discharge @khokana
TRANSCRIPT
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NEPAL ENGINEERING COLLEGENEPAL ENGINEERING COLLEGENEPAL ENGINEERING COLLEGENEPAL ENGINEERING COLLEGE
Changunarayan, Bhaktapur
HYDROLOGY PRACTICAL REPORT
(RIVER DISCHARGE MEASUREMENT)
PERFORMED BY: PREPARED BY:
06-113 Kedar Nath Dahal Mandeep Pokhrel
06-114 Kewal Pandit CRN:06-116
06-115 Kumar Mani Dhakal
06-116 Mandeep Pokhrel SUBMITTED TO:
06-121 Niranjan Pudasaini Asst. Prof. Sanjay Chalise
06-123 Prabhu Ram Thapa
Date: 15-12-09 Date: 18-12-09
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Hydrology Discharge Measurement Mandeep Pokhrel(06-116)
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Objective: Determination of Discharge of a river by measuring area and velocity at various sections
of river.
Instruments Used:
Cup type current meter Measuring tape Rope
Theory:
In hydrology, discharge is the volume rate of water flow, including any suspended solids (i.e.
sediment), dissolved chemical species (i.e. CaCO3(aq)) and/or biologic material (i.e. diatoms),
which is transported through a given cross-sectional area. The unit of discharge is m/s.
A commonly applied methodology for measuring, and estimating, the discharge of a river isbased on a simplified form of the continuity equation. The equation implies that for any
incompressible fluid, such as liquid water, the discharge (Q) is equal to the product of the
stream's cross-sectional area (A) and its mean velocity ( ), and is written as:
Where,
Q is the discharge (m3/s) A is the cross-sectional area of the portion of the channel occupied by the flow (m2) is the average flow velocity (m/s)
Actual velocities vary along the river length, width, and depth. A typical velocity profile is
depicted in Figure 1. Stream velocities also vary similarly along the width, that is the velocity is
near zero at the edges and reach a maximum in the central part of the stream. Deviations from
ideal velocity profiles occur often, especially along bends in the river, around structures that
disrupt the flow, and irregular bottom shapes.
Due to the complex variations in velocities that occur naturally in streams/rivers, it has becomea well established practice to subdivide a river cross-section into several parts (Figure 2),
measure stream velocities at 1-3 depths within each part at a particular cross-section (Figure 3),
and calculate the discharge in each part:
qi= viai
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Hydrology Discharge Measurement Mandeep Pokhrel(06-116)
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The total discharge (Q) and cross-sectional area (A) are obtained from summing theflow andarea respectively, for the individual sections (qiand ai, i=1, 2, ,N=number of individual
sections):
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Hydrology Discharge Measurement Mandeep Pokhrel(06-116)
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The velocities of the river/stream sections can be measured in a variety of ways: weirs, flumes,
orifice plates, surface velocity, tracer velocity, and profile velocity. The float method uses the
velocity of the river surface, which can be measured by timing a prescribed distance traveled by
a floating object. Recently this approach has become more sophisticated using acoustic velocitymeters (e.g., Doppler type). Surface velocities need to be adjusted to obtain the average
velocity.
The velocity profile of the section is usually measured at a particular fraction of the depth (h) of
the measurement area (Figure 3). This approach is the most common.
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Hydrology Discharge Measurement Mandeep Pokhrel(06-116)
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For streams where the depth is less than 2.5 feet, the velocity is usually measured at the 60%
depth (0.6 method ) and this measurement is taken to be the average velocity in the individual
section (vi):
For depths greater than 2.5 feet, then measurements are taken at the 80% and 20% depths and
the two measurements are averaged:
A current meter is mostly used instrument for measuring stream velocity which essentially consists of a
mechanical rotating element. When a current meter is inserted in a flowing water there is an
unbalanced drag on the rotating element which causes rotation. The current meter is calibrated to give
the velocity corresponding to different speeds of rotation.
