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Thread: ICM - 2D - Modelling ditches and canals (connectivity between 1D and 2D)

  1. #1

    Join Date
    Feb 2017

    Question ICM - 2D - Modelling ditches and canals (connectivity between 1D and 2D)


    I've been busy modelling the urban drainage systems of various urban areas lately. The goal of these modelling studies is (generally) to identify where possible flooding of houses etc can occur during heavy rainfall events due to a lack of capacity of the drainage system. Im doing this using IW ICM, using 1D2D models.

    Generally, my model is comprised of the drainage system, with all nodes set to 2D flooding, and all outlets set to 2D outlets. My 2D model is built up using LIDAR data, and for my mesh I utilise voids (buildings), mesh zones (for the dense urban area), and mesh level zones for the large waterways.

    My question pertains to the modelling of the canals, big and small;
    right now I use mesh level zones to burn the water level in the canals into the mesh. This way, i am only modelling the storage capacity of the canals. For small ditches, I use the LIDAR data. For the purpose of my calculations, this generally serves me well. However, problems arise at locations where I try to couple ditches together using 1D pipes (under roads and such). This either doesn't work (I read that this may be due to the mesh element size), or there are too many timestep halvings and the calculation fails.

    My question to you is; how do you model urban waters such as canals and ditches in a combined 1D2D model, and specifically the connection between the 1D and 2D components? There must be a more elegant way than what I do now. I've read up on river reaches and inline banks a bit, but I'm not sure I should invest the time to master those options. Below is an example; red shows the ditch, blue is the pipe connecting the two ditches. In blue the water on the mesh is shown. The left ditch is full, yet the right one is not receiving water.
    1d2d connection.PNG

    Thanks in advance,

  2. #2
    Senior Member
    Join Date
    Feb 2013
    Hi Ruben
    Modelling a canal as an mesh level zone is generally OK. It's slightly hard to see what you're doing, but a) you WILL have to master river reaches to model watercourses satisfactorily and b) are you sure the nodes are correctly set up? Ground level = level in mesh level zone, for example? And if the mesh level zone on the right is your canal with a level above the level in the watercourse? to the left, then there won't be any flow anyway.
    River reaches are fiddly, but not really difficult. You do have to be systematic about setting them up.

  3. #3
    We've modelled lots of small ditches as mesh level zones,or just using LiDAR data, with 2d outfalls connected to 1D links for bridges and the like.
    This seems to have some stability benefits c.f. using 1D river reaches.
    Generally it is necessary to ensure the ditch is locally no higher than the 1D connecting link and that the element size is large enough to accept the 1D - 2D flow transfer. This can be done using a local mesh level zone around the inlet/outlet.
    It's impossible to tell why there's no flow in the ditch on the right without knowing the levels and the node types.

  4. #4
    Senior Member
    Join Date
    Feb 2013
    Martin's right. For small ditches, if you're using the DTM on its own, you do have to watch out for local blips and oddities that can interrupt the flow - ditch widths are often less than the LiDAR resolution. A mesh level zone overcomes that problem. All depends how critical it is in a particular location.
    For larger watercourses, river reaches become necessary at some stage, and are more versatile, and once structures such as bridges are involved and their head losses matter, you do have to use river reaches. Stability issues can be a PAIN!!!! but can generally be overcome by tweaking the bank discharge coefficients and modular limits, and by carefully defining and sampling your bank lines, and using mesh level zones adjacent to troublesome banks - instability is often due to oddities in the DTM.

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