MikeSHE SZ Drainage Codes

I have a Mike 11 model coupled with MikeSHE for hydrology. We are using drainage codes for a number of developed areas to represent efficient surface drainage. We have them set up spatially to cover the whole developed area and then drain to a Mike11 ditch on the boundary. My question is how does MikeSHE calculate the SZ drainage flow rate to send to the ditch?

Does the rainfall first have to enter the SZ (by infiltrating) before the SZ drainage is activated?
Or does all water that hits the grid cell get routed to the boundary, regardless of if it would otherwise infiltrate to the SZ?

I read that the paved runoff coefficient values are used to take a portion of the ponded water between (0 and 1) and route it directly to the selected SZ drainage. This leads me to believe that if you had a paved runoff coefficient of 0, all the water would pond on surface and either infiltrate or runoff overland, and only the infiltrated water would drain through the SZ drainage.

However, I have been reviewing the MikeSHE grid results for "SZ drainage flow from point", and the values here indicate that all rainfall is being routed to the SZ drainage, regardless of the paved runoff coefficient. Ie. cells with a paved runoff coefficient of 0.2 are showing the same drainage flow rates as cells with a paved runoff coefficient of 1.0.

This is not what I intended when the model was setup - I wanted the paved runoff coefficient to take a portion of the runoff and put it directly to the Mike 11 river network, and the remainder would either infiltrate or runoff normally.

I have not been able to find clear wording in the user manuals that describes this, and would appreciate any help or advice.

Thanks!

Dear Spencer
The paved area and the drainage module both works in a way where the water is routed to a receiving cell which could be an internal cell, boundary or a river point. The paved area module works on ponded water while the drainage module works on the saturated zone. Note that both the paved area and drainage module uses the same reference system, meaning that the way the water is routed from one cell to the receiving cell is the same for both drainage and paved area.

For paved area the amount of water removed from a cell during one time step is calculated as the runoff coefficient times the depth of ponded water. A value of 1 will remove all the ponded water in each time step. The calculation sequence is that MIKE SHE first calculates ET losses, then updates the ponded water and calculates the paved area runoff. The paved area runoff is calculated BEFORE infiltration, but after ET losses.

The drainage module works on the saturated zone as a time constant times the difference between the drainage level and the groundwater level. The drainage module is only active when the groundwater level is above the drainage level in the specific cell. So drainage works after infiltration to the SZ.

The best way to verify how the two modules work will be to select two cells having different runoff coefficients but the same rainfall and soil type. Then plot the different components, from the two cells, as time series, and it should hopefully be easier to understand how the paved area and drainage module works.

Feel free to post additional comments on this site or contact DHI directly at mikebydhi@dhigroup.com

Thanks
Oluf (DHI)