Up to 50 revolutions,
v=2.2N+0.03 (ft/s)
Above 50 revolutions,
V=2.22N+0.003 (ft/s) Fig: Cup type current meter
Price & Gurley current meter is a cup type meter which can register velocity in the range of 0.15 to 4.0
m/s. The accuracy of the instrument is about 1.5% at threshold & about 3% at speed of 1m/s & above.
Methodology:
Suspension bridge was used for making discharge measurement of the river because it could notbe waded due to excess polluted river water .
The entire section of the river was divided into several sections of 2.4 meters interval bymarking in bridge.
At the first section, the cup type current meter was connected to a rope & was suspended fromtop of bridge.
A mark was made in the rope by tape to indicate level of bridge at which the sill of currentmeter just touched the free water surface.
Then the current meter was submerged fully inside the river such as it touches the bed level. The depth of water in the river was found out by measuring the length of rope above mark
made by tape in the rope.
Now, at depth of 0.8D from WSL the no of revolutions of cup for specified time was taken. The above step was repeated for 0.2D from WSL. In case, the water depth was less then 1m then only one reading at 0.6D from WSL was taken.
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Hydrology Discharge Measurement Mandeep Pokhrel(06-116)
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The velocity (v) was found by equations specified in theory. Similarly for other sections, the depth and velocity was found out.
Observation Table:
Station no: 550.05 Measurement no:
River name: Bagmati Computed:
Location: Khokana Date:
Date:15-12-009 Checked:
DISCHARGE MEASUREMENT NOTES
Width(m):30.6 Area(m2 ):36.074 Mean velocity(m/s):0.041 Discharge(m3):1.52
Party: Counter no: Meter no: Rated no:
Weight: Distance from current meter to bottom of weight(m):
Location of measurement site: Downstream side of bridge Weather: Sunny
Mean gauge height(m):0.7 Air temperature(0C):20 Water temperature(0C):
LEW start time: 1:30 p.m. REW end point: 2:30 p.m.Remarks:
DISTANCE
FROM
INITIAL
POINT
WIDTH DEPTH
(m)
AREA
(m2)
OBS.
DEPTH
REVO
LUTIO
NS
TIME VELOCITY
(m/s)
MEAN
VELOCITY
(m/s)
DISCHARGE
(m3/s)
0 0
1.8 2.5 0.56 1.4
4.2 1.8 0.98 1.764
5.4 1.8 0.93 1.674 0.6D 2 50 0.036 0.036 0.033
7.8 2.4 1.04 2.496 0.6D 7 43 0.118 0.118 0.123
10.2 2.4 1.18 2.832 0.8D 10 43 0.165 0.182 0.215
10.2 1.18 0.2D 15 53 0.199
12.6 2.4 1.04 2.496 0.8D 20 44 0.314 0.333 0.346
12.6 1.04 0.2D 23 45 0.352
15 2.4 0.92 2.208 0.6D 24 43 0.383 0.383 0.353
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Hydrology Discharge Measurement Mandeep Pokhrel(06-116)
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at all. There were excess deposition of sediments (especially sludge) at some sections due to which flow
was hindered. There may be some instrumental error as well and some data errors but these were
minimized as much as possible.
Conclusion:
Thus, the methodology of measuring discharge by area-velocity measurement of a river was well
acquainted. And the discharge of Bagmati River at Khokana at 0.7m gauge height was found to be
1.52m3/s.
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NEPAL ENGINEERING COLLEGENEPAL ENGINEERING COLLEGENEPAL ENGINEERING COLLEGENEPAL ENGINEERING COLLEGE
Changunarayan, Bhaktapur
HYDROLOGY PRACTICAL REPORT
(RIVER DISCHARGE MEASUREMENT)
PERFORMED BY: PREPARED BY:
06-113 Kedar Nath Dahal Mandeep Pokhrel
06-114 Kewal Pandit CRN:06-116
06-115 Kumar Mani Dhakal
06-116 Mandeep Pokhrel SUBMITTED TO:
06-121 Niranjan Pudasaini Asst. Prof. Sanjay Chalise
06-123 Prabhu Ram Thapa
Date: 15-12-09 Date: 18-12-09
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8/14/2019 Discharge @Khokana
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Hydrology Discharge Measurement Mandeep Pokhrel(06-116)
2 | P a g e
Objective: Determination of Discharge of a river by measuring area and velocity at various sections
of river.
