Flood model of city of beer: Plzen City – Czech Republic

DHI Hydroinform, Czech Republic

Abstract

Plzen City, know also as the City of the world famous beer Pilsner Urquel, was heavily flooded during the catastrophic flood in August 2002. This accident was a major driving force to create mathematical models of the rivers of Plzen City. In general, a hydraulic mathematical model of a city is not an exception in the Czech Republic. However, the mathematical model of Plzen City, thanks to an extremely complicated river systems is a very special hydraulic case. The presented model covers the hydrological and hydraulic conditions of five river bodies and four junctions and their mutual interrelations, which were found as significant. The most interesting part of the Plzen model was a special calibration procedure applied on mathematical models and the simulation of a number of different scenario combinations. A global 1D+ mathematical model was created, covering more than 60 km of river network and a wide area of flood plain using MIKE 11 software. The central part of Plzen City was schematized into a 2D curvilinear mathematical model using MIKE 21C SW. The main official outputs of this project were:

• flood maps • maps of depths, water levels and velocities • longitudinal and cross-sectional profiles with an indication of water

levels • evaluated proposal of the flood protection plan of Plzen City • HTML documentation

The case of Plzen City is also special thanks to its location downstream of the reservoir Hracholusky on the Mze River, which creates a hypothetical risk for Plzen City. That is why the simulation of the Hracholusky dam break was carried out. Last but not least, an important feature of this project was a special way of creating the project documentation in HTML format. The documentation allows not only the viewing and reading of all documents and drawings, but also the preparation of user specific composition of final results and the ability to print them directly to a predefined scale, having installed only Internet Explorer. The finished model is nowadays frequently used for flood protection evaluation and the authorization of flood plain activities. Keywords: flood mapping, mathematical modelling, water management, 1D and 2D modes, dam-break, flood protection measures, HTML documentation.

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M. Svobodová & J. Spatka

1 Introduction

In August 2002 a major part of the Czech Republic was affected by a catastrophic flood. Amongst the most heavily affected urban areas also was the town of Plzen and its surroundings and other suburban parts. The city of Plzen has over 200,000 inhabitants and its world-famous name is based on the brewery production of the world finest brand of beer “Pilsner Urquel.” Just after the catastrophic flood, i.e. during the period of 2003 and 2004, the flood model of the city of Plzen was designed. This model was drawn by DHI Hydroinform acting as a supplier and the customers Vltava River Basin Authority and the municipality of Plzen. Flood models of cities are applied to many locations in the Czech Republic, and DHI Hydroinform produced the vast majority of them. Most flood models of cities in the Czech Republic are used for description and evaluation of historical or potential flood situations in cities, where only one river usually flows through (for example cities like Pardubice, Decin and Usti nad Labem on the Labe river, Turnov or Mlada Boleslav on the Jizera river, Ostrava city on the Odra river and so on) or eventually in cities placed on junction of two rivers (for example the capital of Prague on the Vltava and the Berounka rivers or Ceske Budejovice on the Vltava and the Malse rivers). The flood model of Plzen city is rather special among the others due to very a complicated and atypical situation. Six rivers, which consecutive confluence each other with only one river named Berounka as an outlet from Plzen territory, run in the area of Plzen city. With this respect it is to say that flood model of Plzen city is, among other city flood models, the most complicated mathematical model in the Czech Republic.

2 Area of interest The study area of interest is determined by the river network of six rivers, which are to be found in the area of Plzen agglomeration. Namely we will speak about these rivers the Mze, the Radbuza, the Uhlava, the Uslava, the Berounka and the Vejprnicky creak.

• In the frame of this project the below indicated river branches were taken into account:

• The Mze river from Hracholusky reservoir to its junction with the Berounka river – 21,9 km

• The Radbuza river from Ceske Udoli reservoir to its junction with the Berounka river – 6,9 km

• The Uhlava river from profile Stenovice to junction with the Radbuza river – 14,2 km

• The Uslava river from profile in Koterov village to junction with the Berounka river – 10,2 km

• The Berounka from confluence of the Mze and the Radbuza rivers to weir in Bukovec – 9,7 km

• The Vejprnicky creak from profile in Slovanske valley to junction with the Mze river – 2,9 km

© 2005 WIT Press WIT Transactions on Ecology and the Environment, Vol 83, www.witpress.com, ISSN 1743-3541 (on-line)

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Figure 1: Map of the interest area with indicated rivers being solved.

3 Project aims

Main topics of this study cover namely following activities and targets: • set-up of one-dimensional mathematical model (1D+) of selected rivers

and flood plains and its calibration • set-up of detailed two-dimensional mathematical models (2D) for

selected locations of higher interest and their calibration o drinking water treatment plant Homolka o waste water treatment plant o area of Koterov village o area of Roudna village o technical park of transport concern of Plzen city in Jatecni

street o area around streets Malostranska – Ricni o historical centre and confluence of the Radbuza and the Mze

rives • update or new generation of flood lines for discharges Q1, Q5, Q20, Q50,

Q100, • identification of active zones of inundation areas for discharge Q100, • conception design of flood protection measures and evaluation of their

effects on flow condition under flood situation (using mathematical models)

• definition of flood risk area under specific flood conditions caused by dam-break of Hracholusky reservoir

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• creation of intuitive HTML project documentation comprising all the information about input data, different project phases results and all final project outputs in the form of final report, photo documentation, resulting flood lines and flood maps etc., with possibilities not only to view, but also to print technical maps in the selected scale, all from the single “operative” CD.

