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A Comparison Between 1D and 2D Hydraulic Modeling for Bridge Replacement
ProjectsMark Bailey, PE
Manager, INDOT Office of Hydraulics
1D –vs- 2D
1D –vs- 2D1D 2D
HEC-1 released 1968 (51 years old) HEC-RAS version 5 released 2016 (3 years old)
Cross section survey + Quad maps (or better) Cross section survey + LIDAR
Output – tables, very basic graphics Output – can include animations
Variables that require engineering judgement and experience• Flow paths• Ineffective flow areas• Contraction and expansion coefficients• Flow splits
Many computations
1D –vs- 2D
1D –vs- 2D• Indiana Administrative Code Title 312 Article 10 Rule 2 Chapter 3 “Adversely
affect the efficiency of, or unduly restrict the capacity of, the floodway”
1D –vs- 2D
1D –vs- 2D
2D 1D
1D –vs- 2D
1D –vs- 2D
1D –vs- 2D
1D –vs- 2D
1D –vs- 2D
1D –vs- 2D
1D –vs- 2D
1D –vs- 2D
1D –vs- 2D
1D –vs- 2D
1D –vs- 2D
1D –vs- 2D
1D –vs- 2D
1D –vs- 2D
1D –vs- 2D
Date Total Q (cfs)Eel Q (cfs)
Eel % of Total
Killion Q (cfs) Killion % of Total
2/25/2018 6,691 6,450 96.4% 241 3.6%
5/5/2017 18,236 13,900 76% 4,336 24%
4/4/2018 35,768 27,800 78% 7,968 22%
1D –vs- 2DStorm Date
Total Q (cfs) Eel Q (cfs) Eel % of Total
Killion Q (cfs) Killion % of Total
2/25/18 6,691 6,450 96% 241 4%
HEC-RAS 1D 5,430 81% 1,261 19%
HEC-RAS 2D 6,624 99% 67 1%
SRH-2D 6,584 98% 96 1%
5/5/17 18,236 13,900 76% 4,336 24%
HEC-RAS 1D 14,789 81% 3,447 19%
HEC-RAS 2D 14,907 82% 3,328 18%
SRH-2D 14,435 79% 3,799 21%
4/4/18 35,768 27,800 78% 7,968 22%
HEC-RAS 1D 31,336 88% 4,432 12%
HEC-RAS 2D 26,754 75% 9,014 25%
SRH-2D 27,245 76% 8,520 24%
1D –vs- 2D
measured
HEC-RAS 1-D
HEC-RAS 2-D SRH-2D
-
1,000
2,000
3,000
4,000
5,000
6,000
7,000
Q (
cfs)
Eel River split for a total Q = 6,691 cfs
measured
HEC-RAS 1-D
HEC-RAS 2-D SRH-2D
-
200
400
600
800
1,000
1,200
1,400
Q (
cfs)
Killion Ditch split for a total Q = 6,691 cfs
1D –vs- 2D
measured
HEC-RAS 1-D
HEC-RAS 2-D
SRH-2D
13,200
13,400
13,600
13,800
14,000
14,200
14,400
14,600
14,800
15,000
Q (
cfs)
Eel River split for a total Q = 18,236 cfs
measured
HEC-RAS 1-DHEC-RAS 2-D
SRH-2D
-
500
1,000
1,500
2,000
2,500
3,000
3,500
4,000
4,500
5,000
Q (
cfs)
Killion Ditch split for a total Q = 18,236 cfs
1D –vs- 2D
measured
HEC-RAS 1-D
HEC-RAS 2-D
SRH-2D
24,000
25,000
26,000
27,000
28,000
29,000
30,000
31,000
32,000
Q (
cfs)
Eel River split for a total Q = 35,768 cfs
measured
HEC-RAS 1-D
HEC-RAS 2-DSRH-2D
-
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
9,000
10,000
Q (
cfs)
Killion Ditch split for a total Q = 35,768 cfs
1D –vs- 2D
14%
37%73%
Storms analyzed compared to Q 100
6,691 18,236 35,768
1D –vs- 2DPotential source of error in 2D models Likely impact(s)
Not incorporating channel data into surface Underestimating flow through Eel
Using the wrong manning’s n values Slight overestimation of flow through Killion
Not incorporating bridge pier data into models Slight overestimation of flow through Killion
1D –vs- 2DConclusions
• Using 1D - main bridge +13% and overflow structure -44%
• Using 2D - main bridge -4% and overflow structure +13%
• Using 2D would have shown the flow angle under the overflow structure
• Using 2D to calculate the flow splits and ineffectives would have taken significantly less time, much less experience, and been more accurate than the traditional 1D methodology
• Using 2D to calculate flow splits is very promising, further testing is warranted.
1D –vs- 2D
Special Thanks To
• Nabil Ghalayini HEC-RAS 2D modeling
• Scott Hogan SRH-2D modeling
• Jeff Woods Gage data
