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Journal of Geographic Information System, 2017, 9, 717-751
http://www.scirp.org/journal/jgis
ISSN Online: 2151-1969
ISSN Print: 2151-1950
DOI:
10.4236/jgis.2017.96045 Dec. 28, 2017 717 Journal of Geograph
ic Information System
Assessment of Flash-Flood Hazard in
Arid Watersheds of Jordan
Yahya Farhan1, Atef Ayed2
1Retired Professor, Amman, Jordan
2Geography Department, University of Jordan, Amman, Jordan
Abstract
Flash flood hazard initiated by heavy rainstorms is co
mmon in arid Jordan,
and often has induced immense damage to life and infrastructures. The cu
r-
rent study presents a flash flood assessment for Wadi Rajil (northern Jordan)
and Wadi Wuheida (southern Jordan)
watersheds using ASTER DEM, GIS,
and geomorphic f
ield observation. A total of 23 morphometric parameters of
paramount relation to flash flood risk estimation were extracted and calc
u-
lated using ASTER DEM, GIS, and mathematical formulae developed for this
purpose. Two methods were employed to assess flash
floods and to generate
flooding risk susceptibility maps. The first method is El-
Shamy’s approach,
and the second is the morphometric hazard degree assessment method. Co
n-
sequently, sub-basins with high and extreme flooding susceptibility were d
e-
marcated a
nd displayed spatially using GIS. The maps so produced are meant
to help planners and decision makers to devise appropriate plans to mitigate
harmful flooding impacts, and to deal with flooding hazards.
Keywords
Flash Flood, ASTER DEM, Morphometry, Arid Watersheds,
El-Shamy’s Approach, Jordan
1. Introduction
Morphometric analyses of drainage basins based on geoinformatics techniques
and field collected data, are essential tools for flash floods hazard assessment.
Relevant evaluation parameters can be employed to predict the hydrological be-
havior of the catchment, and geomorphic processes produced by exceptionally
heavy rainstorms, and the resultant flash floods, including erosion type, rate, and
sediment yield [1]. Hydromorphic explanation can be refined regarding the
How to cite this paper:
Farhan, Y. and
Ayed
, A. (2017) Assessment of Flash-
Flood
Hazard in Arid Watersheds of Jordan.
Journal of Geographic Information System
,
9
, 717-751.
https
://doi.org/10.4236/jgis.2017.96045
Received:
November 19, 2017
Accepted:
December 25, 2017
Published:
December 28, 2017
Copyright © 201
7 by authors and
Scientific
Research Publishing Inc.
This work is licensed under the Creative
Commons Attribution Int
ernational
License (CC BY
4.0).
http://creativecommons.org/licenses/by/4.0/
Open Access
Y. Farhan, A. Ayed
DOI:
10.4236/jgis.2017.96045 718 Journal of Geographic In
formation System
conditions of flash floods generation, their destructive impacts, initiation of
landslides and gully erosion, and the provision of abundant sediment supply.
When field data are incorporated with remote sensing and GIS, it will provide
precise information on morphometric properties of sub-basins and flash floods,
and can be employed to assess areas prone to flash flooding, estimating flooding
risk, and delineating the most susceptibility zones for flooding hazard. Rapid
generated flows are frequently assigned to “flash floods” which are defined as
sudden floods with high peak discharges, produced by severe thunder storms
that are generally of limited areal extent” [2] [3] [4]. Although flood hazards are
most common in humid regions, they are equally characteristic events in arid
and semiarid regions in the form of flash floods [5]. Climatic conditions gene-
rating flash floods are common in arid Jordan, and they are initiated due to
heavy rainstorms accompanied by high amount of precipitation falling in a short
period of time. Thunder storms which produce flash floods are a localized
process that takes place over the inland watersheds ranging in area from few ki-
lometers to several hundred square km. Flashfloods in arid Jordan are extremely
dangerous and of disruptive nature. They frequently cause extensive property
damage, loss of life, and slope instability due to severe gully erosion and slump-
ing. Other factors encouraging flash floods in Jordan are the morphological and
ecological setting of arid watersheds (
i.e.
, the presence of long steep slopes, drai-
nage networks, poor vegetation cover, low or negligible infiltration, and high
velocity flows). In addition, human interventions render arid watershed highly
vulnerable to flash flooding [6]. Past experience in arid coastal cities such as
Aqaba and Eilat on the Gulf of Aqaba [7] [8] [9] [10] [11], Gulf of Suez, Red Sea
and Sinai Peninsula [12] [13] [14] reveals that flash floods are classified as the
most recurrent disaster recorded over the last three decades in the Middle East
including Jordan. Available records of flash floods indicate that arid cities such
as Aqaba, Ma’an, and Wadi Musa-Petra have been exposed to several flash
floods of low magnitude (5 - 7 year return periods), medium magnitude (20 - 25
year return periods), and high magnitude (50 year return periods) [15] [16] [17]
[18] [19] and caused immense damage to life and infrastructure. Moreover, un-
predictable flash floods occasionally have caused serious problems to road net-
works located in mountain valleys (
i.e.
, the Aqaba-Amman highway, Wadi Yu-
tum, 15 km north of Aqaba), or on the desert plains (
i.e.
, the Amman-Aqaba
highway west of Ma’an, the lower Wadi Wuheida), or the roads in Qa’a Azraq
(lower WadiRajil). As an alternative to costly prolonged field monitoring of wa-
tersheds to assess flood hazards, remote sensing and GIS techniques combined
with hydro-morphometric analysis of watersheds provide a rapid, low cost and
efficient tool to map and determine areas vulnerable to flood hazards and level
of risk. Based on available free access ASTER and SRTM DEM’s of reasonable
resolution, it is possible to extract and calculate morphometric basic, linear,
areal, shape, and relief parameters [20]-[26]. In the present research, two arid
watersheds: Wadi Rajil (northern Jordan) and Wadi Wuheida (southern Jordan)
were investigated using topographic, geological maps, air photos, field inspec-
Y. Farhan, A. Ayed
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tion, and ASTER DEM. Morphometric characterization and description of the
hydrological pattern were conducted for both watersheds. The aim is also to
generate flood hazards susceptibility maps based on El-Shamy’s approach [27],
and the morphometric hazard degree assessment method [28] [29] [30], in order
to predict and demarcate the most hazardous sub-basins in terms of flooding,
and the sub-basins which are expected to endanger Ma’an city, the Am-
man-Aqaba highway and Qa’a Azraq. The flood hazard maps produced were in-
tended to help the planners understand the spatial distribution of flood hazard
conditions, thus enabling them to prepare appropriate mitigation measures to
reduce the negative impacts of flash floods. This research is also meant to im-
prove the level of awareness among planners regarding flood geomorphology
[31], and the essential role of morphometric analysis/information in the plan-
ning process [32] [33] [34] [35]. Moreover, attention is focused on providing
advice for future planning on how to avoid destructive flood hazards in remote
and data scarce areas. Appropriate measures for planning adjustment towards
flooding hazards and management are recommended to help decision makers
mitigate flash floods through the construction of flood prevention structures
(
i.e.
