Determination of roughness coefficient of the underside of ice cover

The formation of ice cover in winter on rivers, channels and run-of-reservoirs, changes flow characteristics significantly. Calculations of water surface profiles or discharge along a river reach with ice cover by means of 1-D models, requires detailed values of the roughness coefficients of the river bed and underside of the ice cover. Numerous studies dealing with roughness coefficients of the river bed with free surface flow were carried out in the past, however, very few investigations were performed on the determination of the roughness coefficient of the underside of ice cover. Data of calculations indicate that this coefficient may vary considerably with space and time, depending on the character of the ice cover, flow characteristics and meteorological conditions. In 1-D models roughness coefficient of the bed and of the underside of the ice cover must be combined into one composite roughness coefficient. The paper presents determination of roughness coefficient of the underside of ice cover based on velocity distributions in the Vistula River cross-sections with ice cover. It may be assumed that vertical velocity distribution has a logarithmic character in the regions near the bottom and ice cover. Taking into account this assumption the procedure for the calculation of Darcy-Weisbach friction factor f and Manning roughness coefficient n is proposed, basing on the data of velocity measurements in a given vertical. It was found that roughness coefficient of the underside of the ice cover varies in a much larger range than the bottom roughness coefficient and depends very much on the character of the ice cover.