SELECTION OF SITE FOR A STREAM GAUGING STATION

6.4 SELECTION OF SITE FOR A STREAM GAUGING STATION

The following factors have to be considered in selecting a site for a stream gauging station.

(i) The section should be straight and uniform for a length of about 10 to 20 times the width of the stream.

(ii) The bed and banks of the stream should be firm and stable so as to ensure consist- ency of area-discharge relationship, i.e., the cross section should not be subjected to change by

HYDROLOGY

silting or scouring, during different stages of flow; a smooth rock, shingle or clay bed is favour- able, while a fine sandy bed is unfavourable.

(iii) The bed and banks should be free from vegetal growth, boulders or other obstruc- tions like bridge piers, etc.

(iv) There should be no larger overflow section at flood stage. The best cross section is one with V-shape, so that there is sufficient depth for immersing the current meter without being affected by the bed roughness of the stream.

(v) The part of the reach having the most regular transverse section and steady flow with the current normal to the metering section and velocities in the range of 0.3–1.2 m/sec should be selected.

(vi) To ensure consistency between stage and discharge, there should be a good control section far downstream of the gauging site. This control may be in the form of steep rapids, large rocky boulders, restricted passages, crest of weirs or anicuts etc.

(vii) The sites above the confluence of rivers are best avoided if the flow is affected by back water conditions due to the varying discharges in the tributaries.

(viii) The stream gauging station should be easily accessible. Example 6.2 The stream discharges for various stages at a particular section were observed to

be as follows. Obtain an equation for the stage-discharge relationship and determine the dis- charge for a stage of 4.9 m and 12 m.

Stage (m) 1.81 2.00 2.30 2.90 3.70 4.50 Discharge (cumec)

1.00 1.50 2.55 5.60 11.70 20.20 Stage (m)

5.40 6.10 7.30 7.70 8.10 Discharge (cumec)

32.50 44.50 70.0 80.0 90.0 Solution The relation between the stage (h) and discharge (Q) of the stream can be assumed

of the form

Q = K (h – a) n , (similar to Eq. (6.10)

where K, a and n are the constants. Plot Q vs. (h – a) on a lag-lag paper assuming a value for the constant a = 0.6 m (say); the curve obtained is concave downwards, Fig. 6.12. Now assume

a value a = 1.2 m (say) and the curve obtained is concave upward. Now try an intermediate value a = 0.9 m, which plots a straight line and represents the stage discharge relationship. The slope of this straight line gives the value of the exponent n = 2.2, and from the graph for

h – a = 1, Q = 1.2 = K. Now the constants are determined and the equation for the stage- discharge relationship is

Q = 1.2 (h – 0.9) 2.2

It may be noted that the value of a = 0.9, which gives a straight line plot is the gauge reading for zero discharge. Now the abscissa of (h – a) may be replaced by the gauge reading (stage) h, by adding the value of ‘a’ to (h – a) values. For example the (h – a) values of 0.1, 1, 2,

4, 6, 8 and 10 may be replaced by the h values of 1, 1.9, 2.9, 4.9, 6.9, 8.9 and 10.9 respectively. Now for any gauge reading (stage) h, the discharge Q can be directly read from the graph and the stage discharge curve can be extended. From the graph, Fig. 6.12,

for h = 4.9 m, Q = 25.3 cumec

STREAM GAUGING

and for h = 12.0 m, Q = 240 cumec which can be verified by the stage-discharge equation obtained as

for h = 4.9 m, Q = 1.2 (4.9 – 0.9) 2.2 = 25.3 cumec for h = 12 m, Q = 1.2 (12 – 0.9) 2.2 = 240 cumec

Note The equation for the stage-discharge relation can also be obtained by ‘Linear Regression’ by assuming trial values of ‘a’ and computer-based numberical analysis. See chapter—13. Example 6.3 The following data were obtained by stream gauging of a river:

Main gauge staff reading (m)

12.00 12.00 Auxiliary gauge staff reading (m)

11.65 11.02 Discharge (cumec)

9.50 15.20 what should be the discharge when the main gauge reads 12 m and the auxiliary gauge reads

11.37 m?

Solution

∆h 0 = 12.00 – 11.65 = 0.35 m ∆h a = 12.00 – 11.02 = 0.98 m

Q a F ∆ h a I . 15 20 F . 0 98 = I

Eq. (6.10):

Q 0 HG ∆ h 0 KJ . 9 50 HG . 0 35 KJ

n = 0.5125

Again, when the auxiliary gauge reads 11.37 m,

∆h a = 12.00 – 11.37 = 0.63 m Q 0.5125

a . 0 63

= F I . 9 50 HG . 0 35 KJ

Q a = 12.85 cumec

Example 6.4

A bridge has to be constructed over a river, which receives flow from three branches above the site. Compute the maximum flood discharge at the bridge site from the following data:

Branch 1 has a bridge: Width of natural water way

324.0 m Lineal water way under the bridge (with C d = 0.95 for rounded entry)

262.5 m Depth upstream of bridge

4.6 m Depth downstream of bridge

2.8 m Branch 2 has a catchment area of 4125 km 2 Ryve’s C = 10 Branch 3 levelling of cross section (c /s) data: Distance from BM (m)

0 11 24 52 67 79 84 RL on c /s (m)

10.8 9.6 4.2 2.4 5.4 10.2 10.5 Levelling of longitudinal-section (L /S) data: Distance from

1 km bridge site

1 km

at

downstream HFL along L /S (m)

upstream

bridge site

9.60 9.0 8.39 Manning’s n may be assumed at 0.03.

