Landscaping - What is the best cover material for over a septic tank cover?

We are having to have our septic system pumped again due to extremely heavy rain. The septic tank lid has had grass growing over it with an approximately 6 inch layer of "dirt" which must be dug up and removed and then replaced. This layer seems to get thinner every time this must be done(5 times over 15 years) resulting in an unattractive low spot. Is there a better way to cover this area that won't be affected if we have another 28 inches of rain in three days?

Ordinarily, a septic tank is capped with a layer of clay for impermeability. Grading the surrounding area to control the water from a rain might help also.