Figure 2: Example of output - flood map for Q100 and identification of active zones.

4 Mathematical models

Hydrological investigation of the area, where more than one river is placed, is quite specific. Rivers are affected by each other and it is not possible to solve them separately. Therefore each node of two and more rivers has to be solved with respect to complications and non-standard situations, which occur on rivers junction and its close surroundings. Mathematical models in the Plzen area solve six different rivers at once. Four junctions of above mentioned rivers were included into model setup in this project. Such complicated conditions of Plzen area are to be seen, above all, in more complicated process of model calibration, when giving definitions of boundary conditions and during post-processing of model results. Another factor, which plays its important role and which has to be considered, is the flood from August 2002, and consequential anthropogenic activities. Renovation work or prevention activities on flood protection measures and other

© 2005 WIT Press WIT Transactions on Ecology and the Environment, Vol 83, www.witpress.com, ISSN 1743-3541 (on-line)

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activities in flooded areas were, as a result, executed just after the flood. It led to the use of different alternatives of mathematical models used for calibration and for scenario simulations. Mathematical models were calibrated under real topographic conditions during flood 8/2002, but to calculate valid flood lines the most actual topographical data were used in the model setup (actual river cross section shapes and actual relief of flood plain as well). To model such a complicated area, selection of proper mathematical tools played a key role. Finally the mathematical modeling package MIKE 11 was used for 1D modeling and MIKE 21C respectively for 2D modeling. Both packages were developed by Danish Hydraulic Institute (DHI) production. The list of different mathematical models developed under this project can be divided between global 1D+ models of area of interest and detailed 2D models, solving in great detail water flow conditions in seven key areas of Plzen agglomeration.

Figure 3: Horizontal plan of global 1D mathematical model.

4.1 Global 1D model MIKE 11

Mathematical model MIKE 11 was created to cover the entire area of interest. The global mathematical model was designed with respect to the complicated river network and extent of flood plain in project area. This model provides full dynamic simulations on complex branched and looped computational network.

© 2005 WIT Press WIT Transactions on Ecology and the Environment, Vol 83, www.witpress.com, ISSN 1743-3541 (on-line)

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1D mathematical model was primarily used for definition of flood zones and calculation of water level distribution across the area of interest where no detailed 2D mathematical models were available. Results of 1D model were used for flood lines generation and for determination of active flood plain zones. Subsequently those results were utilized for generation of longitudinal and cross sectional profiles with indication of desired water levels of Qn. Finally 1D model was applied to evaluate proposed flood protection measures.

Figure 4: Maps of depths with vectors of velocities for stages before and after construction of inundation culvert in Koterov.

4.2 Detailed 2D mathematical models MIKE 21C

In the phase of defining and contracting this project, the most important locations “key locations”, to which the detailed 2D studies were applied, were identified. Finally resulting from this, three detailed 2D mathematical models were created, covering nearly the whole of the urbanized area of Plzen city. With respect to specific conditions of “key locations” (computation of two and more rivers at once, complicated flow conditions on river junctions, varied extend of flood plain), curvilinear non-equidistant modelling tool MIKE 21C was selected. Curvilinear and non-equidistant computational grid was adjusted to shape and level of importance of different places of project area. In the areas of river channels, riverbanks and in areas close to various objects (like weirs), the computational grid is concentrated into spatial step around 1m. In the areas of intravilan (urbanized areas) the density of computational network is around 3m and in the areas of wide flood plains the maximal spin of computational grid is left up to 8m.

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2D mathematical model was used for investigation of the most exposed areas of Plzen city and the locations of primary interest, i.e. for example selected city districts, historical centre, waste water treatment plant as well as small localities like low-capacity bridges and inundation culverts. In the picture 4 there is an example of a detail study of new inundation culvert on the Uslava River.

5 Conceptual design of flood protection measures

Based on the results gained from mathematical models for current (actual) state of our area of interest, the conceptual design of flood protection measures for different parts of Plzen city was made. Proposed measures were focused on highly urbanized areas, on areas where municipal property is exposed to and therefore put under the flood danger and on areas with the prospect of future development of Plzen city. The partial propositions of the project form a sophisticated concept which forms, in combination of local measures, the final concept of the proposed flood protection plan. Final version of this concept was tested by mathematical models and “side effects” (back water effect, velocity field changes, etc.) were evaluated. In the picture 5 a proposed line of flood protection measure in the most vulnerable areas of Plzen district – Roudna is presented. In the frame of definition of flood protection scenarios, not only identified places of proposed flood protection measures were dealt with, but also their technical specification as a type (earthy dike, mobile protection walls, concrete wall,..) and their dimensions (length, width,..).

