def calc_d50_depth(
    slope,
    flow_depth,
    tau_c_multiplier,
    rho_water,
    rho_sediment,
    tau_c_50,
):
    """Calculate median grain size via dominant flow depth according to
    Pfeiffer et al. (2017).

    Parameters
    ----------
    see above

    Returns
    -------
    ndarray of float
        d50.
    """

    return (
        rho_water  *flow_depth  * slope
    ) / (
        (rho_sediment - rho_water) * tau_c_multiplier* tau_c_50
    )