Search

Preserving hydrological variability is important when developing environmental flow regimes, and a number of tools have been developed to support this process. A commonly applied method is the index of hydrological alteration (IHA), which describes a set of indices that can be used to assess changes in flow regimes. In cold climate regions, river ice can have large effects on flow regimes through frazil and anchor ice formation, ice cover formation, and ice break-up, and the impact of this is usually not included in the commonly used indexes. However, to understand the effect of ice formation and the break-up on the flow regime, the ice effects on the hydrology should be considered when assessing winter alteration indexes. This paper looks at the effects of river ice on winter flow conditions using data from Norwegian rivers, and discusses these effects in relation to hydrological variability. This paper also shows how indexes can be used to classify ice-induced variability, how this should be used to avoid ice-induced effects in the current analysis, and how this can be combined with the current indices to improve the winter flow regime classification. The findings from this paper show that frazil- and anchor-induced raises of the water level have a large impact on the perceived flow in winter, producing higher flow and deeper water than what the open water conditions discharge could do. Corresponding to this, winter lows connected to ice-induced high flows at other locations are also common. Finally, issues related to the assessment of the temporal and spatial effects of ice formation are discussed. © 2017 by the authors. Licensee MDPI, Basel, Switzerland. This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

The study of frazil is often triggered by the problems that it causes. Frazil ice is known to block water intakes leading to shutdown of power and pumping stations and to form large accumulations in rivers, obstructing the flow and inducing flooding. In the downtown St. Raymond reach of the St. Anne River (Quebec, Canada), frazil accumulates yearly in the form of a grounded frazil jam or hanging dam. To adequately investigate ice-induced flood mitigation measures, knowledge is needed on the quantity of frazil produced by the river and deposited in town. Therefore, an intensive field campaign was conducted during the winter of 2014–2015 to evaluate the amount of frazil ice transported by the river and deposited in the hanging dam. This paper presents a method to quantify transported frazil ice, combining field samples and a complete heat budget. This method is compared to the theoretical ice production calculated from the heat budget only and to the spatial and vertical quantification of the hanging dam. Resulting from those methods, the 9.2 km-long hanging dam of 610,000 m3 and 432 kt (kilotons), was estimated to contain about 237 kt of frazil ice, while it was estimated that the river had produced 350 kt (based on the heat budget alone) and transported 66 kt (based on field samples and heat budget) of frazil before mid-winter. Considering the uncertainties inherent to sampling moving frazil and evaluating the spatial river ice coverage, directly measuring frazil in the hanging dam appears to represent the most reliable quantification approach, although very labor-intensive.