YSI SonTek RiverSurveyor, System Manual

Page: 67 / 194

SonTek/YSI Inc
RiverSurveyor System Manual (May 1, 2007)
55
4.1. Stream (River) Gauging and RiverSurveyor Discharge Measurements
The following topics are covered in this section:
•
What do Stream Gauging and Discharge mean?
•
What are the considerations in making a discharge measurement?
•
How does RiverSurveyor make instantaneous discharge measurements?
4.1.1. What do Stream Gauging and Discharge mean?
Stream (river) gauging is the continuous measurement of the river flow rate (in cubic feet per
second [cfs] or cubic meters per second [cms]) past a specific cross-section in a river. In general,
this flow rate is computed by measuring and recording water level and/or water velocity at a
fixed interval in time, and then developing a rating-curve that can be used to compute discharge.
The
discharge of a river is simply the volume of water that flows through it in a given time.
The development of a discharge-rating curve for a site can be very simple or very complex, de-
pending on the site and hydrologic conditions. In any event, the development of the discharge
rating-curve requires several measurements of total river discharge over the water-year along
with concurrent measurements from the continuously recording devices (e.g., water-level re-
corder and/or water-velocity recorder). For the purpose of clarification, continuous discharge
“computed” for a river is based on a rating curve. Instantaneous discharge measurements are dis-
charge measurements collected at discrete points, and are used to develop the discharge rating
for the stream gauging station (location). The
U.S. Geological Survey Water-Supply Paper: WSP
2175, Vol. 1 & 2, by Rantz, 1982 provides an excellent description of the most common tech-
niques and methods used for stream gauging in the United States.
4.1.2. What are the considerations in making a discharge measurement?
Consider a cross-section of a river that has been divided into a grid (
Figure 4-6
). To measure the
total flow passing through this cross-section, we simply measure the flow across each of the sub-
sections defined by the grid and then add them all together. The contribution of each grid subsec-
tion is estimated by multiplying the local component of the mean water velocity perpendicular to
the section by the area of the corresponding subsection.
Traditionally, this method requires sending a crew on a boat, or a rail car, to measure the depth
profile of the river, and the velocities at multiple points within each of the grid subsections. Get-
ting accurate discharge results usually requires a large number of grid points, especially when
the bottom topography is highly irregular, and/or when the velocity field has significant vertical
or horizontal structure (e.g., eddies).
Figure 4-6. Example of a discharge calculation grid
X
Y
Z
Vertical
profile
Depth cells