Sediment Output Variables-What do they mean???

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2009. All rights reserved.
I have to admit, I don't fully understand what all of the sediment output variables mean. These are the variables that are "plottable" in the sediment spatial plot and the sediment time-series plot. The sediment output level (1 through 6) determines how many of the variables are viewable. Some of them are very obscure and have strange names-I think a holdover from HEC-6 vernacular and possibly the internal Fortran code naming conventions. In any case, I've attached a list of all the sediment variables in version 4.0 of HEC-RAS, as well as variables that will be available in the next release, version 4.1. Hopefully this will help you to understand what you are looking at in the output.

HEC-RAS Sediment Output Variables
(courtesy Hydrologic Engineering Center)

1. Ch Invert El (ft) - Minimum elevation of the main channel at each output time step.
2. wsel (ft) - Elevation of the water surface at each output time step.
3. Observed Data - Observed elevation of main channel bed, entered by the user.
4. Invert Change (ft) - delta change in the minimum elevation of the main channel.
5. mass out: all (tons) – total sediment mass, for all grain size classes, going out of the sediment control volume, per individual computational time step.
6. mass out: class 1-20 (tons) – sediment mass leaving the sediment control volume per grain size fraction, per computational time step.
7. flow (cfs) – total flow at the cross section for each output time step.
8. velocity (ft/s) – average velocity of the movable portion of the bed at each time step.
9. shear stress (lb/sq ft) – average shear stress of the movable portion of the bed at each time step.
10. EG Slope (ft/ft) – slope of the energy gradeline at each output time step. This can be a point value at the cross section or an average value between cross sections.
11. mass bed change cum: all (tons) – cumulative mass of the change in the bed elevation over time.
12. mass bed change cum: class 1-20 (tons) – cumulative mass of the change in bed elevation over time, per grain size fraction (Bins 1 – 20). This only displays the size fraction bins that are being used.
13. mass bed change: all (tons) – Incremental total mass change in the bed for the current computational time step.
14. mass bed change: class 1–20 (tons) – Incremental mass change in the bed for the current time step, by individual grain size fraction.
15. mass out cum: all (tons) – cumulative total sediment mass going out of the sediment control volume for a specific cross section, per individual computational time step.
16. mass out cum: class 1-20 (tons) – cumulative sediment mass leaving the sediment control volume per grain size fraction, at a cross section, per computational time step.
17. mass capacity: all (tons/day) – Transport capacity in total mass at the current computational time step.
18. mass capacity: class 1-20 (tons/day) - Transport capacity in mass, by grain size fraction, at the current computational time step.
19. d50 cover (mm) – d50 of the cover layer at the end of the computational increment. Used in the Exner 5 bed sorting and armoring routine.
20. d50 subsurface (mm) – d50 of the surface layer material at the end of the computational time step. Used in the Exner 5 bed sorting and armoring routine.
21. d50 active (mm) – d50 of the active layer of the simple active layer bed sorting and armoring routine.
22. d50 inactive (mm) – d50 of the inactive layer at the end of each computational time step. Used in the Exner 5 and simple active layer bed sorting and armoring routine.
23. cover thickness (ft) – thickness of the cover layer at the end of each computational time step. Used in the Exner 5 bed sorting and armoring routine.
24. subsurface thickness (ft) - thickness of the surface layer at the end of each computational time step. Used in the Exner 5 and simple active layer bed sorting and armoring routine.
25. active thickness (ft) – thickness of the active layer at the start of each computational time step. Used in the simple active layer bed sorting and armoring routine.
26. mass cover: all (tons) – total tons of material in the cover layer at the end of each computational time step. Used in the Exner 5 bed sorting and armoring routine.
27. mass cover: class 1-20 (tons) – tons of material in the cover layer at the end of each computational time step, by individual grain size fraction. Used in the Exner 5 bed sorting and armoring routine.
28. mass subsurface: all (tons) – total tons of material in the surface layer at the end of each computational time step.
29. mass subsurface: class 1-20 (tons) – tons of material in the surface layer at the end of each computational time step, by individual grain size fraction.
30. mass inactive: all (tons) – total tons of material in the inactive layer at the end of each computational time increment.
31. mass inactive: class 1-20 (tons) – tons of material in the inactive layer at the end of each computational increment, by individual grain size fraction.
32. Armor reduction: all (fraction) – fraction that the total sediment transport capacity is reduce to, based on the concepts of a cover layer computation.
33. Armor reduction: class 1-20 (fraction) – fraction for each individual grain size, that the transport capacity is reduce to, based on the concepts of a cover layer computation.
34. Sediment discharge (tons/day) – total sediment discharge in tons/day going out of the sediment control volume for a specific cross section, per individual computational time step.
35. Sediment concentration (mg/l) – total sediment concentration in mg/liter going out of the sediment control volume at the end of the computational time step.
36. Eff depth (ft) – effective depth of the water in the mobile portion of the cross section, at the end of the computational time step.
37. Eff width (ft) – effective width of the water in the mobile portion of the cross section, at the end of the computational time step.
38. Ch Manning n () – main channel manning’s n value.
39. Ch Froude Num () – main channel Froude number at the end of the current computational time step.
40. Shear velocity u* (ft/s) – shear velocity. Used in Shields diagram and several sediment transport potential equations.
41. d90 cover (mm) – d90 of the cover layer at the end of the computational increment. Used in the Exner 5 bed sorting and armoring routine.
42. d90 subsurface (mm) – d90 of the surface layer material at the end of the computational time step. Used in the Exner 5 bed sorting and armoring routine.
43. d90 active (mm) – d90 of the active layer of the simple active layer bed sorting and armoring routine.
44. d90 inactive (mm) – d90 of the inactive layer at the end of each computational time step. Used in the Exner 5 and simple active layer bed sorting and armoring routine.
45. Mean Eff Ch Invert (ft) – Average channel invert elevation computed by subtracting the effective depth of the main channel from the water surface elevation.
46. Long. Cum Mass change (tons) – Total change in bed mass, cumulative in space and time. Spatial accumulation is from the current cross section to the upstream end of the river reach in which this cross section resides.

