1. Modeling River Ice and River Ice Jams with HEC-RAS
07 March 2007
Modeling River Ice and River Ice Jams with HEC-RAS
Dr. Steven F. Daly
USACE ERDC/CRREL
Hanover, NH 03755

2. Overview of Lecture River Ice Hydraulics Entering Ice Data in HEC-RAS
Modifications to Manning’s equation
Available flow area
Composite roughness
Hydraulic radius
standard step backwater procedure with ice
Entering Ice Data in HEC-RAS
Wide river ice jams – Theory
Simulating Ice jam with RAS

3. Manning’s Equation Manning equation for steady flow,
expressed for the flow velocity or the total discharge, Q

4. Manning’s Equation with IceB d
River ice covers always float at hydrostatic equilibrium.
Or more exactly Non-hydrostatic pressure will always be
temporary and relatively short lived
and for practical applications can be ignored.

5. Manning’s Equation with Ice: Area and Hydraulic RadiusB d
Adjust the terms of the Manning’s equation to account for the presence of ice
Recall that for open water R ~ d

6. Composite Roughness value
Ice region
Bed region
Max velocity
Velocity Profile
under steady flow conditions
If we assume that the average flow velocity in the ice region and the bed region are equal;
And we assume Manning’s equation applies to each;
And we assume the energy grade line is the same in both; ni nb
Sabaneev Formula

7. Type of Ice Condition Manning’s n value
Suggested Range of Manning’s n Values for Ice Covered Rivers The suggested range of Manning’s n values for a single layer of ice
Type of Ice Condition Manning’s n value
Sheet ice Smooth to 0.012
Rippled ice to 0.03
Fragmented single layer to 0.025
Frazil ice New 1 to 3 ft thick to 0.03
3 to 5 ft thick to 0.06
Aged to 0.02

8. The suggested range of Manning’s n values for ice jams
Thickness Manning’s n value
ft Loose frazil Frozen frazil Sheet ice
The suggested range of Manning’s n values for ice jams

9. Manning’s Equation with Ice for Rectangular Channels
At least 32% increase in total depth due to ice cover at uniform flow

10. Standard Step Backwater MethodZ2 Z1

11. The energy head loss can be expressed as the product of the mean
Energy Losses with Ice
The energy head loss can be expressed as the product of the mean
friction slope, , and the distance between cross sections, L
The friction slope is found from Manning’s equation. Often it is
Re-written in the following form, using the conveyance, K

12. Energy Losses with Ice K can be expanded as
There are several means of estimating the mean friction slope. The
Simplest is probably the mean of the upstream and downstream slope
Or other combination method

13. Entering Ice Data in HEC-RAS

14. Entering ice data
All ice data is entered using the geometry data window.
The most basic way to enter the ice data is cross section by cross section using the cross section editor

15. Pull down menus
Cross section
editor
Short cut
Buttons
River schematic

16. Add ice cover

17. Enter the ice cover thickness Enter the Manning’s n values
Ice jam data will be
covered in the next
section

18. Ice cover data
Using this window the ice cover properties can be entered for each cross section
However this has been found to be a long and tedious process
So a short cut table has been developed
Return to the main geometry editor

19. Enter ice cover data

20. Ice cover table
This table allows a fast way to enter the ice cover thickness and roughness at all the cross sections at once.
As long as the “ice jam channel” is set to “no” or (n) for all cross sections the program will calculate the flow properties using the ice properties that are entered.

21. Ice thickness in LOB, channel and ROB
Manning’s n value
in LOB,
channel and ROB
As long as all sections are “n” the ice cover properties entered
will be used.

23. Ice data
Once the ice data has been entered the geometry file can be resaved with a new name to separate the open water from the ice covered geometry data. This can be done by selecting Save Geometry Data AS from the “Geometric Data” window under the “file” menu

24. Forces on an Ice Jam (Plan View) (Profile) Shear Resistance at Banks
Internal Shear Strength
(Profile)
Wind stress
Shear From
Water Flow
Downstream Component
of Jam Weight

25. Ice Jam Sections Maximum depth given by equilibrium section Solid Ice
Transition
Uniform
Maximum depth
given by equilibrium section
Solid Ice
Cover
Equilibrium Section

26. Ice Jam Force Balance For Wide River Jam
Longitudinal Force
Gravity component, Sw = water surface slope
Fluid shear stress
Bank Shear Stress

27. Vertical Stress Granular ice mass floating at hydrostatic equilibriumz
Average vertical stress
Longitudinal stress
Transverse stress
Bank shear

28. Estimating the ice jam thickness based on the complete ice jam force balance equation
Starting with the ice jam force balance equation:

29. Ice Jam Force Balance Solution Procedure

30. Modeling ice jams
HEC-RAS simulates river ice jams by adjusting the jam thickness until the ice jam force balance equation and the standard step backwater equation are satisfied.

31. Ice Jam Force Balance Solution Procedure
Ice Jam Force Balance is solved from upstream to downstream
Assume an jam thickness at the next downstream section
Iterate until a solution is found. (25 Iterations max.) Relaxation procedure used.
Ice cannot completely block cross section at assure this a maximum flow velocity allowed under ice (5 fps default)
Minimum ice jam thickness based on the thickness set by the user

32. Global Solution Procedure
Initial Backwater calculations downstream to upstream using entered ice thickness
Ice Jam Force Balance solved upstream to downstream using flow values determined from backwater analysis
Backwater and Ice Jam Force Balance are alternated until a solution is achieved
The ice jam thickness is allowed to change only 25% of calculated required change in each iteration. (Global relaxation)

33. Global Solution Convergence
Water surface elevation at any cross section changes less than 0.06 ft, or a user supplied tolerance, and the ice jam thickness at any section changes less than 0.1 ft, or a user supplied tolerance, between successive solutions of the ice jam force balance equation.
A total of 50 iterations (or a user defined maximum number) are allowed for convergence.

