Waves are also caused by incorrect computational and mapping output time intervals. If waves are occurring in RAS mapper visually i can guarantee that in all cases the model is either unstable or the mass balance is completely off. Both of which are highly manipulated by dt.
As was mentioned above the EG slope used is very important as to how the flow distributes along each cell face. Ive notices for urbanized areas, values upwards of 1E-10 is best used when trying to distribute flow evenly.