, surface water harvesting systems, and groundwater artificial recharge). The
recommended measures are flexible to the extent that these can be carried out by
local administrative officials and the residents.
2. Study Area
Wadi Wuheida is an ephemeral stream which covers an area of 245 km2 (Figure
1). It is located 35˚26' to 35˚41'E; and 30˚00' to 31˚14'N, and extends between
Figure 1. The study watersheds.
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the crest line (rim), runs across Ra En Naqb escarpment and Sharah mountains
in the west to the bridge of Ma’an on the Aqaba-Amman highway in the east,
and discharges in El-Jafr depression. Wadi Wuheida and its tributaries drain
part of Ras En Naqb escarpment (west of Ras En Naqb town), and part of Sharah
mountains at a maximum average elevation of approximately 1719 m (a.s.l). The
watershed rolls through steep to moderate and gentle-flat slopes across carbo-
nate rocks of Upper Cretaceous age [36] towards El-Jafr depression in the east at
an elevation of 1115 m (a.s.l) (Figure 2(a)). The watershed has a mushroom-like
shape. The upper catchment is elongated north-south with an axis of 23 km,
where as the long axis of the wadi is about 27 km and extendsin a west-east di-
rection. A prominent slope variation exists across the catchment (0˚ -
(a)
(b)
Figure 2. DEM of W. Wuheida (a) and DEM of W. Rajil (b).
Y. Farhan, A. Ayed
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Journal of Geographic Information System
to >25˚). Higher slope categories dominated the upper reaches (15˚ - 20˚, 20˚ -
25˚, and >25˚). Whereas, slope categories of 0˚ - 5˚, 5˚ - 10˚ and 1 - 15˚ dom-
inated the lower catchment (Figure 3(a)). The north-south elongation of the
upper watershed locates it within the track of depressions which run either from
west to east (frontal storms) or from southwest to northeast (Red Sea trough
storms). The mushroom shape of the upper catchment comprises 56% of the to-
tal area of the watershed. Thus, it provides a larger headwater area, which con-
sists of highlands of elevations > 1400 m and receives a higher annual rainfall
approaching 160 mm. In this regard, topographic characteristics of the wa-
tershed combined with the higher rainfall and slopes encourage high runoff po-
tential. The catchment is classified as sixth order basin and of dendritic drainage
pattern (Figure 4(a)). The Wuheida watershed-Ma’an-and El Jafr area is part of
the Arabian Surface (the Oligocene peneplain) which has developed in the Eo-
cene sediments [37].
(a)
(b)
Figure 3. Drainage and stream order of W. Wuheida (a) and W. Rajil (b).
Y. Farhan, A. Ayed
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(a)
(b)
Figure 4. Slope categories of W. Wuheida (a) and W. Rajil (b).
Since early Miocene, and through Pliocene and Pleistocene, tectonic distur-
bances have affected the western and southwestern rims of these surfaces, thus
giving rise to Ras En Naqb escarpment and Sharah mountains; uplifting of the
eastern shoulder of the Rift; and basalt flows (Miocene to Pleistocene) to the
south of El Jafr depression and the north eastern part of the Central Plateau in-
cluding parts of Wadi Rajil catchment. Remnants of the Arabian Surface are still
standing around Ma’an (including lower W. Wuheida), where a flat gentle stone
Hamada surface formed and consists of wind-eroded chert residue, the
flint-strewn Hamada desert”. The B2/A7 aquifer of Upper Cretaceous rocks
constitutes the main aquifer in the catchment, and secure recharge at the head-
water within the Asharah mountains and Ras En Naqb escarpment. Towards
El-Jafr depression to the east, it becomes confined. The B2 (the Amman forma-
tion) consists of limestone with chert inter bedded with phosphatic layers and
marls. Further, A7 (the Wadi Sir formation) is comprised of hard crystalline do-
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lomitic limestone, chalky limestone with chert bands and nodules. Um Rijam-
chert limestone formation (B4) aquifer consists of limestone and chalky limes-
tone [36] [38]. The B4 aquifer receives limited water recharge during flash
flooding [39]. The climate of Wadi Wuheida watershed is arid (BW). Rainfall
amounts vary from 120 - 160 mm in the upper part, to 40 mm in Ma’an. Sum-
mer temperatures are hot and dry and winters are cold and relatively wet. Preci-
pitation over the catchment is subjected to remarkable fluctuations. Relatively
high precipitation in the upper catchment is affected by the orographic factor of
Ras En Naqb and the Sharah Mts. Convective thunder storms which frequently
occur in the fall (October-November) and spring (March-April) are influenced
by the effect of the Red Sea trough and result in heavy rainfall events [40]. Sever-
al south-deepening Mediterranean depressions may affect southern Jordan from
December to February. Snow may fall at high elevations (
i.e.
, Ras En Naqb and
Asharah highlands), and freezing temperatures were recorded in January and
February. Average monthly temperatures range from 30˚C (summer maximum)
to about 7˚ (winter minimum). Absolute average yearly maximum is approach-
ing 43˚C, and absolute yearly minimum may reach-8˚C [41]. The low amount of
rainfall results in poor vegetation cover, comprising of scattered acacia, tamarisk
and cenapod, and some annual grasses. The land use/land cover is restricted to
scattered and small patches of olive trees and woods in the highlands, and li-
mited irrigated farming in El-Jafr and Ma’an.
