How large is the manhole?  If there is a way to get a high pressure water wand inside, it will help break up whatever is in there.  Hot water, or hot water washer would help too.

We have had to do this at times in the spring before we put water in our tanks to get the dried algae out.  But it can take several times with the power washer, and then pumping out whatever junk is in the tank.