# Presentation: 056152

## Modeling River Ice and River Ice Jams with HEC-RAS

**Dr. Steven F. Daly**
**USACE ERDC/CRREL**
**Hanover, NH 03755**

**07 March 2007**

## Overview of Lecture

**River Ice Hydraulics**
**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**

## Manning’s Equation

- Manning equation for steady flow,
- expressed for the flow velocity or the total discharge, *Q*

## Manning’s Equation with Ice

- Or more exactly Non-hydrostatic pressure will always be
-  temporary and relatively short lived
- and for practical applications can be ignored.

**Manning’s Equation with Ice**

## Manning’s Equation with Ice: Area and Hydraulic Radius

*Recall that for open water R ~ d*

**Manning’s Equation with Ice: Area and Hydraulic Radius**

## Composite Roughness value

-  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*

**Composite Roughness 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					0.008 to 0.012
- Rippled ice						0.01 to 0.03
- Fragmented single layer				0.015 to 0.025
- Frazil ice New 1 to 3 ft thick				0.01 to 0.03
- 3 to 5 ft thick 						0.03 to 0.06
- Aged 							0.01 to 0.02

- Suggested Range of Manning’s n Values for Ice Covered RiversThe suggested range of Manning’s n values for a single layer of ice

## The suggested range of Manning’s n values for ice jams

-  Thickness 				          Manning’s n value
- 	ft	    Loose frazil			     Frozen frazil 			Sheet ice
- 	0.3		-				 - 			  	0.015
- 	1.0 		0.01 				0.013 				0.04
- 	1.7 		0.01 				0.02 				0.05
- 	2.3 		0.02 				0.03 				0.06
- 	3.3 		0.03 				0.04 				0.08
- 	5.0 		0.03 				0.06 				0.09
- 	6.5 		0.04 				0.07 				0.09
- 	10.0 		0.05 				0.08 				0.10
- 	16.5 		0.06 				0.09

- The suggested range of Manning’s n values for ice jams

## Manning’s Equation with Ice for Rectangular Channels

- At least 32% increase in total depth due to ice cover at uniform flow

**Manning’s Equation with Ice for Rectangular Channels**

## Standard Step Backwater Method

- d1

- d2

- Z1

- Z2

**Standard Step Backwater Method**

## 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*

**Energy Losses with Ice**

## Energy Losses with Ice

- Or other combination method

*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

## Entering  Ice Data in HEC-RAS

## 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**

## Pull down menus

- River schematic

- Cross section
- editor

- Short cut
- Buttons

## Add ice cover

## Enter the ice cover thickness

- Enter the Manning’s n values

- Ice jam data will be
-  covered in the next
- section

## 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**

## Enter ice cover data

## 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.**

## As long as all sections are “n” the ice cover properties entered

- will be used.

- Ice thickness in LOB, channel and ROB

- Manning’s n value
- in LOB,
- channel and ROB

## Slide 22

## 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**

## Forces on an Ice Jam

***Forces on an Ice Jam***

## Ice Jam Sections

***Ice Jam Sections***

## Ice Jam Force Balance For Wide River Jam

**Longitudinal Force**

**Gravity component, S****w**** = water surface slope**

**Fluid shear stress**

**Bank Shear Stress**

## Vertical StressGranular ice mass floating at hydrostatic equilibrium

**z**

**Vertical Stress****Granular ice mass floating at ****hydrostatic equilibrium**

**Average vertical stress**

**Longitudinal stress**

**Transverse stress**

**Bank shear**

## Estimating the ice jam thickness based on the complete  ice jam force balance equation

**Starting with the ice jam force balance equation:**

## Ice Jam Force BalanceSolution Procedure

## 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.**

## 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**

## 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)**

## 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.

## 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

## 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.**

## 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.**

## 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**

## Ice jam confined to channel

**Confined to Channel**

**ROB**

**Ice jam confined to channel**

**LOB**

## 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.  **

## Ice jam in channel and over banks

**LOB**

**ROB**

## Select ice jam option by setting where the ice jam will be located

- Channel

- Channel plus over banks

- By selecting Channel or Over banks
- These boxes will become available

## The ice jam option can also be set in the ice cover table. This is done by

- 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

## 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.**

## Ice Jam Parameters

******** ****= angle of internal friction**
*********’***** ****=  density of ice**
***e *****=  porosity of jam**
***k******1***** = ratio of lateral to longitudinal pressure**

## ’ =  density of ice 

***’* =  density of ice* *
** = angle of internal friction
*e *=  porosity of jam
*k**1* = 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.

## 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.**

## k1 = ratio of lateral to longitudinal pressure

*e *=  porosity of jam
** = angle of internal friction
***’* =  density of ice

- The numeric values
-  can be set globally,
- like the ice
- thickness and
- roughness values

## 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**

## Estimation of Manning’s n for Ice Jamusing Nezhikovsky’s formula

***t >1.5 ft***

***t <1.5 ft***

**Estimation of Manning’s n for Ice Jam****using Nezhikovsky’s formula**

## By un-checking this box, the user allows RAS to estimate the 

- By un-checking this box, the user allows RAS to estimate the
- Jam roughness

- If the box below is
- checked, these values are
- considered fixed, and will be used

## By changing this column from yes to no, the user will allow

- HEC-RAS to estimate the Manning’s n value of the jam.

## 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?**

## 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

## 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. **

## 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.**

## The user can enter the values on the ice cover editor 

- 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.

- Maximum under ice velocity

## The numeric values

-  can be set globally,
- like the ice
- thickness and
- roughness values

- Set the maximum under ice velocity globally

## Slide 58

## 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**

## Use file menu to name and save plan

- Push compute button to perform analysis

## Indicates progress of calculation and if finished normally

- Indicates Number of iterations of the ice jam force balance

## Viewing Results

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

## Profile plot

## X-y-z perspective plot

## Profile Output 

- Profile Output
- Table
- Ice Cover Table

## 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**