Wadi Rajil watershed is located between 31˚45' and 32˚36'N and 36˚45' and
37˚43'E (Figure 1). The catchment has an area of 3085.87 km2 with 7.6% (236.469
km2) of the total surface located in Syrian territory. The wadi is considered one of
the major wadis entering the Azraq depression from the north. Wadi Rajil takes a
NW-SE direction in the source area, then the direction changes from NE to SW,
until it is terminated in the Azraq depression. The general shape of Wadi Rajil is
trapezoidal, with its long axes generally oriented NE-SW direction. Terrain eleva-
tion ranges from 1553 m a.s.l at Salkhad in Syria, to 511 ma.s.l in Qa’a Azraq
(Figure 2(b)). The entire Wadi Rajil watershed was found to be of seventh order
(Figure 3(b)). Topographically, most of the catchment is nearly flat (0˚ - 2˚), and
undulating ( - 10˚) (Figure 4(b)). Wadi Rajil is part of the limestone plateau in
east Jordan. 86.2% (2770.8 km2) of the total catchment area is covered by six basalt
flows, tuff, and volcanic eruption that originated mainly in Jebel el Druz in Syria
from the early Miocene to historical time.
The youngest deposits exist in the watershed, the gravel and sand are of fluvial
origin. These deposits cover 3.07% of the total catchment area, while the out-
cropping formations of Eocene age [42] are: the Rijam and Wadi Shallala forma-
tion in the central and eastern part of the catchment. It occupies 10.7% of the
total catchment area. 16 fourth-order sub basins were demarcated in W. Rajil
(Figure 5(b)).
Heavy thunder storms occur in April and May, causing major floods and in-
undation in the lower catchment and Qa’a Azraq. Flooded water remains in the
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formation System
(a)
(b)
Figure 5. Fourth-order sub-bsins for W. Wuheida (a) and W. Rajil (b).
Qa’a for a few months before evaporation. The surface water potential of Wa-
di Rajil can be utilized for groundwater artificial recharge, where the calculated
flood volumes are: 0.95, 16.5, 36.9, and 61.5 MCM for 10, 25, 50, and 100 year
return periods [43]. Such large amounts of water encourage surface water har-
vesting and groundwater recharge, thus, protecting Azraq town and the irrigated
farms from flooding. Deep long trenches refilled with available basalt boulders
and gravel have been suggested for artificial recharge in Wadi Rajil and for other
comparable watersheds in the northern Badia of Jordan. Annual rainfall ranges
between 50 mm (Azraq town) to 300 mm close to Salkhad in Syria. The average
annual rainfall over Wadi Rajil is about 127 mm. Due to aridity, large seasonal
and diurnal variation in temperature exist from a maximum 45˚ C in August to
−6˚C in January.
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3. Materials and Methods
3.1. Data Source and Methods
Quantitative morphometric analysis of Wadi Rajil and Wadi Wuheida water-
sheds was performed using toposheets (1:50,000, and 20 m contour interval),
ASTER DEM and Arc GIS (10.1) software packages. ASTER DEM is provided
on line cost free to all researchers, and is available in Geo TiFF format and of 30
m spatial resolution. Recent investigation conducted on the Dead Sea (Jordan) to
test the validity of ASTER DEM was compared with SRTM (the Shuttle Radar
Topographic Mission) DEM, and referenced DEM constructed from a 1:25,000
scale topographic map [44]. It was concluded that the overall accuracy of the
ASTER DEM is in line with the reported official accuracy specification [45]. The
prepared topographic sheets were scanned, georeferenced, and converted to
WGS1984, zone 36 N projection system using Arc GIS and the associated pack-
ages.
The two watersheds and the boundaries of the 21 fourth-order sub-basins
were delineated initially using the topographic sheets. The ASTER DEM (v.2)
was employed to demarcate watersheds and sub-basins boundaries, drainage
networks using Arc Hydro tool (Figure 5). A total of 23 morphometric parame-
ters were extracted and calculated using ASTER DEM, Arc GIS software, and the
mathematical equations displayed in Table 1. The results of computation are il-
lustrated in Tables 2-4. Different topographic features such as aspect, slope, and
elevation were generated using the Spatial Analyst module. Basic parameters in-
clude: basin area (
A
) basin order (
Nu
), perimeter (
P
), basin length (
Lb
), number
of streams (
Nu
), stream length (
Lu
), mean stream length (
Lsm
), length of main
channel (
Lm
). Derived parameters are: bifurcation ratio (
Rb
), weighted mean bi-
furcation ratio (WMRb), stream length ratio (
RL
), sinuosity (
SI
), basin shapein-
dex (
Ish
), length of overland flow (
Lo
), RHO coefficient (
ρ
), stream frequency
(
Fs
), drainage density (
Dd
), drainage texture (
Dt
), basin relief (
Bh
), slope index
(
SIn
%), relief ratio (
Rr
), ruggedness number (
Rn
). Whereas shape parameters are:
elongation ratio (
Re
), circularity ratio (
Rc
), and form factor ratio (
Rf
). The stream
ordering for the entire Wadi Rajil and Wadi Wuheida was executed according to
[24]. Thus, Wadi Rajil catchment was found to be of seventh-order, while Wadi
Wuheida is found of sixth-order. Interpretation of parameter values was carried
out according to the methods elaborated by [20], Strahler [21] [23] [24], Miller
[46], and Schumm [25]. Efficient tools of GIS are capable of generating flood
hazard, flood susceptibility, and sediment hazard maps, which illustrate
flood-prone sub-basins. Risk and vulnerability maps help planners to assess the
potential impact of floods [5], and to delimit the appropriate sites for future de-
velopment; and to avoid sites exposed to flooding and sediment discharge. Based
on the analysis of drainage, GIS afford devices which help not only to determine
areas affected by floods, but to predict sites that are likely to be flooded in the
future. Consequently, appropriate measures can be provided by planners to mi-
nimize the negative impact of floods and flooding effects.
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Table 1. Computation of basic, derived, and shape morphometric parameters.
