Barla Dağı Kuşağı'nda moloz akması duyarlılığının ampirik yaklaşımla bölgesel ölçekte değerlendirilmesi
Debris flow  susceptibility assessment at regional scale based on an empirical approach in the Barla Mountain Belt
Yazar: FURKAN KARABACAK
Danışman: PROF. DR. TOLGA GÖRÜM
İstanbul Teknik Üniversitesi / Avrasya Yerbilimleri Enstitüsü / Katı Yer Bilimleri Ana Bilim Dalı / Jeodinamik Bilim Dalı
Derece: Yüksek Lisans, 2024
Konu: Coğrafya = Geography

Özet:
Dünyanın pek çok yüksek dağlık alanlarında olduğu gibi Türkiye'deki dağlık alanlar da heyelanlardan yoğun olarak etkilenmektedir. Ekstrem hava olaylarının, iklim değişikliğine de bağlı olarak, büyüklüğündeki ve sıklığındaki karmaşıklığı bakımından dağlık alanlardaki heyelanlar üzerindeki etkisi, artan nüfusun ve gelişen ekonomik faaliyetlerin heyelan tehlikesine karşı daha duyarlı hale gelmesine neden olmaktadır. Modern dünyadaki insan faaliyet zincirinin her alanda sürdürülebilir bir şekilde devam etmesi açısından, bu tip jeomorfik tehlike değerlendirmesinin yapılması ve duyarlı alanların belirlenerek uygun önlem yapılarının risk yönetimi kapsamında hayata geçirilmesi önem arz etmektedir.
Moloz akmaları, özellikle dağlık alanlardaki yamaçlarda oluşan ve içeriğindeki farklı tane boyutu ve su miktarına bağlı olarak etki alanı bakımından uzun mesafelere ulaşıp fiziksel ortamı değiştiren bir heyelan türüdür. Söz konusu heyelan yoğun nüfuslu alanlarda afet halinde gerçekleştiğinde, bölgedeki insan yaşamını doğrudan ve dolaylı olarak günlerce ve hatta haftalarca olumsuz etkileyebilmektedir. Özellikle yamaçlarında yüksek moloz üretiminin gerçekleştiği dağlık kuşaklarda moloz akma tehlike potansiyelinin araştırılması ve mevcut riskin kabul edilebilir riske indirgenmesine yönelik korunma ve önlem çalışmalarının yapılması yüksek öneme sahiptir. Bu çalışmada moloz akmalarına duyarlı bölgelerin tanımlanması ve potansiyel yayılma alanlarının belirlenmesi yoluyla bölgesel ölçekte moloz akması duyarlılık analizleri gerçekleştirilmiştir. Geçmişte ölümcül moloz akması olayları ile bilinen Isparta ili sınırları içindeki Barla Dağı Kuşağı'nın litoloji, iklim ve bitki örtüsü bakımından yüksek moloz üretimi potansiyeline sahip olan kuzey yamaçlarındaki 41 havza çalışma alanı olarak belirlenmiştir. Bu sahada, bölgesel ölçekte farklı sahalardaki değişkenler dahilinde uygulanabilirliği ve dinamik modellere göre az veri gereksinimi duyan  mekânsal olarak dağıtılmış ampirik bir model olan Flow-R (Flow path assessment of gravitational hazards at a Regional scale) kullanılarak moloz akması duyarlılık analizleri gerçekleştirilmiştir. Analizlerin gerçekleştirilmesinde Harita Genel Müdürlüğü'nden temin edilen 5 m yersel çözünürlüğe sahip Sayısal Yükselti Modeli verileri kullanılmıştır. Model sonuçlarının kalibrasyonu, Harita Genel Müdürlüğü'nden temin edilen 5 set hava fotoğrafı ve CORONA programından temin edilen 1 set uydu görüntüleri aracılığı ile gerçekleştirilmiştir. Geriye dönük analizler ile model çıktıları değerlendirilerek kötü durum ve ekstrem senaryoları belirlenmiştir. Buna göre 7°-15m/s modeli kötü durum; 6°-17m/s modeli ekstrem senaryo olarak belirlenmiştir. Flow-R modelinin konfüsyon matrisi doğrulama sonuçlarına göre, doğruluk, hassasiyet ve pozitif tahmin gücü sırasıyla %87,78, %46,45 ve %23,03'tür. Çalışma sonucunda moloz akmalarının karmaşık yapısı göz önüne alındığında, minimum veri gereksinimi ve kısa hesaplama süresi ile bölgesel ölçekte moloz akması duyarlılık haritaları üretilmiş ve afet öncesi risk yönetimi çalışmalarında ilksel kaynak sağlanmıştır.

