open TELEMAC-MASCARET The mathematically superior suite of solvers

Hi,

I would like to model runoff on an entire watershed wich is completely urbanized. For this I can't use a classic hydraulic model because most of the flow is product on the model areas.
So, I used this method :

- Model the entire watershed topography (with building), friction coef etc with a good resolution mesh ...
- Use a triangular hyetograph to represent rainfall the watershed...

I already run the model and the results looks pretty good (even very good compare to what we can do with classic models). So my question is :

What should be the limits of this kind of model ? assumption we have to do ?

Thanks for any answer.

Hello,

I understand that you want to model rain on a large and urban area and compute the subsequent flows. There was several other experiments to do such applications and the results seem good so far, even if I see a number of practical or theoretical problems :

* theoretical : we deal with very thin layers of water, with Strickler frictions, and it is not really sure that even Shallow Water Equations apply in this case. Infiltrations are not taken into account.

* practical : the wetting/drying algorithms may encounter counter-examples on thin layers, with some quantities of water not flowing because of finite elements considered dry (however our algorithms are very good because they do not depend on a minimum threshold depth that would prevent any flow).

* practical : the rain over buildings is not taken into account as they are not in the mesh. This has been taken into account in a large application in Vietnam and also one application in France, by adding local sources.

With best regards,

Jean-Michel Hervouet

Thanks for you answer JMH !

For practical problem i already found some solution. For example, the buildings are not model as closed lines but they are part of the topography (the mesh is very thin).

The main problem seems to be theoretical as you said is not sure that Shallow Water Equations apply in this case.

- Most of the time we are interested only by depth > 10 cm and water shallow equations apply for this depth. For the other areas, the only interest of using rainfall is to product a concentrated flow, exactly as a production funtion could do in a hydrologic model (conceptuel models like kinematic wave use this kind of assumption in hydrology). So a good test to know if this kind of model is reliable could be to model an existing watershed and to compare the hydrograph at the outlet with the one given by an hydrologic model for the same watershed and if possible with observed flow.

What do you think ?