Morphometric Parameters
Formula/Definition
References
Basic Parameters
1 Basin Area (
A
) (Km2) GIS Software Analysis Schumm (1956)
2 Basin Perimeter (
P
) (Km) GIS Software Analysis Schumm (1956)
3 Basin Length (
Lb
) GIS Software Analysis Schumm (1956)
4 No. of streams (
Nu
) Hierarchical Rank Strahler (1952)
5 Stream Length (
Lu
) (Km)
12un
L LL L=+ ++
Strahler (1964)
6 Mean Stream Length (
Lsm
) (Km)
sm u u
L LN=
(km) Horton (1945)
Derived Parameters
7 Bifurcation ratio (
Rb
)
1b uu
R NN
+
=
Strahler (1964)
8 Weighted mean bifurcation ratio (
WMRb
)
( ) ( )
}
{
11
bb uu
WRu u NRNNM + +=∑∑
Strahler (1953)
9 Stream Length ratio (
RL
)
RL
=
Lu
/
Lu
-1. Strahler (1964)
10 Sinuosity (
SI
)
SI
=
Lm
/
Lb
Gregory and Walling (1973) [69]
11 Basin shape index (
Ish
%)
2
% 1.27
sh b
I AL= ∗
Haggett (1965) [70]
12 Length of overland flow (
Lo
)
Lo
= 1/2
Dd
Horton (1945)
13 RHO coefficient (
ρ
)
ρ
=
RL
/
Rb
Horton (1945)
14 Stream frequency (
Fs
)
Fs
=
Nu
/
A
Horton (1932)
15 Drainage density (
Dd
) Km/Km2
Dd
=
Lu
/
A
Horton (1932)
16 Drainage texture (
Dt
)
Dt
=
Nu
/
P
Horton (1945)
17 Basin relief (
Bh
) m
Bh
=
H
h
Strahler (1952)
18 Slope index (
SIn
%)
SIn
% = (E/0.75
Lm
) × 100
(
E
= E85 − E10), E85 and E10 are the
elevation of points at 85% and 10% of
the main channel from its mouth).
(
Lm
= The length of main channel
from its mouth to water divide).
Majure and Soenksen (1991) [71]
19 Relief ratio (
Rr
)
Rr
=
H
/
Lb
Schumm (1956)
20 Ruggedness number (
Rn
)
Rn
= D × (
Bh
/1000) Melton (1957)
Shape Parameters
21 Elongation ratio (
Re
)
1.128
be
RAL=
Schumm (1956)
22 Circularity ratio (
Rc
)
Rc
=
A
/
P
2 Miller (1953)
23 Form factor ratio (
Rf
)
2
fb
R AL=
Horton (1932)
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Table 2. Morphometric characteristics of W. Wuheida.
Par. no.
Morphometric Parameters
Stream Order
I
II
III
IV
V
VI
I
Basic Parameters
1 Basin Area (
A
) (Km2) 244.97
2 Basin Perimeter (
P
) (Km) 135.33
3 Basin Length (
Lb
) 30.06
4 No. of streams (
Nu
) 490 387 76 19 5 2 1
5 Stream Length (
Lu
) (Km) 386.69 218.62 115.01 53.06 35.23 14.22 17.41
6 Mean Stream Length (
Lsm
) (Km) 2.98 0.56 1.51 2.79 7.05 7.11 17.41
II
Derived Parameters
II/I
III/II
IV/III
V/IV
VI/V
7 Bifurcation ratio (
Rb
)
5.09 4 3.8 2.5 2
8 Weighted mean bifurcation ratio (
WMRb
) 3.8
II/I
III/II
IV/III
V/IV
VI/V
9 Stream Length ratio (
RL
) 0.55
0.53 0.46 0.66 0.40 1.22
10 Sinuosity (
SI
) 0.58
11 Basin shape index (
Ish
) 0.34
12 Length of overland flow (
Lo
) 0.79
13 RHO coefficient (
ρ
) 0.16
14 Stream frequency (
Fs
) 2.00
15 Drainage density (
Dd
) Km/Km2 1.58
16 Drainage texture (
Dt
) 3.62
17 Basin relief (
Bh
) m 604
18 Slope index (
SIn
%) 0.08
19 Relief ratio (
Rr
) 20.10
20 Ruggedness number (
Rn
) 0.95
III
Shape Parameters
21 Elongation ratio (
Re
) 0.59
22 Circularity ratio (
Rc
) 0.17
23 Form factor ratio (
Rf
) 0.27
Table 3. Morphometric characteristics of W. Rajil.
Par.
no.
Morphometric Parameter
Stream Order
I
II
III
IV
V
VI
VII
I
Basic Parameter
1 Basin Area (
A
) (Km2) 3085.87
2 Basin Perimeter (
P
) (Km) 608.61
3 Basin Length (
Lb
) 126.73
4 No. of streams (
Nu
) 1994 1563 331 75 17 6 2 1
5 Stream Length (
Lu
) (Km) 3004.4 1697.6 869.74 437.06 254.59 136.78 31.86 45.37
6 Mean Stream Length (
Lsm
) (Km) 6.13 1.09 2.63 5.83 14.98 22.80 15.93 45.37
II
Derived Parameters
II/I
III/II
IV/III
V/IV
VI/V
V/VII
7 Bifurcation ratio (
Rb
)
4.72 4.4 4.4 2.8 3 2
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Continued
8
Weighted mean bifurcation ratio (
WMRb
)
3.88
II/I
III/II
IV/III
V/IV
VI/V
V/VII
9 Stream Length ratio (
RL
) 0.53
0.51 0.50 0.58 0.54 0.23 1.42
10
Sinuosity (
S
I
)
0.25
11
Basin shape index (
Ish
)
0.24
12 Length of overland flow (
L
o
) 0.49
13 RHO coefficient (
ρ
) 0.14
14
Stream frequency (
F
s
)
0.65
15
Drainage density (
Dd
) Km/Km2
0.97
16 Drainage texture (
D
t
) 3.28
17 Basin relief (
Bh
) m 604
18
Slope index (
S
In
%)
0.02
19
Relief ratio (
Rr
)
4.77
20 Ruggedness number (
R
n
) 0.59
III
Shape Parameters
21
Elongation ratio (
R
e
)
0.49
22
Circularity ratio (
Rc
)
0.10
23 Form factor ratio (
R
f
) 0.19
Table 4. Morphometric characteristics of the 21 sub-watersheds.
