Chapter 4: Types of flood risks on the territory of the Czech Republic

Transcription

1 Chapter 4: Types of flood risks on the territory of the Czech Republic josef hladný 1. Introduction Among various hydrological situations continuously appearing across the Central Europe with uneven frequency, those with amount of flow surpassing, for varying reasons, capacity of the river channel are considered as floods. Usually a sudden rise of flow is a reason, but it can also be a drop in flow capacity of the channel due, for example, to ice constriction, barriers created from floating objects etc. The water level at such conditions rises above riverbanks and water would start to flow over and flood surrounding areas [inundation, flood]. In relation to specific conditions in the economic use of landscape under the risk of such flooding and to development of precipitation and runoff situation, land and buildings can be inundated, sediments can settle, erosion can take place, available drinking water resources can be degraded, residential and industrial buildings devastated etc. [state of danger flood stage]. The recent large floods in Moravia in July 1997 and in Bohemia in August 2002 are the latest examples. Size, frequency and extremity of flood risk creates therefore, together with increasing vulnerability of the environment in the Czech Republic, a number of serious problems in the current flood defence. Response to the number of these problems relates to the level of knowledge about particular type of flood risks on our territory. This Chapter should contribute to such an aim. It presents basic information on origin of particular types of floods, on physical-geographic possibilities for prediction [predictability] and on possibilities of flood defence in reducing their harmful effects. 2. Classification of flood hazards A large majority of floods appear due to precipitation and, in the winter period, due to sudden warming and subsequent melting of snow cover. Appearance and movement of ice mass in watercourses can also cause a flood. In few cases other specific conditions, such as, for example, sudden blocking of the stream following a landslide can lead to creation of a local flood situation. From a combined identification of causes and their seasonal occurrence the following classification developed in practice for individual types of floods:

2 48 josef hladný summer type of floods due to short storm rainfall, regional rainfall, winter and spring type of floods due to snow melt, development and transport of mass of ice in the river channel, floods for other specific reasons. Heavy rainfall or snow melting do not necessarily cause flood in every case. Hydrological situation in the basin is also important, particularly taking into account whether the previous precipitation saturated the basin to a degree to allow its further saturation, or whether snow melting takes place on frozen soil preventing infiltration or otherwise. Flood danger in large basins appears usually only after a certain amount of precipitation is exceeded, or after certain duration of a period with positive air temperatures. In a simplified way, governing effects, apart from weather, can be attributed to the state of four elements of landscape capable, in various ways, to retain [retention] or, in addition, to accumulate [accumulation] runoff from precipitation. This includes the following effects of the landscape on development of a flood: (1) Intercepting effect of vegetation on precipitation given by density, type and development stage of plants in the season and inter-annually [interception]. Direct interception of a part of precipitation on the plants surface affects only initial stage of the flood. Much higher effect is due to interception of snow in the winter season. In the average conditions it can be assumed that retention effects of meadows range around 2 litres/m² and can reach 5 litres/m² in forests. Vegetation has also an ability to delay movement of surface water and to extend the length of possible infiltration. For example, it is estimated that on flat forest soil 60 to 75 litres/m² can infiltrate per hour. (2) Ability to delay runoff by filling empty spaces in rugged terrain and of depressions without outflow by water [detention]. On flat terrain water accumulation can be temporarily higher than on slopes. In the average, it can be estimated that retention effect can range between 1 to 5 litres/m² depending on local conditions. Detention affects only initial stage of the flood. (3) Seepage of water into soil layers and groundwater aquifers [seepage, infiltration]. Soil can accumulate relatively large amount of precipitation water. It depends on the type of soil, its thickness, porosity, content of humus etc., as well as on its saturation by water. If intensity of the rainfall is higher than intensity of infiltration, surface runoff develops. With continuing rainfall the soil saturation increases and surface runoff intensifies. Measures increasing retention capacity of the soil form and important part of preventive measures for flood protection. (4) Filling river channels including water infiltrated into river bank subsurface layers of embankment zones resulting from increase of hydrostatic pressure due to increasing water level [river network capacity] as well as overflow into inundation areas along watercourses [inundation capacity]. Before water overflows the river channel, its capacity must be filled. The channel behaves therefore

