2. Agenda
• Model Speed - TIME_ACCEL Parameter
• Bridge Hydraulics
• Velocity Vectors
• Street Gutter Flow and Storm Drains
• Multiple Channels - Time of Concentration
• Debug Tool
• Tailings Dam Tool
3. Model Speed
• OMP – 95% complete
• Computer resources control model speed –
processor clock speed is more important than the
number of processors/cores
• Cloud applications
• Creating/editing data files for urban detail is limiting
– more efficient to divide up large project areas into
multiple grid systems
4. Key to Model Speed and Numerical Stability
Steep rising hydrographs and small grid elements result in slow models
Q
Time (hrs)
Q
t
Small timesteps = small inflow volume
5. Timestep Control: TOLER.DAT File
2 parameters
TOL DEPTOL
0.004 - 0.1 (ft) 0.0
Courant Numbers
Floodplain Channel Street
C 0.6 (default) - -
Timestep Accelerator
T 0.1 (default – increment by 0.1, max = 1.0)
Reservoir
only
6. Courant Criteria
t = C x / (v + c)
where:
C = 0.3 - 1.0
v = flow velocity
c = wave celerity
t = variable timestep
x = grid element spacing
(default = 0.6, internal spatial variability)
Rearranging: (v + c) = C x / t
7. In one timestep ∆t, a
particle of water has to
fall within the next grid
element.
Courant Criteria (C > 1.0)
x
x
Distance
Traveled
8. In one timestep ∆t, a
particle of water has to
fall within the next grid
element.
Avoid over-steeping the wave front causing numerical surging.
Courant Criteria (C < 1.0)
x
x
Distance
Traveled
9. Timestep Accelerator Parameter
Controls the rate of increase in the computational timestep
• 0.1 gradually increases the timestep
• 1.0 high rate of increase in timestep
10. TIME_ACCEL
0
1
2
3
4
5
6
7
8
9
0 5 10 15 20 25 30
Computational Timesteps as a Function of TIME_ACCEL
T_Line = 0.1
T-Line = 0.25
T-Line = 1.0
Timestep
Simulation Time in Hours
COMPUTER RUN TIME IS : 0.01709 HRS
0.00928 HRS
0.00879 HRS
All 3 simulations had 0.000000 volume conservation error
16. TIME_ACCEL Recommendations
• Start with 0.1 and build the model
• Increment by 0.1 to speed it up until the
model shows some instability
• Reduce by 0.05
17. Optional New FLO-2D Bridge Flow Routine
3 Flow Conditions:
• Free surface flow
• Pressure flow
• Pressure flow plus deck overtopping flow
No longer need external program for rating table.
18. Existing Bridge Method: Hydraulic Structure Rating Table
S BIGBRIDGE 0 1 631 625 1 0.0 0 0
T 0 0
T 1.73 560
T 2.81 1620
T 3.54 3100
T 5.07 5000
T 10.02 7500
T 14.5 13320
T 16.12 17510
T 17.68 21250
T 19.17 24620
T 20.24 26780
T 23.75 43900
Also use a discharge rating curve:
Q = a depth b
Broadcrested weir: Q = CL hb
19. New Routine: Five Types of Subcritical Bridge Flow
Type 1 Flow: Free surface, subcritical flow (Z > Yu > Yd)
20. Free Surface Flow
Q = 8.02 C A2 (∆h/β)0.5
where:
β = 1 - α1 C2 (A2 –A1)2 + 2gC2 (A2/K2)2 (LB +L1-2 K2/K1)
LB = length of contracted reach
L1-2 = length of the reach from cross section 1 to cross section 2 (Figure 7)
K1, K2 = conveyance at cross sections 1 and 2; K1 = 1.486/n A1 R1
0.67; K2 = 1.486/n A2 R2
0.67
n = Manning’s n-value through the contracted reach
A1, R1 and A2,R2 are the cross section flow areas and hydraulic radiuses respectively
C = Cc / (α2 + ke + kp)0.5; Cc = coefficient of contraction, α2 = energy coefficient at cross section 2, ke = eddy loss coefficient, kp = non-
hydrostatic pressure coefficient
21. Select 1 of 4 Bridge Types (from Hamill, 1999 p. 111-126)
Figures hardwired for interpolation in FLO-2D model.
