Tuesday, March 31, 2009

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.

13 comments:

  1. Thanks a lot for this, you'd think they would put these descriptions in the documentation!

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  2. Any idea how mean effective channel invert elevation is determined? You would think that the mean effective channel invert elevation at any given cross section would be equal to the bed elevation if your cross section is a perfect trapezoid, but this is not the case.

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  3. Ah, i see #45 now. Thanks.

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  4. Thanks. its very helpful for Analysis in HEC RAS

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  5. Thank you for this information.

    The Sediment Transport Analysis requires as input the simulation period, associated with a defined date day/month/year. It has been verified that no reasonable modeling results are obtained for certain periods. If using 1985 simulation date, HECRAS delivers reasonable results, but for other periods and for the same input data, the program provides unreasonable results

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  6. Thanks a lot.....!!!!!!!!!

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  7. Thanks Chris. It sure is very helpful.

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  8. Hello.

    I am creating a model for Sediment transport. I see there a lot of output Variables. I´m interesting in several of them, for example, "tons". I have read that metric ton is different from "English" ton.

    My model is in SI, so I wonder if the results are in metric ton or "English" ton.

    Thanks.

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  9. Hi
    I am modelling a river for sediment transport and using sediment load series as sediment boundary condition.. I am using SI unit system. I have found two kinds of units in HEC-RAS. As input sediment boundary load series the unit is tonnes and after we simulate the model we can see the mass in for each cross section and the unit of that is ton.. These values should be same. Since the units are different the values should follow the proportion of tons and tonnes. But they are very different. if we input a sediment load series of 1 tonne per day at a cross section, after simulating the model the mass in for that cross section is equals to 2.43 tons per day.. Why is this ??? is this a bug or am I doing something wrong???

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    Replies
    1. Sounds like a bug. Please send it in to HEC. www.hec.usace.army.mil

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    2. Thanks for the reply. It's a bug and was fixed in the new version of HEC-RAS 5.0 beta.( 2014.10.01)

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  10. Whoa, there sure are a lot more variables than I had initially thought. Thanks for sharing!

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