MODELING WATER LEVEL VARIATIONS OF THE ANDIJAN RESERVOIR BASED ON A DIGITAL TERRAIN MODEL IN GIS “PANORAMA”

УДК 621.37

Abstract

The paper examines the application of the "Panorama" geographic information system for modeling the flood zones of the Andijan reservoir under various water level rise scenarios. The study focuses on developing a digital elevation model (DEM) of the reservoir's surroundings based on a 1:100,000 scale topographic map (sheet K-43-111). Using the "Panorama" GIS 3D analysis complex, flood zones were constructed for four water level marks: 905, 908, 910, and 912 meters above the Baltic Sea level. Measurements of flooded areas were conducted, and the impact on adjacent lands was analyzed. It was determined that with a level rise of 7 meters, the water surface area increases by approximately 59% relative to the normal retaining level. The results obtained can be used to justify permissible reservoir filling modes and for the ecological monitoring of the transboundary hydraulic system.

Аннотация

В статье рассматривается применение геоинформационной системы «Panorama» для моделирования зон затопления Андижанского водохранилища при различных сценариях повышения уровня воды. Основное внимание уделено созданию цифровой модели рельефа (ЦМР) прилегающей территории на основе топографической карты масштаба 1:100 000 (лист K-43-111). С использованием комплекса трёхмерного анализа ГИС «Panorama» были построены зоны затопления для четырёх отметок уровня воды: 905, 908, 910 и 912 метров над уровнем Балтийского моря. Проведены измерения площадей затопления и проанализировано их влияние на прилегающие территории. Установлено, что при повышении уровня воды на 7 метров площадь водной поверхности увеличивается примерно на 59% по сравнению с нормальным подпорным уровнем. Полученные результаты могут быть использованы для обоснования допустимых режимов наполнения водохранилища, а также для экологического мониторинга трансграничной гидротехнической системы.

Keywords: Flood zone, "Panorama" GIS, matrix of heights (MTW), Andijan reservoir, digital elevation model (DEM), normal retaining level, ecological monitoring.

Ключевые слова: зона затопления, ГИС «Panorama», матрица высот (МВВ), Андижанское водохранилище, цифровая модель рельефа (ЦМР), нормальный подпорный уровень, экологический мониторинг.

Introduction

The Andijan (Kempir-Abad) reservoir is located in the Khanabad mountain tract where the Karadarya River enters the Fergana Valley, at the junction of Kyrgyzstan and Uzbekistan. The buttress dam was commissioned in 1974, and construction was completed in 1983. The normal retaining level (NRL) is 905 meters above the Baltic Sea level, with a total volume of 1.9 km ³a useful volume of 1.75 km³ , a projected water surface area of 56 km³, and a maximum depth of 98 meters [1, p. 120].

The reservoir provides irrigation for more than 280,000 hectares of agricultural land in the Fergana Valley and is a transboundary facility, which gives its operational regime interstate significance. Fluctuations in water levels directly affect the state of coastal territories: as the level rises, floodplains are inundated, landslide processes are activated on the banks, and groundwater levels rise on adjacent irrigated lands [2, p. 14-19].

Geographic information systems are the most effective tool for studying such processes. "Panorama" GIS (KB "Panorama") is widely used in relevant departments of Uzbekistan and other CIS countries. It includes a specialized "3D Analysis Complex," which enables working with MTW format height matrices and constructing flood zones based on specified absolute water level marks [3, p. 150; 4, p. 43].

Several studies have demonstrated successful experience in using "Panorama" GIS for similar tasks. Specifically, for the Andijan reservoir, digitization of the bottom relief was performed, a digital model of the bay was constructed, and water levels during filling and drawdown were modeled for fishery monitoring needs [5, p. 115; 7, p. 33-40]. With respect to facilities in the Fergana Valley, such studies are sporadic, which confirms the relevance of this work.

Figure 1. Hypsometric map of the study area: digital elevation model (MTW) with color coding by altitude zones and contour lines of a 1:100,000 scale topographic map. "Panorama" GIS

The purpose of the study is to construct flood zones for the Andijan reservoir (Fig. 2) for several water level rise scenarios based on a developed DEM in the "Panorama" GIS environment and to assess the environmental impact on the surrounding territories [6, p. 45-52].

To achieve this goal, the following tasks were addressed:

• Preparation of the MTW height matrix (sheet K-43-111) and verification of its compatibility with the cartographic layer in "Panorama" GIS;
• Setting the initial flooding point (cross-section) and constructing flood zones for four water level marks: 905, 908, 910, and 912 meters;
• Measuring the areas of the constructed flood zones and performing a comparative analysis by scenarios;
• Overlaying the flood zones onto thematic map layers (land use, settlements, road networks) to identify the most vulnerable areas;
• Assessment of the main ecological consequences of the reservoir level rise.

Figure 2. Dialog box for flood zone construction and the modeling result based on the height matrix

Methodology. The basis of the study was a matrix of heights (MTW) with a grid step of 20 m, developed by the authors during previous work on the vectorization of a 1:100,000 scale topographic map, sheet K-43-111 (Gulcha). The matrix covers an altitude range from 800 to 2000 m above the Baltic Sea level. The coordinate system is SK-42, Gauss-Kruger projection, Zone 13. The vertical datum is the Baltic Height System, which ensures direct compatibility with official hydrological data on the reservoir level marks.

