def _linear_velocity_calculation(velocity, z_start, velocity_bottom):
    """
    Calculation of the velocity assuming a liner increase with depth
    Assume a linear increase with depth where where the velocity is 0 for
    coordines.z > z_start and for coordinates.z[0] < z < z_start.
    """
    lin_velocity = velocity_bottom * (velocity.z - z_start) / velocity.z[0]
    return lin_velocity