RAINFALL-RUNOFF MODELINGOF WADI ZIMAR 

Prepared by

Ahmad Tallal Abu-HamedMohamed Nemir Mohsen

Osama Omar Nazzal 

Under the direction ofDr. Sameer Shadeed

PRESENTATION OUTLINE

Introduction Study Area Literature review Analysis of Rainfall Data Rainfall Runoff Models

Introduction

GENERAL

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

The main objective of this project is to study the storm water drainage of Wadi Zimar.

derive the unit hydrograph of catchment.

DATA 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.

STUDY AREA

GENERAL

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.

0 10 Kilometers

Zeimar CatchmentWest Bank Outline

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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.

Literature review

INTRODUCTION

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 rainfall-runoff are exist.

RAINFALL-RUNOFF MODELING METHODS

SCS METHOD

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.

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.

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.

the time of concentration in hours

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

The time of peak discharge Tp, in hour

Where: Tr: duration of effective rainfallThe peak discharge, m³/s.cm

SNYDER'S METHOD

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:

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.

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,W50 , W75 are also considered

W50: width of unit hydrograph at 50 % of peak discharge in hour.

W75: 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

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 periodm: rank of eventn: number of events

THIS FIGURE PLOTS THE RETURN PERIOD WITH RAINFALL FOR NABLUS STATION

Rainfall Runoff Models

SCS METHOD

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

Dimensionless hydrograph

triangular unit hydrograph

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

#Y

#Y CentroidContoursStreamsZeimar Outline

0 5 10 15 Kilometers

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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.

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

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

The SCS hydrograph for Wadi Zeimar catchment.

SNYDER'S METHODApplying 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 Ct is 1.2, and n is 0.38.

For the mountain drainage area and for the minimum Ct the coefficient Cp is 0.7

For one hour unit hydrograph tr = 1.

The following table shows the area, slope, main length and mean length for the catchment

= 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

RECOMMENDATION

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