3 types of flood risks on the territory of the czech republic 49 as a reservoir. Then similar filling follows of inundation areas. Knowledge of both capacities is an important factor for estimation of development of flood situation in a given river system. Reservoirs built in the basin belong to the fifth, although not natural flood development affecting factor. By retaining a part of the flood volume, the reservoirs can decrease flood discharges, particularly at their peaks [culmination], delay their passage and limit harmful effects of the flood in the downstream river valley. Protection effect of reservoirs depends on volume of their retention capacities. Bypass channels represent additional active measure to affect flood runoff conditions, diverting a part of the flood volume outside stretches where existing flood protection measures could be insufficient, or diverting flood discharges into another basin where situation is less dangerous. Runoff development is also affected by continuously acting physical-geographical factors, such as the shape of basin, altitude, inclination of slopes, inclination of river channel, orography, type of soils, geographical orientation of slopes, location of urban units, vegetation etc. Individual basins differ in view of runoff by these specific conditions. 2.1 Summer type of floods from short storm rainfalls Floods resulting from storms are recently in the Czech Republic often entitled flash floods according to an American equivalent. The reason lies in a typically short duration of a period between precipitation of the main volume of the rain storm and the peak of the intensively developing surface runoff in the affected basin area [concentration time]. The related sudden development of flood situation responds to these conditions, characterized by relatively fast increase of water levels, sometimes even by several meters in 2 to 15 hours until culmination. In mountainous narrow valleys the flood waves have no possibility for transformation, so they move downstream with increased or at least the same extremity of culmination discharges. In flat terrain the intensity of runoff response to storm rain is rather gradual. In urban areas there is a possibility of flooding of built-up areas even without overflow of water from nearby watercourses due to fast accumulation of surface runoff from the storm rain which cannot be in a given time, for various reasons, collected by the sewerage and draining system of the agglomeration. Flash floods in the Czech Republic appear usually from the second half of April until the end of September. They develop after a typical storm rainfall, so called convection torrential rain of high intensity (in extreme cases over 100 mm per hour, i.e. 100 litres/m² per hour), with short duration (less than 2 to 6 hours), hitting by its size-limited core relatively small area (usually several tens of km²). It means that they can usually generate a flood in basins of small watercourses and in any part of the Czech Republic. Flash floods represent the most frequent cases of flood risk. Apart from single isolated storms there can also develop in the atmosphere one or more sequences consisting of several storm systems behaving as a single common system [convection mezo-system], which may hit areas covering medium size river basins

