Environmental Engineering Reference
In-Depth Information
Fig. 3.13 Example of flow-velocity measuring devices that are commonly used in North America:
(a) top-setting wading rod with a meter attached, (b) vertical-axis current meter (Price AA),
(c) vertical-axis current meter (Price Pigmy), (d) horizontal-axis current meter (Global Water
FP101)
eddies, slack water, and excessive turbulence; velocity and depth are within the
range for which measuring devices give accurate results; and the observer can
safely carry out measurements (Rantz
1982
:139; Dingman
2002
:609). If necessary,
the condition can be improved by removing obstructions in, above, and below the
channel section without affecting discharge.
Measurements can be made by wading, from a boat, or from bridges. After the
selection of a suitable section, the first step is to extend an incremented line across
the channel above the water surface and measure the total channel width. A tape
measure, a rope marked at constant intervals, a marked tag line for boat
measurements, or marking constant intervals on a bridge can be used for this
purpose. Except for bridge-based measurements, the line is placed at right angles
to the direction of flow. The next step is to determine the measurement interval, or
the width of individual rectangular subsections in Fig.
3.12
. Rantz (
1982
:140)
recommend 25-30 subsections with no single subsection contributing more than
5 % of total discharge, meaning that smaller widths need to be assigned for
subsections in deeper and faster portion of the channel.
Depth of water and average velocity is determined at the middle of each
subsection. When flow velocity and depth allow measurements to be made while
wading the stream, a flowmeter commonly is mounted to a top-setting wading
rod specifically designed to indicate water depth and easily placed at the proper
depth for each velocity measurement (Fig.
3.13a
). The observer should stand in a
position that least affects the velocity being measured; for example, by standing
downstream of the tag line and facing either bank, or in the case of a sufficiently
