mannings_to_darcy       Convert Manning's n to Darcy-Weisbach f
n_bathurst1985          Compute grain roughness via Bathurst (1985)
n_brownlie1981          Compute form roughness via Brownlie (1981)
n_cowan1956             Compute hydraulic roughness following Cowan
                        (1956)
n_direct                Compute total channel roughness via direct
                        field measurement
n_engelund1967          Compute form roughness following Engelund and
                        Hansen (1967)
n_fischenich2000        Compute hydraulic roughness due to vegetation
                        following Fischenich (2000)
n_freeman2000           Compute hydraulic roughness due to vegetation
                        following Freeman, Rahymeyer, and Copeland
                        (2000)
n_jarrett1984           Compute grain roughness via Jarrett (1984)
n_limerinos1970         Compute grain roughness following Limerinos
                        (1970)
n_maynord1991           Compute grain roughness following Maynord
                        (1991)
n_strickler1923         Compute grain roughness following Strickler
                        (1923)
n_vanrijn1984           Compute channel form roughness following Van
                        Rijn (1984)
n_wong2006              Compute grain roughness following Wong and
                        Parker (2006)