22. Entering the Data
HYSTRUC.DAT
S-line followed by 2 B-lines
S BIGBRIDGE 0 3 631 625 1 0.0 0 0
T 0 0
T 1.73 560
B 1 0. 0. 0. 0. 1. 1. 1. 1. (Free surface flow discharge coefficients)
B 15. 40. 0.05 40. 1378.0 1380.0 22.0 0. 0. 0.50 3.05 0. 0. 1376.5 1377.2
(Bridge geometry and elevations)
Note: The QGIS bridge data dialog boxes are being developed.
Coefficient = 1.0 means that the bridge
condition doesn’t exist or is minor
Bridge routine – not rating curve or table
Remove rating
table pairs
23. Need 2 Cross Sections
Locations:
1) Upstream of the bridge -
normal depth
2) At the bridge contraction
The data is prepared in
BRIDGE_XSEC.DAT file
X 631 (inflow grid element)
0.00 1380.00 1385.00
0.60 1378.70 1378.46
5.00 1377.00 1376.96
5.50 1376.85 1376.68
6.00 1376.75 1376.46
12.65 1376.70 1376.46
15.85 1376.78 1376.51
18.95 1377.20 1377.00
20.65 1378.15 1377.26
22.00 1378.70 1378.44
22.10 1380.00 1385.00
25. Five Types of Subcritical Bridge Flow
Type 2 Flow: Inlet submerged, outlet free surface, partially full,
sluice gate flow (Yu > Z > Yd)
26. Sluice Gate Flow
Discharge mimics a sluice gate
Qp = CAb (2g ∆H)0.5
where:
C = coefficient of discharge (0.3 to 0.6 dimensionless, Figure 12)
Ab = cross section flow area through the bridge opening
g = gravitational acceleration
∆H = energy gradient from upstream to tailwater elevation Yd given by: Yu – Y + Vu2/2g
27. Five Types of Subcritical Bridge Flow
Type 3 Flow: Inlet submerged, outlet submerged, opening full, sluice
gate-orifice transition flow (Yu > Z > Yd)
28. Five Types of Subcritical Bridge Flow
Type 4 Flow: Inlet submerged, outlet submerged, orifice flow
(Yu > Yd > Z)
29. Orifice Flow
Pressure flow where both US and DS WS elevations are above low
chord: Yu > Z, Yd > Z (drowned opening):
Qp = CAb (2g ∆H)0.5
where:
C = Coefficient of discharge
Ab = Bridge opening cross section flow area
∆H = difference in the energy gradient (headwater and tailwater)
30. Five Types of Subcritical Bridge Flow
Type 5 Flow: Inlet submerged, outlet submerged, deck overflow
(Yu > Yd > Z)
31. Weir Flow
Flow over the deck (broadcrested weir flow):
Qw = C Lc ∆H1.5
where:
C = Broadcrested weir discharge coefficient which varies from 2.6 to 3.1
∆H = energy grade line
Lc = crest length
32. Pressure Flow Plus Weir Flow (QT = Qp + Qw)
If tailwater drowns the weir control, existing FLO-2D submergence
factor is applied internally
33. Comparison with HEC-RAS 1-D
• FLO-2D model of the Middle Rio Grande (170 miles)
• Calibrated to a 2005 prescribed dam release
• Model was applied to 30 yrs of historical spring
releases to support COE Upper Rio Grande Water
Operations Model
34. Los Lunas Bridge
• 2 US xsecs and 3 DS xsecs
• Deck is 90 ft wide
• 7 discharges: 100 cfs to 45,000 cfs
35. FLO-2D Model of Los Lunas Bridge
• 6 hrs simulation time for steady flow
• Type 1 bridge w/no abutment or embankment slopes
• Average low chord and deck elevations
• No flow angle of attack
• No Froude number coefficient
• Spatially variable overbank roughness
38. Results
• FLO-2D floodplain storage was not filled to match HEC-RAS
• Some FLO-2D US overbank flow returned to channel
• Free surface flow matched well
• Pressure and over deck flow was not comparable because of
floodplain overbank flow – shows the fallacy of using 1-D model on
2-D flooding
39. Bridge Routine Summary
Objective: Compute Q based on channel and bridge features.
