Home Forums RORB – general use Initial water level in storage

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    Hi There,

    I am trying to run a RORB model with a catchment that drains to a dam. I need to specify an initial existing water level in the dam of 1m. How do I go about doing this? Would I have to alter the H-S tables or is there a more straight-forward way?


    • This topic was modified 2 years, 3 months ago by ThaneshGaneson. Reason: added tags

    Check the discussion of this in section 2.4.4 of the manual. Also item 2.3 in Table 5.2.
    If you look at the storage node in the graphical editor, there is an Initial Drawdown box where you can enter a numeric value – you might be able to use this to get the effect that you need.


    Hi Tony,

    Thanks for the reply. Your blog has been helpful to me many times

    After consulting the manual at the sections you suggested, I can see that I can specify an initial drawdown of 1m to simulate a 1m ‘dead’ storage in the dam.

    Thank you


    Furthering on from this, trying out an initial drawdown level of 1m caused errors. So I tested out different values and found that even a 0.1m initial drawdown was giving me an error claiming a surcharge, which seems highly improbable, as the model was working fine with 0m of initial drawdown. Any advice on how to troubleshoot this?


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    Benson Liu

    Hi Thanesh,

    How are you putting in your initial drawdown?

    In RORB, there are 2 ways of specifying drawdown, one is to specify it in volume (m³) and the other is to input it as water level. If you put it in as volume it needs to be a negative number. Water level is a positive number.

    I think you are putting it in as drawdown in metres which RORB won’t deal with correctly. If you are putting in “1” RORB will assume that it is a starting water level of 1m not 1m drawdown. This will give an error as your H-S relationship is not defined at 1m (it starts at 949.5m)

    Let me know if this helps.

    Kind regards,



    Hi Benson,

    Yes my initial drawdown was specified as 949.6m when I was testing an initial water level of 0.1m
    Since my initial troubles, I’ve moved away from initial drawdown and I altered the H-S table to simulate a dead storage instead

    Thanks for the assistance



    I’m having similar issues – If I use the option of reducing a volume from the cease to flow, it sets my initial water level correctly.
    If I use the correspondent level (m RL), it gives me the surcharge error.

    Ex.: My spillway is at elevation 100 m RL and I want the IWL to be 99 m.
    my stage-storage reads as follows: 99 m RL = 10 m3 and 100 m RL = 17 m3.

    To get the initial water level at 99 I should input -7m3 as initial drawdown.
    for the option of the initial water level at 99 m, it gives me the surcharge error.

    If I put 1 m, it will understand a value that I don’t have in my stage storage, so it will consider that my pond is empty, with 0 m3 initial storage

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