تعیین سهم برآمدگی های ناگهانی در میزان فرسایش خندقی در سطوح شیب دار مطالعه موردی :زیرحوضه اوجان چای

در کوهستان های نیمه خشک آشفتگی دامنه ها که اغلب در اثر فرسایش خندقی پدید می آید ،از رخدادهای مهمی است که در اثر وقوع تغییرات اقلیمی و دستکاری های انسانی تشدید می شود . در چنین محدوده هایی، خندق ها انبوهی از خاک های دامنه ای را بسوی آب های جاری و پای دامنه ها سرازیر می کنند. در دامنه های شرقی کوهستان سهند، سطوح دامنه ها تحت فرسایش خندقی تشدید شده قرارگرفته اند.آبخیز اوجان چای واقع در دامنه های شرقی کوهستان سهند)واقع در شمال غرب کشور(از محدوده های مهم کشت انواع محصوالت دیمی و آبی به شمار آیند که تحت فرسایش خطی تشدید و تسریع شده قرار گرفته است. انبوه خاکی که در اثر فرسایش خندقی از دامنه هاجدا و تلف می شود، می تواند پیامدهای منفی بسیاری برای تاسیسات آبی و محدوده های کشت داشته باشد.از این نظر، بررسی و شناسائی محدوده های تحت فرسایش خاک در اثر توسعه اشکال خطی از نوع فرسایش خندقی ،از اهمیت ویژه ای برخوردار است. در این مقاله با استفاده از روش های کمی، مقدار خاک تلف شده از فرسایش خندقی برآورد گردیده است. با استفاده از شاخص si شکل خندق ها تعیین و سپس با استفاده از معادله فرسایش)e،)میزان فرسایش خاک محاسبه شده است .با عنایت به نقش نیرو زایی برآمدگی ها در مسیر آبکندها، سهم باالآمدگی ها در هر خندق در میزان فرسایش با استفاده از رابطه k تعیین گردیده است .نتایج حاصل از برآورد ها نشان می دهد که شکل خندق های محدوده مورد بررسی در اثر نقش عوامل مختلف بسیارمتفاوت است .حتی در طول یک خندق مجزا نیز این تفاوت دیده می شود. بیشترین تغییرات درمیزان si در مقطع عرضی خندق شماره 4 وکمترین میزان si در خندق 7 و11 مشاهده می شود که حاکی از v شکل بودن خندق ها است.بررسی ها نتایج نشان می دهد که سهم باالآمدگی های ناگهانی در میزان فرسایش در خندق 3 و 10 بیشتر از سایر خندق ها است. این سهم در خندق 1، صفر و در خندق 5، نزدیک به صفر است. نتایج حاکی از آن است که بطور کلی سهم برآمدگی های ناگهانی در سطح شیب ها در تشکیل و افزایش میزان فرسایش خندقی منطقه زیاد است و این میزان در سه خندق بزرگ محدوده مورد مطالعه بیش از سایر خندق ها برآورد شده است

The estimation of uplifting share on gully erosion rates over slopes ,Case study :Ojan Chay ,North estern slopes of Sahand mountain

Soil erosion is a serious environmental problem around the world .The formation of gully systems is a sign of severe soil erosion, and gullies are an important sediment source in dry lands , also reported that gullies can account for a higher portion of sediment yield in semi arid.Gullies are typical erosion forms in semiarid and arid landscapes all over the world where high morphological activity and dynamics can be observed. Semiarid climate conditions and precipitation regimes encourage soil erosion processes through low vegetation cover uplift and recurrent heavy rainfall events.Gully erosion is a threshold phenomenon and occurs only when a threshold in terms of flow hydraulics, rainfall, topography, pedology , or land use has been exceeded .The North estern slopes of Sahand mountain are severely degraded by rill and gully erosion. In headwater streams in steep land settings, narrow and steep valley floors provide closely coupled relationships between geomorphic components including hill slopes, tributary fans, and channel reaches. These relationships together with small catchment sizes result in episodic changes to the amount of stored sediment in channels. Erosion rate estimates are potentially strongly influenced by the estimation method.A total 11 gullies with various soils and land use types were investigated in Ojan catchement. Field data on gully channel geometry were collected in catchement.The obtained data confirmed the existence of the power relationship for rills and gullies, with the exponent varying from 0.44 for small gully) to 0.5 for gullies . The data did not allow deciding whether the exponent varies consistently with channel width or in a stepwise fashion.Annual sediment yield from gully complexes was derived based on their area using empirical equations obtained in the same rock formation and vary formation in headwater catchments of the Ojan River . Major sediment inputs follow high magnitude events. As headwater catchments are major sediment sources, interpretation of sediment delivery processes in these settings is a critical consideration in our understanding of basinscale sediment dynamics.Analysis of these geomorphic features in steep headwater catchments can also be used to characterize the episodic manner of sediment delivery processes. Narrow valley floors in study area allow sediment to be directly transferred from outside the channels to inside. A data set on soil losses and controlling factors for 11 ephemeral gullies has been collected in the Northestern Sahand mountain . Of the observed ephemeral gullies, gullies. and ephemeral gullies developed on slopes. Analysis shows that E is capable of predicting ephemeral gully crosssections well. Rather than revealing E ability of predicting ephemeral gully erosion, this analysis stresses the problematic nature of physically based models, since they often require input parameters that are not available or can hardly be obtained. With respect to the value of simple topographical and morphological indices in predicting ephemeral gully erosion, this study shows that gullies, respectively, over 80% and about 75% of the variation in ephemeral gully volume can be explained when ephemeral gully length is known. Moreover, when previously collected data for ephemeral gullies in study areas and the data for gullies formed in the U form, it appears that one single length volume . A simple procedure to predict ephemeral gully length based on topographical thresholds is presented here. Secondly, the empirical length–volume relation can also be used to convert ephemeral gully length data extracted from aerial photos into ephemeral gully volumes. The evidence showed that gullying was controlled by faulting and uplifting along the slope. It can be seen that the deep gullies clearly plot higher compared to the shallow gullies. This is also reflected by the intercept of the minimal topographical threshold line. Two bank gullies representing different morphological types of gullies (Vshaped and Ushaped) were chosen from a dataset of gully systems in semiarid . Gully erosion generates significant volumes of sediment that is delivered to waterways throughout the world . Quantifying gully erosion rates and associated sediment yields is critical for the effective prioritisation of management efforts aimed at reducing the environmental impact of gully erosion .Soil fabric and rock structure and weathering studies were undertaken to establish the inheritance of soil cracks from the underlying parent material.Two mechanisms of gully development appear to occur in upland catchement. The first produces stream coupled gullies resulting from lateral channel migration and erosion induced at the base of the hill slopes . The second produces more extensive gully networks that are often initiated in the mid slope .In this context, gullies link hill slopes and channels, functioning as sediment sources, stores and conveyors. From a review of gully erosion studies in semiarid and arid regions, conclude that gullies contribute an average of 50 to 80% of overall sediment production in dry land environments .