Ice breakup in small Norwegian streams
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River ice is present parts of the year in cold region environments and is an important component of the flow regime. The river ice is known to produce many extremes and potential floods far exceed those possible under open-water conditions. Thus, predicting the time of a river ice breakup is essential as it concerns environmental impact, emergency flood warning and hydropower production. The available predictive methods are developed and tested only for moderate-gradient medium and large rivers. Their utility for high-gradient small streams is not known. As a first step toward development of a criterion for ice breakups in small streams one of the existing criteria for large rivers is tested. The ice cover thickness is an important parameter considering the river ice breakup, thus a simulation of the ice cover growth is included in the analysis. Extensive data are retrieved from field studies and analysis regarding ice cover growth and ice breakup is conducted. The Stefan formula is proven to give good results for the ice cover growth in the observation sites. The method provides reliable values of the ice thicknesses which then are used as input parameter to the Empirical criterion for onset of breakup. The Empirical criterion has some inconsistency in its simulations of the river ice breakups. However, the criterion was able to simulate three of five ice breakups in small streams. This is evaluated as a promising result and the criterion can thereby be used as a foundation for further research and development.