Abstract:
As in many high mountainous areas of the world, mountainous areas in Türkiye are also heavily affected by landslides. The effect of the complex characteristics of extreme weather events in terms of magnitude and frequency on landslides in mountainous areas, also related to climate change, causes the increasing population and developing economic activities to become more susceptible to landslide hazard. In order for human activities in the modern world to proceed in a sustainable manner, it is important to carry out geomorphic hazard assessment and to identify susceptible areas and to implement appropriate prevention structures within the scope of risk management. Debris flows are a very common hazard in many mountainous regions of the world. They are particularly destructive due to the high velocities of the sediment of various grain sizes and their ability to travel long distances. In our country, debris flow events that cause loss of life and economic loss occur every year. When a landslide occurs as a disaster in densely populated areas, it can directly and indirectly negatively affect human life in the region for days and even weeks. Particularly in mountainous belts where high debris production occurs on the slopes, it is of high importance to investigate the debris flow hazard potential and to carry out protection and prevention studies to reduce the existing risk to acceptable risk. Identification of regions susceptible to debris flows and determination of potential spreading areas enables regional debris flow susceptibility analyses to be carried out. Landslide susceptibility is defined as the susceptibility of a slope to instability and susceptibility is usually expressed in a cartographic form. A landslide susceptibility map shows areas likely to have landslides in the future by relating some of the key factors contributing to landslides to the past distribution of slope instability. These maps are essential tools for land use planning, especially in mountainous areas. Landslide susceptibility mapping relies on a highly complex knowledge of slope movements and the factors controlling them. The reliability of landslide susceptibility maps largely depends on the quantity and quality of available data, the scale of the study and the choice of the appropriate analysis and modeling methodology. A debris flow susceptibility assessment is part of a landslide susceptibility assessment that includes the identification of the source area or commonly referred to as the initiation area of debris flows and the flow event. The runoff event identified in a debris flow susceptibility assessment may consist of the distance, velocity and magnitude of impact of existing or potential debris flow. Debris flow modeling is one of the assessment methods that contribute to the prediction of the area to be affected by the potential future hazard and to reveal its dynamic processes. Debris flow modeling consists of 3 models: physical, empirical and dynamic. Physical modeling is carried out by field observation and further analysis of the flow by laboratory analysis. The development of simulation techniques, especially in recent years, has made dynamic modeling an increasingly important tool for simulating the properties and characteristics of debris flows and their behavior. Empirical relationships are one of the most widely used techniques to estimate the maximum runoff distance of landslides and debris flows. Empirical modeling is often based on well-documented field observations. The modeling parameters derived from well-documented field observations are used for analysis by determining the relationships between each parameter (e.g., the relationship between the impact distance area and the volume of debris flow). Input parameters for the empirical model are volume estimation, topographic profiles, image interpretation and geomorphological studies. The empirical approach, which has been used in various studies on the spread of debris flows, has the advantages of simple data collection, high applicability for large areas and easy adaptability to different regions. The 41 sub-catchment fan systems with high debris production potential with their lithology, climate and vegetation characteristics on the northern slopes of the Barla Mountain Belt in Isparta province, known for fatal debris flow events in the past, were determined as the study area. The study area is located in the north of the Mediterranean Region, 40 km north of Isparta province in the Antalya Department. The Barla Mountains in the north of the Isparta Angle, bordered by the Gulf of Antalya and the Burdur-Hoyran and Beyşehir Lakes, are mostly consisted of limestone series. The Barla Mountains and Hoyran depression basin, which have been structured by the effect of normal faults in the region from the Upper Miocene to the present day in the north of the Isparta Angle, represent a typical horst graben system. With this characteristics, high elevation differences in short distances caused the establishment of steeply sloping slopes. The daily temperature difference, especially in the elevations after the timberline, has a role in the efficiency of physical weathering in the area, which is represented by a hot semi-arid climate characteristic in summers. Considering that the stratigraphic