Measuring Stream Discharge in White Clay Creek
In
this week’s field exercise we will be measuring the volume of water moving
through the channel of White Clay Creek at two sites in southern Chester
County, Pennsylvania. The amount of
water moving through a stream channel is of great environmental importance for
a number of reasons. Flooding and
drought conditions are often defined by stream discharge values, while the
health of vegetation and animal species within a riparian environment is also
dependent upon the amount of water moving through the stream channel. Thus, it is important to have a thorough
knowledge of stream discharge measurement techniques if we are to understand
this important environmental variable.
Measuring Stream Discharge:
Although
the volume of water moving through a stream channel can be measured in several
different ways, we will be using a relatively simple, common methodology. The volume of water moving through a stream
can be calculated using the simple equation:
Stream
Discharge Q = AV
Where A is the cross-sectional area of the
stream channel (ft2 or m2) and V is the velocity of the
stream averaged across that cross-sectional area (ft/s or m/s). Both A and V require careful measurement. The stream discharge then is measured in ft3/s
or m3/s.
The Field Exercise:
We will be measuring the stream discharge at two sites along White Clay Creek in southern Chester County, Pennsylvania. As stated above, in order to calculate the discharge, the cross-sectional area of the stream and the average velocity of flow in the stream must be calculated.
Calculating the Cross-Sectional Area
To calculate the cross-sectional area we will define the “shape of the streambed by measuring the distance across the stream (L) and then measuring the depth of the stream at many locations along that line (d). We will use the simple formula for the area of a rectangle to obtain the cross-sectional area by assuming an average value for the depth. The area (A) is given as:
A = d L
Calculating the Flow Rate
To
calculate the rate of flow we will use a special instrument know as a current
meter. The current meter will
give us the rate at which water is flowing past the instrument at that
site. Thus, we will measure the rate of
flow at several points along the streambed and calculate an average velocity
for the stream.