4 50 josef hladný (with an area of thousands km²). Flash floods sometimes also appear in connection with large precipitation systems of a regional type, if inside conditions develop prone to creation of storm convection cores masked storms. Flash floods represent for population along watercourses potential danger by their relatively fast development, particularly in small basins with steeply inclined terrain. The time from identifying that current rain could generate by its intensity a flood until the instant when water in a given site exceeds flow capacity of the river channel and starts to endanger surrounding areas [time potential for flood protection measures in real time] is often only several tens of minutes. For possible evacuation, emptying of production facilities, removal of obstacles in the watercourse etc. this is too short a period. If the rainstorm, for example at night, is not observed or if there is no warning or endangered population is not previously preventively instructed on how to react in such a situation, there is dangerous surprise. For extension of this time potential through use of meteorological and hydrological forecasting models there are not real conditions, as predictability of the local rainstorms, developing from unstable air masses, is very limited. The storm cores change their position very quickly and precipitation of flash rain takes place with unpredictable intensity, sometimes even with sudden water fall [cloud burst]. The other obstacle in reliable forecasting of development of rainstorms and particularly their localisation on small basins is modelling technology based on macro-synoptic analysis of weather using different space and time steps, obviously bigger than sizes and time frames for development of storm cores of flash rainfall. Better means for specification of a location of the existing storm systems and for monitoring their movement in the area is the meteorological radar which, so far, provides less precise and therefore less reliable data for predictions on rainfall depths. In majority of cases it underestimates its actual amounts. It is expected that certain further improvement in identification of local rainstorms for use by forecasting and warning flood service can be reached by combining data from the network of meteorological radars and from land-based warning precipitation stations, as well as by preparing the data as an integrated input to the meteorological and hydrological forecasting models. Warning against flash floods through the state monitoring system will have, in any case, limited real possibilities, particularly concerning small watercourses, which can endanger by flood flows population in nearby municipalities. This problem is not limited to the Czech Republic, but it concerns also other countries. For example, in the USA or in Australia local warning systems are developed identifying danger of possible flood situation after a given amount of precipitation or water level in a stream or reservoir is reached. Location of automatic measuring stations and setting up of danger signal limits is determined so that maximum possible time lead is ensured. Minimum period to be ensured is at least 30 minutes, which is internationally accepted standard for people to save their lives before appearance of dangerous stages of a flood wave. Signals on reaching the limits are transmitted optically or acoustically to a place under a permanent human control. Installation of similar local warning systems was initiated already also in the Czech Republic.

5 types of flood risks on the territory of the czech republic Summer type of floods due to regional rainfall Rainfall can generally be characterised by three parameters: average depth, affected area and duration. These three parameters are interrelated by a principle according to which rainfall of long duration affects large areas and has low intensity, while short rainstorms with high intensities hit small areas. Continuous regional rainfalls (hitting areas of thousands to hundred thousands km²) develop usually in connection with atmospheric fronts. In the cold front the cold, and therefore heavier, air mass slides in under the warm mass and forces it to sharp and fast lifting. The accompanying cumuliform clouds bring along heavy rainfalls usually of high intensities. In the warm front, after the two masses of different temperatures meet a slow, but continuous lifting transport of warm air takes place with development of rotation around low depression. Accompanying rainfalls have usually medium intensities, are more uniform in the whole of its duration and hit large areas. Persistent rainy weather in regional scale is more prone to flooding of large rivers. If the amount of precipitation in 24 hours surpasses a certain limit, a danger of flood development increases. This threshold limit of precipitation for flood sensitivity of the landscape is however varying and it depends on whether the effective rainfall affects a basin saturated by previous precipitation or whether a large part of its amount fills subsurface layers of an unsaturated basin. The threshold flood value of precipitation is also significantly affected by altitude of the basin. Approximate estimates of the limits of rainfalls necessary for development of flood situation taking into account basic differentiating factors were prepared together with instructions for their application by the Czech Hydrometeorological Institute in the Technical Guidelines for Flood Warning Service (published by the Ministry of the Environment in 1999) with the aim of allowing announcement of levels of flood activities (LFA) based on measured precipitation also in small basins, where the flood runoff concentration time is very short (from several tens of minutes to two hours), see Table 1. Duration of rainfalls generating regional floods in the Czech Republic ranges between 1 and 3 days, in extreme cases it may be longer, as demonstrated by five-day duration of rainfall of the floods from July At flood situations with duration of Table 1 Approximate estimation of threshold values of flood rainfalls [mm]. Altitude of the basin Unsaturated basin Saturated basin 1. LFA 2. LFA 1. LFA 2. LFA Mountainous 60 to to to to 70 Average 50 to to to to 60 Low 40 to to to to 50 Period of validity: May October. Basin altitudes: mountainous above 700 m a.s.l.; average 400 to 700 m a.s.l.; low below 400 m a.s.l. Saturation: a basin is considered saturated (unsaturated) if the average rainfall depths during the recent 10 days are more (less) than 50 mm 1. LFA: caution, 2. LFA: preparedness, 3. LFA: endangerment