structure is generally tilted to the south, it is observed that debris accumulation is concentrated on the slopes where the channels that intersect the vertical faults steeply develop. This region has experienced the most fatal debris flow disaster in the history of the Republic of Türkiye to date. The deadliest known debris flow occurred at 20:20 on July 13, 1995. The debris flow killed 74 people and damaged 90 houses, completely destroying 179 houses. In addition, approximately 1000 people were left displaced. On July 18 and 19, 1996, debris flow disaster occurred again in Senirkent. The citizens living in the region were warned by the Senirkent District Governorate and left their houses the day before and deaths caused by debris flow were prevented. In the July 19 debris flow, the houses could not be entered due to the debris and mud material from the previous day's flow and thus no loss of life occurred. The presence of the material from the 1995 debris flow in the canals increased the volume of debris flows in 1996, but caused less damage due to the measures taken. The presence of traces of previous debris flows in the study area was determined through 5 sets of aerial photographs obtained from the General Directorate of Mapping of Türkiye and 1 set of satellite images obtained from the CORONA program. In this region, debris flow susceptibility analyses were carried out using Flow-R (Flow path assessment of gravitational hazards at a Regional scale), a spatially distributed empirical model that is applicable to different sites at regional scale and requires less data than dynamic models. Digital Elevation Model data with a spatial resolution of 5 m obtained from the General Directorate of Mapping of Türkiye were used for the analysis. Flow-R is a spatially distributed empirical model developed in Matlab®. The implementation of the model requires two different steps based on a digital elevation model (DEM): (1) source areas are identified through morphological and user-defined criteria, and then (2) debris flows are propagated from these sources based on friction laws and flow direction algorithms. The volume and mass of the debris flow are not taken into account, as the exact values cannot be easily assessed over a large region, as they are extremely difficult to estimate due to the significant mass changes that occur through erosion and incision. The software has a graphical user interface that allows the user to specify criteria for the identification of source areas and select algorithms and parameters for the assessment of spreading. When normalized, the values never exceed 1 and thus approach the concept of spatial probability. Source area identification uses an index-based approach. Input datasets can represent different types of spatial information and are treated with user-defined parameters. Accordingly, the grid cells of each input dataset are classified as (1) "source area" when initialization is possible, (2) "excluded" when initialization is unlikely, or (3) "ignored" when no decision can be made about this parameter. Datasets are merged according to the following rule: a cell is a source field if it has been selected as a source field at least once but never extracted. Alternatively, the user can directly import source fields created with another (GIS-based) approach. 
The models performed in the study area were carried out with maximum reach angles between 6°-14° and maximum velocities between 15 m/s and 17 m/s. The most appropriate worst-case and extreme-case scenarios between these values were tried to be determined according to the spreading areas by back analysis of previous debris flows. In the determination of these scenarios, the reports presented after the debris flow events in Senirkent were also utilized. In terms of the results of the study, 7°-15 m/s model output was determined as the worst case scenario and 6°-17 m/s model output was determined as the extreme scenario. In regional scale debris flow susceptibility analyses, it is difficult for the Flow-R model to fully account for the effects of small human structures, walls or mitigation measure. Problems with the model's propagation areas were generally observed in basins with human intervention. For these basins, a Digital Elevation Model with higher spatial resolution is needed to perform debris flow sensitivity analyses in both scenarios. For example, in all Flow-R model scenarios, the check dams constructed as a mitigation measure after the debris flow events in Senirkent were neglected in the Digital Elevation Model with 5 m resolution. In this case, the model assumed that there were no mitigation measure in the debris channel and continued its progress and was able to simulate the 1995-1996 debris flow events. To assess the current situation, a Digital Elevation Model with higher spatial resolution is needed in these areas. According to the precision validation results of the Flow-R model, the accuracy, sensitivity and positive predictive power were 87.78%, 46.45% and 23.03%, respectively. As a result of the study, considering the complex structure of debris flows, regional scale debris flow susceptibility maps were produced with minimum data requirement and short computation time, and a primary source was provided in pre-disaster risk management studies.