RAINFALLRUNOFF MODELING OF WADI ZIMAR Prepared by Ahmad

RAINFALL-RUNOFF MODELING OF WADI ZIMAR

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

PRESENTATION OUTLINE Ø Introduction Ø Study Area Ø Literature review Ø Analysis of Rainfall Data Ø Rainfall Runoff Models

Introduction

GENERAL Ø Ø 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.

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

DATA COLLECTION Ø Ø Ø 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.

STUDY AREA

GENERAL Ø Ø 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.

Ø Ø Ø 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.

Literature review

INTRODUCTION Ø Ø Ø The history of rainfall-runoff is believed to have begun about 335 years ago. The importance of rainfall-runoff Pa-Qa modeling has long been recognized several hundred perhaps thousands models of rainfallrunoff are exist.

RAINFALL-RUNOFF MODELING METHODS

SCS METHOD Ø Ø 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.

Ø Ø o o 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: Each watershed has a unique unit hydrograph. All unit hydrographs can be represented by a single family of curves or a single equation.

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

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

The time of peak discharge Tp, in hour Where: Ø Tr: duration of effective rainfall The peak discharge, m³/s. cm

SNYDER'S METHOD Ø Ø 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.

The basic procedure of Snyder's method for estimating runoff can be summarized as follows: Ø Basin lags in hours tp Where: L: is the main stream distance from outlet to divide (mi). Lc: is the main stream distance from outlet to a point opposite the basin centroid (mi). Ct: a coefficient representing variations of watershed slopes and storage. S: is the weighted channel slope.

Ø Peak discharge Qp(cfs) Where: Cp: the coefficient accounting for flood wave and storage condition. A: watershed size (mi²). tp: lag time.

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.

ANALYSIS OF RAINFALL DATA

ANALYSIS 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.

THIS FIGURE SHOWS THE ANNUAL RAINFALL AND ITS AVERAGE OF NABLUS 1200 1000 800 600 Average 400 Rainfall 200 0 1956 1959 1962 1965 1968 1971 1974 1977 1980 1983 1986 1989 1992 1995 1998 2001 2004 2007 Depth of rainfall , mm 1400 Time, year

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.

FREQUENCY ANALYSIS The return period for each rainfall was calculated using the following plotting position formula: Where: Tr: return period m: rank of event n: number of events

THIS FIGURE PLOTS THE RETURN PERIOD WITH RAINFALL FOR NABLUS STATION 1400 Annual Rainfall (mm) 1200 R 2 = 0. 8635 1000 800 600 400 200 0 0 10 20 30 40 Return Period, Tr (year) 50 60

Rainfall Runoff Models

SCS METHOD q/qp The dimensionless hydrograph and triangular unit hydrograph of the SCS Method are shown in the following figure. 1. 2 1 0. 8 0. 6 0. 4 0. 2 0 0 2 t/Tp 4 Dimensionless hydrograph 6

triangular unit hydrograph

THE FOLLOWING FIGURE SHOWS THE STREAM, CONTOURS AND CENTROID OF WADI ZEIMAR.

For Wadi Zeimar catchment, the following consideration and calculations can be done: Ø Ø The total area of Wadi Zeimar which is under study is about 172. 5 km². tr =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

Length of main stream (ft) Slope % Velocity ft/s 18766. 2 0. 874 1 16896. 1 0. 339 1 25918. 3 1. 58 1 34874. 9 0. 0577 1 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

By applying the calculation we get the following results t/Tb q/qb t (hr) q (m³/s. cm) 0. 0 0. 5 0. 43 8. 285 10. 825 1 1 16. 57 21. 65 2 0. 32 33. 14 6. 928 3 0. 07 49. 71 1. 515 4 0. 025 66. 28 0. 541 25 20 q (m 3/s) 15 10 5 0 0 10 20 30 40 50 60 70 t (hr) The SCS hydrograph for Wadi Zeimar catchment.

SNYDER'S METHOD Applying the Snyder method to the catchment of the Wadi Zeimar, taking in consideration the following assumptions: Ø The total area of Wadi Zeimar under consideration is about 172. 5 km². Ø For the mountain drainage area the coefficient Ct is 1. 2, and n is 0. 38. Ø For the mountain drainage area and for the minimum Ct the coefficient Cp is 0. 7 Ø For one hour unit hydrograph tr = 1. The following table shows the area, slope, main length and mean length for the catchment Area (mi²) 107. 2 Slope (%) 2. 794 Main length, Mean length, (mi) 18. 27 10. 92

= 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.

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. Tp = 3 * tp = 3 * 7. 3 = 21. 9 hr Area Slope Main Mean Tp (mi²) (%) length (hr) (mi) 18. 27 10. 92 107. 2 2. 794 21. 9 t’p (hr) Q’p (m³/s) 7. 3 186. 3 W 50 (hr) 5. 4 W 75 (hr) 3. 08

RECOMMENDATION Ø Ø Ø 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.

Thanks for your attention