Morphometric parameter
Sub-
basin
Basin
Rf
Rc
Re
Rn
Rr SIn
%
Bh
Dt Dd
Fs ρ Lo
ISH
SI
RL
WMRb
Rb Lsm
Lu Nu
Lb P A
0.27
0.09
0.58
1.00
25.98
1.26 843
0.88
1.18
0.64
0.18
0.59
0.34
0.22
1.00 4.65 5.47
17.91
331.61
179
32.45
203.37
280.33
1
Wadi Rajil
0.30
0.07
0.62
0.89
39.59
1.05 790
0.48
1.13
0.60
0.34
0.56
0.38
0.35
1.45 4.55 4.24
11.71
134.11
71 19.96
148.41
119.12
2
0.28
0.15
0.59
0.31
10.51
0.20 290
1.08
1.08
0.68
0.14
0.53
0.35
0.26
0.69 4.48 4.88
9.04 225.30
144
27.60
133.01
210.83
3
0.30
0.17
0.62
0.57
25.22
0.04 498
0.68
1.15
0.54
0.14
0.57
0.38
0.36
0.57 5.35 4.06
6.59 134.20
63 19.74
93.19 116.91
4
0.29
0.15
0.61
0.37
14.48
0.04 320
0.87
1.15
0.67
0.10
0.57
0.37
0.32
0.49 4.99 4.92
5.73 163.41
95 22.11
108.90
142.64
5
0.29
0.17
0.61
0.39
14.87
0.19 333
0.76
1.17
0.53
0.16
0.58
0.37
0.32
0.64 4.34 3.96
7.64 170.44
78 22.39
103.18
145.92
6
0.32
0.11
0.64
0.56
29.34
0.11 434
0.45
1.29
0.58
0.17
0.64
0.41
0.48
0.60 4.75 3.44
4.86 90.56 41 14.79
90.23 70.30 7
0.29
0.13
0.60
0.24
8.61 0.06 204
0.85
1.16
0.65
0.14
0.58
0.36
0.30
0.59 4.17 4.34
6.51 186.90
105
23.69
123.50
161.06
8
0.27
0.15
0.59
0.42
13.07
0.22 376
0.93
1.11
0.57
0.12
0.56
0.35
0.25
0.58 4.79 4.70
9.11 252.71
130
28.77
139.58
226.85
9
0.33
0.24
0.65
0.17
11.68
0.01 148
0.74
1.15
0.73
0.15
0.58
0.42
0.56
0.47 3.89 3.21
2.65 61.76 39 12.67
52.93 53.57 10
0.32
0.11
0.64
0.22
16.35
0.23 248
0.53
0.90
0.65
0.42
0.45
0.41
0.47
1.42 4.14 3.39
4.39 66.05 48 15.17
90.73 73.51 11
0.32
0.12
0.64
0.32
18.34
0.16 261
0.61
1.22
0.76
0.24
0.61
0.41
0.50
0.84 3.87 3.57
6.26 79.93 50 14.23
81.77 65.70 12
0.33
0.28
0.65
0.21
14.61
0.01 185
0.94
1.14
0.86
0.12
0.57
0.42
0.56
0.50 3.59 4.21
1.36 60.72 46 12.66
49.15 53.47 13
0.32
0.15
0.64
0.21
12.52
0.03 187
0.64
1.11
0.70
0.14
0.55
0.41
0.47
0.52 5.18 3.74
3.65 79.35 50 14.94
78.13 71.55 14
0.36
0.32
0.68
0.16
9.55 0.01 89 0.54
1.77
0.61
0.23
0.89
0.46
0.76
0.60 3.30 2.61
3.62 55.34 19 9.32 35.11 31.20 15
0.33
0.25
0.65
0.11
7.04 0.04 91 0.85
1.19
0.81
0.40
0.60
0.42
0.55
1.37 4.25 3.40
3.69 66.23 45 12.93
52.73 55.52 16
0.37
0.23
0.68
0.75
47.59
0.20 413
1.38
1.81
1.93
0.21
0.91
0.46
0.82
0.74 4.09 3.48
2.78 49.89 53 8.68 38.40 27.50 1
Wadi Wuheida
0.37
0.15
0.69
0.68
46.85
0.18 385
0.93
1.77
1.68
0.15
0.89
0.47
0.26
0.52 4.89 3.50
2.34 44.28 42 8.22 45.32 24.98 2
0.35
0.29
0.67
0.64
33.16
0.27 346
1.63
1.85
1.74
0.15
0.92
0.44
0.60
0.58 5.23 3.87
2.69 70.23 66 10.43
40.44 38.03 3
0.38
0.18
0.69
0.65
42.35
0.14 319
1.08
2.04
1.96
1.12
1.02
0.48
1.95
3.94 4.89 3.50
4.43 43.64 42 7.53 39.06 21.43 4
0.33
0.25
0.65
0.61
25.04
0.15 336
2.47
1.83
2.28
0.13
0.92
0.42
0.85
0.61 4.62 4.74
3.99 108.37
135
13.42
54.72 59.22 5
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3.2. Flash Flood Analysis Methods
Quantitative analysis of drainage basin morphometry using remote sensing and
GIS techniques has been employed recently to assess the probability of flash
flood hazard. Various methods were employed in this regard. The most common
is the morphometric assessment method which comprises:
1) El-Shamy’s approach [27]
2) The morphometric ranking method [47]
3) Wahid’s flash flood rating method [48] and the
4) Morphometric hazard degree assessment method [28] [29] [30]
5) Morphometric analysis of drainage basin and curve number (CN) method
to evaluate flash flood hazard [3] [49].
In the present research, two different morphometric analysis methods have
been adopted to evaluate the hazard degree of flash floods, and to generate
flooding susceptibility maps. These methods are: El-Shamy’s approach for as-
sessing flooding hazard probability [5] [27]. The second method is the mor-
phometric hazard degree assessment” [28] [29] [30] [50]. The second method is
a semi-quantitative measure which determined flash flood hazard based on ele-
ven morphometric parameters.
El-Shamy [27] employed three morphometric parameters to assess the hazard
potential of flash floods for different sub-basins. These are: drainage density
(
Dd
), stream frequency (
Fs
), and bifurcation ratio (
Rb
). He elaborated two dif-
ferent morphometric relationships to determine the flood hazard degree for a
catchment or sub-basin; bifurcation ratio (
Rb
) versus drainage density (
Dd
), and
bifurcation ratio versus stream frequency (
Fs
). The empirical diagram estab-
lished by El-Shamy [27] was divided into three zones. The first zone (
A
) is cha-
racterized by high susceptibility for flash flooding and low possibility for
groundwater recharge. The second zone (
B
) is characterized by moderate sus-
ceptibility for flash floods, and moderate possibility for groundwater recharge.
