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Methods of Discharge Calculation

This document discusses methods for assessing peak discharge and calculating design discharge for cross-drainage structures. It describes using the area-velocity method (Manning's formula), rational method, and Dicken's formula to calculate peak discharge. Mannings formula uses cross-sectional area, wetted perimeter, hydraulic mean depth, bed slope, and roughness coefficient to calculate velocity and discharge. The rational method uses rainfall data, time of concentration, catchment characteristics, and rainfall intensity to determine discharge. The recommended design discharge is the maximum value calculated using these different methods with limits on variation between methods.

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Harish Kumar Mahavar
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866 views3 pages

Methods of Discharge Calculation

This document discusses methods for assessing peak discharge and calculating design discharge for cross-drainage structures. It describes using the area-velocity method (Manning's formula), rational method, and Dicken's formula to calculate peak discharge. Mannings formula uses cross-sectional area, wetted perimeter, hydraulic mean depth, bed slope, and roughness coefficient to calculate velocity and discharge. The rational method uses rainfall data, time of concentration, catchment characteristics, and rainfall intensity to determine discharge. The recommended design discharge is the maximum value calculated using these different methods with limits on variation between methods.

Uploaded by

Harish Kumar Mahavar
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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HYDROLOGY AND HYDRAULICS OF THE CROSS – DRAINAGE STRUCTURES

Assessment of Peak Discharge

The peak discharge and the HFL have been calculated by the following methods:

 Area velocity method


 Rational method
 Dickens’s Formula

Area – Velocity Method (Manning’s Formula)

This method has been utilized to calculate the discharge from the stream cross-section and stream
slope/bed slope at the proposed bridge sites, for both major and minor bridges. After plotting the cross
section of the river, and marking the observed HFL, the cross sectional area (A) and wetted perimeter
(P) have been computed. In the absence of the flood slope of the stream, the bed slope of the river has
been estimated along its length.

The velocity and Discharge have been calculated using the Manning’s formula:

Q = A x V = A x [(1/n) x (R)2/3 x (S)1/2]

Where,

Q = the discharge in Cumecs;

A = Area of the cross section in Sq. M;

V = Velocity in M/Sec;

R = Hydraulic mean depth in M. = A / P;

P = Wetted perimeter of the stream in M;

S = Bed slope of the stream; and

n = Roughness Co-efficient.

The value of ‘n’ has been adopted as per soil criteria and river bed characteristics, observed at site and
are based on Table 3 in IRC SP-13 which has been reproduced below

Surface Perfect Good Fair Bad


Clean, straight bank, full stage, no rifts or deep pools 0.025 0.0275 0.030 0.033
Same as (I), but some weeds and stones 0.030 0.033 0.035 0.040
Winding, some pools and shoals, clean 0.035 0.040 0.045 0.050
Same as (3), lower stages, more ineffective slope and
0.040 0.045 0.050 0.055
sections
Same as (3) some weeds and stones 0.033 0.035 0.040 0.045
Same as (4), stony sections 0.045 0.050 0.055 0.060
Sluggish river reaches, rather weedy or with very deep
0.050 0.060 0.070 0.080
pools
Very weedy reaches 0.075 0.100 0.125 0.150
Rational Formula:

In this method, peak runoff is worked out from the rainfall data obtained from the concern
Meteorological department. Also time of concentration based on catchment characteristic is worked
out and then critical Intensity corresponding to time of concentration is obtained which further used
for calculating design discharge by the formula mentioned below:

Q=AI λ 0

Where,

Q = discharge in m /s
3

I = one-hour rainfall in cm/hr


0

A = catchment area in sq km
0 . 056×f ×P
λ=
t c +1
f = factor depend upon catchment area (Ref: Fig. 4.2 of IRC: SP: 13 as shown below)

P = co-efficient of runoff for catchment characteristics (Ref: Table 4.1 of IRC: SP:
13- 2004) which has been reproduced below

Catchment Area Value


Steep, bare rock and also city pavements 0.90
Rock, steep and wooded 0.80
Plateaus, lightly covered 0.70
Clayey soils, stiff and bare 0.60
Clayey soils, lightly covered 0.50
Loam, lightly cultivated or covered 0.40
Loam, largely cultivated 0.30
Sandy soil, light growth 0.20
Sandy soil, covered, heavy brush 0.10

t = time of concentration in hours


c

0. 385
L3
[
t c= 0. 87×
H ]
L = distance from critical point to structure in km.
H = fall in level from critical point to structure in m.

The size of the flood depends on the following factors:


Rainfall:
 Intensity
 Distribution in time and space
 Duration
Nature of Catchment
 Area
 Shape
 Slope
 Permeability of the soil and vegetable cover
 Initial state of wetness

The discharges thus obtained from Dicken’s method or Rational method has been compared with
discharge computed by Slope-Area method. The recommended design discharge would be the
maximum value of the discharges worked out as above (limiting to 50% variation, if the discharge
obtained by one method is greater than 1.5 times the discharge obtained by the other, the design
discharge is limited to 1.5 times of the smaller one). Accordingly, the design discharges has been
established for the drainage structures.

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