Instruments Used:
Cup type current meter Measuring tape Rope
Theory:
In hydrology, discharge is the volume rate of water flow, including any suspended solids (i.e.
sediment), dissolved chemical species (i.e. CaCO3(aq)) and/or biologic material (i.e. diatoms),
which is transported through a given cross-sectional area. The unit of discharge is m/s.
A commonly applied methodology for measuring, and estimating, the discharge of a river isbased on a simplified form of the continuity equation. The equation implies that for any
incompressible fluid, such as liquid water, the discharge (Q) is equal to the product of the
stream's cross-sectional area (A) and its mean velocity ( ), and is written as:
Where,
Q is the discharge (m3/s) A is the cross-sectional area of the portion of the channel occupied by the flow (m2) is the average flow velocity (m/s)
Actual velocities vary along the river length, width, and depth. A typical velocity profile is
depicted in Figure 1. Stream velocities also vary similarly along the width, that is the velocity is
near zero at the edges and reach a maximum in the central part of the stream. Deviations from
ideal velocity profiles occur often, especially along bends in the river, around structures that
disrupt the flow, and irregular bottom shapes.
Due to the complex variations in velocities that occur naturally in streams/rivers, it has becomea well established practice to subdivide a river cross-section into several parts (Figure 2),
measure stream velocities at 1-3 depths within each part at a particular cross-section (Figure 3),
and calculate the discharge in each part:
qi= viai
-
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Hydrology Discharge Measurement Mandeep Pokhrel(06-116)
3 | P a g e
The total discharge (Q) and cross-sectional area (A) are obtained from summing theflow andarea respectively, for the individual sections (qiand ai, i=1, 2, ,N=number of individual
sections):
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Hydrology Discharge Measurement Mandeep Pokhrel(06-116)
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The velocities of the river/stream sections can be measured in a variety of ways: weirs, flumes,
orifice plates, surface velocity, tracer velocity, and profile velocity. The float method uses the
velocity of the river surface, which can be measured by timing a prescribed distance traveled by
a floating object. Recently this approach has become more sophisticated using acoustic velocitymeters (e.g., Doppler type). Surface velocities need to be adjusted to obtain the average
velocity.
The velocity profile of the section is usually measured at a particular fraction of the depth (h) of
the measurement area (Figure 3). This approach is the most common.
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Hydrology Discharge Measurement Mandeep Pokhrel(06-116)
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For streams where the depth is less than 2.5 feet, the velocity is usually measured at the 60%
depth (0.6 method ) and this measurement is taken to be the average velocity in the individual
section (vi):
For depths greater than 2.5 feet, then measurements are taken at the 80% and 20% depths and
the two measurements are averaged:
A current meter is mostly used instrument for measuring stream velocity which essentially consists of a
mechanical rotating element. When a current meter is inserted in a flowing water there is an
unbalanced drag on the rotating element which causes rotation. The current meter is calibrated to give
the velocity corresponding to different speeds of rotation.
Up to 50 revolutions,
v=2.2N+0.03 (ft/s)
Above 50 revolutions,
v=2.22N+0.003 (ft/s) Fig: Cup type current meter
Price & Gurley current meter is a cup type meter which can register velocity in the range of 0.15 to 4.0
m/s. The accuracy of the instrument is about 1.5% at threshold & about 3% at speed of 1m/s & above.
Methodology:
Suspension bridge was used for making discharge measurement of the river because it could notbe waded due to excess polluted river water .
The entire section of the river was divided into several sections of 2.4 meters interval bymarking in bridge.
At the first section, the cup type current meter was connected to a rope & was suspended fromtop of bridge.
