CGU HS Committee on River Ice Processes and the Environment th 20P P Workshop on the Hydraulics of Ice Covered Rivers Ottawa, Ontario, Canada, May 14-16, 2019. An Emerging Picture of Peace River Break-up Types that Influence Ice Jam Flooding of the Peace-Athabasca Delta Part 2: Insights from the comparison of the 2014 and 2018 Break-ups Martin Jasek BC Hydro, 6911 Southpoint Drive, Burnaby, BC, V3N 4X8 [email protected] The 2014 and 2018 highly dynamic break-ups on the Peace River have been well documented but were very different from each other in that the former produced an ice jam flood in the Peace-Athabasca Delta (PAD) and the latter did not. This paper examines the differences and looks at driving environmental variables that produced very different ice jam flooding features and locations. The paper then extrapolates further to outline other possible break-up types to increase the understanding of the driving variables that affect the chances of ice jam flooding of the Peace River sector of the PAD. A conceptual model for determining PAD floods was developed using 4 binary categories: discharge potential, rapid weather warm up or not, break-up resistance upstream of PAD, break-up resistance in PAD reach and upper Slave River, resulting in 16 outcomes. Only one outcome leads to a guaranteed PAD ice jam flood and the second only possibly. The remaining 14 do not lead to a significant ice jam flood of the PAD. 1. Introduction Background information about the location, hydrology, the ice jam flooding mechanism, the high springtime flows of the unregulated Smoky River to drive the dynamic break-up on the Peace River and channel storage release that all play a role in ecologically beneficial flooding of the Peace Athabasca Delta are described in Part 1 in these proceedings (Jasek, 2019a). This paper compares the 2014 and 2018 break-ups of the Peace River, the former resulted in an extensive flood of the PAD and the latter did not. Detailed descriptions of the 2014 and 2018 break-ups can be found in Jasek (2017a, 2017b) and Jasek (2019a, 2019b) respectively. The term “jave” is used throughout this paper and is defined as: when an ice jam releases it forms a wave or what is called a “jave” that can break-up more ice cover in front of it and may eventually create ice jam flooding at the PAD. The term “jave” defined in Beltaos, (2008). 2. Comparison of the 2014 and 2018 break-ups 2.1 USpring snowpacks on the Alberta plains in the Peace River Basin Spring snowpacks or Snow Water Equivalent (SWE) in the portion of the Peace River basin were well above normal in the spring of 2014 and 2018 (Figure 1a and 1b respectively). When these large snowpacks melt they drive run-off that can trigger dynamic break-ups on the Peace River. Figure 1c shows the ratio of SWE of 2018 over 2014. Eight out 13 of the snow survey sites indicated higher SWE in 2018 than in 2014 and five out of 13 showed a lower SWE. This suggests that the snowpacks were similar in the two years, slightly differently distributed and overall slightly higher in 2018 than in 2014. 2.2 USpring air temperatures Figure 2a shows the hourly air temperatures for April – May 2014 and April – May 2018 on the Smoky River at Watino which are a good indicator of the snowmelt occurring in the low elevations of the Smoky River basin and other catchments along the Peace River between the foothills and the Town of Peace River that typically drive the dynamic break-up on the Peace River. There are some differences and similarities in the temperature trends between the two years indicted in Figure 2a that are worth highlighting. In early April 2014 there was above freezing warm spell that lasted about a week were the same period in 2018 had mostly below freezing temperatures. Therefore, some early melt occurred in 2014 where some of the snowpack may had been lost too soon to contribute to the river flows during break-up in late April. This earlier warm spell in 2014 was enough to dynamically break-up the lower Smoky and Little Smoky Rivers from Apr 11-16, 2014 but not the upper Smoky River (Jasek 2017b). From April 12 to 22, the temperatures were fairly similar and mostly positive in the two years. The warm spells between April 18 to 23 in 2014 and 2018 was what drove the upper Smoky River to break-up on Apr 25, 2014 and what started the Lower Smoky to break-up on Apr 23, 2018. In 2018, the warm weather continued after April 22 where in 2014 it cooled for a