6 52 josef hladný effective rainfall longer than lag time of water travelling from the farthest edge to closing site, whole area of large basin affects development of the flood in terms of runoff process. In addition, in such conditions in mountainous and foothill areas, significant orographic intensification of precipitation take place due to windward effects. If the rainfall is persistent and heavy across the whole area of a large basin, flood waves develop predominantly in regions of short runoff concentration time. After filling the volume of the channels of river network and with continuing rainfall, discharges relatively quickly take a form of flood and development of hydrological situation changes into the state of endangerment in relation to urban areas or other production facilities located along watercourses. In a complex large basin affected by precipitation, these areas are usually the first where the sequence of flood events is relatively fastest and which need, in terms of time priority, external help, if the local responsible authorities cannot cope with the flood protection themselves. Extremity of culmination discharge reached in upper parts of the basin would remain the same in lower stretches of the main watercourse if there were no effects of filling bigger channel, decreasing inclination of the channel and, particularly, more possibilities for inundation. With usually increasing effects of channel and valley retention, the flood wave in the main watercourse should become flatter and its movement should slow down [transformation of flood wave] if this basic hydrological scenario would not be affected by runoff situation on tributaries. This scenario can however be changed be these effects significantly. The flood waves from flooded tributaries can either pass or coincide with the wave in the main watercourse. In the second case discharges gradually accumulate [interference of discharge waves] and water from the tributary immediately increases the volume and contributes to increase of culmination discharge in the main watercourse. Relatively worst development takes place when culmination discharges of both waves reach the confluence at the same time. By composition of the waves, the average flood can become extreme. If these discharges arrive with the short time lag, the resulting flood is characterised by long culmination, or, in a case of longer time lag, by two distinct, not so extreme and therefore less dangerous peaks. Fragmented structure of the river network can, with regional rainfall, cause multiple-peak wave on the main watercourse. These cases are necessary to distinguish from runoff response to time differentiated impulses of increased precipitation. For needs of preliminary warning and estimation of future flood situation, knowledge on transport time of flood discharges is necessary. From the above discussion it is clear that by appropriate time delaying of a part of flood wave volume, water levels downstream can decrease, probability of meeting of culmination phases of waves from tributaries can decrease and the resulting wave becomes flatter, so it cannot make too high damage. Such retention effect can be reached through the following measures: by manipulation on reservoirs with retention volumes; by using natural inundation volumes in the flood plain if these exist and are not urbanised; by implementing measures increasing ability of the landscape to retain water.

7 types of flood risks on the territory of the czech republic 53 A question on how big flood waves with a given extremity of maximum discharge and relevant volume can still effectively be affected in specific conditions of individual basins using an integrated implementation of the three above approaches remain open in the Czech Republic it is one of the key tasks of future development of flood protection. It is however certain in advance that these measures will necessarily have their final limits and that there is a necessity to take into account possibility of occurrence of such floods, the size of which will exceed framework of all possible protection measures. Therefore, responsibility in these circumstances of flood warning and forecasting service in dealing with regional floods is even bigger. If reports from upstream stretches of the river network are used for forecasting, the time lead of forecasts is given by velocity of transport of discharge waves through channels of tributaries and of the main stream. For the CR watercourses it is possible to forecast only several hours in advance in relation to size of the basin. For example on the biggest main Elbe River the longest lead time for the closing site ranges between 24 to 36 hours with estimate of tendency for the next day. The maximum possible forecast depends on the shortest travel time of discharges on some of the nearest tributaries. If the maximum travel time from the hydraulically farthest place of the basin is to be used, it is already necessary to prepare a forecast using information on precipitation fallen in the basin. By introducing information on observed precipitation into the forecasting procedure the lead time can increase twice, forecast precision, however, decreases in the average from the initial 5 % to 20 % of the actual parameter. In a case when quantitative precipitation forecast from a meteorological model is used as an input to the hydrological model, i.e. forecast for forecast, it is obvious that it concerns precipitation water that has not yet fallen in time of issuing the forecast of discharges and that it is water outside the space of basin in question. As stimulation forces for regional precipitation belong to dynamic processes of the atmosphere, attempts for longer lead time, and therefore for longer warning times, will have to rely on development in predictability of these meteorological circulation processes. Regional floods are usually accompanied by large inundations. In view of fragmented structure of altitudes in the Czech Republic, the inundated areas along the upper stretches of watercourses are of smaller extent than at their middle and lower stretches. In terms of surface area, the biggest inundations can reach several kilometres of width and appear along the middle Elbe, lower Ohře, middle and lower Morava and lower Dyje rivers. If the inundation areas are urbanised or used for economic activities, flood damage is usually rather high, often supply services are interrupted and shortage of drinking water is particularly painful due to flooded drinking water resources etc. Precipitation of long duration can also provoke landslides, even after the flood. 2.3 Winter and spring type of floods due to snow melt For melting of snow cover it is necessary that its temperature increases above 0 C. The relevant heat energy may be supplied by sun radiation, air temperature, wind and

