Streamflow Measurement
Although precipitation, evaporation, evapotranspiration are difficult to measure and suggested methods have considerable amount of limitations, streamflow is the only part of the hydrologic cycle that can be measured accurately. Streamflow measurements techniques can be broadly classified into two categories, which are further divided into subcategories: i)
Direct determination of stream discharge a. Area-Velocity Method b. Dilution Techniques c.
Electromagnetic Method
d. Ultrasound Method ii)
Indirect determination of streamflow a. Hydraulic Structures (weirs, flumes, sluice gates, etc.) b. Slope-Area Method
In maximum cases, direct measurement of discharge is difficult, time consuming and costly procedure. Hence, a two step procedure is followed. First, the discharge in a given stream is related to the elevation of the water surface (stage) through a series of careful measurements. In the next step, the stage of the stream is observed routinely in a relatively inexpensive manner and the discharge is estimated by using previously determined stage-discharge relationship. Measurement of Stage: [self study] 1) Manual Gauges a. Staff gauges b. Wire gauges 2) Automatic stage recorders a. Float-gauge recorder b. Bubble gauge Measurement of Velocity: Current meters are used to measure velocity of the any stream at desired depth. The velocity distribution in a stream across vertical section is logarithmic in nature. As it is time consuming to determine the average velocity of the stream accurately, certain simple procedure has been evolved.
In shallow streams of depth up to about 3.0 m, the velocity measured at 0.6 times the depth of flow below the water surface is taken as the average velocity, ( ῡ) in the vertical. This is called single-point observation method.
In moderately deep streams, the velocity is observed at two points; i) at 0.2 times the depth of flow below the free water surface (v ( v 0.2 0.2) , ii) at 0.8 times the depth of flow below the free surface ( v 0.8 0.8). The average of these two readings is taken as the average velocity of the stream.
In rivers having flood flows, only the surface velocity ( v s) is measured within a depth of about 0.5 m below the surface. The average velocity, ( ῡ) is obtained by using a reduction factor K as as ῡ
The value of K is obtained from the observations at lower stages and lie in the range of 0.85 to 0.95.
AREA-VELOCITY METHOD
This method of discharge measurement consists essentially of measuring the cross-sectional area of the river at a selected section (gauging ( gauging site) site) and measuring the velocity of flow through the cross-sectional area. Following criteria are adopted for selecting gauging site.
The stream should have a well defined cross-section which does not change in various season
It should be easily accessible all through the year
The site should be in a straight, stable reach
The gauging site should be free from backwater effects in channel
In this method the cross-section of the river is sub-divided into several segments. The following are some of the guidelines to select the number of segments
The segment width should not be greater than 1/15 to 1/20 of the width of t he river
The discharge in each segment should be less than 10% of the total discharge
The difference of velocities in adjacent segments should not be more than 20%
[Figure: stream section for area-velocity method] Example 4.15: self study Problem 4.3: The
following are the data obtained in a stream gauging operation. A current meter with a calibration
equation V = (0.32N +0.032) m/s, m/s, where N = revolutions per second was used to measure the velocity at 0.6 depth. Using Mid-section method, calculate the discharge i n the stream. Discharge from right bank (m) Depth (m) Revolutions Observation time
0
2
4
6
9
12
15
18
20
22
23
24
0 0 0
0.50 80 180
1.10 83 120
1.95 131 120
2.25 139 120
1.85 121 120
1.75 114 120
1.65 109 120
1.5 92 120
1.25 85 120
0.75 70 150
0 0 0
Solution: Distance from right bank (m)
0 2 4 6 9 12 15 18 20 22 23 24
Average width
0 2.25 2 2.5 3 3 3 2.5 2 1.5 1.125 0
Depth (m)
Revolutions
Observation time
0 80 83 131 139 121 114 109 92 85 70 0
0 180 120 120 120 120 120 120 120 120 150 0
0 0.5 1.1 1.95 2.25 1.85 1.75 1.65 1.5 1.25 0.75 0
Ns
0 0.444444 0.691667 1.091667 1.158333 1.008333 0.95 0.908333 0.766667 0.708333 0.466667 0
Area, ∆A
Discharge, ∆Q
Velocity, ∆v
0 0 0 1.125 0.174222 0.196 2.2 0.253333 0.557333 4.875 0.381333 1.859 6.75 0.402667 2.718 5.55 0.354667 1.9684 5.25 0.336000 1.764 4.125 0.322667 1.331 3 0.277333 0.832 1.875 0.258667 0.485 0.84375 0.181333 0.153 0 0 0 3 Discharge = 11.86373 m /s
3
The discharge in the stream is 11.86 m /s.
() W1 =
() W10 =
= 2.25
= 1.125
