University of Twente Student Theses


The influence of snow initial conditions on ensemble flood forecasting for the Bow river.

Schippers, I. (2021) The influence of snow initial conditions on ensemble flood forecasting for the Bow river.

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Abstract:Many parts of the world experience flooding, which can have catastrophic results. Flood forecasts give insights in the probability that such a flooding will occur and because of that one is able to take measures to reduce streamflow or diminish the impacts. However, there is still a lot of uncertainty in those flood forecasts. In cold-region mountain basins it is unknown how much snow-melt contributes to flooding. This is partially a result of the uncertain conditions of the snowpack, which in models translates to the quality of the snow initial conditions and the ability to model the physical processes in those areas accurately. During the period of 19 to 21 June 2013 intense rainfall and rapid snowmelt resulted in flooding in the Canadian Rocky Mountains and its downstream areas. The flood caused five casualties, monetary damage of approximately six billion Canadian dollars and 200.000 people to evacuate their homes. It is hard to estimate the contribution of the rain-on-snow mechanism, which could potentially have large impacts on floods caused by heavy rainfall and rapid snowmelt. Earlier research showed that especially improved predictions upstream of Calgary could decrease the damages resulting from flooding, like the 2013 Alberta flood. The research objective for this research is to assess the influence of snow initial conditions on ensemble flood forecasts for different lead times for the Bow river by simulating the 2013 Alberta flood. This is divided into two steps, namely; investigating what the hydrological differences between the different sub-catchments in the study area are and determining how adjusting the lead time affects the influence of snow initial conditions on ensemble flood forecasting for the Bow river during each day of the 2013 Alberta flood. Both parts of this research employ Structure for Unifying Multiple Modelling Alternatives (SUMMA) for modelling the hydrological processes in the study area and the hill-slope routing and mizuRoute for the routing of the river network. The hydrological differences between the sub-catchments are assessed based on the snow water equivalent, the precipitation and the streamflow. The snow water equivalent and precipitation, in the hydrological year in which June 2013 falls, are compared with climatology. Furthermore, representative sub-catchments are selected, based on soil type, elevation and land cover, to assess if there is a certain type of sub-catchmentsthat acts different during the 2013 flood, compared to earlier years. Besides that, the value for each of the variables a few days before, during and after the flood are plotted into maps to visually assess the differences between the sub-catchments. From snow water equivalent analysis it shows that the days before the flooding rapid snow melt occurs in the most upstream areas of the upper Bow. When comparing the hydrological year from September 2012 till September 2013 with previous hydrological years, starting from September 2001, it seems that the snow water equivalent is not necessary higher than previous years, but that the snow melt starts earlier in the year and goes more rapidly. In the same days as the rapid snow melt, heavy precipitation occurs in the front ranges of the study area. This suggests that the important factor in the unfolding of the 2013 flood event was the timing of snowmelt and precipitation. For each day of the simulated flood curve (22nd of June – 26 th of June) flood hindcasts are issued, with lead times ranging from 1 day till 8 weeks. Those forecasts are assessed both qualitatively and quantitively. A qualitative assessment of the overall capabilities of the flood forecast is performed by visually comparing all the different forecasts that are made, with each other and with historic discharges. The quantitative assessment is performed using the Continuous Ranked Probability Skill Score, Dichotomous skill scores and the reliability diagram. For each method only the streamflow is assessed. When looking at the flood forecasts for different lead times and different days of the flood peak, it stands out that the flood forecasts only scores well when the forecast is initialised one day before the simulated flood starts. This could mean that the flood forecasts only becomes better because of the improved accuracy of the flow in the river channel. However, a slight differentiation in quality of the flood forecasts can be seen in the river segments that contain the streamflow that resulted from the rapid snowmelt and river segments that contain the streamflow that resulted from heavy precipitation. Therefore, the expectation is that the conditions of the snowpack do have influence on ensemble flood forecasting, however, little. Further research needs to be performed to assess if this improved quality of the flood forecasts for sub-catchments that contain streamflow resulting from rapid snowmelt is indeed caused by the improved snow initial conditions or by different factors.
Item Type:Essay (Bachelor)
Faculty:ET: Engineering Technology
Programme:Civil Engineering BSc (56952)
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