Hydraulic roughness of breakup ice jams

Lack of field data on the thickness and roughness of river ice jams inhibits reliable calibration of ice jam models and can result in erroneous evaluation of important physical parameters. Detailed thickness data that have been obtained on several ice jams by means of a remote profiling technique reveal a highly irregular underside. Statistical analysis points to a normal probability distribution of the irregularities and allows direct assessment of the absolute roughness of each jam. The latter varies linearly with the average jam thickness, at least to a thickness value of 3 m, and is shown to be consistent with previously reported roughness values deduced from purely hydraulic considerations. Using the present findings in conjunction with past data and modelling experience, simple equations are developed to compute composite-flow resistance characteristics in terms of jam thickness and flow depth.