بررسي آزمايشگاهي تاثير رسوب معلق بر جابجايي آلودگي و مقايسه با روش گشتاورگيري زماني و نرم‌افزار OTIS-P

Experimental Investigation of the Suspended Sediment Effect on the Contamination Transport and Comparison with the Temporal Moment Analysis and OTIS-P software

ارزيابي كيفيت محيط زيست و ايمني بهداشت اغلب نياز به پيش­بيني انتقال املاح به ويژه آلاينده ­ها در رودخانه ­ها و آب راه ه­هاي طبيعي دارد. فرايند انتقال و پراكنش رسوب معلق در جريان­هاي مجاري باز و نحوه تبادل آن­ها در داخل محيط متخلخل سنگدان ه­اي (بستر رودخانه) از بسياري جهات شبيه به آلودگي عمل مي­كند. در اين پژوهش، مدل نگهداشت موقت (TSM) در شرايط جريان همراه با رسوب معلق (غليظ) توسط مدل عددي OTIS-P و همچنين بهينه­سازي معادلات گشتاورگيري زماني (TM) با الگوريتم ژنتيك (GA) حل و منحني­ هاي رخنه (BTC) شبيه­سازي شدند. به منظور ايجاد بستر رسوبي از مصالح شني با قطر ميانه (D50) 11/85 ميلي­متر و تخلخل (n) 0/28 استفاده شد. آزمايش­هاي ماده ردياب (NaCl) در يك كانال آزمايشگاهي ﺑﺎ ﻃﻮل 12 ﻣﺘﺮ، ﻋﺮض 0/5 ﻣﺘﺮ و ارﺗﻔﺎع دﯾﻮاره 0/7 ﻣﺘﺮ در ﺳﻪ دﺑﯽ ﺟﺮﯾﺎن 10 ،12/5 و 15 ﻟﯿﺘﺮ ﺑﺮ ﺛﺎﻧﯿﻪ اﻧﺠﺎم ﺷﺪﻧﺪ. ﺑﻪ ﻣﻨﻈﻮر اﯾﺠﺎد ﺷﺮاﯾﻂ ﺟﺮﯾﺎن ﻏﻠﯿﻆ، رﺳﻮب ﻣﻌﻠﻖ ﺑﺎ ﻏﻠﻈﺖﻫﺎي اوﻟﯿﻪ 187500 )SC1( و 375000 )SC2( ﺑﺨﺶ در ﻣﯿﻠﯿﻮن )ppm( ﺗﻬﯿﻪ و ﺑﻪ ﺻﻮرت آﻧﯽ در ﺑﺎﻻدﺳﺖ ﺑﺴﺘﺮ رﺳﻮﺑﯽ ﺗﺰرﯾﻖ ﺷﺪﻧﺪ. ﻧﺘﺎﯾﺞ آزﻣﺎﯾﺸﮕﺎﻫﯽ ﻧﺸﺎن دادﻧﺪ ﮐﻪ وﺟﻮد رﺳﻮب ﻣﻌﻠﻖ در ﺟﺮﯾﺎن )ﺷﺮاﯾﻂ ﺟﺮﯾﺎن ﻏﻠﯿﻆ( ﺳﺒﺐ اﻓﺰاﯾﺶ ﻣﺪت زﻣﺎن ﻣﺘﻮﺳﻂ ﻣﺎﻧﺪ آﻟﻮدﮔﯽ در ﻧﺎﺣﯿﻪ اﺻﻠﯽ ﺟﺮﯾﺎن )MRT( ﻣﯽﺷﻮد. ﻧﺘﺎﯾﺞ ﺣﻞ ﻋﺪدي ﻧﺸﺎن داد ﮐﻪ ﺿﺮﯾﺐ ﺗﺒﺎدل ﻧﺎﺣﯿﻪ ﻧﮕﻬﺪاﺷﺖ )α( در ﺷﺮاﯾﻂ ﺟﺮﯾﺎن ﻏﻠﯿﻆ 1 ﺗﺎ 3/2 ﺑﺮاﺑﺮ ﺿﺮﯾﺐ ﺗﺒﺎدل ﻧﺎﺣﯿﻪ ﻧﮕﻬﺪاﺷﺖ )α( ﻧﺴﺒﺖ ﺑﻪ ﺷﺮاﯾﻂ ﺟﺮﯾﺎن زﻻل ﺑﻮد. ﻧﺘﺎﯾﺞ ﺣﻞ ﻋﺪدي ﻧﺸﺎن داد ﮐﻪ ﺿﺮﯾﺐ ﭘﺮاﮐﻨﺶ ﻃﻮﻟﯽ )Dx( در ﺷﺮاﯾﻂ ﺟﺮﯾﺎن ﻏﻠﯿﻆ 2 ﺗﺎ 7 ﺑﺮاﺑﺮ ﺿﺮﯾﺐ ﭘﺮاﮐﻨﺶ ﻃﻮﻟﯽ )Dx( ﻧﺴﺒﺖ ﺑﻪ ﺷﺮاﯾﻂ ﺟﺮﯾﺎن زﻻل ﺑﻮد. ﻣﻨﺤﻨﯽ ﻫﺎي رﺧﻨﻪ ﺷﺒﯿﻪ ﺳﺎزي ﺷﺪه ﺗﻮﺳﻂ ﻣﺪل ﻋﺪدي OTIS-P و روش ﮔﺸﺘﺎورﮔﯿﺮي زﻣﺎﻧﯽ )TM( ﺗﻄﺎﺑﻖ ﺑﺎﻻﯾﯽ ﺑﺎ ﻣﻨﺤﻨﯽ ﻫﺎي رﺧﻨﻪ آزﻣﺎﯾﺸﮕﺎﻫﯽ ﺑﺎ ﺷﺎﺧﺺ ﻧﺶ ﺳﺎﺗﮑﻠﯿﻒ )Nash-Sutcliffe( ﺑﯿﻦ 0/89 ﺗﺎ 0/97 و 0/89 ﺗﺎ 0/95 داﺷﺘﻨﺪ. در آﺑﺮاﻫﻪﻫﺎي ﻃﺒﯿﻌﯽ داراي ﻏﻠﻈﺖ ﺑﺎﻻي رﺳﻮب ﻣﻌﻠﻖ، ﺗﺒﺎدﻻت ﻫﺎﯾﭙﺮﯾﮏ در ﺟﺎﺑﺠﺎﯾﯽ آﻻﯾﻨﺪهﻫﺎ ﻧﻘﺶ زﯾﺎدي را ﺑﺮ ﻋﻬﺪه دارﻧﺪ. ﺑﻨﺎﺑﺮاﯾﻦ ﭘﯿﺸﻨﻬﺎد ﻣﯽﺷﻮد از ﻣﺪل ﻧﮕﻬﺪاﺷﺖ ﻣﻮﻗﺖ )TSM( ﺑﻪ ﺟﺎي اﺳﺘﻔﺎده از روش ﺣﻞ ﺗﺤﻠﯿﻠﯽ ﻣﻌﺎدﻟﻪ اﻧﺘﻘﺎل-ﭘﺮاﮐﻨﺶ )ADE( اﺳﺘﻔﺎده ﺷﻮد.

