better estimation of flood wave propagation time …...hydropower'15 stavanger, norway 15-16...

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Better estimation of Flood Wave Propagation Time in Meandering Reaches by using 2D-modelling

J. Persson M. Jewert N. Isaksson

Norconsult AB, Sweden Norconsult AB, Sweden Fortum Generation AB, Sweden

ABSTRACT The time aspect and the location of safe areas are fundamentals in emergency preparedness planning. Nationally-coordinated emergency preparedness plans regarding dam break flooding are made for all large rivers in Sweden. This is accomplished by performing numeric calculations for river systems, with focus on flood wave propagation time and flood inundation. Dam break calculations are made with conservative assumptions regarding model parameters in order to avoid underestimation of the flooding. Flood inundation maps are then generated for the river system. The maps are presented in a GIS-tool intended for use primarily by the national rescue service and the dam owners as common basis for the emergency preparedness planning. A meander is a bend in a sinusoidal watercourse. If a river has meanders, the water can take shortcuts during high flow and/or dam break flood waves. The shortcuts could possibly cause significant reduction of propagation times, which could be crucial for when and how necessary em scribed as a channel, by cross sections. It is difficult to predict where shortcuts may appear, why ergency measures should be done. Previous calculations have been only one-dimensional, i.e. the river is de it is difficult to pre-define the flow paths in a 1D-model. Using 2D-modelling the flow paths are not pre-defined and new paths can appear dynamically in the calculation. In the current project comparisons have been made between 1D and 2D results for the meandering river reach. Dam break calculations for two different dams have been made, where one of them is one of the largest rock-filled dams in Sweden. The distance along the river through the meander reach is about 85 kilometers and the straight distance is about 65 kilometers. In other words, if a flood wave takes all possible shortcuts across the meanders, the distance decreases about 20 kilometers. Quantitative results from these comparisons are presented in this paper, together with a discussion about their qualitative implications.

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INTRODUCTION The national rescue services fight nature´s forces in a daily basis. Fires, storms and heavy rainfall are some examples. Heavy rainfall can cause very high discharges along rivers e.g. severe damage on buildings and bridges etc. and may even cause casualties. A flood wave caused by a dam break can cause even worse consequences. Dam safety is therefore on top of the agenda for most dam owners. Dam break on dams that may cause very severe consequences must be prevented by all possible means. In case of a dam failure on a very large dam the consequences can be of such magnitude that they could be described as a national crisis. This implies many casualties and severe damage on central functions as power grid, transportations and industries, i.e. loss of massive economic values. Thus, it is of large importance that society is as well prepared as possible for such a disaster. It is of large importance knowing what flood wave propagation time and water levels to expect after a dam failure (dam break). If the current river system is complicated and/or irregular the propagation time can be difficult to calculate with acceptable accuracy using conventional analyzing methods.

Nationally coordinated emergency preparedness plans for dam failure In Sweden, as well as many other countries, national coordinated emergency preparedness plans for dam failure are made primarily for all the large hydropower rivers. Along these rivers high-consequence dams are situated as well as populated places and/or large economic values such as transportations, industries etc. These plans should provide the rescue service the necessary information about the possible consequences of the dam failure, so e.g. people can be evacuated in time and in the right areas. The most important information is thus propagation time of a flood wave caused by a dam break and the flood inundation along the river. This information is provided by performing numeric calculations for the current river system combined with GIS analysis.

In 2006 a pilot project regarding nationally coordinated emergency preparedness plans for dam failure, “Pilot Ljusnan” (Elforsk, 2006), was performed for river Ljusnan. This project has set the guidelines for how this kind of projects should be performed in Sweden. Papers describing the Swedish model for emergency preparedness planning have been presented in previous ICOLD meetings: in 2009 (Engström-Meyer et al, 2009) in Brasilia and in 2014 in Bali (Söderström et al, 2014). Conservative assumptions are made regarding the available discharge capacity of the spillways and regarding the development of the dam break, in order to avoid underestimation of the effects. During extreme floods the discharge capacity can be reduced for some reason; e.g. one of the spillways can be blocked by floating debris or the access road may be destroyed by heavy rain. The possible maximum dam breach area should not be underestimated. Combinations of unlikely events are formed to different scenarios and are simulated in the numeric calculations. Results from the calculations are then used for GIS analysis and flood-inundation maps are being made in pre-defined scales, following the standard of the nationally-coordinated emergency preparedness plans. Flood-inundation maps are made for scenarios without any dam breaks as well as dam break scenarios, in order to be able to visualize the margin effect of the dam break. Figure 1 shows a flood inundation map created in the project that will be presented in this paper.

