Designing a Rain Garden Infiltration Rate with Sand Amendment

Targeting 1–2″/hr infiltration in silty loam requires 25–50% coarse sand by volume, tilled 12–18″ deep.

Sand Specification

• Coarse sand (concrete/play sand), not fine mason sand.
• 30–40% sand in top 12″ is a practical target.
• Verify no clay layer below 12″ that would create a bottleneck.

Surface Protection

• Landscape fabric between mulch and soil prevents clogging of pore space as mulch decomposes.
• Mulch with shredded bark or gravel.

Verification

Test infiltration: dig 12″ hole, fill with water, measure drop per hour. Adjust sand percentage if needed.