity bridge. 



Determination of the volume of- water 

 to be displaced in the standpipe is governed 

 by the height of the coliunn of water in the 

 standpipe which is in turn governed by the 

 stream depth at the location of the test. 

 By measuring the depth of water in the 

 standpipe, of known diameter, the volume of 



-^. 



Direction of flow in sfrea 



m 



^^^^om^mwsss^ 



Point A 



Point B 



Figure 4. — Showing the complex nature of "true" or "absolute" velocity and its 

 relation with "apparent" velocity of water between points A and B. 



