## Discontinuities

MOHID Land forum. Questions and discussion related to MOHID Land
MOHID Beginner
Posts: 8
Joined: Tue Oct 18, 2011 12:48

### Discontinuities

Hello,

I am a new Mohid user. First, I used Mohid Studio and I made a MohidLand project.
I had no problem of the program compilation, but the results obtained when I take into account the river only are strange :
-The function of the channel flow is discontinuous in time if I consider a constant discharge with a value of 200 m3/s for instance. If this value is reduced, the instabilities of the channel flow function decreases. But, I observed that, for low or high constant values, some river nodes have different channel water depth whereas the strahler coefficient and the channel geometry are the same for all these nodes. I tried to reduce the time step but it didn't change anything.

Best regards,

frank.maretec@ist.utl.pt
MOHID Guru
Posts: 151
Joined: Thu Jan 01, 1970 00:00
Location: Technical University of Lisbon
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### Re: Discontinuities

if you discharge flow along a channel, flow should be constant, if, and only if, all water is routed through the channel. It may happen, in MOHID Land, that the flow rate exceeds the channel transport capacity and water is partially routed "overland". You can verify if this occurs, taking a look at a property call percentage max volume. If this property exceed 100%, water is routed overland + channels.

Channel water depth will not be constant along a profile, since it depends also on the bed slope...

To avoid instability, please also use a time series to discharge water, starting at 0 and raising to a value you want to use. Then it can remain constant. Starting from 0 to 200m3/s in one single iteration will result in numerical instability.

Hope this helps,
Frank
In order to understand recursivity you must first understand recursivity.

MOHID Beginner
Posts: 8
Joined: Tue Oct 18, 2011 12:48

### Re: Discontinuities

Best regards,

MOHID Beginner
Posts: 8
Joined: Tue Oct 18, 2011 12:48

### Re: Discontinuities

Hi,

I have two last questions concerning the drainage network :

1- As you said above, it is normal to find different values of water depth in the channel for nodes along the river. In the case of a same strahler coefficient, the bed slope has also a role in the water depth. But, how the slopes of the cells (Cellslope. dat) is computed in Mohid ? What is the formula used ?

2- In my study case, when the watershed delineation is computed, the simulated river (the Garonne river) is not exactly the same than the real Garonne river (i.e., most nodes are not located to their "real" space location). This is probably due to the accuracy of my xyz file. So, is it possible to constrain the watershed delineation process in order to obtain a more precise simulated river ? Maybe it would be possible to impose some nodes in which the main river has to reach...

davidbrito
Posts: 187
Joined: Thu Jan 01, 1970 00:00

### Re: Discontinuities

1. The cellslope is computed from the DTM and in each cell is given the higher slope computed from the 8 neighbours. (AltitudeCell - AltitudeNeighbour)/[Dist to neighbour center]

2. We are thinking of a burn in process but for now the only way is to change topography since draianage network will always follow the higher slope direction.

David