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Figure 1: Flood inundation map created for emergency planning.

The emergency plan projects are funded by the Swedish hydropower industry and by Swedish National Grid which is a state-owned public utility that has many different areas of work. Together with transmission of electricity from power stations to regional electric grids dam safety is an important task for the Swedish National Grid (www.svk.se, 2015).

NUMERIC DAM BREAK CALCULATIONS In hydrodynamic modelling e.g. dam break flood analysis, a numeric program such as Mike 11 or HEC-RAS may be used as an analytic tool to estimate propagation times and inundation levels. In the current study Mike 11 were used. The hydrodynamic theory used by Mike 11 is described in the reference manuals (DHI, 2011).

Conventional dam-break analyzing tool In the very most numeric dam break analysis being made, one-dimensional modelling is used for describing the river hydraulics. In the model´s dam break module a number of dam break parameters are set in order to describe the dam break (breach size and time to develop etc.) often according to some guideline. One-dimensional (1D) modelling often is a fast, effective and sufficient tool for dam break analysis. One-dimensional implies that the river is described as a straight channel built-up from cross-sections and that the flow paths are thus pre-defined. This is often true for many river systems; i.e. a possible flood wave is likely to follow the main river course. In 1D-modelling the user must pre-define all possible new flow paths, in order to describe possible diversions of the river during extreme floods such as in case of a dam break. This is suitable if a major or complete diversion of the river is likely to occur. In a 1D- model an average velocity is calculated for every cross-section in the model and the direction of the flow is always perpendicular to the cross-sections, i.e. pre-defined. In some cases new possible flow paths are hard or even impossible to anticipate and the geometric extents of new flow paths does not necessarily have to be easy to define. A well-defined river could

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e.g. flood into a fairly shallow flood plain where the flow direction is not obvious. In addition, an even more difficult problem in 1D-modelling occurs if the river is meandering, i.e. if the river has sinusoidal bends and it is possible for shortcuts through the bends during high flow. Then a different method is needed that does not pre-define the flow paths and that allows for an infinite number of new flow paths to be created dynamically in the simulation. This can be accomplished using multi-dimensional modelling (2D or 3D). In the current project 2D-modelling was used.

Two-dimensional modelling as a part of the dam-break analysis 2D-modelling implies that the river bathymetry is represented by a three-dimensional space but the model grid is a horizontal two-dimensional grid (rectangular) or mesh (triangular). In the current project mesh was chosen, in order to be able to differentiate the mesh element sizes. When using grid it is not possible to have different grid cell sizes. A velocity is calculated for each mesh element and the direction of the velocity is the resultant of the x and y vectors in the horizontal plane. In a 3D-model the velocity vector can also vary in the z-direction (vertical). The magnitude of the velocity depends on the bed slope and on the bed level. Thus, in 1D-modelling the river bathymetry is represented by cross sections and in 2D-modelling it is represented by a number of mesh elements. This implies that in 2D-modelling the water velocity can vary across the river, as in a real river. A 2D-model makes no difference between the river and the bathymetry (terrain) around the river – all is just a whole continuous bathymetry, i.e. the flow paths are an infinite number and are not pre-defined. However, it is possible to assign different characteristics to different terrain types. When using 2D-modelling in a dam break analysis the dam break parameters are still set in a 1D-model and the dam break hydrograph will be an input (inflow) to the 2D-model, since 2D-modelling programs usually do not have dam break modules.

THE CURRENT PROJECT The project to be presented is called “Coordinated emergency preparedness plan for river Klarälven”, which was performed according to the methodology described in the introduction. The Finnish energy company Fortum owns almost all of the dams and power plants in the river system, why the project is funded by Fortum and Swedish National Grid.

