Manning’s roughness coefficient in small scale nature-like fish passages

Abstract: Fishway construction remains a crucial measure in environmental adaption of flow barriers. A well-functioning fishway should be able to pass all naturally occurring species of fish and other aquatic fauna. A common approach when designing and dimensioning a nature-like fishway is to calculate flows with Manning’s equation but the selection and impact of Manning’s roughness coefficient lacks a robust scientific background. In this study, field-data from four different nature-like fishway passages are collected to evaluate roughness coefficients for 19 different cross-sections. The average roughness coefficient was M = 10.5, spanning from M = 3.7 to M = 20.7. The method of field measurement was compared to two other ways of estimating roughness coefficients from (Cowan, 1956) and (DVWK, 2002). Limitations included measurement difficulties and assumptions of uniform flow for easier application of Manning’s equation. Based on the results, Manning’s roughness coefficient should be lowered from a typical design value of 15 to around 10 when designing small scale nature-like fishways or when estimating low-flow conditions in regular sized fishways. Future studies should seek to build data sets for greater varieties of fishways, evaluate the effect of varying flow within a fishway and thoroughly estimate the impact of perturbation rocks.