few days. The continuing warm weather in 2018 caused the upper Smoky River to break-up on Apr 26, 2018, only three days after the lower Smoky River broke up. By contrast, there was a 9 day delay between the lower Smoky completed break-up and the upper Smoky break-up. However, in both years, the Peace River dynamic break-up for long distances towards Fort Vermilion and the PAD did not start until the upper Smoky River had broken up. 2.3 UDischarge Figure 2b shows the discharge at the Town of Peace River in April – May 2014 and April – May 2018. The traces start after there is no more ice affected backwater at the Town of Peace River. Even though the water volumes available from snowpack were similar in the two years, the run- off came in three pulses in 2014 and in one pulse in 2018 due to differences in air temperatures. The first pulse in 2014 in Figure 2a is not shown because the Peace River at the town of Peace River were backwater affected by an ice cover but it did break-up the lower Smoky River and 3 Little Smoky Rivers. However, both the 2014 and 2018 break-ups exceeded 4000 mP P/s at the Town of Peace River on about the same date (Apr 24-25) which started the dynamic break-up of the Peace River in both years. In 2014, there were two discharge peaks at the Town of Peace River that drove the break-up 3 3 further downstream, about 4700 mP P/s on April 25, 2014 and another of about 4000 mP P/s on May 3-4, 2014. In 2018 there was one prolonged and much higher discharge peak between 6000 and 3 8000 mP P/s from Apr 26 to May 1, 2018. These two very different hydrographs drove the two break-up sequences in 2014 and 2018 described next. 2.4 UBreak-up sequences, ice jams and flooding in 2014 and 2018 Figure 3 shows break-up front locations and observed ice jams for the 2014 and 2018 break-ups. Horizontal lines indicate stalled break-up fronts with vertical lines terminating at the horizontal lines indicating locations and lengths of observed ice jams. Small sloping lines are indicative of slow moving break-up fronts, typically thermal break-ups. Steep lines indicate faster moving dynamic break-up fronts. Lines that are dotted are approximate break-up fronts to a few hours based on indirect data and rate of travel assumptions. Indirect data includes subsequent observations of ice runs the next day, timing of gauge increases and prior observations of intact ice cover locations a few hours to a day before. More detailed similar plots indicating intact ice covers, open water and ice runs on similar figures to Figure for the 2014 and 2018 break-ups can be found in Jasek (2017a) and Jasek (2019a) respectively. The starting date of the dynamic break-up on the Peace River in both 2014 and 2018 was Apr 26 as noted by the change in slope of the break-up front celerities in Figure 3 on that day. The break-ups in both years transitioned from thermal to dynamic not only on the same date but also a similar location, just downstream of Sunny Valley, about 700 km upstream of the PAD. Both break-ups created ice jams around Fort Vermilion and Vermilion rapids but the 2014 break- up had stalled ice jams in place for 3 days where the 2018 break-up had stop and release of ice jams from Thompkins Landing to Vermilion Rapids on a more or less daily interval. Figure 3 shows the break-up of the ice cover between Vermilion Rapids and the PAD was very sequential in 2014 lasting only about 1 day. By contrast the break-up of this reach in 2018 experienced multiple break-up fronts over about a 2 day period. The most important difference between the two break-up sequences relative to PAD flooding is that in 2018 the ice in the PAD reach and well downstream on the Slave River melted out thermally prior to the arrival of the main break-up jave from upstream, where there was competent ice there prior to the 2014 break-up. In 2018 an early thermal break-up started from Rocky Point sometime on Apr 25 and 26 when the more forceful dynamic break-up was still 700 km upstream. This thermal break-up continued downstream on the Slave River and was about 76 km downstream of the PAD on the Slave River on May 2 when the jave from the main dynamic break-up from upstream arrived in the PAD reach. This was too far downstream of the PAD for the ice volume available from upstream to create an ice jam through the PAD reach; the head of the ice jam on the Slave River was apparent in a satellite image on May 4 to be 36 km downstream of the PAD.