watershed - Does the common flow accumulation techniques still work over flat area?

My terrain is a highly developed area--which means that

1. It involves lots of flat area ( gradient=0) because they have been properly paved.


2. And it involves sudden drop, such as retaining wall, or drain drop.

These two features make me wonder whether the usual flow accumulation algorithms ( like D8) can actually work on such terrain?

I want to do streamline analysis on it, but I am quite unsure whether the usual flow accumulation algorithms ( like D8) can actually work on such terrain? Or, as my previous question frames it, is there any open source flow accumulation algorithm in C++/C# on Windows that handles these features nicely?

Note: I would have to conduct watershed analysis at modelling stage ( because we need to study the waterflow path even before the development begins, to avoid flooding at unwanted area), which means that the DEM is based on my modelling and not something out there on the actual terrain (yet).