purpose of drop manhole 3

Drop manholes are used to allow the use of reduced slopes in sewers. Reduced slopes are desirable to minimize the scouring effect of high velocities caused by high slopes. Drop manholes are also used to reduce turbulence in the manhole. Turbulence will cause gases to be released in the manhole.

These situations do not occur with storm water.

Also, maintenance people want exterior drop pipes for the occasions when they have to work in the manhole.

bimr has nailed this, but I guess just to state it another way there are occasions (due to topography etc.) when sewer segments laid at normal slopes (to create general, normal velocities etc.) intersect some manholes at significantly different elevations. The use of "drop manholes" avoids the unappealing "shower" (to any folks entering the manhole) that would otherwise occur, and also as mentioned above other undesirable effects of cascading/aerosoling sewage flow. Because of potential blockage problems, I believe drops are used sparingly and/or also sometimes configured with crosses and/or risers in lieu of simple tees to provide for cleanouts (you may wish to refer to ASCE MOP #60).

I have another reason for drops: For a design concept, I had the invert of an 8" entering a 24" at the spring line. The City engineer had me change that so the invert of the 8" is now at the top of the 24" pipe. The reason for this is that the flow of the 8" will not be inhibited by any back pressure from the 24" (Under normal conditions). The 8" flow simply drops on top of the 24" flow and away it goes.