Multiple drainage areas outlet to lake. 1

[orshii]

I have 3 drainage areas that outlet to a lake at 3 separate points. How do I model the lake runoff? There is a dam outlet structure at the end of the lake.

Thanks,

 

[psmart]

Are you asking about the direct rainfall on the surface of the lake? This could be modeled as a sub-area within one of the contributing subcatchments, but it's generally better to model it as a separate subcatchment with a Tc of zero and a CN of 98. You will find a CN entry for water surfaces in the HydroCAD lookup table.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[psmart]

...If you're asking about the three separate drainage areas, theses would generally be modeled as three separate subcatchments, each routed to the pond.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[orshii]

The direct rainfall on the lake is not my question. I've modeled 3 drainage areas as 3 separate subcatchments. They outlet to the lake in 3 different spots. I have the Tc for each. There is a dam at lake outlet. The runoff from one subcatchment outlets to the lake at the far end away from the dam and runoff from the others enters at the sides of the lake. I just need to know how to handle modeling the subcatchments with the lake runoff in Hydrocad. You suggested modeling the lake as a separate subcatchment with a time of conc. of 0 and a curve number of 98. Can you clarify a bit?

 

[psmart]

Although the travel time from each subcatchment to the pond outlet may be slightly different, all the flows create a level pool when they enter the lake, so the Tc calculation normally stops at that point.

This would be different for a river channel with a sloped water surface. But a pond routing assumes a level water surface, so the specific point of entry is not considered.

In most cases, the travel time through the reservoir will be very small in comparison to the other flow segments. You can verify this by setting up an additional flow segment (in the subcatchment Tc screen) using the Lake/Reservoir option.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[orshii]

I have one last question. Do I need to add an additional Lake/Reservoir flow segment for each subcatchment area?

 

[psmart]

You could, but it's usually not done because (a) the lake travel time is generally very small in comparison to the other Tc segments, (b) the lake/pond effect is already being simulated by the pond routing, so you would be effectively double-counting the effect.

If in doubt, try it, compare the results, and draw your own conclusions.



Peter Smart
HydroCAD Software

http://www.hydrocad.net