The third zone (
C
) is characterized by low susceptibility for flash floods and high
possibility for groundwater recharge. The data from the diagram represent bi-
furcation ratio (
Rb
) vs. stream frequency (
Fs
), and the diagram illustrates that
bifurcation ratio (
Rb
) vs. drainage density (
Dd
) are used to determine the overall
hazard degree. If a sub-basin is plotted in zone B of the first diagram (moderate
susceptibility of flash floods), and located in zone C of the second diagram (low
susceptibility for flash floods), the overall hazard degree for this sub-basin will
be low possibility for flash floods which represents the more conservative” situ-
ation [5].
The morphometric hazard degree assessment method employed eleven mor-
phometric parameters with a direct effect on flash floods [30]. Eight parameters
are characterized with a direct proportional relationship with the degree of risk,
whereas three parameters have an inverse proportional relationship with the de-
gree of risk (Table 5). A hazard scale number was designed, starting with (1)
representing the lowest hazard up to (5) indicating the highest hazard, and has
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Table 5. Effective morphometric parameters based on the kind of relationship with the
degree of risk.
Relationship with hazard
Direct proportional relationship Inverse proportional relationship
1) Area (
A
)
2) Drainage density (
Dd
)
3) Stream frequency (
Fs
)
4) Basin shape index (
Sf
)
5) Relief ratio (
Rr
)
6) Ruggedness number (
Rn
)
7) Slope index (
SIn
%)
8) Drainage texture ratio (
Tt
)
1) Mean bifurcation ratio (
Rb
)
2) Length overland flow (
Lo
)
3) Sinuosity (
SI
)
been assigned to all parameters. The distributions of the hazard degrees for the
sub-basins of Wadi Rajil and Wadi Wuheida have been calculated in order to:
1) Determine the minimum and maximum values for each morphometric pa-
rameter for all sub-basins of Wadi Rajil and Wadi Wuheida,
2) Assess the actual hazard degree for all the parameters which are located
between the minimum and maximum values, depending on a test to extract the
empirical relation between the relative hazard degree of a sub-basin with respect
to flash floods and the morphometric parameters, and
3) The equal spacing or simple linear interpolation between data points pro-
cedure was chosen [51].
For parameters which show a direct proportional relationship to the degree of
risk, the hazard degree was calculated using the following equation [28] [29]:
( )
min
max min
4
Hazard deg ee 1r XX
XX
+
=
(1)
Likewise, for parameters which show an inverse proportional relationship to
degree of risk, the hazard degree is calculated using the following equation:
()
max
min max
4
Hazard deg ee 1r XX
XX
+
=
(2)
where
X
is the value of the morphometric parameters to be assessed for the ha-
zard degree for each sub-basin,
X
min and
X
max are the minimum and maximum
values of the morphometric parameters of the two basins and all sub-basins re-
spectively. The summation of the hazard degree (1 + 2) for each sub-basin
represents the final flood hazard of these sub-basins.
An integration of the results was carried out using the morphometric hazard
degree assessment method, and El-Shamy’s model. Such a procedure was per-
formed through superimposition of flood risk maps generated through the ap-
plication of the two estimation methods. The product enables us to recognize the
common sub-basins falling under each category of flooding risk; and to identify
the most risk-prone sub-basins with reference to flash flooding. It also, allows
the possibility to demarcate the most vulnerable sub-basins against flooding.
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4. Morphometric Analysis of Watersheds
Quantitative analysis was conducted for W. Rajil and W. Wuheida catchments
and the 21 fourth-order sub-catchments connected to both watersheds, in order
to evaluate the morphometric properties of the drainage networks, based on
twenty-five morphometric descriptors classified as: basic, derived, and shape
parameters. These variables are utilized to characterize the two arid watersheds,
and to improve our understanding of arid watershed development in relation to
essential controlling factors,
i.e.
, geology (lithology and structure), tectonic and
geomorphic processes, and rejuvenation. The results of morphometric analysis
for the two watersheds (Table 2 and Table 3) and the 21 sub-basins are illu-
strated in Table 4. The drainage pattern is dendritic to sub-dendritic and
sub-trellis in the southern part of W. Wuheida, whereas, it is trellis to sub-trellis
in the southwestern and northeastern parts of W. Rajil, then changing to
sub-dendritic in the northwestern part of the watershed. Referring to the ratio
between basin area (
A
), and perimeter (
P
), the ratio is found 5.071:1 for W. Rajil,
and 1.810:1 for W. Wuheida. Such figures indicate that the borderline of W. Ra-
jil is a highly irregular water divide compared to W. Wuheida which is a slightly
irregular water divide. The noticeable difference in the ratios between (
A
) and
(P), has been verified by a striking differences in stream frequency (
Fs
) and
drainage density (
Dd
) values. Although the number of streams and the area (
A
)
of W. Wuheida are smaller compared to W. Rajil, the drainage network of W.
Wuheida is more developed and integrated. Conversely, 86.2% of Wadi Rajil is
covered by six basalt flows ranging in age from Oligocene to Pleistocene [52];
thus, the drainage network of W. Rajil was impeded from becoming integrated
due to the progression of volcanic eruptions.
4.1. Basic Morphometric Parameters
The basic morphometric parameters computed for W. Rajil and W. Wuheida
and the 21 sub-basins comprising basin area (
A
), basin perimeter (
P
), basin
length (
Lb
), stream length (
Lu
), mean stream length (
Lsm
), and length of main
channel (
Lm
).
4.1.1. Basin Area (A), Basin Length (Lb), and Basin Perimeter (P)
The area of a drainage basin is an essential morphometric parameter for hydro-
logical data processing, analysis and interpretation. Larger basins and sub-basins
with high local relief generally have greater discharge, thus directly influencing
runoff and peaks magnitude. For this reason, basin area is an important compo-
nent in hydrological processes [53]. In this regard, Chorley
et al.
