Coğrafi bilgi teknolojileri yardımıyla şehirleşmenin taşkın alanlarına etkilerinin araştırılması

Abstract

Şehirleşme nedeniyle arazi kullanım tiplerinin hızlı ve kontrolsüz değişimi, iklim koşullarını, kentsel yağış-akış sürelerini ve taşkın riskini olumsuz etkilemekte ve çözüm bekleyen bir konu olarak karşımıza çıkmaktadır. Bu nedenle, Kağıthane Deresi'nin mansap kısmı pilot bölge seçilerek şehirleşme ve taşkın konuları Coğrafi Bilgi Sistemleri (CBS) kullanılarak değerlendirilmiştir. Buradaki esas amaç taşkın yönetiminin klasik sistemler ile münferit olarak değerlendirmeyip, artan şehirleşme karşısında bütüncül yönetilmesini sağlayan CBS'nin önemini ortaya çıkarmaktır. Bu kapsamda CBS ortamında hazırlanan mekansal veritabanı ile taşkın analizlerinin yapıldığı HEC-RAS programının entegrasyon sağlanmış olup farklı senaryolara göre taşkın haritaları oluşturulmuştur. Geliştirilen senaryolar: Senaryo1(SN1) mevcut durum, Senaryo 2 (SN2) kısmı şehirleşme olması, Senaryo 3(SN3) tam şehirleşme olması, Senaryo 4 (SN4) şehirleşmenin olmaması durumunu içermektedir. Bu senaryolar CBS'nin veri tutma, düzenleme, sorgulama, analiz ve dinamik yapısından dolayı oldukça hızlı değerlendirilmiştir. CBS de geniş bir veri tabanı oluşturulduğu için şehirleşme ve taşkın konularına ek olarak taşkın riskinin nasıl azaltılacağı konusu da sürdürülebilir kentsel drenaj çözümlerini esas alarak CBS aracılığıyla değerlendirilmiştir.

