Finding the correct roughness height

[thangaji117]

I am trying to validate experimental results from a hydraulic flume study using ANSYS. The flume is 12 m long, 1 m wide, and 0.8 m deep, with a 20 cm thick sediment bed. For the numerical model, I am representing the sediment bed using sand-grain roughness and have tested different roughness height values, but I am unable to obtain good agreement with the experimental results. The sediment has a median grain size (d50) of approximately 3.4 mm. Also I am using VOF model to validate my results. Could someone advise on an appropriate sand-grain roughness height for this sediment bed, or suggest recommended correlations or best practices for selecting this value?