Journal of Geographic Information System, 2011, 3, 153-159
doi:10.4236/jgis.2011.32012 Published Online April 2011 (http://www.SciRP.org/journal/jgis)
Developing GIS-Based Unit Hydrographs for Flood
Management in Makkah Metropolitan Area, Saudi Arabia
Gomaa M. Dawod1,2, Nabeel A. Koshak3
1Survey Research Institute, Giza, Egypt
2Umm Al-Qura University, Makkah, Saudi Arabia
3Center of Research Excellence in Hajj and Omrah, Umm Al-Qura University, Makkah, Saudi Arabia
Received March 6, 2011; revised March 31, 2011; accepted April 2, 2011
Abstract
Unit hydrographs (UH) are either determined from gauged data or derived using empirically-based synthetic
unit hydrograph procedures. In Saudi Arabia, the discharge records may not be available either for several
locations or for long time scales, and therefore synthetic unit hydrographs are crucial in flood and water re-
sources management. Available metrological, geological, and land use datasets have been utilized in order to
apply the US National Resources Conservative Services (NRCS) methodology in a Geographic Information
Systems (GIS) environment. Furthermore, NRCS unit hydrographs have been developed for six watersheds
within Makkah metropolitan area, southwest Saudi Arabia. The accomplished results show that the UH time
to peak discharge vary from 1.15 hours to 4.47 hours, and the UH peak discharge quantities range from
10.14 m3/s to 16.74 m3/s. It is concluded that the third basin in Makkah city may be considered as the most
hazardous catchment. Hence, it is recommended that careful flood protection procedures should be taken in
this area within Makkah city.
Keywords: GIS, Unit Hydrograph, NRCS, Flood Management, Saudi Arabia
1. Introduction
Flood modeling usually involves approximate descrip-
tions of the rainfall-runoff transformation processes, based
on empirical, or physically-based, or combined descrip-
tions of the physical processes involved. The resulting
models are quite useful in practice since they are simple
and provide adequate estimates of flood hydrographs.
Sherman [1] first proposed the unit hydrograph (UH)
concept. The UH of a watershed is defined as the direct
runoff hydrograph resulting from a unit volume o f excess
rainfall of constant intensity and uniformly distributed
over the drainage area. The UH approach is applied for
several engineering designs and environmental studies
[2,3] Detailed descriptions of UH typ es and formulas can
be found in several literatures [4,5]. Moreover, the Geo-
graphic Information Systems (GIS) technology has been
applied for flood management based on utilizing unit
hydrographs [6-8].
2. Objectives
The current research study aims to:
 Utilize the unite hydrograph approach for flood
characterizing in Makkah city, Saudi Arabia.
 Compute the requited quantities of unit hydro-
graphs for six hydrological basins within the study
area.
 Apply the US National Resources Conservative
Service (NRCS) methodology to develop unit hy-
drographs, for the first time in Saudi Arabia.
 Perform flood assessment computations within a
GIS environment as a precise, effective, and fast
technological tool.
3. Previous Works
Sets of observations of effective rainfall and direct runo ff
are required for the derivation of unit hydrographs. When
no direct observations are available, or when UH’s for
other locations on the stream in the same watershed or
for nearby watersheds of similar characteristics are re-
quired, Synthetic, or conceptual, Unit Hydrograph (SUH)
procedures must be used [9]. SUH procedures can be
categorized as [10]: 1) those based on models of water-
shed storage; 2) those relating hydrograph characteristics

