Thursday, September 17, 2009

Sediment Mass Plot/Invert Change Issues

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2009. All rights reserved.

A very common "head-scratcher" for sediment modeling is the situation where your mass plot is showing a lot of accumulation of sediment at a given cross section (mass plot curve rises with time, indicating deposition), but the channel invert shows no change over the same time period. Usually this is due to the placement of the moveable bed limits.

Keep in mind the spatial plot only shows the minimum elevation node for each cross section. As a result, a depositional zone may not be captured correctly in that plot, if the mobile bed stations are missplaced. This example shows the mobile bed limits placed low in the channel. As the bed aggrades, the low point moves out to the mobile bed limit and its elevation remains constant throughout the simulation. The Sediment Spatial plot will indicate the cross section is stable, when in reality it is depositing a lot of sediment. A check of the mass plot will help to figure this out. One solution would be to move the bed limits out onto the banks, so that the minimum cross section point moves up with the deposition . Also, you could move the bank stations inside of the moveable bed limits, if that is acceptable to your conveyance distribution (remember, the minimum bed elevation is reported inside the bank stations). Or, you could leave it as is, and just recognize that the channel invert plot is misleading.

7 comments:

  1. I have another head scratcher associated with ras sediment modelling. I am an intermediate ras user but novice sediment modeller. I have entered all the data for a "point-source" sediment model but every time I run the model it gives me a zero flow value error. I am not sure where I am going wrong?

    Joel B

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  2. Is your quasi-unsteady flow file correct?

    ReplyDelete
  3. This is an important tip. I have also seen this go poorly if you dredge outside of your mobile bed limits. Dredging outside of the mobile bed limits (without turning on the option to deposit outside of the limits) can result in oddly shaped cross sections and a static thalweg even if your XS is depositing. This used to be much more confusing before the XS plotter. Now you can generally see what is going on by animating the XS.

    I like to look at the bed change as one of my first output variables but Tony Thomas (author of HEC 6) has talked about how it is a deceiving variable for a number of reasons. Mass is a much more reliable (if less intuitive) variable.

    ReplyDelete
  4. HOLA:
    ALGUIEN ME PUEDE AYUDAR
    AL MODELAR EL TRANSPORTE DE SEDIMENTOS EN HEC RAS ME SALE UN ERROR EN LA PRIMERA SECCION AGUAS ARRIBA.

    ERROR:
    River: "Yanaso" Reach: "unico" RS=889.4622 - has a zero flow value.


    POR FAVOR SI ALGUIEN ME PUEDE AYUDAR.
    SALUDOS

    ARTURO

    ReplyDelete
  5. HOLA:
    ALGUIEN ME PUEDE AYUDAR
    AL MODELAR EL TRANSPORTE DE SEDIMENTOS EN HEC RAS ME SALE UN ERROR EN LA PRIMERA SECCION AGUAS ARRIBA.

    ERROR:
    River: "Yanaso" Reach: "unico" RS=889.4622 - has a zero flow value.


    POR FAVOR SI ALGUIEN ME PUEDE AYUDAR.
    SALUDOS

    ARTURO

    ReplyDelete
  6. Arturo - yo trabajo en la oficina con Chris G.

    Primero, ?Qual tipo de sedimento "input" tiene en la primera seccion? ?"Rating Curve"? o ?"Sediment Load Series"? no le gusta a HEC-RAS un "Sediment Load Series" en la primera seccion. no se porque. Tipicamente yo asigno un "Rating curve" en la primera seccion con pequeno valores de sedimentos y un "sediment Load Series" a la segunda seccion con los valores reales por el modelo.

    espero que eso le ayuda.

    ReplyDelete