Comparing flat and dome shaped drain covers.
The hypothesis that a flat drain coverS will more easily become clogged by debris is correct.
Drain hole covers are also found in kitchen sinks, bathrooms and roof gutters. They are there to prevent any large debris or rubbish from entering the plumbing pipes and causing blockages. Even though the surface of the drain hole cover needs regular cleaning to remove the debris, this is much easier to do compared to clearing blockages that occur inside the plumbing.
Try to repeat this science fair project using different type of debris like paper, rocks, broken branches or sand.
The experiment can also be repeated using cone shaped, pyramid shaped or box shaped drain hole covers.
How to cover and open a drain hole - http://www.doityourself.com/stry/how-to-cover-an-open-drain-hole
Blocked drain and blocked drainage - http://www.articlesbase.com/home-improvement-articles/blocked-drains-and-blocked-drainage-394320.html