G. M. DAWOD ET AL. 154
to watershed characteristics [11]; and 3) those based on a
dimensi onl ess uni t hy drograph [12,13].
In Kingdom of Saudi Arabia (KSA) the Snyder UH is
the most common method in a variety of geomorpho-
logic and flood literatures [14,15]. However, a crucial
issue in this model is the existence of two parameters
(namely: Ct a coefficient represents variations in water-
shed slopes and storage characteristics; and Cp a coeffi-
cient represents the effects of retention and storage) that
need to be determined from actual observations for the
specific watershed or can be taken from some other wa-
tersheds that have similar topographic and morphometric
characteristics. Hence, empirical approach is suggested
as an alternative for constructing a dimensionless UH for
ungauged basins in southwest region of KSA [16,17].
The Soil Conservative Service (SCS) method is seldom
utilized, particularly in few academic studies in KSA
[18].
4. Materials and Methods
4.1. NRCS Hydrographs’ Method
The National Resources Conservative Service (NRCS),
formally SCS, UH approach represents an optimum di-
mensionless UH method, that are extensively utilized in
the last few years in several countries [19-22]. The NRCS
dimensionless UH was developed based on an extensive
analysis of measured data for a large number of actual
watersheds and then made dimensionless by dividing all
discharge ordinates by the peak discharge and the time
ordinates by the time to peak. The time base of the di-
mensionless UH was approximately 5 times the time to
peak, and approximately 3/8 of the total volume occurred
before the time to peak; the inflection point on the reces-
sion limb occurs at approximately 1.7 times the time to
peak, and the UH has a curvilinear shape [23]. The dis-
charge ratios for selected values of the time ratios are
given in Table 1.
The mathematical formulas of the requited parameters,
to construct the NRCS UH for watersheds, are:
qp = qu A Q (1)
where,
qp = peak discharge (m3/s)
A = drainage area (km2)
Q = depth of runoff (m m)
qu = unit peak discharge (m3/s/km2/mm) that can be
interpolated from a specific charts (e.g. NRCS, 1986) or
computed from corresponding tables [23, pp. 5-28].
The time of concentration, tc, is the time needed for a
drop of water to move from the most distant point in the
watershed to the design point downstream (taken as the
basin outlet in the current study). There are numerous
Table 1. Some ratios for NRCS dimensionless UH.
Time Ratio
t/Tp Discharge Ratio q/qp Time Ratio t/Tp Discharge Ratio q/qp
0 0.000 0.9 0.990
0.1 0.030 1.0 1.000
0.2 0.100 1.5 0.680
0.3 0.190 2.0 0.280
0.4 0.310 3.0 0.055
0.5 0.470 3.6 0.021
0.6 0.660 4.0 0.011
0.7 0.820 4.5 0.005
0.8 0.930 5.0 0.000
After [23, p. 6-59].
empirical equations to calculate tc, such as: Jaton for-
mula [24], kirpich formula [25], and kirpich/Ramser
formula [8]. However, the NRCS formula for time of
concentratio n [18] is given by [26, pp. 3-9] :

0.7
0.8 0.5
1.671 1900*tcL SSL




 


(2)
where,
tc = concentration time (minutes),
L = length of basin main stream (feet)
SL = average watershed land slope in percentage.

2
0.2 0.8QP SP S (3)
where,
Q = depth of direct run o ff (m m)
P = depth of precipitation for a specific return period
(mm)
S = maximum potential retention (mm):




25.41000 CN10S (4)
where CN is the curve number, a coefficient determined
based on geological, soil, and land use properties for
each basin.
4.2. Materials
Makkah city is located in the south-west part of KSA,
about 80 Km east of the Red Sea (Figure 1). It extends
from 39˚35'E to 40˚02'E, and from 21˚09'N to 21˚37'N.
The area of the metropolitan region (the study area)
equals 1593 square kilometers approximately. The to-
pography of Makkah is complex in nature, and several
mountainous areas exist inside its metropolitan area. The
winter is considered as the main rainy season in Saudi
Arabia. The annual rain ove Makkah city, for a period r

G. M. DAWOD ET AL. 155
Figure 1. Study area.
extends from 1966 to 200 9, varies from 3.8 mm to 318.5
mm, with an average of rainfall equals 101.2 mm. Due to
the complexity of Makkah’s topography, flash floods
occurs periodically with significant variations in magni-
tude [27]. The rain intensity in a single extreme storm
may exceed the annual rain average in that year.
Several datasets have been collected for the cause of
flood assessment. The main data set, of the current study,
is a Digital Elevation Model (DEM) for the study area.
The acquired DEM produced by the by King Abdulaziz
City of Sciences and Technology (KACST) with a spa-
tial resolution equals 5 meters. A window covers Mak-
kah metropolitan area has been provided through the
Center of Excellence in Hajj and Omrah, Umm Al-Qura
university. Mirza et al. [28] confirm that that national
DEM is 3 times more accurate than published global
DEMs (ASTER and SRTM 3). The other collected data-
sets include digital geological, soil, and land uses maps
of the study area. The Arc GIS v.10 software has been
utilized, in the current study, to delineates the main
catchments in Makkah based on the available DEM. Six
main basins are identified those area ranged from 74.3 to
360.6 square kilometers, and lengths of their main streams
vary from 16.50 to 48.55 kilometers (Figure 2). Tabl e 2
presents statistics of some accomplished hydrological
parameters of these catchments.
5. Results
Dawod et al. [29] computed CN, runoff depth, peak dis-
charge, and time of concentration for the six basins in
Makkah metropolitan area. In that study, a GIS-based
methodology has been developed for quantifying and
spatially mapping the flood characteristics. The core of
that approach is integrating several topographic, metro-
logical, geological, and land use datasets in a GIS envi-
ronment that utilizes the NRCS method of flood model-
ling for ungauged arid catchments. The computations
have performed using the depth of precipitation (P)
equals 200 mm for a return period of 50 years. Addition-
ally, the calculations of flood quantities, such as depth
and volume of runoff (Equations 3 and 4), were per-
formed in the attribute tables of GIS layers, in order to
assemble all results in the same environment. Table 3
presents these quantities.
The NRCS UH methodology, as described above, has

