Tool Options for Terrain Model has the option for setting intervals of existing "flat" contours. This is done by increasing the topo interval based upon the sequence of contour picks using the SHIFT key.
Using the "Set New: Interval" only increases in the "positive" direction from the Site (Starting) Height. This makes multiple instances of hills and valleys in a large site plan slightly difficult. One must use the MOVE tool to tediously rise each contour to the correct height.
My suggestion is that the Terrain Model Tool Options include the ability to repeat an interval and change to a negative interval.