شبيه‎سازي فرسايش و رسوب ناشي از زمين‎لغزش ها با استفاده از مدلGeoWEPP (مطالعه موردي: حوضه گرمچاي ميانه)

Simulation of Landslides Effects on Erosion and Sediment Yield by GeoWEPP Approach (Case Study: Garmcahey Basin)

فرسايش آبي و زمين‎لغزش ها، هر دو از منابع اصلي توليد رسوب در حوضه هاي آبخيز هستند، ولي تا كنون مدل جامعي براي تعيين سهم حركات توده اي در بار رسوبي حوضه هايي كه در آن زمين‎لغزش هاي زيادي اتفاق افتاده، ارايه نشده است. پژوهش پيش رو با اين فرض انجام گرفته است كه عوامل موثر در فرسايش‎پذيري و افزايش بار رسوبي حوضه ها، تاثير مشابهي در ايجاد و تشديد حركات توده اي زمين داشته و توان توليد رسوب در دامنه هاي ناپايدار همبستگي معناداري با سطوح گسيختگي دارد. با توجه به اينكه هميشه تمامي مواد جابه‎جا شده لغزشي، امكان انتقال به داخل شبكه زهكشي را نداشته و در اغلب موارد، بخشي از آن در سطح دامنه ها همچنان باقي خواهند ماند. در اين پژوهش، از مدل فرايندي WEPPدر محيط نرم‎افزاري ArcGIS با عنوان GeoWEPP، براي شبيه‎سازي مقادير رسوب و ته‎نشست متاثر از وقوع زمين‎لغزش ها در حوضه گرمچاي ميانه استفاده شده است. نتايج پژوهش نشان داد كه بار رسوبي سطوح ناپايدار دامنه اي در ارتباط با مورفولوژي دامنه و مديريت حاكم برآن در قالب كاربري زمين و اقليم منطقه متغير بوده و مدل ياد شده به خوبي قادر به شبيه‎سازي مقادير رسوب و ته‎نشست در ارتباط با عوامل تاثيرگذار برآن بوده است. قابليت مدل يادشده در اين زمينه، پيش از اين در مطالعات مشابه ديگري ثابت شده است. از آنجاكه در حوضه آبخيز گرمچاي اطلاعات رسوب اندازه گيري شده موجود نبود، لذا نتايج شبيه‎سازي مدل فقط از طريق انطباق مناطق بحراني رسوب با سطوح لغزشي مشاهده اي، به‎صورت كيفي ارزيابي شده است. رابطه بين مناطق لغزشي با بار رسوبي، از طريق ارايه مدل رگرسيوني دو متغيره بررسي شد. نتايج آن وجود رابطه معناداري با ضريب تبيين 71/0 درصد را نشان داد كه اين رقم معيار مناسبي از موفقيت مدل ياد شده است و نشان مي دهد كه مي توان از اين مدل در برآورد بار رسوبي مناطق لغزشي حوضه هاي فاقد آمار رسوب استفاده كرد.

Introduction Water-induced soil erosion and shallow landslides are the main sources of sediment supply in steep hilly catchments. In previous studies of water-induced landslides, some efforts have been made to quantify the influence of landslide-driven sediment yields and to compare these with annual sediment budgets. The landslide-driven sediment yields depend on how the failed material is transported to a stream network. It is generally hypothesized that the sediment yield is significantly high if the material evolves into debris flow after breaking up and mixing with water because it can travel a considerable distance and discharge directly into the stream network. In many cases, however, not all sediment dislodged from the landslides reaches the main stream: much of it remains within the hillslope. If the material deposits immediately downslope area from a landslide scar, then sediment can enter the stream network only if the landslide is adjacent to the stream network or by soil erosion processes. To quantify changes in long-term soil erosion rates and sediment yields, it is important to understand landslide initiation, evolution, retrogression, mobilisation, and deposition processes. Monitoring soil erosion and landslides and their mobilisation can be a highly expensive, labour-intensive, and time consuming activity that results in limited data. Models are therefore used to estimate potential soil erosion and shallow landslides and to quantify their influence on sediment yields. However, there have been relatively few efforts to model shallow landslides and soil erosion in an integrated approach. The principal aim of this research was to study and model soil erosion and shallow landslides in an integrated way in Garmchay basin. Obtained results can be helped to understanding the interactions between landslides and soil erosion in an integrated approach. Methodology In this research, landslide effects on sediment yield have been investigated in Garmchay basin on Meyaneh city in East Azerbaijan. The GeoWEPP approach was proposed for catchment-scale modeling. The approach simulates soil erosion using the Hillslope and Flowpath methods in WEPP. WEPP model was runned as GeoWEPP program based on some required spatial layers in ASCII format such as: DEM was created based on 1:25000 topo maps and 3D analysis. Then, hilshed and stream networks were created by TOPAZ. Landuse was created based on Object-Oriented method by eCognition software on SPOT5 (2005) satellite image. Soil data was perfomed by field observation and laboratory analysis. Climate data was generated by CLIGEN program based on Meyaneh synoptic meteo-station.Landslides have been discriminated based on aerial photo interpretation and field observation. Its relationship has been investigated by bivariate regression model. Results and Discussion Results showed Garmchay basin is a landslide prone region in East Azerbaijan. Due to this research, around 130 landslides were discriminated based on aerial photo interpretation and field observation. Most part of observed failure planes was adjacent to the stream network. Therefore, it can be concluded the failed material can be transported into a Garmchay stream network. Landslide-driven sediment yields have been computed in case of Avin landslide by GeoWEPP. Results showed there is a good agreement between failure plane and critical erosion area which is simulated by GeoWEPP. In this regards, the soil loss in Avin landslide prone was estimated 40464 ton/year while the observed sediment yield on stream network was 9419 ton/year. In other word, only 23% of failed materials were transported into a stream network. Remained parts of material deposited on the hillslope so well simulated by GeoWEPP approch. The results indicate a coefficient of determination of 0.71 for a bivariate regression model. Conclusion In this research, landslide effects on sediment yield have been investigated in Garmchay basin on meyaneh city in East Azerbaijan based on WEPP modelling in GIS as GeoWEPP model. While the simulated results were not validated quantity by observed data, hence, there is a good agreement between area of failure plane and critical erosion area which is simulated by GeoWEPP. Similar efforts have been made by other researches inform of morphometric investigation, MPSIC model and volumetric estimation of landslides based on landslide shapes and area. It is suggested to apply GeoWEPP model for simulation of landslide effects on sediment yield separately in pre and post failure events by more accurate data inform of different scenarios.