Floods and urban floods are the main natural disasters in recent years. The causes of these natural disasters can be defined as changes in land use, construction of structures on natural stream beds, climate factor and increase in urbanization. With the increase in urbanization, especially with unplanned urbanization, the areas covered by hard impermeable surfaces such as asphalt roads and parking areas increase and the precipitation flows directly, the underground waters cannot be fed sufficiently, thus the risks of flood and overflow increase and flood and overflow are an issue waiting for a solution. In order to examine how urbanization has an effect on floodplains, the downstream part of Kağıthane Stream was selected as a pilot region within the scope of this thesis, and urbanization and flooding issues were evaluated using Geographic Information Systems (GIS). The reason for choosing the downstream part of Kağıthane Stream as the study area in the study is to show with the help of GIS how the flood risk will change under increasing urbanization if the densely populated residential areas and industrial structures that are important for the country's economy have increased in the downstream region and the areas that are not open to settlement in the upstream part are opened to settlement. The purpose of using GIS is to consider hydraulics, hydrology, urbanization issues holistically and to reveal the importance of future studies to be carried out with this approach. In addition, since the effect of urbanization on flood areas is examined not only in the current situation but also in different scenarios in the study, GIS provides flexibility due to its dynamic structure. Scenarios developed in this context include: Scenario 1 (SN1) current situation, Scenario 2 (SN2) partial urbanization, Scenario 3 (SN3) full urbanization, Scenario 4 (SN4) no urbanization. Scenario 1; It reflects current land use. It reflects the situation where the building and road areas are impermeable and the flow coefficient value is 0.95, the open areas are permeable, and the flow coefficient is 0.40. Scenario 2; Part of the current land use reflects the urbanization situation. The average value of the concrete and green area flow coefficient value of 0.60 was chosen since it is accepted that the covering type of the buildings, roads, green areas, soil floors etc. within the basin area is partly urbanization. The main issue to be evaluated here is to consider how much the flood will be affected if open areas and non-residential areas are opened to settlement. Scenario 3; It reflects the complete urbanization of the land use within the basin border. In this case, the flow coefficient value is taken as 0.95, assuming that the buildings, roads, green areas, soil floors, etc., within the catchment area have become concrete. Scenario 4; It reflects the situation where there is no urbanization in the land use plan. The flow coefficient value was taken as 0.25, assuming that the areas such as buildings, roads, green areas, earthen ground etc. within the basin area are uncultivated soils. In scenario 1-2-3-4, different flow coefficient values are given to building area, road area and open areas. The main reason why these coefficients can be given in such detail and easily is that the spatial database is prepared in the GIS. In the preparation of the spatial database, the existing maps were used and 60 existing maps with a scale of 1:1000 within the study area basin were digitized in the GIS environment. The integration of the prepared spatial database and the HEC-RAS program, in which flood analyzes are made, has been ensured, and flood maps have been created according to different scenarios. As the results of the flood maps were evaluated, it was determined that there were spatial and volumetric differences. For example, Scenario 1 flood area reflecting the current situation is 86.58 hectares, Scenario 2 flood area is 102.76 hectares. The reason for the 18.69% increase in the flood area in scenario 2 is the partial urbanization. In the scenario established, the definition of partial urbanization reveals the situation where the percentage of impermeable area increases in case of open areas in land use or areas that are not open to settlement, and in this case the water cannot meet the soil and the risk of flooding will increase. When evaluated as the number of buildings under flood, it was determined that there would be an increase of 40% in the number of buildings. When the Scenario 3 comparison, which covers the full urbanization setup after the urbanization evaluation, from Scenario 1 to Scenario 2 is evaluated, it is observed that the spatial and volumetric differences increase with urbanization. The urbanization term mentioned here is the planning of open areas or places planned to be opened to settlement according to the completely impermeable ground structure. The result that the flood area will increase by 66.74% when scenario 3 is passed from Scenario 1, which is the current situation, has been concretely demonstrated with the prepared model. In addition, when the flooded building area is evaluated, it was determined that the number of buildings under flood in Scenario 1 increased by 100% in the transition to Scenario 3. Scenario 2 and Scenario 3 deal with the urbanization setup of the current situation. The improvement evaluation scheme, which is the opposite of these constructs, that is, if there was no urbanization in the basin area, is Scenario 4. In Scenario 4, which is examined as to what would happen if every place was permeable in the current situation, it was determined that the flood area decreased by 48.16% compared to the current situation. This situation has made a great contribution to our evaluation of the fiction of how the flood increases unknowingly while urbanization is being carried out. For this reason, in the city plans to be planned, the construction of permeable areas next to the impermeable building areas or the reuse of rainwater will greatly reduce the risk of flooding. When the flood risk maps prepared according to the acceptance of Scenario 2 and Scenario 3 are compared, there are spatial and volumetric differences. Scenario 2 flood area is 102.76 hectares, Scenario 3 flood area is 144.36 hectares. When the flood risk increased by 40.48% from Scenario 2 to Scenario 3 and when the flooded area is evaluated, the number of buildings under flood in Scenario 2 is 406, while the number of buildings under flood in Scenario 3 is 600. It has been determined that the reason for these increases is partial urbanization, that is, the transition from semi-permeable ground structure to urbanization to be planned with a completely impermeable ground structure. When the flood risk maps prepared according to the acceptance of Scenario 2 and Scenario 4 are compared, there are spatial and volumetric differences. Scenario 2 flood area is 102.76 hectares, Scenario 4 flood area is 44.88 hectares. When the transition from scenario 2 to 4 was made, it was determined that the impermeable areas decreased and accordingly the flood risk decreased by 56.33%. In addition, when the flooded building area is evaluated, the fact that the number of buildings under flood in Scenario 2 was 406, while the number of buildings under flood in Scenario 4 decreased to 177, is another concrete indicator of the decrease in the flood risk. When the flood risk maps prepared according to the acceptance of Scenario 3 and Scenario 4 are compared, there are spatial and volumetric differences. Scenario 3 flood area is 144.36 hectares, Scenario 4 flood area is 44.88 hectares. It was determined that when the transition from Scenario 3 to Scenario 4 was made, the permeable soil structure increased, and the flood risk area decreased by 68.91%. In addition, when the flooded building area is evaluated, the number of buildings under flood in Scenario 4 is 177, while the number of buildings under flood in Scenario 3 is 600. The numerical data revealed by the analyzes emphasizes that the greater the increase in the urbanization rate, the more the flood risk will increase. Evaluation of all these scenarios was provided by integrating the spatial database prepared on the GIS and the HEC-RAS program, in which flood analyzes were made. If the GIS was not used within the scope of the thesis work and the work was done individually, the integrity and flexibility in terms of time, storage, analysis and management would not be provided. The effect of urbanization on floodplains was evaluated according to different scenarios on the GIS, and the results were evaluated within the scope of the thesis, and the importance of GIS in evaluating such issues was once again revealed. Considering what to do more with the quality spatial database prepared with GIS, the issue of how to solve the flood risk is also discussed using the same spatial database. In this context, how to reduce the flood risk was evaluated with sustainable urban drainage solutions. In order to determine the sustainable urban drainage solutions (SKDÇ), flow rate calculations were made again by giving the flow coefficients required for sustainable urban drainage suitable for building areas, road areas and open areas within the spatial database installed in the system. All of the GIS and hydraulic model integration steps discussed in the above-mentioned urbanization schemes were applied here, and the model was run again, and flood maps were created again according to this result. If we talk briefly about this definition before opening sustainable urban drainage solutions, it can be defined as increasing the infiltration of the soil by imitating the natural water cycle and imitating the natural drainage processes, ensuring that the rainwater reaches the underground, reducing the amount of surface flow and flow rate, reducing the number of pollutants in the surface flow, and improving the hydraulic functions of the cities definable. In the scenarios discussed in this context, only SKDF was applied to buildings in SKDF 1 and the flood risk was 15.45% compared to the current situation, SKDF solution was applied to buildings and roads in SKDS 2 and the current flood risk area was reduced by 18.93%. Although SKDS 3 was applied to both roads and buildings, flood mitigation was reduced at approximately the same rate with SKDS 1. This is because the percentage of impermeable areas such as road area and building area is close. Making such comments reveals that GIS will also help us in decision-support. By making use of the flexibility of the model established in the GIS environment, the existing infrastructure was evaluated and the effect on the existing infrastructure was interpreted. In this context, it was concluded that the carrying capacity of the pipes decreased when SKDÇ was applied to the existing stormwater infrastructure system. This finding is an advantage in terms of both the improvement of the infrastructure, the safety of the pipes that were previously laid without considering the climate factor, and the reduction of city floods. With the flexibility of the model established on the GIS, both the effect of urbanization on floodplains and the result of urban drainage solutions proposed for flood risk are tried to be explained within the scope of this thesis. Although this thesis study was made for a pilot region, the study can be reflected in their systems by all government institutions. Thus, while new settlements are built in areas with intense urbanization, flood risk areas can be examined according to different scenarios and possible flood risks can be minimized and solution proposals can be applied.