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PM_2009

channel flow /the overland flow problem?

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Hi,

I have a question about relationship between the channel flow and the overland flow in GSSHA:

In my simulation (overbank flow allowed) I have noticed that in a place, where water rises above the channel bank level and floods to the floodplain, the water level on the flood plain does not rise even close to the same level as the water level in the channel. The channel depth and thalweg elevations are such that the channel bank should be a few centimeters below grid elevation (but I have tried with the channel bank elevation above the grid elevation, with the same problem). The water level in the channel stays above the channel bank elevation (and overland water level) for about 9 days (the time step is 180 sec, max. flow in the channel is 8 m3/s). It seems that the overland water elevation starts to rise only after the channel water elevation has been above the grid elevation for two days or so. The max. overbank water elevation is about half a meter above the channel water elevation. It does not seem to make a difference if Manning’s coefficient on the floodplain is greater than in the channel. Infiltration rate is assumed to be very small.

Any ideas why there seems to be this boundary between the channel and the floodplain (is this because the channel flow is calculated as 1D and the overland flow as 2D?) Is there any way to stop this from happening?

I got the overland water elevations by adding the water depth on a cell to the grid cell elevation and the channel water level by adding the channel water depth to the node z-value , is this a correct way to get these elevations?

Thanks again for any answers!

Pirkko

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Pirkko,

I don't know whether this helps much, but there are a 3 different modes for handling the interaction between overland flow and channel flow in GSSHA simulations. These modes are explained on the following page on gsshawiki.com

http://gsshawiki.com/gssha/Surface_Water_Routing:Overland/Channel_Interaction

WMS does not have an interface to the OVERLAND_BACKWATER mode, but it does have an interface to the OVERBANK_FLOW mode in the Diffusive wave channel routing parameters. I think this is the mode you want to use to simulate overbank flow from your streams. If this is turned off, water from the streams does not flow from the streams to the overland flow plane.

Chris

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I have had the overbank flow mode on,

but thanks for the answer! :) Do you know if it would make any difference to use ADE-PC (now using ADE) as overland flow computation method?

Pirkko

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Hi,

I realised that my model actually seems to be working like "overland_backwater": the water does not go to the stream when the water level in the stream is higher than the grid/water surface on the grid, but the water does not come out of the channel either, even if I have "allow overbank flow" chosen in the job control. Any idea if there has been this kind of problems with gssha? If this problem were be caused by too high/low channel depth, which one would be more likely to cause this kind of problem (I have tried to define the streams so that the channel bank is about at the same level as the grid, but as the grid is not totally smooth, in some places the channel is "too shallow".)

Grateful for any answer, as usual! :)

Pirkko

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