The river system The Klarälven river system is the lower part of a transnational waterway located in mid-Sweden and the province of Värmland (near the Norwegian border). The source of the river is located in the border-land of Norway and Sweden and the middle stretch of the river runs through Norway (known as Klara, Trysilelva and Femundelva depending on what stretch referred to) and the complete river system has a total basin size of approximately 11 850 km2.The main river is about 460 km and finally reaches Lake Vänern which is Sweden´s largest lake. The average discharge entering Vänern is approximately 165 m3/s. There are 41 dam facilities located along the river system, including tributaries. Figure 2 shows an overview of the river system. River Klarälven is known for its meandering reach which is protected by law from hydropower work. The meandering reach is a very popular for tourists, where kayaking is the most popular activity.

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Figure 2: Left: overview of Sweden and the area of interest marked. Right: overview of the Klarälven river

system and the meandering river reach marked.

The two largest dams, Höljes and Letten, are located far upstream in the river system. In this paper conclusions are drawn only for the calculations regarding the Höljes dam. A dam failure there could possibly cause the most serious consequences compared to all of the other dams in the river system. Figure 3 shows the Höljes dam and a part of the meandering reach. About 40 km downstream of Höljes the meandering reach starts. From this point the river meanders quite heavily for about 85 km. If a flood wave takes every possible shortcut through the meander bends the distance can be reduced by about 20 km. Fortum decided that a 2D approach would be the most appropriate approach.

Figure 3: Left: the Höljes dam, about 80 m high at most. Right: a part of the meandering reach.

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Combining one and two dimensional modelling

The program Mike Flood provides a link between the 1D model (Mike 11) and the 2D model (Mike 21). This link can either be lateral, i.e. the main river is modelled in 1D and the areas around the river are modelled in 2D, or standard which implies that a continuous 2D model is used. Then 1D models can be linked to the 2D model in the upstream or downstream end or as tributaries, or even completely inside the 2D area. Figure 4 illustrates the two link types. In this project the standard link was chosen because it is the most straight-forward of the two types.

Figure 4: Illustration of lateral (upper) vs standard link (lower) in Mike Flood (DHI).

The hydrodynamic theory used in the Mike programs is described in the reference manuals (DHI, 2011)

Figure 5 illustrates the mesh from the 2D model and the colors display the level of the bathymetry, which implies that the river is located in a valley. A very high flood (more than 100 years return period) has no choice but to follow this long valley that stretches along the whole meandering reach; there is no alternative flow path.

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Figure 5: A minor part of the meandering river reach, modelled in 2D.

In this project the dam break flood from Höljes dam is routed through the upstream 1D model and then into the 2D model along the meandering river reach and finally into the downstream 1D model via two standard links. Parallel to this configuration with a combination of 1D and 2D models a separate 1D model covering the whole river system including the meandering reach was set up as a comparative model.

COMPARISON BETWEEN 1D AND 2D RESULTS In order to evaluate the possible benefit of using a two-dimensional model as a part of the dam break analysis and emergency planning a comparison to the one-dimensional model for the same reach is necessary. The most important parameter to be compared is in this case the flood wave propagation time. If the propagation times are shown to be significantly shorter, that is important data in emergency planning. The consequence of shorter propagation time is that the villages along the meandering reach needs to be evacuated earlier than expected from previous investigations. Figure 6 and figure 7 illustrates a comparison between the 1D model and the 2D model for a location within the meandering reach of Klarälven. The diagrams show the hydrograph from the dam break flood wave from Höljes dam, expressed in percent of the maximum dam break discharge, routed about 50 km from the dam into the meandering reach. The current scenario shown is the probable maximum flood hydrograph (PMF) as an inflow to the Höljes reservoir and a dam break discharge adding to it.

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Figure 6: Diagram illustrating results from Mike 11 and Mike 21 – discharge at a location in the downstream

regions within the meandering reach, expressed in percent of the maximum dam break discharge.

Figure 7: Diagram illustrating results from Mike 11 and Mike 21 – discharge at a location in the downstream

regions within the meandering reach, expressed in percent of the maximum dam break discharge. Zoomed-in to a smaller time period compared to figure 6.