A mark was made in the rope by tape to indicate level of bridge at which the sill of currentmeter just touched the free water surface.
Then the current meter was submerged fully inside the river such as it touches the bed level. The depth of water in the river was found out by measuring the length of rope above mark
made by tape in the rope.
Now, at depth of 0.8D from WSL the no of revolutions of cup for specified time was taken. The above step was repeated for 0.2D from WSL. In case, the water depth was less then 1m then only one reading at 0.6D from WSL was taken.
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Hydrology Discharge Measurement Mandeep Pokhrel(06-116)
6 | P a g e
The velocity (v) was found by equations specified in theory. Similarly for other sections, the depth and velocity was found out.
Observation Table:
Station no: 550.05 Measurement no:
River name: Bagmati Computed:
Location: Khokana Date:
Date:15-12-009 Checked:
DISCHARGE MEASUREMENT NOTES
Width(m):30.6 Area(m2 ):36.074 Mean velocity(m/s):0.041 Discharge(m3):1.52
Party: Counter no: Meter no: Rated no:
Weight: Distance from current meter to bottom of weight(m):
Location of measurement site: Downstream side of bridge Weather: Sunny
Mean gauge height(m):0.7 Air temperature(0C):20 Water temperature(
0C):
LEW start time: 1:30 p.m. REW end point: 2:30 p.m.
Remarks:
DISTANCE
FROM
INITIAL
POINT
WIDTH DEPTH
(m)
AREA
(m2)
OBS.
DEPTH
REVO
LUTIO
NS
TIME VELOCITY
(m/s)
MEAN
VELOCITY
(m/s)
DISCHARGE
(m3/s)
0 0
1.8 2.5 0.56 1.4
4.2 1.8 0.98 1.764
5.4 1.8 0.93 1.674 0.6D 2 50 0.036 0.036 0.033
7.8 2.4 1.04 2.496 0.6D 7 43 0.118 0.118 0.123
10.2 2.4 1.18 2.832 0.8D 10 43 0.165 0.182 0.215
10.2 1.18 0.2D 15 53 0.199
12.6 2.4 1.04 2.496 0.8D 20 44 0.314 0.333 0.346
12.6 1.04 0.2D 23 45 0.352
15 2.4 0.92 2.208 0.6D 24 43 0.383 0.383 0.353
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Hydrology Discharge Measurement Mandeep Pokhrel(06-116)
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17.4 2.4 0.92 2.208 0.6D 18 44 0.283 0.283 0.261
19.8 2.4 1.2 2.88 0.6D 5 43 0.087 0.087 0.105
22.2 2.4 1.57 3.768 0.6D 4 60 0.054 0.054 0.085
24.6 2.4 2.09 5.016
27 1.8 1.93 3.474
28.2 1.2 1.64 1.968
29.4 1.35 1.4 1.89
30.6 0
TOTAL: 36.074 0.041 1.520
Limitations:
There are always some limitations while measuring area or velocity of a river in field condition. The
velocity could not be measured by wading in the river because of excess pollution and sewage
deposition. The velocity at all sections were not laminar but rather in some sections there was flow from
downstream to upstream which is not desired at all .So the velocity in such sections were not measured
0
0.5
1
1.5
2
2.5
0 1.8 4.2 5.4 7.8 10.2 1 0.2 12.6 12.6 15 17.4 19.8 2 2.2 2 4.6 27 28.2 2 9.4 3 0.6
Depth(m)
Distance from Left Edge(m)
RIVER CROSS SECTION
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Hydrology Discharge Measurement Mandeep Pokhrel(06-116)
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at all. There were excess deposition of sediments (especially sludge) at some sections due to which flow
was hindered. There may be some instrumental error as well and some data errors but these were
minimized as much as possible.
Conclusion:
Thus, the methodology of measuring discharge by area-velocity measurement of a river was well
acquainted. And the discharge of Bagmati River at Khokana at 0.7m gauge height was found to be
1.52m3/s.