Floodplain lower than channel – levee or ineffective flow?

[Igloo_79]

I am modelling a channel where parts of the floodplain are lower than the channel – e.g. construction of embankments or flood storage areas. I only want these floodplain areas to fill up with water once the banks have been overtopped and would appreciate advice on selecting the best approach. (Note that floodplain flows will be modelled using storage areas + lateral weirs). I am thinking of either:

1. Setting a levee at the left bank – however, does this mean that the LOB area will suddenly fill up to the same water level as the channel once the levee is overtopped?
2. Using an ineffective flow area
3. Cropping the LOB of the XS to delete the low elevation points

Thanks in advance.

[Anonymous]

1. Yes, this will cause the LOB to suddenly fill with water.

2. Ineffective area by itself won’t keep the LOB from filling up before the banks have been overtopped. You could do both a levee and ineffective flow.

3. If you don’t need to model the amount of water storage that is in the LOB, this is a simple solution. Hydrographs attenuate as they move downstream. The more area that the water has to fill up the more it attenuates. So if the LOB fills up after the bank is overtopped, it would be more accurate to keep it in the cross section. In the big picture, whether you do so or not may not matter. Depends on the model.

[Igloo_79]

Many thanks for your reply.

2. Re: Ineffective flow areas – I don’t they are appropriate for this example.

3. I would like to model the amount of storage in the LOB, however, I don’t see how this can be done with standard channel XS since (as far as I understand) it’s not possible to model the LOB area gradually filling up with water after the channel overtops. Thus, my intention is to model out of bank flows via storage areas and lateral weirs.

[Anonymous]

3. Correct. For 1D models, there is no way to get the LOB to gradually fill up. To have that area fill up, you would need a lateral structure at the high point. The lateral structure could be connected to a storage area just to account for volume, or you could create a second reach to represent the flow in the LOB and connect the lateral structure to that.

If you have good enough terrain data, you could, of course, consider using 2D.

[Author]

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