Discussion: Estimates of the Manning's coefficient for ice-covered rivers

Abstract

Li, 2012 has plotted a huge range of composite Manning's n values, between about 0 . 01 and 0 . 05, derived from analysis of winter velocity measurements in ice-covered rivers in Canada. It is difficult to believe that composite values near the lower end of this range could be realistic, since even smooth concrete has an n value of about 0 . 012 and river beds seldom have values much below 0 . 020. The abstract states that 'the slope of the energy grade line is difficult to measure. . . it appears to be about 30% of the water slope'. This statement suggests that the studied reaches were highly non-uniform, in which case the determinations of the n value are unlikely to be reliable.Another questionable statement is that 'the composite Manning's coefficients reported. . . are useful for modelling ice-covered river flow and determining winter discharges. . . particularly when sitespecific data are unavailable'. Even if the reported values were correct, how could they be used for modelling unless they were linked to expected ice conditions and geometric characteristics of the river bed and ice cover, which the author does not discuss at all? The underside of ice covers can exhibit widely different geometries ranging from nearly smooth to highly rough and irregular, depending on the processes of ice accumulation and consolidation during the freeze-up period and on subsequent under-ice transport, accumulation and erosion.In a previous article (Neill and Andres, 1984) the present contributors analysed the hydraulics of a thick, irregular winter ice cover in 1982 on the Peace River in northern Alberta, Canada. This cover had resulted from the consolidation of a newly formed ice cover by fluctuating discharges released from a reservoir far upstream. The winter cover, which had an average thickness of about 4 m, consisted mainly of slush with embedded ice floes. The slope of both the river bed and the ice surface was approximately 0 . 32 m/km. The composite Manning n was about 0 . 043, near the upper end of the author's range, and the bed roughness under open-water conditions was about 0 . 032, indicating a roughness of about 0 . 053 for the underside of the ice cover. Author's replyThe contributors are correct that a finished concrete surface has a Manning's n value of about 0 . 012. It is, however, important to note that roughness characteristics differ between concrete and river ice cover. On the underside of ice covers, downward protruding elements (or vertical deviations from the mean position of underside ice cover) can be eroded by water flowing underneath when water temperature rises above a certain threshold; depressing elements can be filled due to temperature fluctuations. This possible mechanism will lead to a reduction in ice-cover roughness. The associated time scale can be short. At the same time scale, water flow in natural rivers is unlikely to reduce the texture of a concrete surface or surface roughness. Thus, it is possible that underside ice cover has a lower Manning's n value than concrete.On the bas...

Editorial

“…Their work has important implications for researchers and practitioners interested in the pathways followed by raw water during and after rainfall events, and the timing of resulting flow hydrographs and so on. Of particular usefulness to researchers in this area is a summary given in Table 1 of the controls that generally apply in the isotopic composition of rainfall and surface waters, containing references to pertinent literature.A discussion by Li et al (2014) …”

“…A discussion by Li et al (2014) concludes this issue of Water Management, containing a contribution by Neill and Andres, together with Li's comment on this contribution. The discussion relates to how to treat Manning's n in an ice-covered river.…”