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Solution (i) Discharge from Branch 1, i.e., Q 1 under bridge openings, Fig. 6.15, from Eq. (6.3)

Q 1 =C d A 1 2gh ( ∆+ h a )

If L, d, V and L 1 , d 1 , V 1 refer to the length, mean depth and velocity of the normal stream (upstream of bridge site) and those under the contracted section of the bridge and also

A 1 =L 1 d, Q 1 = LdV, h a =

Afflux Ñ h

d d Flow

a. Cross section

Superstructure Parapet

Lineal water way

Foundation on

hard rock b. Flow through bridge openings

Fig. 6.15 Flow through bridge openings (Example 6.4)

Eq. (6.3) after substitution and simplification yields the expression for afflux (i.e., head- ing up of water on the upstream face of the bridge openings) as

Afflux ∆h =

2 g HG CL d 1 KJ

If the Branch 1, flow under bridge openings

V 2 F 324 2 I

− HG 1

× 262 5 . 2 KJ

V = 7.16 m/sec Q 1 = LdV = 324 × 2.8 × 7.16 = 6500 cumec

(ii) Discharge from Branch 2: From Ryve’s formula (see Eq. 8.2)

Q = CA 2/3 2

= 10 (4125) 2/3 = 2580 cumec

STREAM GAUGING

(iii) Discharge from Branch 3 (from slope-area method): See Table 6.2 for computation C/S area and wetted perimeter, Fig. 6.16.

Table 6.2 Computation C/S area and wetted perimeter (Fig. 6.16)

Area Area Area, A 1 Wetted perimeter, P 1 no.

13 H.F.L.

Dist. 0 11 24 52 67 79 84 R.L.

Fig. 6.16 Cross section of river at bridge site (Example 6.4)

A 280 05 .

Hydraulic mean radius,

Water surface slope,

By Manning’s formula, the velocity of flow

1/2 = 2.16 m/sec

Q 3 = AV = 280.05 × 2.16 = 605 cumec

Discharge at bridge site Q =Q 1 +Q 2 +Q 3 = 6500 + 2580 + 605 = 9685 cumec

QUIZ VI

I Choose the correct statement/s The method of measuring discharge in a turbulent stream is

(i) by measuring the drop in water surface under bridge openings and canal falls.

HYDROLOGY (ii) by adding common salt into flowing water and determining its concentration downstream.

(iii) by slope-area method. (iv) by area-velocity method. (v) by measuring head over weirs or anicuts. (vi) by measuring the upstream depth in a standing wave flume. II Match the items in ‘A’ with the items in ‘B’

(i) Area-velocity method

(a) Afflux

(ii) Slope-area method (b) Turbulent streams (iii) Bridge openings

(c) Mean velocity

(iv) Salt-concentration method

(d) v = aN + b

(v) Surface and subsurface floats

(e) Q = K(h – a) n

(vi) Velocity rods

(f) AWLR

(vii) Current-meter rating (g) Control section downstream (viii) Stage-discharge curve

(h) Peak flood where no gauging

station exists

(ix) River stage (i) Current-meter gauging (x) Stream gauging spite

(j) Surface velocity

III Say ‘true’ or ‘false’, if false, give the correct statement: (i) The staff-gauge reading corresponding to zero-discharge in a stream is always a positive

number. (ii) The mean velocity in a vertical stream can be calculated by measuring the velocities at one-

fifth and four-fifths of the depth of the stream in that vertical. (iii) While the surface and subsurface floats measure the mean velocity of the stream, the veloc-

ity rods measure the surface velocity. (iv) Subsurface floats will not give the velocity accurately since they are affected by wind. (v) Discharge in a river can not be determined by measurement near bridge openings. (vi) The salt-concentration method can best be used to determine the discharge of non-turbulent

rivers. (vii) The hydraulic turbine can be used as a good water meter. (viii) The stream gauging site should be on the upstream of the confluence of rivers.

(ix) There should be a good-control section immediately upstream of the gauging site.

(x) The calibration of the current meter is called the current meter rating while the stage-dis-

charge is the relation between the staff-gauge reading and the stream discharge. (xi) If the stage-discharge relation is governed by the slope, size and roughness of the channel

over a considerable distance, the station is under channel control. (xii) A pressure transducer may be used to obtain: (a) a stage-hydrograph. (b) a discharge–hydrograph.

(xiii) The slope-area method is often used to estimate peak floods near the existing gauging station. (false: ii, iv, v, vi, viii, ix, xiii)

STREAM GAUGING