During breakup, severe ice jams form at Fort McMurray, Alberta because of the dramatic change in the character of the Athabasca River at that location. Such jams, which produce water levels in the order of 10 m above the normal open water stage, were documented in 1977, 1978, and 1979. Additional channel surveys and improved estimates of discharge made since the initial analysis have redefined the ice jam characteristics. The Manning roughness coefficient of the underside of the ice jams was found to be 0.072. The new discharge estimates, which were up to twice those previously reported, result in a calculated coefficient of internal friction of 0.8–2.7. This is 30–100% greater than previous estimates, but still similar to values determined for ice jams at other locations.Even with the variation in the coefficient of internal friction, the river stage due to an ice jam at Fort McMurray could be computed with reasonable accuracy for a range of given discharges. If jams form downstream of the mouth of the Clearwater River at discharges greater than 800 m3/s (considerably less than the 1-in-2-year open water flood), flooding will occur within lower Fort McMurray. Unfortunately, the frequency of such an event is unknown because the probabilities of both the discharge being exceeded and the jam occurrence cannot be defined. Key words: ice, breakup, ice jam, ice roughness, flooding, hydraulics.

The effects of ice on the conveyance characteristics of the Mackenzie River at the outlet of Great Slave Lake are modeled on the basis of cross section surveys, discharge measurements, and water surface profiles taken during open water and ice covered conditions. The calibrated bed roughness values, expressed in terms of Mannings n, range from 0.020 to 0.030. Based on measured ice thicknesses ranging from 0.6 to 1.2 m in the study reach upstream of Providence Narrows, the calibrated roughness of the 1992 late winter ice cover is 0.015. Discharge estimates, based on this late winter ice cover calibration, measured water surface profiles, and documentation of major ice movements during April and May of 1992, show relatively good agreement with the discharge measurements taken at the same time. The analysis indicates that flow in the channel just downstream of Great Slave Lake is uniform under both open water and ice covered conditions. However, stage–discharge relationships at the Water Survey of Canada gauging station are affected by variable backwater conditions, particularly when an ice accumulation develops in Providence Rapids. Key words: ice, breakup, backwater curves, hydraulic resistance, river.

is highly sensitive to the Froude number, F r , and tends towards infinity as F r approaches 1 . 0, so that the equation is meaningless for near-critical or supercritical flows. In natural rivers, F r generally correlates fairly well with grain size and does not exert an important independent influence; in fact, many experimentally based pier scour relationships discount it completely.The inadequacy of Equation 12 as a predictor is also demonstrated by the authors' Figure 7, in which the predicted values of their ratio d s /Y can range from about 0 . 25 to 2 . 5 times the observed value. It thus appears that Equation 12is not a useful predictor of design scour depths for river bridge piers and could lead to gross under-prediction or over-prediction in practical cases. Much more reliable predictive relationships, generally based on experiments but not significantly challenged by field experience, have been available for many years. The assertion made by the authors in their conclusions, regarding the utility of their model to engineers and planners, is not supported by the examples provided above. Authors' replyIn response to the first of the contributors' comments, namely that using d s /Y as primary variable will not result in better models, the authors developed a GEP-based model using d s /b as primary variable (resulting equation presented as Equation 13 below) and found that the values of the coefficient of determination, R 2 , were very low (0 . 35 for training and 0 . 25 for the validation data set) when compared with the GEP-based models developed for the same data sets using d s /Y as primary variables, for which the R 2 values were 0 . 76 and 0 . 74 for training and validation data sets, respectively. The model that was developed

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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...