CEE 599: MODELING SEDIMENT TRANSPORT USING HEC-RASSTABLE CHANNEL DESIGN

Roxanne J CariniSpring 2016

Choices for computing

n

• Manning• Kuelegan• Strickler• Limneros• Brownlie• Soil Conservation Service

Manning

KueleganRecall Chezy C, related to n:

Strickler

Upper Regime v.

Lower Regime

LimnerosUpper Regime (Gravels & Cobbles) only!!

Brownlie

Soil Conservatio

n ServiceManning n versus VRH for 5 grass classes

Stable Channel

Design Methods

• Copeland– based in Brownlie’s work (~7,000 lab and field records)

• Regime theory– from study of irrigation canals in Pakistan and India

• Tractive Force method– analytical shear stress balance

Copeland • Predicts channel parameters and whether bed is aggrading or degrading (eroding) based on the following variables:

– Grain-related Froude number– Critical grain –related Froude number– Slope– Bed hydraulic radius– Median grain size (D50)– Sediment gradation coefficient (standard

deviation of gradation) – Kinematic viscosity– Manning “n” for side slopes

Regime Theory

(Blench)

Mostly works for the uniform steady flow sand bed trapezoidal channels for which it was developed.

Tractive Force

Method

• Calculates uniform shear on a “slice” of the channel.

• Compares this with a critical tractive force – Either from Shield’s analysis:

Tractive Force

Balance from Lane

Tractive Force from

Lane

HEC-RAS Tutorial

Stable Channel

Design Functions

1. Allow users to easily compute the hydraulic parameters of a given cross section.

2. Use that information to design a stable channel with regard to its size and armoring.

3. Determine the sediment transport capacity of that cross section.

Uniform Flow Computations• Open Hydraulic Design Functions

window• Select Type Uniform Flow• HEC-RAS will automatically select a

cross section (XS) from the geometry file to display. User can choose any XS from the dropdown menu.

• S/Q/y/n tab: calculate normal slope, discharge, depth, and roughness for the current XS

• Width tab: calculate bottom width for uniform flow in a user-defined compound channel

Solve for S/Q/y/n

• Enter 2 out of 3 fields (Slope, Discharge, W/S Elev) and solve for the third.

• Roughness, n: Automatically taken from geometry file, but can be changed. Function to define roughness can be chosen (6 options).

• Do not need to define n everywhere, only where it changes.

Solve for S/Q/y/n

• Roughness options: Manning, Keulegan, Strickler, Limerinos, Brownlie, Grass-lined channels

• Limerinos & Brownlie require gradation distribution (only applied to main channel) d85, d50, d16

• Brownlie requires sediment specific gravity =2.65• Kuelegan requires temperature • COMPUTE button will not become active until all

necessary parameters have been entered!• To solve for roughness, click on and delete only one

roughness value in the table. HEC-RAS will then compute a Manning’s n value to satisfy the uniform flow equation for the portion of XS that is desired. Then the roughness value is back-calculated to match the selected roughness function.

• Computed value will appear in bold.

Solve for Bottom

Width• Only occurs when user defines

compound channel.• Only trapezoidal compound channel

supported, with up to three levels: low flow channel, main channel, overbank channel.

• Bottom width, B, of main channel or overbank channel may be solved for.

• Subtraction or addition of width may be applied to right of centerline, left of centerline, or equally to both sides.

Solve for Bottom

Width• SSL/ SSR: Side slope of left/ right of channel• WL/ WR: Bottom width of left/ right side of channel from

centerline to the toe of side slope• Height: Distance from to top of side slope of a respective

channel (low flow channel, main channel, overbank channel)• Invert: invert of a respective channel• Click Apply Geometry to plot the data.• Default Manning’s n applied in Station-Elevation table below.

User may change these as before, but do so in the Width tab.• Click Apply Geometry after making any changes.

Solve for Bottom Width

• Enter energy slope, discharge, and water surface elevation in appropriate fields.

• Select Compute Widths and choose how: – Solve for main channel or overbank channel – Apply computed width left of CL only, right of CL only,

or centered equally (Total)• When all required data entered, COMPUTE

button will become active. Click it!• Unrealistic Geometries: Bottom width of upper

channel cannot become less than top width of the channel below it.

• Acceptable Geometries: Top width of lower channel can become greater than bottom width of channel above it. HEC-RAS automatically increases the upper channel’s bottom width to compensate.

Solve for Bottom Width

• Click Copy XS to Geometric Data.• Enter the river station you want this XS to be

applied to. If the river station already contains a XS, will be asked if you want to replace it. If not, XS will be added and distances between the XSs will be adjusted.

• Must check bed elevations to ensure all are referenced to the same datum.

– Go to Geometry window, click Cross Sections button.

– Select Options Adjust Elevations… • File SAVE!!! in the Hydraulic Design

Uniform Flow window.

Stable Channel

Design

• Open Hydraulic Design Functions window• Select Type Stable Channel Design

Copeland Method

• Choose Copeland tab.• Set Design Discharge.• Fill in other Required Input, including

Gradation.• Choose Manning or Strickler to compute

roughness n or k.• Optional Input includes choosing Default,

Upper, or Lower Regime. HEC-RAS will report if Transitional Regime found in calculations, although it will still use whichever method you chose (upper or lower).

Copeland Method

• Once all Design section inputs complete, click Inflow Sediment button to add information about the upstream sediment concentrations that will enter your Design section.

• Option 1: Set Inflow Sediment Concentration.• Option 2: Ask HEC-RAS to calculate Inflow

Sediment Concentration. – Enter necessary inputs for the Supply Reach.– Click OK.• When all required data entered, COMPUTE

button will become active. Click it!• Will receive 20 different options for Stable

Channel Design!

