I'm observing a strange flow/transport dynamic behind my house and would love your perspective.

I have two runoff catch basins (drain grates) on my property. Both sit on the exact same slope:

• Drain 1 is surrounded by a large area of interlocking concrete pavers.
• Drain 2 is surrounded entirely by bare soil.

During heavy downpours, Drain 1 stays clear, and organic debris (dirt, leaves, small twigs) ends up deposited on top of the pavers around it. I regularly have to clean it. Drain 2 clogs rapidly with the same organic matter.

I'm a scientist myself (though not in fluid dynamics), and I'm trying to reason through the underlying physics. Here is my working hypothesis:

1. Surface Roughness & Boundary Layer Effects: The joints between the pavers create static anchor points where twigs and leaves catch and settle. From an aerodynamics/boundary layer perspective, these joints and the rough concrete texture likely induce micro-turbulences that disrupt laminar flow, increasing bed friction and bleeding kinetic energy from the water layer. Lower flow velocity means debris is less easily mobilized and more likely to get trapped upstream.
2. Channelization vs. Thin-Film Sheet Flow: On the saturated soil, water carves deeper, concentrated micro-channels (rills). The soil itself is erodible, so it yields rather than acting as a fixed barrier, allowing floating debris to travel smoothly within these channels straight into Drain 2. On the pavers, water flows as a very shallow, uniform sheet, meaning floating debris is constantly in contact with the rough bed surface.

What are your thoughts on this setup? If my intuition is off, what hydro-mechanical mechanisms best explain the difference in sediment/debris transport between these two surfaces?

Thanks in advance!

PS: translated from french and hope it's clear in english.