Active layers

If moderately or widely graded sediments are present, then the SORTED algorithm should be used and an active layer will be modelled.  The thickness of the active layer is set as a factor on the local D95 bed material size.  This factor should be set carefully.  A value of 2.0 can be used, unless:

• bed features, such as dunes, are present; in which case the factor can be set to produce an active layer thickness comparable to the dune height (bed features will be expected in most sand bed channels)
• the active width of the channel is significantly less than the entire width (and method 2 or 3 is used for bed updating); in which case the factor should be reduced by the ratio of the expected active width to the entire width.  This adjustment is required because the width of the active layer is kept constant and it is assumed to extend across the entire channel section.
• all size fractions are cohesive, in which case the factor should be set to ensure an active layer thickness of about 0.1m
• additional damping is required in the sediment sorting computations, which may improve stability should problems arise: try increasing the factor tenfold to introduce damping.