Dispersion values (K_h) specify the longitudinal spreading of water quality constituents during transport through river reaches in HEC-ResSim.
Values may either be specified as user-entered constants (allowing for spatial variation in those constants), or may be calculated dynamically at cell faces based on the equation
K_h = 0.011 \frac{v^2 W^2}{dv^*} \quad v^* = v \sqrt{gd}
where
v* is shear velocity, W is the stream top width, g is gravitational acceleration, d is hydraulic depth.
This is the same formulation used in the HEC-RAS 1D water quality model, and its derivation is described in Fisher et al. (1979).