Most commonly used methods for stream gauging are: 1. Area-Velocity Method 2. Area Slope Method 3. Discharge Measuring Structures.
1. Area-Velocity Method:
It should be noted that depth of flow varies in a section of a stream across the main flow direction. Likewise, the velocity of flow too varies from point to point within a cross-sectional area of the stream.
The best method for stream flow measurement would, therefore, be to determine the velocity variation at a channel section and then integrate (i.e., sum) the products of velocity in a small individual elemental area with that area for the whole flow cross-section.
However, this may not be possible most of the times and, hence, other simpler measurements are made to estimate the stream flow. For example, a flow section can be divided into a suitable number of vertical strips the average depths of which are measured. In each of these strips, velocities at 0.2 h and 0.8 h depths below the free surface are measured using a current meter. Here, h is the depth of flow in the strip.
The average of these two values for any given strip gives approximately the average velocity of flow in that strip. The total discharge in the stream can be obtained by obtaining the sum of products of the average velocity and area of the strip for all the individual strips, i.e.,
in which Bi, hi, vi and Qi, are width, average depth, average velocity and discharge of ith strip and the flow cross-section has been divided into N strips. For obtaining the average velocity in a strip, one can, alternatively, measure velocity only at 0.6 h depth below the free surface and take it equal to the average velocity in the strip. Or, one can measure surface velocity within the strip and multiply it by 0.9 to get an approximate average velocity.
2. Area-Slope Method:
During floods, it is generally difficult to identify peak flow conditions and then make velocity measurements within peak flow duration. It is relatively simpler to install two gauges with the same datum at some distance apart. During flood, water surface levels in these gauges are read. At times, flood level marks on the banks can also be used for this purpose.
Knowing the river cross-section, one can determine the area of flow cross-section at the two gauge locations and an average value of area A is determined. Likewise, the hydraulic radius R can also be obtained.
Water surface slope S can also be computed using the gauge level readings and the distance between the two gauge stations. Assigning a suitable value to the Manning’s n, one can predict discharge Q in the stream by using the Manning’s equation,
3. Discharge Measuring Structures:
The most commonly used structure for the purpose of discharge measurement in a river is a weir which is, generally, either triangular (for relatively small discharges) or rectangular (for relatively large discharges) in shape.
These weirs are in the form of an obstruction constructed across a river. The discharge Q flowing over a weir is related to the head over the weir, i.e. H and the weir characteristics. For example, for a triangular weir having its notch angle Ɵ, the discharge Q is expressed as
Likewise, for rectangular sharp-crested weir
in which Cd is the Coefficient of discharge, B the length of weir and g is the acceleration due to gravity.