Environmental quality and health safety assessment often require prediction of solute transport in rivers. The process of moving suspended sediment in rivers acts like contamination. In this study, the transient storage model (TSM) was used to simulate the contamination transport in dense flow conditions (with suspended sediments). OTIS-P numerical model and Temporal Moment analysis (TM) were used to solve the transient storage model (TSM) and the breakthrough curves (BTCs) were simulated. Grain material with an average diameter (D50) of 11.85 mm and the porosity (n) of 0.28 were used to create a sedimentary bed. Experiments of tracer material (NaCl) were performed in a flume with a length of 12 m, a width of 0.5 m and a height of 0.7 m applying three different flow discharges (10, 12.5 and 15 l/s). In order to create dense flow conditions, suspended sediments with initial concentrations of 187500 ppm (SC1) and 375000 ppm (SC2) were injected. Experimental results showed that the existence of suspended sediment in the stream (dense flow conditions) increased the medium residence time (MRT) of contamination in the main stream. The results of numerical solution showed that storage zone exchange coefficient (α) in dense flow conditions was 1 to 3.2 times the storage zone exchange coefficient (α) compared to clear flow conditions. The results of numerical solution showed that the longitudinal dispersion coefficient (Dx) in dense flow conditions was 2 to 7 times the longitudinal dispersion coefficient (Dx) compared to clear flow conditions. The BTCs simulated by the OTIS-P numerical model and the temporal moment analysis (TM) were highly agreement with the laboratory BTCs with the Nash-Sutcliffe index between 0.89 to 0.97 and 0.89 to 0.95.In natural rivers with high concentrations of suspended sediment, hyporheic exchanges have an important role in the transport of contamination. Therefore, the use the transient storage model (TSM) is recommended instead of the analytical solution of the advection-dispersion equation (ADE).