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R AINFALL -R UNOFF M ODELING O F W ADI Z IMAR

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Prepared by Ahmad Tallal Abu-Hamed Mohamed Nemir Mohsen Osama Omar Nazzal Under the direction of Dr. Sameer Shadeed

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P RESENTATION O UTLINE Introduction Study Area Literature review Analysis of Rainfall Data Rainfall Runoff Models

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Introduction

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G ENERAL The demand of water is increasing since the population of the world is increasing. Studies of water and its management in the Palestinian Territories are essential. Hydrology is the science that deals with the occurrence and movement of water on and over the surface of the earth. hydrology studies of precipitation and runoff linked with problems associated with design and management of water resources projects.

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O BJECTIVES The main objective of this project is to study the storm water drainage of Wadi Zimar. derive the unit hydrograph of catchment.

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D ATA COLLECTION Contour map of Wadi Zeimar. Nablus and Tulkarm Metrological stations. Other sources were consulted to collect information about the study area. This include Nablus and Tulkarm Municipalities.

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S TUDY A REA

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G ENERAL The Wadi Zeimar catchment area is hilly, mountainous in the eastern side, and approximately flat in the western part. Surface and subsurface water drains towards the coastal aquifer in the direction to the Mediterranean sea.

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The climate in the study area is Mediterranean, with moderate summers and warm winters. The annual rainfall is very unevenly distributed over the year. Land use in the catchment varies from agricultural area to Industrial area and Built up area.

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Literature review

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I NTRODUCTION The history of rainfall-runoff is believed to have begun about 335 years ago. The importance of rainfall-runoff P a -Q a modeling has long been recognized several hundred perhaps thousands models of rainfall-runoff are exist.

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R AINFALL -R UNOFF M ODELING M ETHODS

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SCS M ETHOD The SCS synthetic unit hydrograph is the dimensionless unit hydrograph developed by the soil conservation service. The SCS synthetic unit hydrograph was developed based on the analysis of a large number of natural unit hydrographs from a wide range of catchment sizes and geographic.

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SCS method is used, since it requires geographical parameters which can very easily be obtained by using GIS techniques. Synthetic unit hydrographs are developed along two main concepts: o Each watershed has a unique unit hydrograph. o All unit hydrographs can be represented by a single family of curves or a single equation.

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The basic procedure of the SCS method for estimating runoff can be summarized as follows: Determine the watershed area and slope in percent. The flow velocity could be estimated.

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the time of concentration in hours Where: L: length of flow path in feet. V: average velocity in feet per second. T c : time of concentration in minute.

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The time of peak discharge T p, in hour Where: T r : duration of effective rainfall The peak discharge, m³/s.cm

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S NYDER ' S M ETHOD Snyder's method is a very straight forward, mostly empirical method for deriving the time to peak and peak flow of a unit hydrograph. Snyder method was developed using the analysis of a large number of hydrographs from catchment region.

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The basic procedure of Snyder's method for estimating runoff can be summarized as follows: Basin lags in hours t p Where: L: is the main stream distance from outlet to divide (mi). L c : is the main stream distance from outlet to a point opposite the basin centroid (mi). C t : a coefficient representing variations of watershed slopes and storage. S: is the weighted channel slope.

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Peak discharge Q p (cfs) Where: C p : the coefficient accounting for flood wave and storage condition. A: watershed size (mi²). t p : lag time.

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To construct Snyder's unit hydrograph,W 50, W 75 are also considered W 50 : width of unit hydrograph at 50 % of peak discharge in hour. W 75 : width of unit hydrograph at 75 %of peak discharge in hour.

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A NALYSIS OF R AINFALL D ATA

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A NALYSIS OF ANNUAL RAINFALL The average annual rainfall data are obtained from the meteorological station in Nablus for the years from 1954 to 2008. The average of the average annual rainfall of Nablus data is calculated at 584.0684 mm. The standard deviation equals 233.26 and the maximum and minimum annual rainfall is 1195 mm (1992), 149.7 mm (1970) respectively.

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T HIS FIGURE SHOWS THE ANNUAL RAINFALL AND ITS AVERAGE OF N ABLUS

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From the figure, it is noted that the deviation of the annual rainfall from the average varies from year to year and that the relatively wet year are 26 years out of 55 studied years. The dry years are 29 years out of the 55 studied years.

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F REQUENCY A NALYSIS The return period for each rainfall was calculated using the following plotting position formula: Where: T r : return period m: rank of event n: number of events

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T HIS FIGURE PLOTS THE RETURN PERIOD WITH RAINFALL FOR N ABLUS STATION

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Rainfall Runoff Models

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SCS M ETHOD The dimensionless hydrograph and triangular unit hydrograph of the SCS Method are shown in the following figure. Dimensionless hydrograph

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triangular unit hydrograph

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T HE FOLLOWING FIGURE SHOWS THE S TREAM, CONTOURS AND CENTROID OF W ADI Z EIMAR.

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For Wadi Zeimar catchment, the following consideration and calculations can be done: The total area of Wadi Zeimar which is under study is about 172.5km². t r =1 hour (assume one hour unit hydrograph). Description of water course is natural channel not well defined. the length, slope and velocity for the rainfall bath was found. The following table shows the results

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By using this formula the time of concentration can be calculated = 1/60(18766.2+16896.1+25918.3+34874.9)=1607.6 min = 26.79 hr = 0.5 +0.6 *26.79 = 16.57 hr = 2.08(172.5/16.57) = 21.65 m³ /s.cm

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By applying the calculation we get the following results The SCS hydrograph for Wadi Zeimar catchment.

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S NYDER ' S M ETHOD Applying the Snyder method to the catchment of the Wadi Zeimar, taking in consideration the following assumptions: The total area of Wadi Zeimar under consideration is about 172.5 km². For the mountain drainage area the coefficient C t is 1.2, and n is 0.38. For the mountain drainage area and for the minimum C t the coefficient C p is 0.7 For one hour unit hydrograph t r = 1. The following table shows the area, slope, main length and mean length for the catchment

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= 1.2((18.27*10.92)/2.794^½)^0.38 = 7.386 hr = 18.125 * 0.7 * 107.2/7.386 = 184.15 m³/s = 7.386/5.5 = 1.343 hr this is the duration of unit hydrograph.

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To obtain one unit hydrograph, the adjusted formulas become: = 7.386 + 0.25(1 – 1.343) = 7.3 Q’ p = 184.15(7.386/7.3) = 186.32 m³/s. T p = 3 * t p = 3 * 7.3 = 21.9 hr

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R ECOMMENDATION Install an instrument at Wadi Zeimar to measure the flow and to estimate the real hydrograph. This will enable the measurement of actual flow, and thus the evaluation of the applicability of the UH model on the catchment. Develop a model to estimate the relationship between rainfall and runoff in semiarid region. Develop the available GIS-system in the Palestinian Authorities and justification to account for catchment characteristics.

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Thanks for your attention

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