The study consists of two parts. The same workflow is used in both:

Transcription

1 Results following the workflow The study consists of two parts. The same workflow is used in both: Climate impact indicators of hydrology in a future climate Data collection and hydraulic modelling Identify areas and objects at risk and determine the water level at which they are flooded Plan of measures The first, Part A, deals with river Storån that is a meandering stream with polluted areas/sites mainly in the community of Hillerstorp. For the study a model was built in the hydraulic modelling toll HEC-RAS. The second, Part B, deals with Tabergsån and Lake Vättern. Tabergsån is a steep stream that has earlier been modelled by the author in the hydraulic modelling toll MIKE11HD. For the study the model is updated and recalibrated. There are several polluted areas/sites along Tabergsån between the lake Vederydssjön where the model starts and Lake Vättern, the end of the model. Further the tilting of Lake Vättern in combination with changed discharges from the lake will be considered. Results following the workflow 1 Climate impact indicators of hydrology in a future climate The most interesting scenario for the client is the worst case the RCP 8.5 scenario for the year The parameter River flow (daily, seasonality and mean) was extracted from the SWICCA platform for the Storån catchment. Under the RCP 8.5 scenario, the mean annual discharge will increase by 11% by The largest monthly increase will occur in January (+80%). In Hillerstorp this would mean that the mean discharge in January would increase from approximately 7 m³/s (present day) to approximately 13 m³/s (2080). A flow of 14 m³/s is equivalent to the present day 2-years flood. Data collection and hydraulic modelling To obtain the flooded areas, a one dimensional hydraulic model, HEC-RAS, was built. The model was extended 30 km downstream from the study area in order to get a proper downstream boundary condition. Bridges and dams were included in the model using drawings from the Swedish Transport Administration. The model was calibrated with a modelled water level from flooding that occurred in 2004, which was equivalent to a 25-year flood. These water levels were obtained from Gnosjö commune. Detailed cross section information was not available for the site, and instead channel bathymetry was calculated using an iterative approach that modifies the channel bottom level until the correct water level is obtained from the available observed level in the elevation data. However, no discharge stations are available along Storån and instead modelled

2 data was used. Data was extracted from SMHI s Vattenweb, from the SHYPE hydrological model, for same period in It would additionally be useful to consider the potential for increased erosion along the study area. However, velocity data obtained from a one dimensional model is not robust and therefore this aspect was not considered further in the present study. Identify areas and objects at risk and determine the water level at which they are flooded. Figures 3 and 4 show sites that will be flooded in the baseline (present day) climate, according to the simulations performed here. Only one risk classed site will be affected by the 25-year and 100-year floods. In Hillerstorp, the 100-year discharge is 34 m³/s and the discharge of the 25-year flood is 22 m³/s. A discharge less than 22 m³/s will therefore not affect the polluted site (orange triangle). The recurrence interval of this critical discharge (22 m³/s) was subsequently examined under a future climate. A frequency analysis, using the Gumbel distribution, demonstrated that the present day 25-year return period flood will by 2080 and under the RCP8.5 scenario have a recurrence period of 7.3 years. This is shown in Figure 5. Finally, the hydraulic model was re-run for the period 2080 in order to see whether the 100-year flood would increase in extent. Figure 6 clearly shows that no significant increase is expected. Figure 7 shows a cross section at a location just downstream of the industrial site in figure 4 to demonstrate the reason for this. Figure 3. Red triangles are polluted areas with very high risk (Riskklass 1), orange triangles are polluted areas with high risk (Riskklass 2), yellow triangles are areas with moderate risk (Riskklass 3) and green triangles are areas with little risk (Riskklass 4). White triangles are areas without classification. Dark brown squares are industries. The orange field is the area flooded by the 100-year flood in today s climate while the blue field is the area flooded by the 25-years flood in today s climate.

