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Thread: Modelling of informal woody dams - ICM

  1. #1

    Question Modelling of informal woody dams - ICM

    We're trying to represent a reach in ICM which contains some informal woody dams - build ups of branches and woody debris which impede flow but may get dislodged by higher flows. Is there any guidance on modelling these?

  2. #2
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    No formal guidance that I know of. I'd go for a screen with some plausible made-up numbers for bar (trunk) diameter and spacing. If it matters a lot about the possibility of the dam being dislodged, you could go for a porous wall with a suitable collapse depth, but that would have to be in the DTM rather than river reach (I think) so you'd have to create a 2D outfall upstream and a 2D manhole downstream or some such arrangement. No idea how you'd model the resulting debris building up somewhere else........

  3. #3

    Thanks

    Quote Originally Posted by Kristian Ravnkilde View Post
    No formal guidance that I know of. I'd go for a screen with some plausible made-up numbers for bar (trunk) diameter and spacing. If it matters a lot about the possibility of the dam being dislodged, you could go for a porous wall with a suitable collapse depth, but that would have to be in the DTM rather than river reach (I think) so you'd have to create a 2D outfall upstream and a 2D manhole downstream or some such arrangement. No idea how you'd model the resulting debris building up somewhere else........

    The ones on this reach didn't seem to have much flow going through them, with the gaps between the larger branches blocked by twigs, leaves etc. I'm inclined to think porous walls may be the best representation. The worst case may be to assume they don't collapse at all - my research would suggest that if they do, they will rebuild themselves somewhere else.

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    I'm not aware of any formal guidance. This is something I personally researched 102-15 years ago for my PhD research. The two questions I would ask is whether they are permanent features within the model and how do they behave hydraulically?


    With regards the former, I know of some that have been in situ for 30 years+ in which case they can be permanent features within most models. This would simplify things as then there is no time-varying component. If there is a time-varying component, then you would be looking at RTC.

    With regards the second, my research showed that there were a number of different types of accumulations which had their own hydraulic behaviour, with some deflecting flow, some partially blocking flow and others fully blocking flow and causing de facto weirs. The specific type and hydraulic behaviour might then govern their representation.

    You could, as Kristian says, represent as a screen although the equations are better suited for clean screens. These would probably be larger branches which are not infilled by smaller debris. Alternatively, you could also use a user-defined link and a head-discharge relationship to represent the blockage. Perhaps the simplest would be to represent their effect using the Mannings' n roughness parameter with different values for the different types of accumulation. Finally, for those which introduce weir type effects, an irregular weir or inline bank could be used. The above approaches could be used in the 1D domain.

    Within the 2D domain, a porous wall or base linear structure could be one approach, or again a roughness zone.

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    Sounds like an interesting PhD, Duncan. Either way, we're into made up numbers territory, which means ideally "calibrating" them form observed performance if possible or doing some sensitivity testing. All depends how much it matters. Interesting stuff.

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    Absolutely, couldn't agree more. The difficulty is that the places you would stick monitoring equipment is precisely the location these types of accumulations would be removed.

    It would be worth checking out Simon Dixon's page, in particular discussions around and links to his papers:-
    https://therivermanagementblog.wordpress.com/blog/

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    I was thinking more of visual observations, flood extents and so on. Monitors would be at high risk and the data would probably be highly erratic.
    That's an interesting blog, excellent for displacement activity disguised as professional development. I shall consult it regularly. Thanks, Duncan.

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