Sand input from northern boundary not appearing in water column (Clamped boundary)

Dear COAWST developers and users,

I am trying to reproduce an idealized seagrass growth test case (Kalra et al., 2020, GMD). My model setup includes a 90×98 grid, 100 m resolution, uniform 1 m depth, and a tidal forcing at the northern boundary (amplitude 0.25 m, period 12 h). I use COAWST v3.8 (ROMS 4.1) with the sediment module (non-cohesive sand, NNS=1). The northern boundary for sand_01 is set to Clamped in sediment_veg_growth_test.in:

LBC(isTvar) == Clo Clo Clo Cla

In my bry.nc, I have variable sand_north_01 with time‑varying values: 0.5 g/L during flood tide and 0.02 g/L during ebb tide. The log file shows that the boundary condition is read successfully:

GET_NGFLD_NF90 - suspended noncohesive sediment northern boundary condition ... (Min = 2.00E-02 Max = 5.00E-01)

The variable definition in `bry.nc` is:

$ ncdump -h Projects/Veg_growth_test/bry.nc | grep -A5 "sand_north_01"
double sand_north_01(bry_time, s_rho, xi_rho) ;
sand_north_01:time = "bry_time" ;

However, in the history output veg_growth_test_his.nc, the suspended sediment concentration sand_01 is always zero at all vertical levels at the northern boundary grid points (eta=0, xi=46). The bed mass sandmass_01 also remains at its initial value (10 kg/m²). The sediment only appears in the interior (e.g., eta=45, 60, 80) due to local erosion, not from the northern boundary.

What I have already @tried:
Reduced bottom drag (RDRG=1e-5, RDRG2=0.001) and viscosity (VISC2=0, AKV_BAK=1e-6)

Lowered settling velocity (SAND_WSED=0.005 mm/s → 0.0001 mm/s) and erosion threshold (SAND_TAU_CE=0.02 → 0.001)

Changed northern velocity boundary from Flather to Radiation

Increased tidal amplitude to 0.5 m

Confirmed mask_rho=1 at the boundary

Checked variable name (sand_north_01) and dimension (bry_time, s_rho, xi_rho)

None of these allowed sand to enter the water column from the northern boundary.

Am I missing something? Is there any additional setting required for a Clamped sediment boundary to actually impose the concentration? Could this be a known issue in ROMS/COAWST?

Any help would be greatly appreciated. I can provide my input files if needed.

Thank you.

Start with sediment_veg_growth_test.in and the LBC(isTvar) setting, then compare the sand_north_01 definition and values in bry.nc with sand_01 and sandmass_01 in veg_growth_test_his.nc. Trace the sediment boundary-condition handling for the northern grid points; done means the configured nonzero northern concentration is reflected in the history output rather than remaining zero.

Written by the indexing model from the issue text.