8 54 josef hladný rainfall. If, after a change of meteorological situation, some of the factors dominantly or in their combination induce snow melt, subsequent development of the flood situation depends primarily on snow depth, water capacity of snow, degree of soil freeze, altitude and orientation of the basin. Snow melt during thaw spreads from the surface of the snow layer downwards and also from its bottom upwards. Deep snow cover behaves as a suction sponge. It can retain melted or rain water in its free cavities between flocks of snow crystals and it has therefore, until its retention capacity is exhausted, rather slowing effect in development of flood discharges. On the other hand, thin snow layer melts faster. If snow melt is accompanied by rainfall and if soil is frozen and prevents infiltration of water, water from snow outflows together with rain water and a flood danger increases. In mountainous regions, in relation to differences in altitudes, large snow volumes are accumulated that are not susceptible as often and are not as sensitive to snow melting as snow in hilly and low lands. Snow on mountainous ridges, after seasonal warming melts usually slowly and increases water levels in watercourses in the spring season. Release of water from melting snow in the basin proceeds in altitude belts from the lowest places along the watercourses upwards as far as to zero degree isotherm (an imaginary line joining points of zero degree temperature), or to the basin boundary. In the time sequence, firstly water from melted snow from foothills runs off, provoking often, in periods of several day duration of temperatures above 0 C, individual discharge waves that can also reach flood levels. During sudden invasion periods of warm air accompanied by intensive rainfall of long duration and when subsequently all altitudes in mountainous regions with snow cover are involved in snow melting, relatively intensive runoff can develop. For this flood situation fast increase of water levels is typical in foothill regions with increased extremity, particularly in volume of the flood wave. In lowlands and hilly lands, in relation to frequent alternation of snowfall and subsequent thawing, the snow cover is not continuous during winter. If a continuous cover develops, it is much thinner than in the mountainous regions. However, thawing can, in view of small differences in altitudes, affect simultaneously large areas. Melting of snow cover with small thickness induces relatively fast development of surface runoff in large areas and related increase of a number of flood endangered stream stretches, particularly if these experienced ice phenomena. Such situations can appear not only in the spring season, but also in typically winter months December, January and February. They are generated by warm and humid air masses flowing predominantly from the west. The influence of warm air speeds up the process of thawing. Amount of water accumulated in snow cover depends on its physical state For example, one centimetre of fresh powder snow corresponds to 1 mm of water depth, i.e. 1 litre of water per 1 m². By gradual melting, freezing and pressure of its own weight the crystal structure of the snow looses its cavities and becomes compressed, so that 1 centimetre of settled old snow contains in the average up to 4 mm of water. Flood waves generated by spring melting reach usually the biggest volumes throughout the year, they are typical by flat peak and long duration.

