15 Floods, high water levels and climate change: An overview
Climate change will lead to some significant variations in several meteorological variables that are central in hydrology. These variations will have major consequences on floods, whether they are fluvial (river-related), pluvial (rain-fed), coastal, or caused by ice break-ups and ice jams. It seems clear that the increase in intense rainfall in the summer and fall will lead to more frequent and significant pluvial flooding in urban areas due to the inability of current systems to evacuate these large volumes of water (see Fact Sheet 17). The situation is more complex for fluvial flooding and coastal submergence, where several factors come into play1. The fact sheets in Module C focus more specifically on fluvial and pluvial flooding.
The highest water level of the year generally occurs in the spring in the large watersheds of Quebec and Canada, and is known as the spring freshet. The magnitude of the spring freshet depends on the amount of snow that accumulated during the winter (the snowpack), the speed at which this snow melts, and the occurrence of rain-on-snow episodes during the melting period. However, current projections indicate an increase in total precipitation during the winter accompanied by a marked increase in temperatures during the same season. This rise in temperatures means that episodes of melting and rain will be more frequent in the winter. The impact on the snowpack available at the time of melting is therefore very uncertain, since part of the additional winter precipitation could be in liquid form. Similarly, an increase in average temperatures could result in more frequent and intense rain-on-snow episodes in spring. The cumulative impact of these factors on the spring freshet is therefore very uncertain and will depend on the geographical location of the watershed, its altitude, its surface area and some of its physiographic characteristics. It’s likely that the spring freshet in basins located further north will increase due to an increase in the snowpack available for melting, while the opposite situation could occur for basins located further south, where alternating periods of snow accumulation and thawing will lead to a decrease in the snowpack in the spring. In every case, the spring freshet will be earlier.
The intensification of summer and autumn rainfall will have major impacts on water levels, particularly for small watersheds where heavy rain can fall across the entire watershed and where concentration times are shorter. The most extreme high water events will be the most impacted, because the rainfall will also be more extreme and will generate greater volumes of runoff. The effect on larger watersheds is more uncertain and will depend on the future changes in soil moisture conditions.
In sum, the impacts of climate change on flood patterns depend on many factors, several of which are still poorly or incompletely represented by the models. Overall, there could be an increase in the occurrence of annual maximum floods in the summer or fall, to the detriment of the spring freshet, in several small watersheds located further south. Similarly, the most significant high water events are likely to be the most impacted by climate change. These high water events will be more sudden and will be associated with convective systems or extratropical storms that end their journey in Canada. It’s essential to understand that the flooding of the future also depends significantly on future changes in land use in these watersheds.
The findings in this fact sheet are drawn from the Hydroclimatic Atlas of Southern Québec (see Fact Sheet 16) and from Sharma, Hettiarachchi, and Wasko (2021).↩︎