Modelling of Flood Hazard Zone for the Łęg River * * * *

Floods and their intensification in terms of the frequency of occurrence and the power of high water rises during the last century all over the world cause that they are becoming a global problem. More and more institutions are established that deal with the issue of floods. The main objective of those institutions is to minimize the negative results of floods as regards to the economic, social, and nature-related aspects. One of such institutions is the European Exchange Circle on Flood Mapping (EXCIMAP). It was established in 2006, with its main objective being the exchange of information between particular European states about producing flood maps. Also, legal action has been initiated in order to prepare Member States for coping with flood disasters. In November 2007 the so-called ′′Flood Directive′′ was enacted on the assessment and management of flood risks [2]. The directive determines schedules and scopes of operations and action to be undertaken by EU Member States within the framework of flood prevention. It imposes, among other things, an obligation to prepare planning documents concerning flood risk management in line within a coherent approach, uniform for the whole Europe, and to provide public access to information and results of all flood-related assessments, maps and plans. It obligates Member States to prepare and publish preliminary flood risk


Introduction
Floods and their intensification in terms of the frequency of occurrence and the power of high water rises during the last century all over the world cause that they are becoming a global problem.More and more institutions are established that deal with the issue of floods.
The main objective of those institutions is to minimize the negative results of floods as regards to the economic, social, and nature-related aspects.One of such institutions is the European Exchange Circle on Flood Mapping (EXCIMAP).It was established in 2006, with its main objective being the exchange of information between particular European states about producing flood maps.Also, legal action has been initiated in order to prepare Member States for coping with flood disasters.
In November 2007 the so-called ″Flood Directive″ was enacted on the assessment and management of flood risks [2].The directive determines schedules and scopes of operations and action to be undertaken by EU Member States within the framework of flood prevention.It imposes, among other things, an obligation to prepare planning documents concerning flood risk management in line within a coherent approach, uniform for the whole Europe, and to provide public access to information and results of all flood-related assessments, maps and plans.It obligates Member States to prepare and publish preliminary flood risk

Introduction
Despite the continuous improvement of survey methods and advances made in survey equipment technology, the elimination of outliers still remains an issue today.When performing an adjustment one often assumes a very simple probability distribution of errors, such as a normal distribution.In classical statistics the correctness of the results relies on the assumption, that the chosen errors distribution model is strictly true.This is, in fact, often not the case, as the large errors occur considerably more often than the normal distribution would suggest.Even the high-quality samples analysed in astronomical research, containing several thousands of measurements each, do not follow the normal probability distribution.Deviations from the model may occur due to e.g.blunders in measuring, incorrect point numbering, errors made during data copying etc. [12].
Although there exists a wide range of literature concerned with gross errors detection and elimination, this surveying problem is still being discussed.There are many so-called methods robust against the in uence of gross errors, which can generally be divided into two groups.
The  rst group includes methods based on the criteria of so-called robust estimation.These methods minimise the in uence of the outlying observations on the  nal result of the computations by modifying of the observation weights.
The second of them consists of methods where results, obtained by the least squares adjustment are analysed with the use of statistical tests.In these methods an identi ed outlier is removed from the dataset.If multiple outliers occur, the iterative process of least squares adjustment is conducted and followed by tests.The observations suspected of gross errors are discarded from the dataset [1].A few commonly used methods of these groups are presented below.http://dx.doi.org/10.7494/geom.2013.7.4.31 assessments, flood hazard maps, flood risk maps, as well as flood risk management plans [2,7,8].
As far as Poland is concerned, tasks related to the preparation of flood hazard maps are governed by the Regulation of the Minister of Environment, the Minister of Infrastructure, and the Minister of Internal Affairs and Administration of 22 January 2013 Concerning the Preparation of Flood Hazard and Risk Maps.According to the provisions of that regulation, areas endangered with flooding are shown as surface objects with occurrence probability values assigned to them [15].Those areas are outlined based on water elevations calculated as a result of hydraulic modelling with the use of GIS and DTM tools.The hydraulic modelling is split into single-dimensional models, in which the velocity vector has a single component, and two-dimensional models, in which the velocity vector has two components.Two-dimensional models are produced for province and county capitals, as well as cities inhabited by more than 100,000 people.For the remaining terrains, singledimensional models are provided [9,15].
According to the said regulation, maps should show, among other things, the following: flood hazard zone limits, maximum water table elevations calculated through hydraulic modelling, natural water courses and canals with their names, water reservoirs, flood banks, names of localities, background map (prepared on the basis of a topographic map or an orthophotomap of terrain pixel not bigger than 50 cm) [15].This paper presents a process of working up a flood zone for the Łęg river in the Podkarpackie province of Poland.The zone provided a basis for the preparation of a flood hazard map, and for the performance of an analyses based on which it became possible to preliminarily assess the flood risk for the selected area.

