def METHOD_NAME():
    """
    Make sure the diffusivity-vs-depth function is correct.
    This test checks that the diffusivity should equal the shallow water
    diffusivity at the shoreline (when tidal range is zero), and at the wave
    base depth (here 50 m). It should be about half the nominal value at a
    depth of 84.657359 m (1 - ln(1/2) times the wave base depth). It should be
    near-zero for negative depths.
    When the tidal range is nonzero, the value at a depth of zero (shoreline)
    should be half the shallow-water diffusivity.
    """
    grid = RasterModelGrid((3, 3), xy_spacing=40.0)
    topo = grid.add_zeros("topographic__elevation", at="node")
    topo[3] = -84.657359
    topo[4] = -50.0
    topo[5] = 50.0
    ssd = SimpleSubmarineDiffuser(
        grid, tidal_range=0.0, wave_base=50.0, shallow_water_diffusivity=100.0
    )
    ssd.calc_diffusion_coef()
    assert_array_almost_equal(grid.at_node["kd"][2:6], [100.0, 50.0, 100.0, 1.0e-20])
    ssd = SimpleSubmarineDiffuser(
        grid, tidal_range=2.0, wave_base=50.0, shallow_water_diffusivity=100.0
    )
    ssd.calc_diffusion_coef()
    assert_array_almost_equal(grid.at_node["kd"][2:6], [50.0, 50.0, 100.0, 1.0e-20])