تعيين گستره‌ي سيل رودخانه‌ي مرغك در محيط HEC-GeoRAS

Determining the Flood Prone areas of Morghak river Using HEC-GeoRAS

در حال حاضر، تكنولوژي به آن درجه از تكامل نرسيده است كه از بروز سيلاب‌هاي زيان‌بار جلوگيري و در عوامل و عناصر جوي تغييري ايجاد كند. بنابراين، هرگونه راه‌ حل اصولي و چاره‌ساز را بايد در روي زمين و خصوصاً در عرصه‌ي حوضه‌هاي آبخيز جستجو كرد. در اين ‌باره اولين اقدامي كه براي كاهش خطر سيل مطرح مي‌شود مهار سيل در سرمنشأ آن، يعني زير حوضه‌هاي آبخيز، است. به اين جهت شناسايي مناطق سيل‌خيز در داخل حوضه داراي اهميت فراواني است. از اين رو، بايد مناطقي كه پتانسيل بالايي در توليد سيل دارند شناسايي شوند تا امكان بهينه‌سازي عمليات اجرايي در سطوح كوچك‌ و خطرساز فراهم شود و از هزينه‌هاي اضافي طرح‌هاي كنترل سيل جلوگيري گردد. هدف از اين تحقيق نيز ارائه‌ي روشي است تا با استفاده از آن بتوان ضمن در نظر گرفتن آثار متقابل عوامل مؤثر بر سيل‌خيزي مناطق خطرساز و سيل‌خيز را در داخل حوضه تعيين كرد. محدوه‌ي مطالعه اين تحقيق بازه‌اي از رودخانه‌ي مرغك در حوضه‌ي تالاب انزليِ استان گيلان بين دو ايستگاه هيدرومتري امام‌زاده شفيع و كتمجان به طول سي كيلومتر است. در اين پژوهش، هدف معرفي يكي از روش‌هاي پهنه‌بندي سيل با استفاده از تلفيق نرم‌افزارهايARCVIEW وHEC-RAS و الحاقيه HEC-GEORAS-431 است تا ضمن تعيين حريم سيل در قسمتي از رودخانه‌ي مرغك جديدترين، باصرفه‌ترين و كوتاه‌ترين روش پهنه‌بندي سيل از نظر اقتصادي و زماني معرفي و مزايا و توانايي‌هاي آن بررسي شود. از اين رو، مقاطعي از بازه‌ي رودخانه كه بيشترين عرض و گستره‌ي سيل را با دوره‌ي بازگشت 25 ساله دارد مشخص شده است. نتايج حاصل از تحقيق نشان مي‌دهد كه اين مقاطع در پايين‌دست رودخانه حد فاصل جاده‌ي اصلي فومن به پونل تا بازه‌ي انتهايي رودخانه (ورودي به تالاب انزلي) در هشت كيلومتر پاياني از مجموع حدود سي كيلومتر بازه‌ي رودخانه قراردارد.

Flood pron areas of rivers are generally hazardous. Regionalizing these hazardous areas in terms of the degree of hazard they produce is very important for regional flood management, insurance companies and land users. Therefore, this research has tried to regionalize the potential hazard of the flood prone areas of the Morghak River using HEC-GeoRAS model as an example for all flood plains of Gillan province. In order to develop the hydrolic model of the river, the following data were prepared. The river profile, roughness index of the river channel and flood plain and river bank conditions were obtained from 1:2000 TIN maps. The data were entered into the HEC-RAS model. Then the data of the river banks and flood discharge amounts were entered and hydraulic computations were carried out. The model results were entered into the GIS. After the requested processing in the extension of HEC-GeoRas431, the final maps of depth of river, water movement velocity, shear velocity and the flow intensity along the river channel were produced. The maps were moved into the Google Earth and the flood area with different return periods were plotted. The results showed the areal expansion of the 25-year return period floods of the river basin. This plain is narrow in the upper areas of the river and widens in the lower areas of the area. In the areas that there are constructions, the basin gets wider and its higher discharges causes severe hazards in the settlements around the river. The widest part of the flood plain is over the lowlands around Anzali swamp. In these lowlands the flood spreads over the vast area and making problems for the farmers and dwellers. According to the results of this research some adaptation measures are needed in the areas where people have moved to the river channel and have built some structures. Some of these measures include vegetation planting, cement and rocky barriers and cleaning all extra wastes. The results of the study also indicate that in most of the branches the building of the channel has narrowed the channel and caused flood in the settled areas. The physiographic parameters of the river have seriously been changed and caused the severe floods in the river especially in the lower areas. The flow speed of the river changes from 4.1 m/s in the maximum discharge to .2 m/s in the very low discharge. The width of the channel has also changed from 281 meters in the maximum to 11 meters at the low discharge period. The discharge stress was between .3 to 357 newtons the overall results of the research indicate that the human interference in the river basin has caused all these hazards. an‎d the only solution is that the humans should go out of the risky areas of the river basin. The continuation of this process in this river or in the other rivers will worsen the present hazards,