The temporal constraint on deposition is the limiter based on the simplest and most robust theory. Fall velocity controls how fast particles can drop out of the water column and deposit. By comparing the vertical distance a particle has to travel to reach the bed surface and the vertical distance a particle can travels in a time step (fall velocity * time), HEC-RAS computes the percentage of a sediment surplus can actually deposit in a given control volume in a given time step. The model computes a deposition efficiency coefficient for each grain class (i) :
C_d = \frac{V_s \left( i \right) \times \Delta t}{D_e \left( i \right)}
Where: Cd is the deposition efficiency coefficient, Vs(i) is the fall velocity for the grain class, t is the time step, and De is the effective depth of the water column over which the grain class is transported.
The coefficient is a fraction, which will reduce deposition if the product of the fall velocity and the time step is less than the effective depth. If the time and fall velocity are sufficient for the grain class to fall the entire effective depth (i.e. the numerator is greater than the denominator), all of the surplus sediment deposits. This ratio requires two parameters (in addition to the time step, which HEC-RAS provides automatically): fall velocity and the effective transport depth.