Study Area
The area, for which the flood hazard and flood risk map was prepared, covers a strip of the Łęg river valley (ca. 4 km long) below the water reservoir dam in the village of Wilcza Wola, in the district of Kolbuszowa (the Podkarpackie province) (Fig. 1).Land within the small distance from the river bed is used mainly for agricultural purposes, or is overgrown with bushes and single tree clumps.The river bed is unregulated.There are two bridges within the examined river section: one is a wooden bridge on reinforced-concrete supports with metal cross-beams, while the second is a wholly reinforced concrete bridge, with an asphalt surface course.Houses and buildings can be found at the whole length of the area under consideration, within average distances of 150-500 m from the river bed.However, there are a few farms located in a distance smaller than 100 m.The Digital Terrain Model was obtained from resources held by the State Centre for Survey and Topographic Documentation (CODGiK).Altitude data were recorded in the PUWG 1992 Cartesian coordinate system and Kronsztad 86 altitude system.The DTM was made based on analogue aerial photos, scale 1:14000, subjected to the process of scanning with 14-μm pixels.The DTM was recorded as a regular grid of squares (25 m), supplemented with characteristic structural lines.The mean square error calculated based on model control profile and photo control points amounted to 0.28 m and 0.41 m, respectively.
In order to examine the area under consideration in terms of risk of flooding of selected surface objects, the Topographic Data Base (scale 1:10000, sheet M-34-56-D-d-3) was used.The Data Base was obtained from the resources owned by the regional centre for survey and topographic documentation (WODGiK).
The following layers were utilized for analyses: -buildings, -grassy and arable lands, -permanent farming lands, -forests or wooded lands, -compact, dense or dispersed development lands, -residential complexes, -roadway sections, -water areas, -river and canal sections.
The project made also use of the open-source software: QGIS and GRASS, which enabled us to prepare hydrological analyses that employed both vector and raster data, as well as Geomedia Professional software.The said programs are able to cooperate with external databases, to provide 2D and 3D visualizations, and to present analysis results. of cross-sections established.Flood zones were generated for the following high water levels: 186.50 m; 184.30m; 182.28 m; 181.18 m; 180.08 m.Particular partial zones were plotted based on wave damming elevations at particular cross-sections, that is why it was assumed that the error of determination of flood zone limit point at the intersection of the cross-section with its corresponding partial zone equalled zero.The so generated "partial zones" of various flood wave altitudes provided the basis for the interpolation of the total flood zone.Flood limits between neighbouring cross-sections were interpolated based on overlapping partial zones.During interpolation, also distances between interpolated points of flood limits and the adjacent were considered.The way of interpolating is shown on the longitudinal profile of the river (Fig. 3).
provided basis for the interpolation of the total flood zone.Flood limits between neighbouring cross-sections were interpolated based on overlapping partial zones.During interpolation, also distances between interpolated points of flood limits and the adjacent were considered.The way of interpolating is shown on the longitudinal profile of the river (Fig. 3).The framework of research work as presented herein also included performing an analysis to assess flood risk in terms of inundation hazard posed to selected land surface objects [11].The GIS analyses for the outlined flood zone were performed with the utilization of the Topographic Data Base.
Following a detailed study of topography of the area under consideration, all operations were repeated in relation to three zones there were created as a result of the basic zone extension by 25 m, 50 m, and 100 m, respectively (Fig. 5).As a result of operations performed, surfaces of inundated areas were obtained, with division into specific land surface forms (forests, agricultural land, developed areas).Additionally, a risk analysis was performed in relation to developed areas in terms of need to evacuate local residents [1,10].To that purpose, the number of real estates and buildings  The framework of research work as presented herein also included performing an analysis to assess flood risk in terms of inundation hazard posed to selected land surface objects [11].The GIS analyses for the outlined flood zone were performed with the utilization of the Topographic Data Base.Following a detailed study of topography of the area under consideration, all operations were repeated in relation to three zones there were created as a result of the basic zone extension by 25 m, 50 m, and 100 m, respectively (Fig. 5).As a result of the operations performed, surfaces of inundated areas were obtained, with division into specific land surface forms (forests, agricultural land, developed areas).Additionally, a risk analysis was performed in relation to developed areas in terms of need to evacuate local residents [1,10].To that purpose, the number of real estates and buildings was analysed, for which there exists a direct exposure to flood.Taking into account their functions, the buildings were divided into three types: residential, farming, and other buildings (including public buildings).provide auxiliary material for the preparation of crisis management plans in case of flood [16].Those combined data may also be used in the preparation of local area development plans.It should be stressed that the correctness of assumptions, the proper correctness of data, as well as the correctness of performing calculations and operations related to the flood zone determination highly affect results of analyses the of the potential flood risk occurrence.

Fig. 3 .
Fig. 3. Longitudinal profile of the Łęg river section below the water reservoir dam in the village of Wilcza Wola showing flood wave pattern, as well as levels of high water rises and ranges of determined partial flood Jones Source: [12] Longitudinal profile of the riverLevels of high water risesCross-sections [river kilometre]

Fig. 3 .
Fig. 3. Longitudinal profile of the Łęg river section below the water reservoir dam in the village of Wilcza Wola showing flood wave pattern, as well as levels of high water rises and ranges of determined partial flood zones Source: [12]

Table 1 .
Maximum flood wave elevations in particular localities for the area of flood hazard map preparation