External program for stage-Q relationships is no longer required
• A smooth transition between flow regimes is attempted
• User has complete control of all coefficients
40. Vector resolution is not used
directly by the model, but a
resolved vector is reported.
FLO-2D is a finite volume conservation model. Velocity
and discharge are computed in all eight flow directions for
each timestep.
Resolved Velocity Vectors
41. Momentum Equation
Sf = So - h/x - ( Vx /g)* (β Vx)/x - (1/g)* (αVx)/ t
Kinematic wave
Diffusive wave
Full dynamic wave
convective
acceleration
uniform flow = 0
local
acceleration
steady = 0
steady uniform flow friction slope = bed slope
pressure term
uniform flow = 0
β ~ 1.07 - 1.33 α ~ 1.0
47. Curb and Gutter
T
Curb Height = h
Sidewalk
Sx = h/T = 0.02
Flow Width T
Sidewalks
Street
FLO-2D Grid Elements
Flow from street to
sidewalk overtopping curb
Flow from sidewalk
to street
Flow from gutter element to
street element
Flow from street
element to street
element
Flow from gutter
element sidewalk to
floodplain element
Flow from gutter
element to gutter
element
Street Crown
48. Curb and Gutter
• Flow from street to sidewalk
• Flow from sidewalk to street
Curb height = h
Sidewalk
F
P
d = flow depth
FPD
FPE
Curb height = h
Sidewalk
F
P
d = flow depth
FPD
FPE
50. Curb and Gutter Results
• Concentrates flow
• Faster street velocities
• Higher head on storm drain inlets
• Forces more volume downstream
51. Multiple channels (rills and gullies) concept
• Concentrate the overland flow instead of sheet flow
• Higher depths = higher velocities and less infiltration
52. Shallow rectangular channels – rills and gullies
Purpose: Improve timing of overland sheet flow
Draw polyline – select width, depth and n-value
Slope is based on cell topography
Option: At bankfull Q multiple channels expand to
accept more Q (limit: cell width)
Multiple Channel Details
53. Multiple Channels Improve the Time of Concentration
Small rectangular channels – assign width, depth, n-values in MULT.DAT
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
Discharge,
cfs
Time
Observed
FLO-2D Saxton 2005
HEC-1 2005
RAINBOW WASH at SR-85
MEASURED DATA COMPARED WITH FLO-2D RESULTS
Storm: August 9, 2005; 5:00pm - 6:45pm
NEXRAD Locally-Adjusted Precipitation: 4.15" max, 2.43" Avg
Rainbow Wash Gage Measurement: 1.90" total
NOAA Atlas 14:
Frequency for Watershed Avg: 250-yr, 2-hr
Frequency for Gage: 50-yr, 2-hr
Observed: V = 169 ac-ft, Q = 1,919 cfs
FLO-2D 2005: V = 226 ac-ft, Q = 1,788 cfs
HEC-1 2005: V = 774 ac-ft, Q = 4,201 cfs
Volume Comparisons:
a. HEC-1 1993 Runoff Volume: 458 % of measured
b. Saxton 2005 Runoff Volume: 138 % of measured
59. Tailing Dam Tool
• Screening tool for mining engineers, civil engineers and regulators to identify
potential issues based on site and historical information.
• Use risk analysis to predict failure and subsequent release volume of stored
tailings.
60. Mount Polley Mine Tailing Dam Breach, 2014
Tailing Dam Breach Tool
1. Screening tool to identify potential issues based on site and historical data
2. Uses risk analysis to predict failure and release stored tailings
61. QGIS FLO-2D – Plugin Tool
#DamSafety19
• QGIS plug-in calls Tailing Dam Tool
• Generates the breach hydrograph
including tailings sediment volume
68. Tailing Dam Tool – Final Product
• INFLOW.DAT : Water and sediment breach hydrograph
for routing the mudflow downstream with FLO-2D
• Delineate the potential downstream area of inundation
for the worse case flood hazard for assessment
