Environmental Engineering Reference
In-Depth Information
Fig. 3.12 Schematic diagram showing a channel cross section divided into rectangular subsections
for the velocity-area method of discharge measurement (Adapted from Rantz
1982
:81. Published
with kind permission of the U.S. Geological Survey. Figure is public domain in the USA. All Rights
Reserved)
monitoring period should be avoided, but if change is unavoidable, the relation
between the new and old datum needs to be clearly established and recorded.
Details regarding installation, operation, and maintenance of stage-measuring
devices are described in the Sect.
3.3.1
on wetland stage. For stage measurements in
streams, a water-level sensor can be protected from flowing debris by installing it in
a stilling well, which also dampens waves generated by wind or turbulence and
provides a more stable reading. A stilling well is a vertical pipe or culvert that is
hydraulically connected to the stream water level. It can be installed in a stream
bank and connected to the stream by a subsurface horizontal pipe, or installed
directly in a channel. It must be secured to a stable anchor so that its elevation does
not change over time. The bottom of a stilling well should be at least 0.3 m below
the minimum stage and its top should be above the peak flood level. Details
regarding construction of stilling wells and their shelters is described by Rantz
(
1982
:41-52) and Sauer and Turnipseed (
2010
:6-11).
3.6.4 Discharge Measurement
3.6.4.1 Velocity-Area Method
Among several methods for discharge measurement, the most commonly used is
the velocity-area method, which involves direct measurement of flow velocity
using a current meter at successive locations along a channel cross section
(Fig.
3.12
) and summation of measured values to calculate the total discharge.
The ideal stream cross section is located within a straight reach having streamlines
parallel to each other; the streambed is relatively uniform and free of numerous
boulders and extensive aquatic vegetation; flow is relatively uniform and free of
