d50

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

$$d_{50}=exp⁡(0.41*\frac{m_c}{100}+2.71*\frac{m_{silt}}{100}+5.7*\frac{m_s}{100})$$

where $d_{50}$ is the median particle size of the sediment (µm), $m_c$ is percent clay in the surface soil layer, $m_{silt}$ is the percent silt in the surface soil layer, and $m_s$ 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