Figure 5: Proposed line of flood protection measure of Plzen city district

Roudna.

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6 Extraordinary flood - dam-break of Hracholusky reservoir

Within the project of “Flood Model of Plzen City”, the “special” flood caused by Hracholusky reservoir dam-break was investigated. Hracholusky reservoir is located on the Mze River, approximately 20 km upstream from Plzen city and thanks to its position it creates a potential risk of dam-break flood not only for Plzen city itself. The dike of Hracholusky reservoir is an earthy dike with inner clay seal. Calculated scenario of dike failure depicts a hypothetical situation when the water inflow into reservoir is equal to extreme flood wave of Q10 000. Consequently the dike is corrupted by water erosion on its aerial side due to overtopping the dike crest. Generated flood hydrogram has following parameters:

Peak discharge QZPV = 9514 m3/s Hydrogram volume WZPV = 112.8 mil. m3

For simulation the global 1D mathematical model, adjusted for such dynamic conditions as dam-break, was used. What results from this part of project is the definition of longitudinal profile of maximal water level for above mentioned hydrograph and identification of potentially flooded area identified by flood lines (flood maps) of dam-break flood.

7 HTML documentation

For presentation of project results, the intuitive off-line (capable to run from CD) web pages were created. This presentation offers information about all project phases, used input data, gained results, applied solution methods, and used software and so on. To explore these pages, running alone from a CD, no special software except for IE 6.0 or higher is required. HTML documentation is divided into two principal parts, Flood model of Plzen and Dam-break of Hracholusky reservoir. The whole system of both parts is designed keeping its style to be user-friendly, i.e. on the left side of the screen the navigational menu is located, making easy orientation and movement over single pages of HTML presentation possible. The right frame of HTML presentation is dedicated to data, maps, documents… For comfortable exploration of stored results (mainly map results), simple and intuitive GIS environment was developed inside the HTML. This GIS environment provides the opportunity of viewing, zooming and printing of selected maps in user-defined scale. Selected maps (maps of depths, velocities, water levels and flood lines – flood maps) can be interactively modified by user in terms of displayed layers (different map background, ortho-photo, different flood lines, different line and colour properties…). Location plan offers the extended graphical information about river structures, water levels for different discharges in all cross-sections and much more valuable information. The basic idea and major aim of this type of project result presentation was to make the most important results accessible to all parties of interest. It means not only people in charge with water board and city municipality, but also

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politicians, river administrators, affected villages, crisis and flood committees etc. for their decision-making processes and last but not at least to the general public. At present the HTML documentation is available not only on a CD and the Intranet of Plzen city and the Vltava River Basin Authority, but in a limited version also on the official Internet pages of Plzen city.

8 Conclusion

The project “Flood Model of Plzen City” was finished and handed over to everyday use in November 2004. At this time mathematical models are frequently used in authorization and permition processes of new constructions proposed in flooded areas and areas close to the rivers. Another frequent application of created mathematical models is evaluation of new and revised flood protection scenarios. During the year of 2005 several connected studies have already been executed, which either approved or disproved convenience of proposed activities in the area of interest. For example, the studies of new Plzen city highway circle, evaluation of impact of changes on movable weir or investigation of different scenarios how to increase river capacity with most positive effect rivers in Plzen have been made. These are concrete examples how to utilize mathematical models for optimization of extremely high investments into realization of flood protection measures to be the most effective, efficient and at the lowest price possible. This is also a clear demonstration that, fortunately, this project has not ended up deep in the drawer after the official hand of authorities over the project results was drawn back, but it is, and in the future will be, widely used in daily practice.

References

[1] Barnes, H. H., (1967). “Roughness Characteristics of Natural Channels”, U.S. Geological Survey Water-Suplay Paper 1849, United States Government Printing Office, Washington, 1967.

[2] Jan Špatka a kol., „Methodology for determining active zones of flood plains“, methodology, 2003-2004.

[3] Marcela Svobodová, Jan Špatka, Jiří Cabrnoch, Jan Cihlář, Methodology of flood damages evaluations “, final methodology, 2002.

[4] J. Smetana. „Hydraulika 1 a 2“, Nakladatelství České akademie věd, Praha 1957.

[5] Jan Špatka, Viktor Hrnčíř, Marcela Svobodová, Filip Mateásko, Marek Maťa, Pavel Tachecí, „The methodology for analysis of consequences of forecasted floods”, final methodology, 2002

[6] CHMI, “Preliminary report on hydrometeorological situation of the flood in August 2002” CHMI, Prague 2002.

[7] Ven Te Chow, McGraw-Hill, (1959). "Open Channel Hydraulics", (pp. 180-185), International Book Company, Hamburg, London, Sydney, 1959.

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[8] V. Kolář, C. Patočka, J. Bém. „Hydraulika“, SNTL – nakladatelství technické literatury, Praha 1983.

[9] Danish Hydraulic Institute, (1989). "MIKE-21", Manual 1. a 2.Horsholm, 1994.

[10] Danish Hydraulic Institute, (1989). "MIKE-11", Manual 1. a 2.Horsholm, 1994.

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