Stopping sinking rate 'flux' term from being negative (#131)

* Stopping sinking rate 'flux' term from being negative

* Adding comment to sinking rate

Co-authored-by: Helen Powley <78801223+hpowley@users.noreply.github.com>

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Co-authored-by: Helen Powley <78801223+hpowley@users.noreply.github.com>

src/pelagic_base.F90

@@ -331,7 +331,8 @@ subroutine do_bottom(self,_ARGUMENTS_DO_BOTTOM_)
 
          ! Retrieve sinking rate (at centre of cell closest to the bottom)
          ! NB by ERSEM convention, this rate is returned in m/d, positive for sinking, negative for floating!
-         w = self%get_sinking_rate(_ARGUMENTS_LOCAL_)
+        !  Avoid negative sinking rate across the pelagic-benthic boundary to stop mass being moved from benthos to pelagic as conceptually incorrect for vertical movement
+         w = max(0._rk,self%get_sinking_rate(_ARGUMENTS_LOCAL_))
 
          ! Store near-bed vertical velocity. Use FABM convention (m/s, negative for downward) to
          ! allow this custom fuctionality to be ultimately be replaced by a FABM API.