5.11
Result of subcritical flow analysis
511
Plan: Plan 01 11/9/2006
s ome river creek
465
Legend
EG PF 1
460
WS PF 1
Crit PF 1
Elevation (ft)
455
Ground
450
445
440
435
430
0
1000
2000
3000
4000
5000
Main Channel Dis tance (ft)
Figure1. Water surface profile from subcritical analysis
Table 1.
River Sta Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev E.G. Slope Vel Chnl Froude # Chl
(cfs)
(ft)
(ft)
(ft)
(ft)
(ft/ft)
(ft/s)
6000 10000
450.2
463.24
459.99
463.63 0.001286
6.65
0.34
4000 10000
448.2
458.38
457.99
459.81
0.00587
11.76
0.7
3000 10000
442.2
454.29
451.99
454.85 0.002012
7.86
0.42
1500 10000
439.4
449.19
449.19
450.97 0.007486
12.89
0.79
1000 10000
433.4
443.2
443.2
444.97 0.007421
12.85
0.78
Comment
The profile shows that the flow condition between 0 and 500 ft is critical flow. HEC-RAS does
not find water surface elevation for subcritical flow given a channel geometry and slope and flow
rate (HEC-RAS sets water surface to critical depth by default if the solution is not converged).
This implies that this slope is hydraulically steep slope. All other reach sections are expected to
be hydraulically mild since flow is subcritical. This is verified by mixed flow analysis.
Result of mixed flow analysis
511
Plan: Plan 01 11/9/2006
s ome river creek
465
Legend
EG PF 1
460
Crit PF 1
WS PF 1
Elevation (ft)
455
Ground
450
445
440
435
430
0
1000
2000
3000
4000
5000
Main Channel Dis tance (ft)
Figure 2. Water surface profile from mixed flow analysis
Table 2.
River Sta Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev E.G. Slope Vel Chnl Froude # Chl
(cfs)
(ft)
(ft)
(ft)
(ft)
(ft/ft)
(ft/s)
6000 10000
450.2
463.24
459.99
463.63 0.001286
6.65
0.34
4000 10000
448.2
458.38
457.99
459.81
0.00587
11.76
0.7
3000 10000
442.2
454.29
451.99
454.85 0.002012
7.86
0.42
1500 10000
439.4
449.19
449.19
450.97 0.007486
12.89
0.79
1000 10000
433.4
442.5
443.2
445.2 0.011921
15.36
0.98
Comment
In the mixed flow situation, both subcritical and supercritical conditions occur. HEC-RAS can
simulate water surface profile for the mixed flow by performing both subcritical analysis from
downstream to upstream and supercritical analysis from upstream to downstream. The result of
this analysis clearly shows that the flow is a subcritical flow followed by supercritical flow.
5.13
River geometry was set up as follow
513
.05
Plan: Plan 01
11/9/2006
.035
.06
.03
10
Legend
EG PF 1
WS PF 1
Crit PF 1
Ground
Elevation (ft)
9
Bank Sta
8
7
6
0
20
40
60
80
100
120
Station (ft)
River bank is defined at station 40 for left side bank and at station 80 for right side bank.
Uniform flow with this geometry and slope 0.0008 are solved for flow in hydraulic design,
uniform flow option, producing 279 cfs. Go back to HEC-RAS geometry (see geometry setting),
and try subcritical flow, and supercritical flow. At this point, flow condition is not known. The
boundary condition for subcritical flow analysis was set as follows
 Flow = 279 cfs
 Normal depth downstream = 0.0008 or Know WS downstream = 9.8 ft
Either of downstream boundary condition produces the same result because of uniform flow at
this section. The WS profile shown below indicates the uniform flow is subcritical flow. The
hydraulic variables at downstream (station 0) are given in the table. The kinetic energy flux
correction coefficient (alpha) is 1.33. Specific energy is sum of water depth (pressure head) and
velocity head, 3.88 ft. This is given by energy grade elevation (9.88 ft) minus channel bottom
elevation (6 ft)
513
Plan: Plan 01 11/13/2006
Roaring Creek main
10
Legend
EG PF 1
WS PF 1
9
Crit PF 1
Elevation (ft)
Ground
8
7
6
5
0
200
400
600
800
Main Channel Dis tance (ft)
Figure. Water surface profile from Subcritical analysis
1000
Table. Hydraulic variables at station 0 (downstream)
E.G. Elev (ft)
Vel Head (ft)
W.S. Elev (ft)
Crit W.S. (ft)
E.G. Slope (ft/ft)
Q Total (cfs)
Top Width (ft)
Vel Total (ft/s)
Max Chl Dpth (ft)
Conv. Total (cfs)
Length Wtd. (ft)
Min Ch El (ft)
Alpha
Frctn Loss (ft)
C & E Loss (ft)
9.88
0.08
9.8
7.76
0.000801
279
106.63
1.97
3.8
9860.8
100
6
1.34
0.08
0
Element
Wt. n-Val.
Reach Len. (ft)
Flow Area (sq ft)
Area (sq ft)
Flow (cfs)
Top Width (ft)
Avg. Vel. (ft/s)
Hydr. Depth (ft)
Conv. (cfs)
Wetted Per. (ft)
Shear (lb/sq ft)
Stream Power (lb/ft s)
Cum Volume (acre-ft)
Cum SA (acres)
Left OB
Channel Right OB
0.05
0.035
0.049
100
100
100
14.08
114.05
13.34
14.08
114.05
13.34
6.63
265.95
6.43
33.61
40
33.02
0.47
2.33
0.48
0.42
2.85
0.4
234.2
9399.4
227.1
33.63
42.17
33.04
0.02
0.14
0.02
0.01
0.32
0.01
0.32
2.62
0.31
0.77
0.92
0.76
Supercritical flow analysis was also tried using upstream boundary condition (normal depth
slope = 0.0008). The water surface profile computed by HEC-RAS shows that critical flow,
indicating no solution to the depth for supercritical flow.
513
Plan: Plan 01 11/13/2006
Roaring Creek main
8.5
Legend
EG PF 1
8.0
WS PF 1
Crit PF 1
Elevation (ft)
7.5
Ground
7.0
6.5
6.0
5.5
5.0
0
200
400
600
Main Channel Dis tance (ft)
Figure. Water surface profile from Subcritical analysis
800
1000