بررسی تاثیر سازۀ پل در مسیر رودخانه بر رفتار هیدرولیکی جریان (مطالعۀ موردی: رودخانۀ ولیان)

وجود سازه‌ها در مسیر رودخانه مانند پل، بند، اعیانی‌ها و... باعث تغییر خصوصیات هیدرولیکی جریان رودخانه شده و تغییر در این خصوصیات جریان ممکن است بستر رودخانه را تغییر دهد. در پژوهش حاضر، اجرای مدل هیدرولیکی رودخانۀ ولیان و نتایج حاصل از آن و نقش پل‌ها (چندار، اسکول ‌دره، آجین دوجین و ولیان) در شرایط طبیعی بستر مورد بررسی قرار گرفته است. با در اختیار داشتن نقشه‌های توپوگرافی با دقت مناسب (مقیاس 1:500) و نرم‌افزارهای gis شامل arcgisgeoras و امکان ساخت مدل gis از این محدودۀ مطالعاتی، از مدل ریاضی hec-ras استفاده شد. رودخانۀ مورد مطالعه در شرایط طبیعی و در وضعیت موجود (وجود سازه‌ها)، برای سیلاب‌های با دورۀ بازگشت‌های مختلف 2، 2/33، 5، 10، 25، 50، 100، 200، 500 و 1000 مدل گردید. نتایج نشان داد که وجود پل‌ها  بیشترین تاثیر روی مساحت مقطع عبوری جریان (11/6 %) و عرض سطح آزاد آب (16/5 %) و کمترین تاثیر روی سطح ارتفاع آب و تراز گرادیان انرژی (حدود صفر درصد ) دارند. عدد فرود و سرعت جریان در تمام پل‌ها به‌جز پل ولیان کاهش داشته است. ارتفاع گرادیان انرژی متغیر بوده است. کمترین تغییرات مربوط به سطح ارتفاع آب، گرادیان انرژی و بیشترین اختلاف مربوط به پارامتر عرض سطح آب و مساحت است.  با توجه به نتایج حاصل از خصوصیات هیدرولیکی با دورۀ بازگشت 25 ساله برای اتخاذ تصمیمات مدیریتی لازم، پیشنهاد گردید پل اسکول‌دره به‌دلیل بیشترین تغییرات در سطح مقطع جریان و عرض سطح آب در اولویت اول و پل ولیان در اولویت آخر مدیریتی قرار گیرند.

investigation of the impact of bridge structures on the river route on the hydraulic behavior of the flow (case study: velian river)

introduction: as the main source of water and life for humans and other creatures, rivers may sometimes bring about destruction and irreparable damage. therefore, it is necessary to study the hydraulic properties of the flow and the riverbed’s area to set a safe zone for human activities around it. on the other hand, constructions built along the river’s route such as bridges, weirs, superstructures, etc. alter the river flow’s hydraulic behavior, which, in turn, could make changes in the riverbed. thus, this study sought to examine the implementation of a hydraulic model of the valian river and its results and to investigate the role of bridges (chandar, eskol darreh, ajin dojin, and velian) in the riverbeds’ natural conditions. the study area for designating the river’s bed limit and riparian zone included the upstream of eskol darreh village, all the routes which pass along the eskol darreh, velian, ajin dojin, khorvin, and chandar villages, and finally, the entrance of kordan river at the downstream of chandar village. the length of the main river is about 17.5 km, whose upstream coordinates located, according to the utm coordinate system, at 485090 east and 3990600 north, and its downstream coordinates are located at 481050 east and 3977000 north.material and methods: to simulate the river under study, hydraulic parameters were taken into account, including the flood discharge with different return periods, manning coefficient, the river’s geometry in the form of cross sections, and the data regarding the constructs built along the river’s route. moreover, the flow’s hydraulic conditions were simulated and the necessary hydraulic analysis over the studied periods was performed using the hecras model. according to the applicable laws, the flood zone is designated under the river’s natural conditions in terms of 25year flood discharge. in this study, the final riverbed’s limit was designated by identifying the hydraulic bed and considering the effect of morphological, legal, riverbed land use, and socioeconomic studies. the riparian zone is normally located at a certain distance away from the riverbed, which was considered 4 meters for the valian river. also, the water surface profile was calculated for the constant flow mode according to the standard stepwise method using the hec_ras software. the calculations were performed based on solving onedimensional energy equations (bernoulli equation). the manning equation was also used to calculate the energy loss in successive periods. furthermore, other types of losses such as the periods’ expansion and contraction coefficients were taken into account, and the momentum equation when applied for periods when fast variable currents occurred (in mixed currents, including a combination of supercritical and subcritical currents, hydraulic jump, the current passing through the bridges’ underpasses and the junction of branches, etc.). in addition, this model can also analyze the effects of riverrelated constructs such as bridges, culverts, weirs, lateral weirs, and organizing constructs discussed in this study. taking the possibility of developing a gis model for the study area, by having and according to the width of the river bank mapping, some cross sections were prepared along the river from the main route and the flood plain, designated at certain distances away from each other based on the mapping of the river’s edges using topographic maps with appropriate accuracy (scale 1: 500) and gis software, including the georas and the arcgis software.