G. M. DAWOD ET AL. 156
Figure 2. Catchments and their main streams in Makkah.
Table 2. Statistics of morphometric quantities.
Item C1 C2 C3 C4 C5 C6
Basin Area (km2) 252.7 122.3 74.3 109.9 360.6 200.2
Basin Premier (km) 134.6 69.13 50.23 89.09 134.76 102.03
Length of Main Stream (km) 42.48 23.64 16.50 29.70 48.55 38.13
Table 3. NRCS-based hydrological results in the study area.
Item C1 C2 C3 C4 C5 C6
CN 84 84 93 89 84 83
Time of concentration (hours) 5.69 3.76 1.73 2.63 6.72 4.17
Runoff depth (mm) 152 152 179 167 152 149
Peak discharge (m3/s) 1554 1063 1307 1234 4489 1514
been applied, for the first time in Saudi Arabia, for the
six watersheds of Makkah metropolitan area. The ac-
complished results are presented in Table 4 and depicted
in Figure 3. It can be seen that the elapsed time from
rainfall start to peak discharge vary from 1.15 hours (in
catchment C3) to 4.47 hours (for catchment 6). Secondly,
it has been found that the UH peak discharge quantities
range from 10.14 m3/s (for catchment 2) to 16.74 m3/s

G. M. DAWOD ET AL.157
Figure 3. NRCS Unit Hydrographs of Makkah’ Catchments.
(for catchment 5). Furthermore, the total runoff time va-
ries from 5.75 hours (for catchment 3) to 22.34 hours
(for catchment 5). The same results can be visualized
graphically from Figure 3.
Moreover, a correlation analysis has been performed
between the main morphometric and NRCS UH parame-
ters of the six basins. The results (Table 5) showed that
the basin area, with a positive correlation equaling to
0.74, is the most effective element that influences the UH
peak discharges. In addition, the time of concentration
also resulted in moderate positive correlation values, 0.55.
6. Discussion
Results in Table 4 indicate that the smallest elapsed time
is 1.15 hours (in catchment C3). Recall from Table 3,
Figure 4. Geology and land use of Makkah’ c atc hments.

G. M. DAWOD ET AL. 158
Table 4. NRCS UH quantities for Makkah’ catchments.
Item C1 C2 C3 C4 C5 C6
Time to Peak (hours) 3.78 2.50 1.15 1.75 4.47 2.78
UH Peak discharge (m3/s) 13.86 10.14 13.40 13.04 16.74 14.96
Total Time (hours) 18.92 12.51 5.75 8.74 22.34 13.88
Table 5. Correlation between main morphometric and UH
parameters.
Item UH Peak
Discharge Basin area Time of concentration
UH Peak Discharge 1
Basin area 0.74 1
Time of
concentration 0.55 0.97 1
this catchment has the higher CN value, since it mainly
constitutes of residential areas (Figure 4) which have the
least permeability property. Hence, it can be concluded
that the third basin in Makkah city may be considered as
the most hazardous catchment. Also, the total runoff time
for this basin reaches 5.75 hours, which is another evi-
dent that the third basin in Makkah city may be consid-
ered as the most hazardous catchment. The same conclu-
sions can be drawn from the inspection of Figure 3,
where the UH of this particular basin has the least
time-to-peak and total runoff time. That leads to the fact
that there is no enough time, in case of floods, for both
the residents and the governmental authorities to evacu-
ate people or apply precaution procedures. Hence, care-
ful flood protection policies should be taken in this area
within Makkah city. Moreover, it has been found that the
maximum UH peak discharge equals 16.74 m3/s (for
catchment 5). Although this catchment produces the
highest peak discharge, its time to peak is relatively large
(4.47 hours), which might gives suitable enough time for
residents to get ready and receive some governmental
assistance.
7. Conclusions
Unit hydrographs graphically represent the direct runoff
resulted from a unit volume of excess rainfall of constant
intensity and uniformly distributed over the drainage area.
Out of several methods of unit hydrographs develop-
ments, the NRCS represent an optimum approach for
ungauged watersheds, since it incorporates several data
types of the area of interest. The current research study
has utilized a high-resolution DEM in order to apply the
NRCS approach for flood assessment. NRCS- based unit
hydrographs have been developed for basins within
Makkah metropolitan area, southwest of Saudi Arabia.
The attained results show that the time to peak discharge
vary from 1.15 hours to 4.47 hours, and the UH peak
discharge quantities range from 10.14 m3/s to 16.74 m3/s.
It is concluded that the third basin in Makkah city may
be considered as the most hazardous catchment, since it
has the least UH time-to-peak and total runoff time.
Hence, it is recommended that careful flood protection
procedures should be taken in this area within Makkah
city.
8. Acknowledgements
The authors would like to acknowledge the financial
support offered by the Center of Research Excellence in
Hajj and Omrah, Um Al-Qura university, Saudi Arabia.
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