Environmental Engineering Reference
In-Depth Information
3.6 Surface Water Inflow and Outflow
Surface water flow is commonly the largest component of a water budget for wetlands
in humid regions, making it an important component to measure accurately. Accurate
measurements are possible if flow is confined to well-defined channels. Measurement
uncertainty can be very large, however, in wetland settings with exceptionally low
topographic gradients where flow commonly occurs through a network of poorly
constrained channels. Furthermore, the distinction between surface-water flow and
diffuse overland flow can be difficult to determine. In this section, we limit discussion
to settings where a well-defined channel exists and briefly discuss low-gradient
settings in the section on flow estimation using indirect methods.
Surface-water flow in terms of volume per time (m 3 /s) is often referred to as
discharge in the stream hydrology literature. A more precise definition is the
volume rate of flow through a stream cross-section at a right angle to the flow
direction. Since it is impractical to manually measure stream discharge with 15-min
to daily temporal resolution over long periods of time, it usually is calculated from
stage measured at a stream-gaging station installed in a channel, or stage measured
in an artificial control structure (i.e., flume or weir) using a pre-established stage-
discharge relation. Often referred to as a stage-discharge rating curve, this empirical
function is determined with direct measurements of stream discharge over a range
of stages or by calibration of theoretical formula. In the following section, we will
briefly describe establishment of a stream-gaging station, measurement of stream
discharge and stage, development of a rating curve, and the associated potential
problems and sources of error.
3.6.1 Stream Gaging Station
The accuracy of stream discharge measurement is strongly dependent on the
accuracy of the rating curve, which is dependent on the degree of flow control.
Therefore, whenever possible, it is best to establish gaging stations in a stream
section with good natural flow control (see below) or in a specifically designed
control structure such as a weir or flume. Flow control can be defined as a feature
some distance downstream of a gaging station that controls the stage-discharge
relation upstream at least as far as the location of the gaging station.
Ideally, a gaging station should be located in a place where: (1) the channel is
straight about 100 m upstream and downstream, (2) the total flow is confined to one
channel at all stages, (3) the streambed is not subject to scour and fill and is free of
aquatic vegetation growth, (4) banks are permanent, free of brush, and high enough
to contain floods, (5) the downstream flow control is unchanging, (6) a pool is
present upstream from the control, (7) the site is far enough upstream from the
confluence with another stream that flow in the tributary does not affect flow at
the gaging station, and (8) a suitable stream section for making manual discharge
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