356 fry. SEDIMENTATION IN RESERVOIRS [Ch. 20 



fixed pivot point on the bank from which measurements are taken. The 

 second sheave is made of casehardened steel to maintain a fixed rim 

 diameter and operates a three-digit counter that indicates the distance 

 of the boat from the pivot point. 



The party is equipped with transit, plane table, 200 feet of steel 

 chain, chainman's plumb bobs, signal flags, Abney level or clinometer, 

 machete, hand axes, and iron pins. 



One of the most useful pieces of equipment is a Walkie-Talkie radio 

 which enables the boat party and the instrumentman on the bank to 

 maintain conversational contact with one another, thereby eliminat- 

 ing hand signals and misunderstanding. 



Sounding Range 



Prior to the actual sounding, ranges should be located and marked 

 by a red or white flag, about 2 feet square, on a stick behind the silt- 

 range monument. Red is used where the background is light, white 

 where it is dark as seen from the water. 



If the hydrographic party moves up to a range in the boat, the boat 

 lands on one bank. The distance from the monument to a point on 

 the shore is measured with the steel tape and the vertical angle taken 

 from that point to the range monument by the hand clinometer. The 

 transit is set up over the point so located. The boat with the other 

 members of the party then crosses to the opposite bank in order that, 

 during sounding, the boat operator will be facing the instrumentman. 

 Here a point is set on range about 5 to 8 feet above the water where 

 an iron pin is driven into the ground. The distance from this pin to 

 the range monument on that bank is measured, and the vertical angle 

 is taken. The distance from the pin to the distance wheel on the boat 

 is then measured. The end of the distance wire is secured to the iron 

 pin and the counter set to read the distance from the pin to the wheel. 

 The projector is lowered, the chart operator turns on the sounding 

 recorder, and the boat moves across the river on the range at a speed 

 of about 3 miles per hour. The transitman keeps the boat on range 

 by giving directions over the Walkie-Talkie to the boat operator, who 

 wears headphones connected to the radio on the boat. At the end 

 of the run, the distance from the boat to the instrument is taped. 



The recorder is equipped with a fix button which, when pressed by 

 the operator, prints a line on the moving chart. At intervals of ap- 

 proximately 25 or 50 feet, the distance-wheel operator calls out the 

 distance, the chart operator pushes the fix button and notes the dis- 

 tance on a sheet of paper. These distances are transferred to the 

 chart immediately on the completion of the range. 



