Median particle size of suspended and benthic sediment
SWAT+ calculates the median sediment particle diameter for impoundments located within a landscape unit using the equation
where is the median particle size of the sediment (µm), is percent clay in the surface soil layer, is the percent silt in the surface soil layer, and is the percent sand in the surface soil layer.
Because reservoirs are located on the channel network and receive sediment from the entire area upstream, defaulting the sand, silt, and clay fractions to those of a single LSU or HRU in the upstream area is not appropriate. Instead, the user may set the median particle size diameter to a representative value.
The table below lists the D50 values of different sediment classes:
| Sediment class | Size [µm] | Approximate size |
|---|---|---|
Boulders
> 256,000
> Volley ball
Cobbles
> 64,000
> Tennis ball
Pebbles
> 2,000
> Match Head
Coarse Sand
1,500
Medium Sand
375
Fine Sand
94
Coarse Silt
47
Medium Silt
11.7
Not visible to the human eye
Fine Silt
4.9
Clay
1.95