اثرات هيدرولوژيك احداث سد چاشم در پايين دست حوزه آبخيز رودخانه تالار

Hydrological Effects of Chashm Dam on the Downstream of Talar River Watershed

احداث سدهای مخزنی موجب بروز مشكلات هیدرولوژیك و زیست محیطی در پایین دست خواهد شد. احداث سد چاشم با هدف تأمین آب شرب استان سمنان بر روی سرشاخه‌های حوزه آبخیز رودخانه تالار مازندران در دستور كار وزارت نیرو قرار گرفته است. تحقیق حاضر به بررسی آثار هیدرولوژیك و زیست محیطی (نیاز آبی) احداث سد مذكور در پایین دست حوضه پرداخته است. ابتدا، خصوصیات فیزیكی حوزه آبخیز، سطح اثر ایستگاه‌های باران‌سنجی منطقه و درصد اراضی غیرقابل نفوذ با بكارگیری داده‌ها و نقشه¬های توپوگرافی 1:25000 و قابلیت¬های سیستم اطلاعات جغرافیایی تهیه شدند. سپس، با بكارگیری روش SCS برای شبیه¬سازی هیدروگراف جریان، روش كمبود اولیه و ثابت، برای برآورد ارتفاع رواناب و روش فروكش نمایی برای برآورد آب پایه در محیط مدل HEC-HMS، یك مدل بارش- رواناب برای شبیه¬سازی رفتار هیدرولوژیك حوضه ارائه شد. سپس، بهینه¬سازی و اعتباریابی مدل انجام پذیرفت. در مرحله بعد، شبیه¬سازی خصوصیات فیزیكی سد چاشم و مخزن آن در مدل اعتباریابی شده به منظور روندیابی در مخزن و شبیه¬سازی عملكرد هیدرولوژیك مخزن سد انجام شد. همچنین، نیاز آبی منطقه با میزان مشاركت مخزن سد در تأمین آب شهر سمنان و جریان خروجی از مخزن سد مقایسه و بررسی شد. نتایج نشان داد كه حجم آبدهی سالانه حوضه سد چاشم در حالت نرمال كمی كمتر از مجموع نیاز آبی نه میلیون متر مكعبی در نظر گرفته شده است.

Introduction: In the last century, dams have constructed with the objective of water supplies for agriculture, drinking water and industry. However, the results from the performance review of dams show adverse effects on the downstream environment and the availability of water resources. The purpose of the Chashm dam construction on the TalarRiver 's tributaries is the water supply for Semnan city. Materials and Methods: This study was conducted in TalarRiver watershed. TalarRiveroriginatesfrom AlborzMountains in Mazandaran province, in the southern Caspian Sea basin, in north of Iran and flows parallel with the Firouzkooh-Ghaemshahr road and it arrives to the Caspian beach area in the Malek Kala village. In order to supply the water requirements of Semnan city, the construction of Chashm dam on the TalarRiver 's tributaries placed on the agenda of the Ministry of Energy. However, because of the uncontrolled exploitation of agricultural streams and invasion of privacy riverbed, the TalarRiver has acute and critical conditions from the point of hydrologic and environmental. To study the hydrological impacts of Chashm dam, Talar watershed was considered with an area of approximately 1057 square kilometers of the Pole Sefid gauging station using a rainfall-runoff model. Results and Discussion: Simulation of the study area hydrological behavior shows that the Chashm Dam average water discharge is near to 8.6 million m3. This figure will be significant changes during wet and droughtperiods. The minimum and maximum monthly discharge of the Chashm Dam watershed in August and February is equal to 0.31 and 0.55 m3/s respectively. The minimum and maximum monthly water demand in turn in October and August is equal to 0.015 and 0.4 m3/s respectively and this shows that the river discharge in June is lower than the downstream water demand. Based on confirmed studies of the Kamandab Consulting Engineers, drinking water requirement of Semnan province, water rights users ' requirement and downstream environmental requirements are 4.54, 2.164 and 2.448 million m3, respectively. This is despite the fact that the volume of annual input water is slightly lower than this figure in normal. Conclusion: Simulation of the study area hydrological behavior shows that the Chashm Dam average water discharge is near to 8.6 million m3. This figure will be significant changes during wet and drought periods. The minimum and maximum monthly discharge of the Chashm Dam watershed in August and February is equal to 0.31 and 0.55 m3/s respectively. The minimum and maximum monthly water demand in turn in October and August is equal to 0.015 and 0.4 m3/s respectively, and this shows that the river discharge in June is lower than the downstream water demand. Based on confirmed studies of the Kamandab Consulting Engineers, drinking water requirement of Semnan province, water rights users ' requirement and downstream environmental requirements are 4.54, 2.164 and 2.448 million m3, respectively. This is despite the fact that the volume of annual input water is slightly lower than this figure in normal. In addition, the Chashm Dam area is about 110 hectares and given the minimum annual actual evaporation equal to 700 mm, about seven hundred thousand cubic meters of water stored in the reservoir will be lost. Due to the simultaneous occurrence of the maximum water requirement, maximum evaporation and a minimum of water inlet to the Chashm Dam reservoir in warm seasons, it seemsthat, it is not possible to provide needs based on these studies and no doubt, in the case of water supply in Semnan province, we have to stop the flow of the river in downstream of the dam. The results of this study suggest that on many rivers large headwater dams have reduced the frequency and duration of floodplain inundation downstream and these changes lead to changes in downstream ecosystems. The results from the simulation and analysis of the Chashm Dam in downstream are as follows: a) stop of the river flow in downstream of the dam site, b) the sharp decline in river discharge in minimum (varied) flows, c) reduce in the rate and volume of maximum flows, d) changes in the hydrological regime of the river such as base flow, flow stop, the frequency of the river full section and competency which will make dramatic changes in the morphology of the river and downstream ecosystems. Note that is not verified by modeling and forecasting studies, is how to manage the reservoir. The amount of water stored in the reservoir and discharge to downstream is directly a function of the reservoir management.