9 types of flood risks on the territory of the czech republic 55 For floods from snow melt it is possible to forecast their main generating factor, i.e. air temperature better than precipitation. Also, increase of discharge is usually slower than for floods from summer precipitation, because even very fast melting is similar in intensity to the effects of only medium rainfall. An exemption is represented by snow floods generated by flow of warm air with simultaneous appearance of rainfall. If the snow flood is accompanied by ice movement, forecast quality is lower due to higher noise (i.e. summary effect of factors not known when preparing the forecast). 2.4 Winter and spring type of ice floods Characteristic feature of floods occurring due to ice phenomena on watercourses is always decreased flow capacity of the channel and subsequent increase of water to the flood level. Ice phenomena start to appear usually only after the amplitude of daily fluctuation of air temperature (difference between the maximum and minimum air temperature during a day) drops completely to the negative temperature zone, i.e. below 0 C. The ice crystals created in water join in these conditions into bigger structures, leading, during long freezing periods, to appearance of closed ice cover, even in flowing waters, growing gradually from banks to the flow line. Its fragmentation, or transport along the channel [ice drift], or definite freeing of the channel from ice phenomena [ice shove] take place during thawing, particularly if accompanied by intensive rainfall. Ice cover starts to break at stretches where it is, due to strong water flow or warm water, thinnest. Stretches with intact ice cover prevent smooth movement of ice sheets or slabs [ice blocks]. At its upper edges, but also at shallow places, bends, channel narrowing etc. ice blocks slide under each other, pile up, block free channel, rise water and create ice jams, growing gradually in height and length. After their breaking, new and usually bigger barriers develop and the process is repeated until the last phase, i.e. pile up of ice floes in lower part of the watercourse. The resulting free flow carrying ice mass is usually characteristic by its destroying force. Ice jams are dangerous particularly by making relatively low discharge, which is rather safe in normal conditions, suddenly endangering in a changed conditions after development of ice barriers and by their ability to provoke floods comparable with highly extreme discharges, sometimes higher than a discharge occurring in the average once in a hundred year period. It is known from history, that among highest water levels those were included belonging to ice floods of this type. Ice floods can also develop during a period of freezing in watercourse stretches with low depth and high inclination of river bed. In their cold flow particles of frazil develop, crystals of which are transported by turbulence to the bottom where they can occasionally attach to its rugged surface and gradually grow into a form of bottom ice. In a similar way river bank ice can develop, slowly growing up to a stage when water level of narrow streams freezes up. If a strong frost lasts for a long period, the channel is extremely filled by ice and its flow capacity decreases. When discharge increases during sudden thawing, conditions appear for ice flood danger.

10 56 josef hladný In increased water depths and low flow velocities, particles of frazil can join to flocks that form, after they reach water surface, slush ice. In frozen stretches transport is interrupted and inflow of ice blocks the channel. Freeze-up ice jam develops that can generate ice flood even in a period of frosts if it rises water so it flows out of the channel. Predictability of ice floods is very difficult, unlike other commonly forecasted hydrological parameters (water level, discharge, etc.). Ice phenomena are affected by a number of random and locally differing factors, therefore it is not possible to adopt a common methodological approach for forecasting. To a certain degree it is possible to rely on a tendency that at certain meteorological conditions ice floods occur repeatedly in characteristic stretches of river network. For this, it is necessary to systematically record and analyse in detail development of floods and ice phenomena at all places where ice and freeze-up jams appear. 2.5 Floods for other specific reasons Decrease of flow capacity of a channel and subsequent, often fast, raise of water up to a flood level or direct inundation can be induced also by: sudden blocking of a watercourse by a landslide (due to erosion of the slope foot by lateral flow or due to saturation of nearby slopes by persistent rainfall), or also by a landslide from loosen rock or snow mass and pulled down material [avalanche floods], inundation due to raise of water at lower reaches of tributaries due to increased water level at the main river [inundation from backwater], flush of sediments following intensive precipitation or snow melt at forest free slopes of mountainous regions when motion energy of surface runoff increases to a degree that it starts to pull down to the stream gradually larger and bigger particles of eroded rock a resulting mixture of water, mud, gravel and stones destroys on its way to valley everything in its way [sediment floods], inundation due to extremely strong winds (storms etc.), which push high waves of water on banks of large lakes or reservoirs [inundation from wind waves]. To the first category of flood hazards, blocking of bridge openings, culverts or channels with flow of obstacles (tree trunks, bushes, woods and other carried free objects) can be included. This situation usually appears on small watercourses where these barriers often burst open and the generated break wave with large amount of obstacles increases its tendency to block the channel and create another blocking downstream. This phenomenon can repeat itself in cascades and intensify extremity of maximum discharge. The resulting discharge culmination in lower stretches then does not correspond to initial natural causes. It follows that subsequent indirect identification of maximum discharge using traces left by the highest water level in a given site cannot be done without examination of the whole reach of the watercourse and its tributaries. The other two types of flood hazards are not common in physical-geographic conditions of the Czech Republic. Water flush and rush of water from extreme wind