Copeland Output

• Select one to view plot.

• Click OK.

Copeland Output

• Once computation run, these buttons will activate.

• Click to see Table again.

• Click to see Stability Curves.

• Click to Copy results into Geometry File.

Regime Theory

• Choose Regime tab and fill in Required Input.– Side Factor based on Blench work: 0.1 for friable

banks, 0.2 for silty, clayey, or loamy banks, or 0.3 for tough clay banks. Default value is 0.2.

• When all required data entered, COMPUTE button will become active. Click it!

• The Stable Channel Regime values for depth, width, and slope will be solved for and will appear in their appropriate fields.

• Plot window will display resulting XS.• Click Copy XS to Geometric Data to add XS as before.

Tractive Force Method• Choose Tractive Force tab and fill in Required Input.– Angle of Repose: see RM Figure 12-9 for suggested

values.• Choose method with which to solve or critical shear:

Lane or Shields.• Remaining values are dependent variables. Only two

can be solved for at a time. Must provide other two. (Note: all three fields for particle diameter are considered just one variable.)

• When all required data entered, COMPUTE button will become active. Click it!

• Plot window will display resulting XS.• Click Copy XS to Geometric Data to add XS as

before.

Next Time...

1. Allow users to easily compute the hydraulic parameters of a given cross section.

2. Use that information to design a stable channel with regard to its size and armoring.

3. Determine the sediment transport capacity of that cross section.

Sediment Transport Capacity

Example Setup

• Open BEAVCREK.prj project file in HEC-RAS.• Run Steady Flow Analysis with: – Geometry File: Bvr. Cr. + Bridge – P/W: New Le, Lc– Steady Flow File: Beaver Cr. – 3 Flows• Run subcritical steady flow analysis.• Save Plan File.• View water surface profile plot with all 3

flows.

Hydraulic Design

• Click HD button.• Choose Type Sediment Transport

Capacity.• Sediment Reach: Series of cross-sections for

which sediment transport capacity is computed.

– Can have multiple Sediment Reaches within one River Reach, but they cannot overlap.

– Cannot have a Sediment Reach span more than one River Reach.

• File New Sediment Reach: Name the reach and define its spatial extent (river, reach, US RS, DS RS)

Save and Define

Sediment Reach• File Save Hydraulic Design

Data As... And name the file.• River = Beaver Creek• Reach = Kentwood• US RS = 5.99 (US most XS)• DS RS = 5.49 (just US of bridge)• Profiles = select all 3 flows• Temperature = 55 F• Specific Gravity = 2.65• Concentration Fines (optional)

Define Sediment

Reach• Bed Stations: XS stations that

separate LOB from main channel and ROB from main channel for sediment transport computations.

– Default = main bank stations– Values can be changed for every XS

in sediment reach– Appear as yellow nodes and

bracketed by “Mobile Bed” (MB) arrows at top of plot

• Bed Station Left = 866• Bed Station Right = 948

Define Sediment

Reach

• Functions = Check boxes for whichever functions you’d like HEC-RAS to use to compute sediment transport.

• When you select a function, the dialog box below lists its specifications and assumptions. Really helpful for deciding which is most appropriate for your river!!!

Define Sediment Reach

• Gradation = User can enter up to 50 particle size/ percent finer relationships. Right-click to expand the chart. Typically 5-10 gradation points is sufficient. If a 0% and 100% diameter are not specified, HEC-RAS will use first and last specified diameters for those percentages, respectively.

• Enter Gradation from HW 3 and 4 into the LOB, Main Channel, and ROB charts.

• Plot Gradation = graphical representation of % Finer versus grain size curve

Compute Sediment Transport

• Choose Compute for this Sediment Reach, or Compute for all Sediment Reaches, if you’ve created more than one and they all have the same conditions.

• Options Menu: – Fall Velocity: Default chooses method used in the

research of the selected function(s).• Options = Toffaleti, Van Rijn, Rubey– Depth/Width: Default chooses depth/width

combination used in the research of the selected function(s).

• Options = Effective Depth/ Effective Width, Hydraulic Depth/ Top Width, Hydraulic Radius/ Top Width

– Compute for Small Grains Outside Applicable Range: Default for HEC-RAS to perform calculations for grain sizes which are smaller than the applicable range for a given transport function. Select “No” to override and only make computations within the applicable range for each transport function (Table 12.7 in RM).

Compute Sediment Transport

• Click Apply anytime to save current changes to the file.

• Click Compute once all specifications are made.

• Click Close once computations finished.• Use Sediment Rating Curve Plot button

to view plot of sediment transport capacity rates for a selected cross section within a sediment reach.

• Use Sediment Transport Profile Plot button to view sediment transport capacity rates along a selected sediment reach.

Sediment Rating

Curve

Sediment

Transport Profile

Report • Click Report button within each plot window to get table of results with description.

• Report will show only those results selected and plotted on the graph.

• River Station: choose amongst those within the sediment reach (5.99 to 5.49 here)

• Sediment Reach: choose sediment reach (only 1 here)

• Profiles: choose all or only select certain flows (3 flows here)

• Functions: choose from those computed

• Subsections: Total, LOB, Main, ROB

• Grain Size: All, or only selected ranges

In Class Exercise

1. Create 2 Sediment Reaches: one from US-most RS to just US of bridge, the other from just DS of bridge to DS-most RS.

2. Choose 3 Sediment transport functions to run and compare.

3. Answer these questions:• Which station has the highest sediment

transport capacity? Which has the lowest?• Is this what you would expect based on the

results for velocities, shear stress, stream power etc.?

• If you had to actually assess a value for sediment transport capacity, which of the methods would you choose? Why?