PCWA-L 467.pdf - PCWA Middle Fork American River Project ...

5. DISCHARGE-CURRENT-METER METHOD 153
bridged above the water may be weak, even though relatively thick.
At the cross section selected for measurement three holes, one at
each quarter point of the width, are cut to check on the possible
preseuce ofslush ice or possible maldistribution of flow. Ifpoor conditions
are found, other cross sections are similarly investigated to find
one that is free ofslush ice and has a favorable horizontal distribution
of flow. After finding a favorable cross section, at least 20 holes are
cut for the current-meter measurement ofdischarge. The holes should
be spaced so that no subsection carries more than 10 percent of the
total discharge. On narrow streams it may be simpler to remove all
the ice in the cross section.
The effective depth of the water (fig. 94) is the total depth of water
minus the distance from the water surface to the bottom of the ice.
The vertical pulsation of water in the holes in the ice sometimes
causes difficulty in determining the depths. The total depth of water
is usually measured with an ice rod or with a sounding weight and
reel, depending on the depth.
The distance from the water surface to the bottom of the ice is
measured with an ice-measuring stick (p. 125), unless slush is present
at the hole. In that situation the effective depth is the total depth
Streambed
0= Water surface to bottom of ice
b= Total depth of water