11 types of flood risks on the territory of the czech republic 57 waves endanger mainly shore regions of seaside countries and gravel bearing floods are a fear of population in mountainous valleys of such mountain ranges as, for example, Alps. On the other hand, raise of water in tributaries due to increased water level in the main watercourse, causing inundation in flat terrain of lowlands, is often an accompanying effect of floods on large rivers. Apart from natural impulses, flood situation can be worsen or contributed to its development by a sudden random accident or damage of some of the controlling elements of a water scheme, so that the retention volume of a reservoir cannot be fully exploited because of its forced restriction or elimination. In such cases, it is sometimes necessary to empty the reservoir, affecting runoff situation downstream the water scheme. Water management schemes exposed to effects of floods are built with a certain level of safety against a risk of overflowing. Overflowing equipment and outlet installations of significant schemes retaining relatively large volumes of water, and endangering in a case of destruction of its dam urbanised landscape, are designed is such a way that this risk is as low as possible. In the Czech Republic, maximum discharge occurring in the long-term average once in 10,000 years (Q₁₀ ₀₀₀) at a site of the dam is used as a design value for water schemes of the Ist and IIⁿd category. Maximum design discharge for other categories of water schemes is given by the Methodological guideline for assessment of safety of dams during floods published by the Department of Water Protection of the Ministry of the Environment (Official Journal of the Ministry of the Environment No. 4, 1999). Currently, research projects are carried out on determination of how big catastrophic precipitation is possible at all in our natural conditions. 3. Conclusions From the previous sections a generally valid strategy of current flood defence can be derived aiming at possibly controlled way of opening as large space as possible for inundation, free river valley as far as possible, prevent further urbanisation of inundation zones and in economically and socially legitimate cases try to move buildings placed formerly at risky zones. Other areas, where such an approach cannot be applied, should be protected through technical measures, which should, however, form an integrated complex together with natural capacity of landscape to retain water. Absolute defence of landscape against floods is certainly not possible and it is necessary to rely on socially and economically accepted level of flood defence, differentiated according to the specific conditions of individual elements of the landscape. The flood defence in the physical-geographical conditions of the Czech Republic must take into account a risk of possible occurrence of such a flood risk, which will surpass protecting capability of all existing flood defence measures.

12 58 josef hladný References HABERSACK, H., MOSER, A. et al. (2004): Peakform Hochwasser. Ereignis Dokumentation Hochwasser August Zentrum für Naturgefahren und Risikomnagement, Universität für Bodenkultur, Wien, 172 p. VÚV T.G.M. et al. (2003): Vyhodnocení katastrofální povodně v srpnu 2002 a návrh úpravy systému prevence před povodněmi. MŽP, Praha, 156 p. ŠERCL, P. et al. (2003): Hydrologické vyhodnocení katastrofální povodně v srpnu etapa, ČHMÚ, Praha, 134 p. MKOL (2004): Dokumentace povodně v srpnu Internationale Kommission zum Schutz der Elbe, Magdeburg, 207 p.