برآورد مقدار سيلاب حداكثر به روشهاي مشاهده‌اي ، كريگر و اس-سي-اس(مطالعه موردي: رودخانه قره‌سو در كرمانشاه)

Determination of Maximum Flood Flow by use of Sampling (observe) Creager and SCS Method (Case study: Gharasoo river, Kermanshah, Iran)

برآورد مقدار سيلاب حداكثر حوزه آبريز، بخش مهمي از مطالعات هيدرولوژي در ساماندهي رودخانه است. در اين ارتباط لازم است دبي پيك سيل و شكل هيدروگراف آن براي دوره‌هاي بازگشت مورد نظر مشخص باشد. انجام اين كار مستلزم وجود آمار طولاني مدت و دستگاه ليمينگراف در محل مورد نظر است. در مطالعه حاضر با مقايسه آمار موجود در ايستگاههاي آب‌سنجي و با استفاده از روشهاي مشاهده‌اي،كريگر وSCS اقدام به برآورد ميزان سيلاب طرح در حوزه آبريز رودخانه قره‌سو در استان كرمانشاه شد. ارتباط بين ژيومرفولوژي و هيدرولوژي اين امكان را فراهم مي‌آورد تا در حوزه‌هايي كه از نظر ژيومرفولوژي تشابه وجود دارد با ايجاد روابط بين دو حوضه آبريز، برآورد آمار هيدرولوژيكي را در حوزه‌هايي كه فاقد آمار به‌دست آورد. بدين منظور هيدروگراف‌هاي طبيعي و مصنوعي با استفاده از داده‌هاي مرفولوژيكي، باران سنجي و هيدرومتري حوضه آبريز و تجزيه و تحليل هيدرولوژيك تامين شد و هيدروگراف واحد مصنوعي با هيدروگراف‌هاي واحد طبيعي مورد مقايسه و بررسي قرار گرفت. با توجه به دقت مناسب روش SCS كه از مشخصه‌هاي مختلف اقليمي، زمين شناسي و فيزيوگرافي استفاده مي‌كند، نتايج اين روش به عنوان سيلاب طرح پيشنهاد شد. براي برآورد سيلاب طرح به روش SCS از نرم افزار HEC-HMS استفاده شد كه از داده‌هاي سيلاب اسفندماه 1376 جهت كاليبراسيون نرم افزار فوق استفاده شد. نتايج مبين مقدار دبي حداكثر 1520m3/s در ايستگاه پل‌كهنه است. روش كريگر براي برآورد دبي در حوزه‌هاي فاقد آمار به‌كار گرفته شد كه نتايج دبي حداكثر به‌دست آمده از اين روش براي ايستگاه دوآب قزانچي،1530 m3/s برآورد شد.

Introduction Estimation of design floods in ungauged catchments is frequently required in hydrological practice and is of great economic significance Hydrologists consider artificial unit hydrograph as one of applicable methods to estimate peak discharge for watersheds for which hydro climatology data is not available. In general, there are three methods for estimating peak discharges, including; Schneider, the Soil Conservation Service (SCS) and triangular methods. All three methods give the same results in regards to the confidence level. However, SCS method estimates flood hydrographs with less error than other methods. The aim of this research is to suggest the best peak discharge estimation which has the most compatibility with natural hydrographs method. Moreover, natural and artificial methods and unit hydrograph will be compared with each other. In addition, the best estimation method will be proposed for calculation of coefficients of artificial method. Finally, the flood peak and design flood will be determined. The study area Kermanshah province with area of 24856km2 is located in west of Iran which its average altitude is around 1200m above sea level. Gharahso catchment is located in longitudes 45° 22ʹ to 47° and latitudes 34° and 34° 55ʹ. Gharahso River is one of the most important tributary of Saymareh River. It collects water from Kermanshah and Kurdistan provinces and delivers it to Saymareh River. Materials and methods using flood data in gauged stations In general, recorded data have some limitations. However, most of methods require complete statistical data from under study catchment to estimate the peak discharge and to calculate more accurately runoffs. In observational method, to estimate flood in the study area, the recorded data in gauged stations are been used. Estimated maximum instantaneous discharges in selected stations based on different return periods are given in table 1. Flood estimation using Kriging method E-mail: karkuti@yahoo.com Krigingʹs formula is a world widely used formula to determine maximum flood discharge for big and ungauged catchments. Krigingʹs formula is as follows: Corresponding author: Tel: 09195077184 Q = 46CA(0ʹ894A"°ʹW8) (1) Where: Q = peak flood discharge (ft3/s) A = watershed area (mile) C = coefficient of watershed which its amount depends on climatic and physiographic of under study watershed. Table 1: Maximum estimated and observed instantaneous discharge in selected stations based on various return period (m3/s) Station Return period(year) Maximum observed instantaneous discharge (m3/s) 100 50 25 10 5 2 Pole kohne 1587 1083 729 419 264 127 1255 Hojat abad 663 472 335 210 145 85 407 Khers abad 212 178 147 108 81 46 198 Doab merek 409 320 243 159 106 49 288 Estimating flood using Soil Conversation Service (SCS) method Estimating rain water discharge using Curve Number (CN) is an internationally known method. Soil Conversation Service of untied states presented this method as SCS or dimensionless unit hydrograph in year 1957. Results showed that this model can be used in each urban area, natural and mixed water shed. Required in formation fore estimation in this soft ware are: Physiographic of watershed. Permeability status of watershed Numerical amount of CN parameter. Selecting design pattern and continuity of raining floodʹs trajectory in the river Results and discussion Results of Kriging Method Peak flood in Gharahso River in Doab Ghazanchi station was estimated using Kriging method. Thus using this method and estimated peak flood (for different return periods) in Pole Kohne gauge station the coefficient of C was determined. Therefore, the estimated coefficient C was suggested for the study area and Pole Kohne station. The coefficient C for different return periods are given in table 2. Table 2: Floodʹs peak in Gharahso River in Doab Ghazanchi by Kriging method for different return period m3/s Area (Km2) Place name 100 10 25 return period(year) 50 1587 11.0 1530 1083 7.5 1045 729 5.1 703 5026 4585 419 2.9 403 264 1.8 254 127 0.9 122 Pole kohne Creagerʹs C Doab Ghazanchi SCS method results Considering the geographical characteristics of catchments and gauged stations in the study area, one representative rain gauge station was determined for each sub basin. Using maximum precipitation for different return periods, the volume of peak flood for 2 to 100 years retuning periods were estimated. However, the flood hydrograph for 100-year return period for Gharaso River in Doab Ghazanchi and Pole Kohne stations are shown in figures 1 and 2. As it is observed, the maximum volume of peak flood of Gharaso River in Doab Ghazanchi and Pole Kohne are estimated as 1479m3 and 1520m3. Fig.l: 100-year return period for peak flow in Gharaso River, Doab Ghazanchi station Fig.2 : 100-years return period peak flow in Gharaso river, Pole Kohne station Conclusion For Doab Ghazanchi (ungauged station), Kriging method was used to estimate the peak flood. The peak flood was estimated to be 1530m3/s for 100-year return period. Moreover, at the same station, the peak flood equaled to 1479m3/s for 100-year return period, using SCS method. Consequently considering possible mistakes in recorded data and lack of determined and perfect hydrograph (useful for data collection) as well as weakness in observational methods in one hand and the accuracy of SCS method which considers different climatic, geological and Physiographic and precipitation specifications on the other hand led us to accept the results of SCS method as the design flood.