[54] argued that
the maximum discharge of flood per unit area is inversely related to the area of
the drainage basin. The total drainage area for W. Rajil is 3085.87 km2, and for
W. Wuheida is 245 km2; for the 21 sub-basins, it varies from 21.4 to 280.33 km2.
The basin length represents the maximum length of the watershed and
sub-watersheds measured parallel to the main drainage line. The length of Wadi
Rajil basin is 126.73 km and Wadi Wuheida 30 km, while the lengths of the
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sub-basins range from 7.53 to 28.77 km. The perimeter of W. Rajil is 608.61 km,
and W. Wuheida is 135.33 km. The perimeters of the sub-basins vary from 35.11
to 203.37 km (Table 4). Sub-watershed no.15 of W. Rajil, represents the shortest
one, whereas, sub-basinno.10 of the same watershed has the longest perimeter.
Equally, sub-watershed no.1 is the longest, but with the highest perimeter. With
reference to the area, sub-basin no. 1 of W. Rajil is the largest, and sub-basin no.
4 of W. Wuheida is the smallest.
4.1.2. Stream Order (u) and Stream Number (Nu)
Stream ordering has been assigned based on the number and type of tributary
junction. The two watersheds and the sub-basins were ranked according to the
stream ordering system developed by Strahler [24]. Stream order represents a
preparatory indicator of stream size, drainage area and discharge. The total
number of streams in W. Rajil watershed is 1995, and the first order streams ac-
count for 78.3% of the total stream numbers. The total number of streams in W.
Wuheida is 490, and the first order streams constitute 79% of the total streams.
The details of stream characteristics validate Horton’s [20] first law, or, the law
of stream number,” which stated that the number of steams of different order in
a given drainage basin tends to closely approximate an inverse geometric ratio
[55]. W. Rajil consists of 16 sub-basins which are assigned as fourth-order ba-
sins, and W. Wuheida is composed of 5 sub-basins of the same order. Applica-
tion of Strahler’s ordering procedure through Arc GIS (10.1) reveals that W. Ra-
jil is classified as seventh order; and W. Wuheida is sixth order.
4.1.3. Stream Length (Lu) and Mean Stream Length (Lsm)
Stream length is calculated from the mouth of stream to drainage divide.
Lu
is a
dimensional parameter utilized to understand the characteristics of the drainage
network elements and its contributing basin surfaces [24].
Lu
is an important va-
riable employed to examine the hydrological characteristics of the drainage ba-
sin,
i.e.
, surface runoff and the hydrological properties of the underlying bedrock
such as permeability. Streams of short length generally are characteristic of ter-
rain with greater slopes and highly dissected with very fine textures. Longer
stream lengths are often developed where the bedrocks are permeable. Details of
stream length characteristics of W. Rajil and W. Wuheida support Horton’s [20]
“law of stream length” which states that “the average length of streams of each of
the different orders in a drainage basin tends closely to approximate a direct
geometric ratio”. Variation in order and size of the tributary basins are largely
attributed to morphological (relief and slope), structural, tectonic and geomor-
phic evolution of the drainage basin. The total stream length is generally high in
the first order streams and decreases as the stream order increases. The total
stream length of W. Rajil is 3004.4 km, and for W. Wuheida is 386.7 km, and the
first order streams account for 56.5% of the total stream length of both water-
sheds. Mean stream length is calculated by dividing the total stream length of
order (
u
) and number of stream segments of the same order (
u
). The mean
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stream length for W. Rajil varies from 1.09 km for the first order streams to
45.37 km for the seventh-order stream (Table 3), whereas the mean stream
length for W. Wuheida ranges from 0.56 km for the first order streams to 17.41
km for the sixth-order stream (Table 2). The
Lsm
value for any given order is
greater than that of the lower order and less than that of its next higher order.
For the 21 sub-basins, the
Lsm
values vary from 0.56 for the first-order streams,
to 22.8 for the fourth-order streams.
4.2. Derived Morphometric Parameters
4.2.1. Bifurcation Ratio (Rb) and Weighted Mean
Bifurcation Ratio (WMRb)
The bifurcation ratio
(Rb
) refers to the ratio of the number of streams of a given
order (
Nu
) to the number of streams in the next higher orde (
Nu
+ 1). Horton
[20] elaborated
Rb
parameter as a morphological index of relief and dissection.
For flat or rolling catchments, the
Rb
value is 2, while the value for dissected
catchments approaches 4. To achieve a more illustrative bifurcation value,
Strahler [56] employed a weighted mean bifurcation ratio (
WMRb
) calculated
according to the following equation:
( ) ( )
}
{
11
bb uu
WRuu NRNNM + +=
∑∑
Later, Shumm [25] adopted this method to determine the mean bifurcation
ratio (
Rb
) of a drainage basin at Perth Amboy, New Jersey, where the value is
found to be 4.87.
Rb
values vary from 2 to 4.72, and the weighted mean bifurca-
tion ratio (
WMRb
) is 4.6 for W. Rajil (Table 3). Likewise, the
Rb
values range
from 2 to 5, and the
WMRb
value is 3.8 for W. Wuheida (Table 3 and Table 4).
Higher values of
Rb
and
WMRb
for W. Rajil indicate that the wadi is largely af-
fected by progressive tectonic activity during the eruption of successive basalt
flows. Whereas W. Wuheida experienced less structural distortions, and mainly
affects the water divide area at Ras En Naqb. The trend of
Rb
values in W. Rajil
from the second-order stream to seventh-order stream is relatively irregular due
to the tectonic activity mentioned above. By contrast, the trend of
Rb
values in
W. Wuheida decreases regularly from the second-order streams to the
sixth-order stream due to the presence of homogeneous surface rock strata in
Ras En Naqb in the south, and El-Jafr depression in the north.
4.2.2. Stream Length Ratio (RL) and Sinuosity (SI)
Stream length ratio (
RL
) refers to the ratio between the individual lengths of
steam in a given order and the total length of streams in the next order [20].
RL
parameter reveals a significant relationship with surface flow discharge and the
erosional stage and geomorphic development of the drainage basin [53] [57]. As
a result of morphological variation (
i.e.