Also the maximum water level is important output, as well as the maximum water depth, which is a function of the water level. These parameters are needed to be able to locate safe areas for evacuation of people, routes that may be taken in rescue operations etc. Figure 8 shows that in addition to a reduced time to reach safe areas the area itself along the river becomes smaller depending on The higher water levels predicted by the 2D calculation.

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The diagrams illustrate that the hydrograph is more damped in the one-dimensional model, probably due to the significant difference in travelling distance for the flood wave. The propagation time in 2D is between 6 and 8 hours shorter compared to 1D, depending on how the propagation time is defined. The time when the flood wave arrives can be defined in many different ways. One definition could be when the water level has risen e.g. 10 % above the initial water level and another one could be when the maximum water level occurs. The diagram in figure 8 illustrates the calculated water depth in 1D and in 2D. The calculated depth is about 0.5 m higher in the 2D model, due to that the water level in the large area being flooded is not so easily raised despite a quite higher discharge in 2D.

Figure 8: Diagram illustrating results from Mike 11 and Mike 21 - water depth at a location in the

downstream region within the meandering reach.

Figure 9: Diagram illustrating results from Mike 11 and Mike 21 – water depth at a location in the downstream regions within the meandering reach, zoomed-in to a smaller time period compared to figure 8.

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DISCUSSION AND CONCLUSIONS Neglecting the effects of meandering along rivers may lead to significant overestimation of the time available for warning and evacuation. One important result from the presented project is that significantly shorter flood wave propagation times was obtained by using 2D modelling instead of 1D modelling, for a meandering river reach. The dam break flood wave is more dampened in the 1D model than in the 2D model, due to about 20 km longer distance for the flood wave. The terrain surrounding the meandering river is a long valley without sharp bends, which implies that the dampening effect is modest compared to in 1D. The long valley could be described as large-scale almost straight river bed. If the surrounding terrain instead should have been more heterogeneous both in direction and in level, i.e. more flat and maybe even with more alternative flow paths, it is possible that the difference in propagation time would have been smaller between the models. When considering 2D modelling it is of great importance to study in beforehand what resolution of the mesh (or grid) that is appropriate for the purpose and goal of the actual river reach. The resolution that is considered minimum still may cause simulation times that are hard to handle, especially if the model area is big. At worst the number of required mesh elements (or grid cells) can be more than the program limit. In the presented project the model area was indeed big and thus the simulation times were long but manageable. An interesting additional study from this project would be a comparison between results from one Mike Flood configuration with at lateral link and from the one described in this paper, i.e. with a standard link. One phenomenon in the real world that a 2D model cannot handle is if the water split-up in layers when reaching the flood plain. This is an effect that needs to be described with a three-dimensional model. Although 3D modelling theoretically may be the most suitable option, it is not realistic for a model of this size due to extensive computational time.

ACKNOWLEDGEMENTS The authors wish to thank Fortum and Swedish National Grid for funding the work presented in this paper.

In addition, the authors especially like to thank Fortum for thinking outside the box at an early stage by suggesting 2D modelling as a tool for the analysis. It was a very clear-sighted thinking from Fortum to really think through what was the most appropriate approach when about analyze this particular river system. Although 2D modelling involves a higher cost than 1D modelling Fortum still considered it to be of great importance to the project.

REFERENCES

1. Elforsk (2006). Dammsäkerhet. Beredskapsplanering för dammbrott – Ett pilotprojekt i Ljusnan, Sweden.

2. Engström Meyer A. et al. (2009). Coordinated emergency preparedness planning in Swedish rivers. ICOLD congress, Brasilia.

3. Söderström A. et al. (2014). Coordinated Emergency Preparedness Planning in Sweden. A follow up on coordinated emergency preparedness planning in Swedish rivers. ICOLD congress, Bali.

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4. www.svk.se (2015, official homepage of Swedish National Grid)

5. DHI (2011). Mike 11 - A Modelling System for Rivers and Channels, Reference Manual

6. DHI (2011). Mike 21 Flow Model – Hydrodynamic Module, Scientific Documentation

7. DHI (2011). Mike Flood, 1D-2D Modelling, User manual

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