Stage-discharge curve for Dam Breach modeling

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2009. All rights reserved.
In the NWS DAMBRK model, a single stage-discharge curve could be constructed external to the program and used as a method for defining the reservoir. Though this is an easy and stable way to define a reservoir, it lacks the ability to account for submergence effects, which could significantly affect the results once the breach has fully developed. My guess is this is why HEC-RAS does not allow for a stage-discharge curve to be used to define a reservoir.

If you use cross sections to define your reservoir (full dynamic hydraulic drawdown routing), then you can use an inflow hydrograph at the upstream-most cross section. If you use a storage area to define your reservoir (level pool drawdown routing), then a simple stage-storage curve is required. A storage area can also have a lateral inflow hydrograph attached to it. You also have the option to determine your breach hydrograph external to HEC-RAS and just enter it as the upstream boundary to a cross section. That way you could avoid modeling the reservoir in HEC-RAS altogether.

A family of stage-discharge curves (rating curves) CAN be used at the inline structure to define flow through the structure prior to the breach. This can be used to describe a complicated gate strcture or spillway-one that is not available explicitly in HEC-RAS. However, the weir equation will be used to define flow through the breach-in addition to any other outflow you may have at the dam. And this is not the same as using a stage-discharge curve to define the reservoir.

Bug in Sediment Transport Boundary Condition

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2009. All rights reserved.
In the sediment data, rating curve boundary condition editor, there is a bug in "saving" the total load. The first time you enter it, everything is fine-works great. However, if you change anything in the rating curve editor, save it, close the sediment data editor and then reopen it, you'll see that the original data is back in. RAS won't save new edits to the rating curve window in the current version (version 4.0). An easy way to get around this is to save RAS, close RAS completely, then open up the *.s01 (or *.s02, *.s03, whatever sediment file you are using) in a text editor. If you have an XML editor, it will be easier to sift through the data and find what you want to change. Then, find the data you wish to change and manually change it in the editor, external to HEC-RAS. Then reopen HEC-RAS, and you'll see the changes have been made. Caution! Be careful when making edits in the text editor. If you accidentally change any of the syntax, you could corrupt the sediment file, making it partially or totally unreadable by HEC-RAS. To be safe, it's not a bad idea to make a backup copy of the original file, before you make any edits to it.

Make changes in a text editor

or make changes in an XML Editor (easier)

Caution with interpolating cross sections

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2009. All rights reserved.
The interpolation feature in HEC-RAS makes it very easy to add cross sections to satisfy numerical spacing requirements. Althought the routines are very robust and easy to use, caution should be used when interpolating between cross sections with multiple blocked ineffective flow areas. While normal ineffective flow areas are interpolated quite well (provided there are ineffective flow triggers on both bounding cross sections), multiple blocked ineffective flow areas are NOT interpolated.

So if you have multiple blocked ineffective flow areas on either of the bounding cross sections, you must manually add the multiple blocked ineffective flow areas to the interpolated cross sections. I suggest using the graphical cross section editor when doing this, especially if you have a lot of cross sections to fix.

Minimum Flow requirements

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2009. All rights reserved.
It's common knowlege that HEC-RAS cannot go "dry" at any time during a simulation. However, very low depths can be problematic as well. Very low depths can lead to compounding errors that can cause instabilities, and eventually your model to crash. There are a couple of tricks to get around this problem. Pilot channels and added base flow. None of you inflow hydrographs should start with 0 inflow, unless there is sufficient backwater in you system to prevent the upstream cross section from going dry. The trick with adding base flow is to keep it as small as possible, while providing the necessary stability. Try 1% of the peak of the hydrograph. You definately do not want to add a significant amount of volume to your model before the flood wave arrives-that will limit the extent of hydrograph attenuation you'll get.

To add base flow to an inflow hydrograph, simply add a minimum flow value in the box at the lower left corner of the flow hydrograph editor. Then, for that hydrograph, RAS will use the base flow amount any time the hydrograph flow is less than the minimum flow value.

HEC-RAS Short Course-Basic and Sediment Transport!

Next month I will be teaming with Gary Wolff to teach an HEC-RAS short course through River Restoration Northwest. The course will be in Tacoma, Washington, March 23-27. Gary will be teaching Basic RAS the first 3 days, and I'll be teching Sediment Transport with RAS that last 2 days. If you're interested, don't wait too long, last year we completely booked up the same course. You can get more information and register at http://www.rrnw.org/index.html.

Wide Flat Cross Section Bottoms

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2009. All rights reserved.
A common problem with the import of cross sections from GIS that were developed with a terrain model generated from LIDAR is the cross section with a flat bottom. This is a result of LIDAR's inability to penetrate the water surface. As a result, you get a flat bottom that represents the water surface elevation in the channel at the time the LIDAR was taken.

This will cause problems right from the beginning of your simulation, particularly if you are running low flows (low depths), and your model will most likely crash right away. You can see the result of the lack of bathymetric data in the profile plot.

To fix this, you'll need to get some definition to the bottom of your cross section(s), either by approximating the channel bottom, or by field surveys.