Discharge @Khokana

8/14/2019 Discharge @Khokana

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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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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):


4/16


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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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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.


6/16


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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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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.


9/16

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


10/16


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


11/16


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):


12/16


4 | P a g e

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.


13/16


5 | P a g e

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.


14/16


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


15/16


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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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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.

Discharge @Khokana

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