4" Loamy Sand over 6'+- Sandy Clay Loam - What rate should you use for drawdown?

[meatymatt]

Existing soil is sandy clay loam, rawls rate 0.17

A grassed infiltration basin will be placed in the area, topped with and 4" loam and seed (assume loamy sand rawls rate 2.41)

What rate do you use for drawdown? 0.17 or 2.41? I think the 0.17 but am being told otherwise

 

[Pinwards]

Ask whoever is telling you that, how does the 4" topping loam magically increase the infiltration rate of the underlying soil that receives the water?

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[swazimatt]

if it is an infiltration basin I assume it will be holding water, in which case the top section will be come fully saturated pretty quickly and your soakage rate will then depend on the material below that