HEC-RAS deposits cohesive sediment based on Krone (1962). Krone's observed that suspended sediment decreased logarithmically, in his experiments, for concentrations less than 300 mg/l, quantifying the deposition rate with the equation:

(\frac {dC}{dt})_d = -(1-\frac{\tau _b}{\tau _c})\frac{V_sC}{y}

where:

C = sediment concentration
t = time
τb = bed shear stress
τc = critical shear stress for deposition
Vs = fall velocity
y = water depth (Effective Depth in HEC-6)

This equation yields an exponential deposition relationship, where the deposition rate increases non-linearly as bed shear drops farther below the critical shear:

\int \frac{dC}{C} = \int-(1- \frac{\tau_b}{\tau_c})\frac{V_s}{y}dt \rightarrow C = C_oe^{-(1-\frac{tau_b}{tau_c})\frac{V_st}{y}}

Because of the logarithmic assumption the Krone equation only requires one empirical coefficient, the critical shear (τc).
If the calculated bed shear (τb) is less than the critical erosion shear (τc) HEC-RAS will deposit transporting cohesive sediment based on this equation. The equation is not applicable for shear stresses greater than the depositional threshold.

As Krone (1962) recognized, the cohesive deposition rate is also dependent on the flocculation rate, which is a function of the sediment concentration and the water chemistry. Many sophisticated coupled flocculation-deposition models account for these processes. However, HEC-RAS does not attempt to compute flocculation. Therefore, the grain size distribution should reflect the distribution of flocculants rather than discrete grains, even though standard particle sized distribution methods tend to report the latter.