The technological workflow included four sequential stages:

1. Data Preparation. The MTW height matrix was linked to the map via the File → Open Matrix menu. A check was performed for the coincidence of the coordinate systems of the map and the matrix, along with visual control of the correspondence between the contour lines on the map and the relief on the DEM. A hypsometric coloring of the height matrix was constructed with a color scale ranging from dark green (lowlands) to brownish-red (mountain peaks).
2. Setting Modeling Parameters. In the 3D Analysis Complex, the "Flooding" module was activated. A point within the reservoir area near the dam was specified as the starting point (cross-section). For each of the four scenarios, the absolute water level mark was set according to Table 1. The construction algorithm is based on the hydraulically connected propagation of the zone from the starting point across the DEM relief up to the specified mark.
3. Constructing and Saving Flood Zones. Each scenario was executed separately, and the resulting flood zone polygon was saved to a separate named map layer. Different color fills with 40% transparency were assigned to the layers to ensure the clarity of the joint display.
4. Area Measurement and Thematic Analysis. The areas of the flood zones were measured using geometric analysis tools (Tasks → Measurements → Object Area). To assess the environmental impact, the flood zones were overlaid onto thematic map layers: land use, settlements, and the road network.

Table 1. Modeling scenarios for the water level rise in the Andijan reservoir

Results and Discussion

The main result of the work was the construction of four flood zones for the Andijan reservoir, corresponding to the specified water level marks. The mountain ranges in the north of the region, depicted in yellow-brown tones, contrast sharply with the green areas of the low-lying valleys, which are the first to be subjected to flooding as the water level rises [8, p. 200; 9, p. 180].

Figure 2. Flood zone at 905 m in "Panorama" GIS

The measured areas of the flood zones for each scenario are presented in Table 2. With a level rise of 5 m, the water surface area increases by approximately 18% compared to the NRL. At the surcharge level of +10 m (Scenario 3), the increase is about 35%, while at the extreme level of +12 m (Scenario 4), the flood zone covers additional lateral valleys of the tributaries, with an area increase of approximately 59%.

Figure 3. Flood zone at the surcharge level of 910 m (Scenario 3). Dark blue polygon with dot hatching represents the additional flood zone relative to the NRL. "Panorama" GIS.

Table 2/ Flood zone areas by modeling scenarios

Flood zone areas by modeling scenarios

Map analysis allowed for the establishment of several spatial patterns. When the water level rises, the most intensive expansion of the water area occurs in the western and southwestern directions, where the tributary valleys have a gentle relief with a slope of less than 5°. In these zones, even when the NRL is exceeded by 3 m, flooding spreads to a distance of 2–4 km deep into the lateral valleys. Conversely, the eastern and northern shores are constrained by steep mountain slopes (slopes over 20°) and show almost no change in their position, even under the extreme scenario [10, p. 61; 11, p. 22-27].

Figure 4. Flood zone at the extreme level of 912 m (Scenario 4). Significant expansion of the water area in the western and southwestern directions. "Panorama" GIS

Analysis of Impact and Discussion

Based on the overlay of flood zones onto thematic map layers, it was established that under Scenario 3 (elevation 910 m), irrigated lands along the banks of tributaries with a total area of up to 8–12 km², as well as areas of tugai vegetation along the Karadarya floodplain, fall into the additional flood zone. Under Scenario 4 (912 m), certain unpaved roads and outbuildings in the lateral valleys enter the risk zone.

Thus, the provided data on the modeling of flood zones for the Andijan reservoir in "Panorama" GIS indicate that the available 1:100,000 scale digital elevation model (DEM) provides sufficient accuracy for the scenario analysis of water level changes. The developed methodology allows for the prompt assessment of flooded areas and the identification of the most vulnerable sections without conducting expensive field work.

It was established that the western direction of the flood zone expansion, where irrigated lands are located, is the most hazardous from an environmental perspective. The flooding of these lands carries a risk of soil salinization due to the rise in groundwater levels and the capillary rise of mineralized waters, which is characteristic of the irrigated territories of the Fergana Valley [11, p. 131]. The periodic wetting and drying of coastal slopes composed of loams and loess-like deposits contribute to the activation of landslide processes and an increase in water turbidity in the reservoir.

Existing global models such as SRTM and ASTER GDEM are not linked to local topographic maps and do not provide uniform accuracy for the mountainous terrain of the study area. The DEM developed in previous work based on the topographic map K-43-111 is free of this drawback and ensures a reliable reproduction of the coastal morphology [12, p. 7-12].

In the future, it is advisable to refine the modeling results using a DEM with higher spatial resolution and to verify the constructed flood zones using archival Landsat/Sentinel-2 satellite imagery from periods of maximum reservoir filling. The developed GIS model is recommended for application in the water management monitoring system of the Andijan hydraulic system.

Conclusion

The conducted study on modeling the level changes of the Andijan reservoir based on a digital terrain model allows for several conclusions.

1. The application of the "3D Analysis Complex" in "Panorama" GIS together with a 1:100,000 scale MTW height matrix showed high efficiency for constructing flood zones. The algorithm of hydraulically connected zone propagation from a starting point allows for correct results without filling isolated topographic depressions.
2. The analysis confirmed that under any scenario of level rise, the most intensive flooding occurs in the western and southwestern directions, where the gentle valleys of tributaries form zones of maximum vulnerability. With a level rise of 7 m, the water surface area increases by 59% relative to the NRL.
3. The developed GIS model serves as a predictive modeling tool that allows for calculating the risk zones for flooding of lands and infrastructure, justifying permissible reservoir filling regimes, and planning environmental protection measures in the coastal zone.

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