3 Figure 4. The only site in Hillerstorp that will be affected by severe flooding. Figure 5. Frequency analysis by Gumbel distribution for the period 2071 to 2100 under RCP8.5. The floodplain of the meandering stream is flat with steep flood plain borders, as is common in fluvio-glacial dominated areas. The top two blue lines are the simulated 100-year floods. The line on top is the simulated 100-year water level in a future climate (year 2080) and the blue line beneath is the simulated water level in today s climate. The difference in level of the two lines is 0.1 m, showing that additional flow volume cannot have a significant effect on flood extent or depth. In general therefore, the most significant change expected at the Hillerstorp study site is increased recurrence of critical discharges, and not increased extent or severity of the flooded area. This is an important finding when considering how to adapt to future flood risk in the area. Plan of measures The most important issue is the recurrence interval of a 22 m³/s discharges in Hillerstorp. As can be seen in figure 3, there is only one coloured triangle that touches the flooded area which means that there is only one industrial site in Hillerstorp that will be affected by flooding. This site is the only one that is situated on the lower flood plain. Since the other polluted sites are found on the higher flood plain they will not be affected by any flood below the 100-year return period. In the only site that was found to be at risk of flooding the toxic chemicals can be relocated to a higher level during flooding. In the future when planning new industries in the area the results from this study will be taken into account. Erosion due to increased water velocities is an aspect that should also be considered when considering future mitigation. However, velocity estimates from

4 a 1D model are not robust and therefore this issue was not considered further in the current study. Figure 7. Cross section from the HEC- RAS model which shows the flood plain of the meandering stream Storån in Hillerstorp. Figure 6. A comparison between the extent of flooding with two 100-year discharges. One in today s climate (red) and the other in a future climate, 2080, (blue) with the RCP 8.5 scenario. Results following the workflow - 2 The narrow and steep stream Tabergsån lies in Jönköping County in central southern Sweden, as shown in figure 1. Tabergsån is a part of the Motala river system that drains into Lake Vättern which in turn drains into the Baltic. A number of polluted areas are present along the identified watercourse, which the client must manage under future climate conditions. The aim of the study is therefore to analyse how the frequency of flooding of these areas will change in a future climate, and to provide potential mitigation measures for the future Lake Vättern is a 130 km long and 20 km wide lake and is the second biggest lake in Sweden. The lake extends from the city of Jönköping in the south in a north-north-east direction. Jönköping is the ninth biggest city in Sweden. Due to uplift Lake Vättern is tipping over towards Jönköping causing an increase of the water surface there.

5 Indicators used will be: River flow (daily, seasonality and mean) for the stream Tabergsån and outflow from lake Vättern, extracted from SWICCA at catchment resolution and 5 km grid resolution for reference period and 2080 (RCP 8.5, mean ensemble range). Flood recurrence for the RCP 8.5 scenario, calculated separately using methods described in a later report. Climate impact indicators of hydrology in a future climate The most interesting scenario for the client is the worst case the RCP 8.5 scenario for the year The parameter River flow (daily, seasonality and mean) will be extracted from the SWICCA platform for the Motala river catchment. On one hand under the RCP 8.5 scenario with catchment resolution, the mean annual discharge from Lake Vättern will decrease by 74% by The largest monthly decrease will occur in September (-99%). On the other hand the RCP 8.5 scenario with 5 km grid resolution the mean annual discharge from Lake Vättern will increase by 28% by The largest monthly increase will occur in March (34%).The increase in September is 24%. Data collection and hydraulic modelling To obtain the flooded areas, a one dimensional hydraulic model, Mike11HD, was used i.e. an existing model was improved and recalibrated. The calibration was done using water levels from a digital elevation model with high accuracy and resolution. The discharge on the day when the laser scanning took place was used. As detailed cross section information is not available for the stream, instead channel bathymetry was calculated using an iterative approach that modifies the channel bottom level until the correct water level is obtained from the available observed level in the elevation data. A discharge station Norrefors in Tabergsån was used. The station is approximately half way between the beginning and end of the model. It would additionally be useful to consider the potential for increased erosion along the study area. However, velocity data obtained from a one dimensional model is not robust and therefore this aspect was not considered further in the present study. Identify areas and objects at risk and determine the water level at which they are flooded. Figure 2 show the polluted areas in the uppstream part of Motala river drainage basin. Red triangles are polluted areas with very high risk (Riskklass 1), orange

6 triangles are polluted areas with high risk (Riskklass 2), yellow triangles are areas with moderate risk (Riskklass 3). The objective of the study is to identify the risk of inundation of areas with high risk and very high risk. These areas are mainly found just upstream Jönköping city. Another objective is to study the risk of lake flooding of the city of Jönköping in a future climate combined with the effect of land uplift. Figure 1. The situation with the stream Tabergsån and the south part of Lake Vättern. The blue lines are the sections used in the hydraulic model. Figure 2. Red triangles are polluted areas with very high risk (Riskklass 1), orange triangles are polluted areas with high risk (Riskklass 2),yellow triangles are areas with moderate risk (Riskklass 3).