264
P. Sreeja and S. Singh
of attack of the flow, K 3 is the correction factor for bed condition, K 4 = 0.4(V R )
0.15
is the correction factor for armouring by bed material size, and a is the pier width.
Froehlich’s pier equation [9] is given by
y s = 0.32∅
a
0.62 y
0.47
1 Fr
0.22
1 D
−0.99
50
+ a
(5)
where y s is the pier scour depth; ∅ is the correction factor for pier nose shape; a
is the projected pier width with respect to the direction of the flow; y 1 is the depth
immediately at upstream of the pier; Fr 1 is the Froude number at the upstream of
pier; D 50 is the bed material grain size at which 50 per cent is finer; a is the pier
width.
Gumbel’s method is used for determining the design flood for different return
periods, and the corresponding flood values are used in the river flow modelling for
the determination of scour depth.
x T = ¯
x + K σ n−1
(6)
where x T is the value of T year flood event; ¯
x is the mean of the maximum
instantaneous flow; σ n−1 is the standard deviation of the maximum instantaneous
flow.
3 Results and Discussion
The various results obtained from the work done for the determination of depth of
scour around bridge piers of Saraighat Bridge in Brahmaputra river for a high flood
flow condition at different sections using Hec-Ras software.
The peak flow in the Brahmaputra River from a single gauging station at
Pandu Port has been determined using the Gumbel’s method and found to be
74,856.736 m
3 /s corresponding to return period of 50 years. Water surface profile and
velocity profile of the river corresponding to these flows have been computed using
Hec-Ras for three conditions, namely (i) without the bridge, (ii) with the existing
bridge and (ii) with two parallel bridges.
3.1 River Flow Analysis for the Existing Data
Velocity profile of the river without any bridge, with the existing bridge and the two
bridges together are determined for the existing data obtained from Gammon India
Ltd and return period 50 years. Figure 1 represents the comparison of velocity profile
of the river without bridge, with the existing bridge and the two bridges together.
P. Sreeja and S. Singh
of attack of the flow, K 3 is the correction factor for bed condition, K 4 = 0.4(V R )
0.15
is the correction factor for armouring by bed material size, and a is the pier width.
Froehlich’s pier equation [9] is given by
y s = 0.32∅
a
0.62 y
0.47
1 Fr
0.22
1 D
−0.99
50
+ a
(5)
where y s is the pier scour depth; ∅ is the correction factor for pier nose shape; a
is the projected pier width with respect to the direction of the flow; y 1 is the depth
immediately at upstream of the pier; Fr 1 is the Froude number at the upstream of
pier; D 50 is the bed material grain size at which 50 per cent is finer; a is the pier
width.
Gumbel’s method is used for determining the design flood for different return
periods, and the corresponding flood values are used in the river flow modelling for
the determination of scour depth.
x T = ¯
x + K σ n−1
(6)
where x T is the value of T year flood event; ¯
x is the mean of the maximum
instantaneous flow; σ n−1 is the standard deviation of the maximum instantaneous
flow.
3 Results and Discussion
The various results obtained from the work done for the determination of depth of
scour around bridge piers of Saraighat Bridge in Brahmaputra river for a high flood
flow condition at different sections using Hec-Ras software.
The peak flow in the Brahmaputra River from a single gauging station at
Pandu Port has been determined using the Gumbel’s method and found to be
74,856.736 m
3 /s corresponding to return period of 50 years. Water surface profile and
velocity profile of the river corresponding to these flows have been computed using
Hec-Ras for three conditions, namely (i) without the bridge, (ii) with the existing
bridge and (ii) with two parallel bridges.
3.1 River Flow Analysis for the Existing Data
Velocity profile of the river without any bridge, with the existing bridge and the two
bridges together are determined for the existing data obtained from Gammon India
Ltd and return period 50 years. Figure 1 represents the comparison of velocity profile
of the river without bridge, with the existing bridge and the two bridges together.