34. Modeling Wide River Ice Jam
User specifies (globally or at each section)
Extent of the jam
Limit jam to channel or include overbanks
Material properties of the jam
Internal friction angle (45º)
Jam Porosity (0.4)
K1 (.33)
Maximum flow value under the jam (5 fps)
Manning’s n or let RAS estimate

35. Selecting the ice jam location
The user must select where an ice jam will be located. HEC-RAS cannot determine on its own where a jam will be.
The ice cover editor available from the cross section data window, or the ice cover table, available from the geometry data window can be used to locate the jam.

36. Selecting the ice jam location
The user must also determine if the jam will be confined to the channel or be allowed to extend into the over banks.

37. Selecting the ice jam location
Confining the jam to the channel is appropriate where
the water levels do not reach the overbank
The river ice is confined to the channel by trees
The overbank areas are very broad and an ice jam could not form in these areas

38. Ice jam confined to channel
LOB
ROB

39. Selecting the ice jam location
If the jam is allowed to enter the overbank area, then the flow properties of the overbank and the channel are combined to determine the average flow properties acting on the jam.

40. Ice jam in channel and over banks
ROB
LOB

41. Select ice jam option by setting where the ice jam will be located
By selecting Channel or Over banks
These boxes will become available
Channel plus over banks
Channel

42. Channel plus over banks
The ice jam option can also be set in the ice cover table. This is done by
changing the no’s, n, to yes’s, y, in the appropriate column. The user must
select if the ice jam is confined to the channel or will include both the
channel and the over banks.
Change each cross section individually. The values cannot be set to
y or n all at once, as the numeric values can.
Channel
Channel plus over banks

43. Selecting ice jam location
Remember: there must be section with fixed ice thickness at the upstream and downstream end of the jam.
Therefore, every cross section can not be set to yes. There must be at least one section with no at the upstream and downstream ends of the jam.

44. Ice Jam Parameters  = angle of internal friction ’ = density of ice
e = porosity of jam
k1 = ratio of lateral to longitudinal pressure

45.  = angle of internal friction
’ = density of ice
 = angle of internal friction
e = porosity of jam
k1 = ratio of lateral to longitudinal pressure
The user can enter the values on the ice cover editor
for each cross section individually. Note that default
values of the parameters have already been entered.

46. Ice jam parameters
The user can also set the ice jam parameters globally using the ice cover table. Note that default values of the parameters have already been entered.

47. k1 = ratio of lateral to longitudinal pressure e = porosity of jam
The numeric values
can be set globally,
like the ice
thickness and
roughness values
k1 = ratio of lateral to longitudinal pressure
e = porosity of jam
 = angle of internal friction
’ = density of ice

48. Manning’s roughness of jam
The user can either fix the Manning’s n value for the jam or let HEC-RAS estimate the value based on Nezhikovsky’s formula

49. Estimation of Manning’s n for Ice Jam using Nezhikovsky’s formula
t >1.5 ft
t <1.5 ft

50. checked, these values are considered fixed, and will be used
If the box below is
checked, these values are
considered fixed, and will be used
By un-checking this box, the user allows RAS to estimate the
Jam roughness

51. By changing this column from yes to no, the user will allow
HEC-RAS to estimate the Manning’s n value of the jam.

52. Maximum under ice velocity
Note that there is one parameter that we have not discussed. This is the maximum under ice velocity. Where did this come from?

53. Maximum under ice velocity
Recall from the early lecture that the stresses in the cover were developed assuming that the ice cover is floating at hydrostatic equilibrium.
Assume zero stress
at bottom of jam

54. Maximum under ice velocity
As a result, the stress analysis is not valid when the ice cover contacts the bed of the channel.
Therefore the calculations must assure that the ice cover does not contact the bed.
When the ice cover contacts the bed, the under ice area approaches zero.

55. Maximum under ice velocity
Recall by continuity
Q=VA or
V=Q/A=Q/(dB)
Therefore, by setting a maximum under ice velocity, we are assuring that the area under the ice cover does not become too small.
The value can be increased by the user if necessary. The result will be that the ice jam will become thicker and the area beneath the jam less.

56. Maximum under ice velocity
The user can enter the values on the ice cover editor
for each cross section individually. Note that a default
value has already been entered.

57. The numeric values
can be set globally,
like the ice
thickness and
roughness values
Set the maximum under ice velocity globally

59. Running the ice jam simulation
Once a geometry file has been created with the ice jam option selected, RAS will do an ice jam simulation.
Select the steady flow analysis button from the main interface

60. Use file menu to name and save plan
Push compute button to perform analysis

61. Indicates progress of calculation and if finished normally
Indicates Number of iterations of the ice jam force balance

62. Viewing Results
We can view results using the profile plot, the cross section plot, the x-y-z perspective plot, and the rating curve.

63. Profile plot

64. X-y-z perspective plot

65. Profile Output
Table
Ice Cover Table

66. Modeling Ice Jams Jam Location Volume of Ice in Jam
RAS will calculate volume but will not limit jam length based on volume.
Volume = (Reach length x width x thickness ) x % of ice that reaches jam location
Make sure thickness is not limited by max velocity
If channel is dry – there will be problems modeling ice
Appropriate Flows – Not high flows (2-10 year range)
Bridges