بررسي تغيير مورفولوژي بستر رودخانه پيش و پس از احداث سد با استفاده از HEC RAS و GIS (مطالعه‎ي موردي: منطقه‎ي پايين‎دست سدّ ستارخان اهر)

Morphologic Change Assessment of Riverbed Before and after Dam Construction Using HEC RAS Modeland GIS (Case Study: Downstream of SatarkhanDam)

رودخانه‎ها از منابع عمده‎ي تامين آب شيرين در جهان به‎شمار مي‎روند. كاهش منابع آب و كمبود آب شيرين در جهان، لزوم مطالعه و بررسي تغييرات ايجادشده در ويژگي‎هاي آن را آشكار مي‎كند. هدف از اين بررسي، مطالعه‎ي تغييرات ايجاد شده در مورفولوژي بستر رودخانه و جابه‎جايي شكل گرفته در مسير رودخانه و نيز، بررسي مقاطع عرضي رودخانه و عوامل موثّر در ايجاد تغييرات رخ داده‎اي همچون دبي و رسوب و... به‎دليل احداث سد است. در بررسي تغييرات بستر رودخانه، به‎دليل احداث سد، از دو سري داده‎هاي زماني براي پيش و پس از احداث سد استفاده شده است كه شامل دبي و رسوب مي‎شوند. داده‎هاي دبي و رسوب روزانه از ايستگاه‎هاي هيدرومتري اشدلق و تازه‎كند به‎دست آمد و مقادير رسوب سالانه، بر اساس مدل رگرسيون خطّي به‎روش كمترين مربّعات محاسبه شد. همچنين مقاطع عرضي رودخانه با استفاده از نقاط كنترلي موجود در آمارهاي ايستگاه‎هاي هيدرومتري مورد نظر، به‎كمك نرم‎افزار HEC RAS ترسيم و با بررسي داده‎هاي محاسبه شده، ميزان اين تغييرات مشخّص شد. جابه‎جايي مسير رودخانه در محيط نرم‎افزار ARC MAP و از طريق تصاوير ماهواره (2006) SPOT، عكس‎هاي هوايي 1:40000 سال 1374 و نقشه‎هاي توپوگرافي 1:25000 سازمان نقشه‎برداري بررسي و نتايج به‎صورت نقشه ارايه شد. نتايج حاكي از كاهش ميزان رسوب حمل شده پس از احداث سد، كاهش ميزان دبي و ثبات تقريبي آن ـ كه به‎دليل هدايت جريان آب به داخل شبكه‎هاي آبرساني براي كشاورزي و مصارف آشاميدني بوده ـ پايداري تقريبي بستر رودخانه، به‎دليل جلوگيري از جريان يافتن سيلاب‎هاي بزرگ توسط سد و نيز، جريان يافتن دبي تقريبي ثابت طيّ سال‎ها است.

Introduction Rivers are important sources of water supply in the world. So reduction of their water canimpact on human life. This water supply is facilitated by dams as hugestructures that save water and make big reservoirsbehind. Dams are well known for influencing channel and vegetation dynamics downstream, but little work has focused on distinguishing effects of land use and channel responses to the impoundment. The impact of construction of dams and reservoirs on alluvial rivers extends both upstream and downstream of the dam. Downstream of dams, both the waterand sediment supplies can be altered leading to adjustmentsin the river channel geometry and ensuing changes in riparian and aquatic habitats.Construction oflarge water structuressuch as dams across rivers may lead to considerable decrease in the flood discharge proportionate to structure size which will in turn result in lower flood probability. Assessmentofthe vulnerability of river boundary is of great importance due to their rich soil and easy access to irrigationwater which makes such riverbank farms economically valuable especially in dry areas. The results of suchassessment can be used in land use planning.In this paper, we examinedthe interacting effects of Satarkhan dam on downstream changes in channel morphology of Aharchai River. Satarkhan dam was constructed on the main stem of the Aharchai in 1998 for water supply, flood control and sediment retention. This dam is located between Ahar and Varzaghan city, eastern Azarbaijan, northwest of Iran. Methodology Base line data including mean annual sediment, mean daily discharge and cross sections data located downstreamwere collected from several sources.Dams influence the two primary factors—waterand sediment—that settle the shape, size, and overallmorphology of a river and represent fundamental interventionsin the fluvial system.These datawere calculated and estimated for two period asbefore (1989-1997) and after (2003-2009) dam construction.To measure the cross sections changes, HEC RAS model and two hydrometric station’s data (Oshdologh and Tazekand) were used to estimate mean discharge of the river and annual sediment transportation. Also satellite images SPOT 2006), aerialphotos 1995 (scale, 1:40000) and 1:25000 map were used to study the changes in river path. These datawere calculated in Arc map and the results represented as maps. Results and Discussion Prior to the construction of the Satarkhan, the total mean flow discharge measured at Tazekandand Oshdologh stations varied along the whole year, and ranged between 2.3 m3/s to 9.3 m3/s and 1.8 m3/s to 7.3 m3/s.The total annual sediment for this stations varied between 287000 to 100000 t/y and 275000 to 51000 t/y. As Satarkhan was completed, a complete control of the Aharchai River discharges was achieved and the channel has no longerbeen subjected to high floods.The flow rate of water also became under full control. Accordingly, the average annual discharge has been reduced by 63% and 61% at Tazekand and Oshdologh stations. Moreover, a substantial change in the sediment regime of the river downstream of the damoccurred, which in turn disturbed the stability of the hydraulic geometry of the river. Conclusion Changes in the release schedule for Satarkhan dam have led to a decline in the frequencyand greatness of peak flows that trigger channel changes. The river is morestable some distance downstream from the sedimentsources because of the decline in channel avulsion.But far from the dam, this influence is weak. On the other hand, agreat change in the flow regime of the Aharchai River occurs downstream of the Dam, with is captured all upstream flows early in the rainy season,most low flows, many larger flows and the reduction in magnitude of some floods that arereleased downstream.The change in flow regime is also likely to have caused a change insediment load. Changes in the water and sediment regimes of the river resulted channel adjustments inboth the vertical and lateral dimensions. The channel pattern has shifted from a wide, braided shape with mid channel bars, to a single-thread straight and meandering planform. The channel pattern change was studied by comparing the SPOT satellite image and the aerial pictures for prior and after dam construction.