, slope and relief) over a watershed,
RL
values vary considerably. The
RL
value for W. Rajil is 0.3, and for W. Wuheida is
0.55, whereas
RL
values for the 21 sub-basins vary from 0.51 for the first-order
streams to 0.66 for the fourth-order streams. Sinuosity (
SI
) is defined as the ratio
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of the maximum length of the main channel (
Lm
) to basin length (
Lb
) or,
I mb
S LL
=
Generally,
SI
values range from 1 to 4. Streams having
SI
value of 1.5 are de-
scribed as sinuous. When
SI
values exceed 1.5, streams are described as mean-
dering [58]. Sinuosity (
SI
) is considered a significant morphometric parameter
which helps in interpreting the geomorphic evolution of a watershed.
4.2.3. Basin Shape Index (Ish) and Slope Index (SIn%)
The basin slope index (
Ish
) describes the relation between the basin area, and the
length of the drainage basin. W. Rajil and W. Wuheida attain basin shape index
values of 0.24 and 0.34 respectively. These values reveal an elongated drainage
basin; thus a reasonable chance is available for groundwater recharge. Slope in-
dex (
SIn
%), or the slope of main channel of a given basin is considered to be of
hydrological significance [59]. Steep channels occasionally have high surface
runoff values, and low infiltration rates, which in turn accelerate soil erosion.
Therefore, sediment load production tends to be high in arid watersheds, where
slopes are overgrazed and barren [11] [60]. Slope index (
SIn
%) is also an indica-
tor for the channel slope from which an assessment of the runoff volume can be
estimated [30]. The two arid watersheds under consideration are characterized
by medium to high relief, where the slope index (
SIn
%)values are 0.02 for W. Ra-
jil, and 0.08 for W. Wuheida.
4.2.4. Length of Overland Flow (Lo) and RHO Coefficient (ρ)
RHO coefficient (
ρ
) is defined as the ratio between the length ratio (
RL
) and the
bifurcation ratio (
Rb
) [20].
ρ
parameter is influenced by physical (geological,
geomorphic, climatic and biologic) and anthropogenic factors [61]. The rela-
tionship between drainage density (
Dd
) and the geomorphic evolution of a drai-
nage basin is determined by the RHO parameter. Consequently it helps to assess
the storage capacity of the drainage network [20]. A high RHO value of a wa-
tershed is indicative of a high hydric storage during flooding; therefore, the ero-
sion effect is decreased during the raised discharge [59]. The RHO values for W.
Rajil and W. Wuheida are 0.14 and 0.16 respectively (Table 2 and Table 3).
Whereas RHO values for the 21 sub-basins range from 0.10 to 0.42 (Table 4).
Length of overland flow (
Lo
) is defined as the length of water over the ground
before it gets concentrated into stream channels or permanent drainage channels
[62]. The shorter the length of overland flow the quicker surface runoff will en-
ter the stream. In the current study, the length of overland flow (
Lo
) for W. Rajil
is 0.49 and for W. Wuheida is 0.79. Such values indicate that
Lo
for W. Rajil is
shorter than W. Wuheida, due to variation in slope, lithology, land cover, rain-
fall intensity and infiltration capacity [53]. The slope index value for W. Rajil is
0.02 which is noticeably less than the slope index value of W. Wuheida (0.08).
These values denote that surface water concentration in W. Wuheida is faster
compared with W. Rajil. The larger value of average
Lo
is close to half the aver-
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age of the distance between the stream channels. Thus, it is equal to approx-
imately half of the
Dd
value [20]. The long-term evolution of a drainage basin
and landforms development is also affected by Lo parameter. The
Lo
values for
the 21 sub-basins vary from 0.45 to 1.02 (Table 4).
4.2.5. Stream Frequency (Fs), Drainage Density (Dd), and
Drainage Texture (Dt)
Stream frequency (
Fs
) refers to the ratio of the total number of streams (
Nu
) of
all orders in a basin and the basin area [20]. Stream frequency is affected by li-
thology and underlying materials; therefore, it is an expression of drainage tex-
ture of watersheds and sub-basins. The values of
Fs
are positively correlated with
Dd
values of the catchment. Consequently any increase in stream segments is
associated with that of drainage density [63]. High
Fs
values indicate low im-
permeability and low infiltration rate of surface water; thus, increased runoff is
noted. Such hydrological conditions make the watersheds and the sub-watershed
more prone to flooding, surface erosion, and landslide activity. The
Fs
value for
W. Rajil is relatively low (0.65), while
Fs
value for W. Wuheida is moderate (2.0)
which denotes that fractured and weathered basalt covered most of W Rajil
compared with more solid carbonate rocks exposed across W. Wuheida. The
values of
Fs
for the 21 sub-basins range from 0.53 to 2.28. Drainage density (
Dd
)
is defined as the total length of stream per unit area divided by the area of the
watershed [20].
Dd
value refers to the closeness of spacing of channels; therefore,
it is a quantitative measure for relief dissection, runoff potential, and thus, in
turn the drainage efficiency of watersheds. High
Dd
values denote high runoff
and low infiltration rate due to the presence of impermeable underlying mate-
rials, spare vegetation, and hilly relief. Conversely, low drainage density implies
low runoff, high infiltration and groundwater recharge. The achieved value of
Dd
for W. Rajil is 0.97 km / km2 (Table 3), and for W. Wuheida 1.58 (Table 2).
High
Dd
value for W. Wuheida implies a high potential runoff from large head-
water area over the Ras En Nagb highlands, and consequently high flooding po-
tential down the wadi [17].
Dd
values for the 21 sub-basins range from 0.90 to
2.04 (Table 4). Drainage texture (
Dt
) represents the total number of stream seg-
ments of all orders per unit perimeter of the basin [20]. Drainage texture is con-
trolled by: lithology, soil, relief, vegetation, infiltration-capacity and climate. Dt
reflects the relative channel spacing within a drainage network. According to
Smith [64],
Dt
tends to be coarse (2 - 4) in initial and early stages of the erosion
cycle, and fine (6 - 8) in maturity stage. Barren soft rocks (
i.e.
, Lisan marl or the
“Kata” in the Jordan Rift) produce a fine (6 - 8) and very fine (>8) texture al-
though it is developed under arid climate. Whereas massive limestone rocks in
northern Jordan produce a coarse texture though landforms were developed
under a dry-Mediterranean (semi-humid) climate. The
Dt
value for W. Rajil is
3.28, and for W. Wuheida 3.62. These values imply that the texture of both cat-
chments is coarse, while the
Dt
values for the 21 sub-basin range from 0.45 to
2.47.
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4.2.6. Basin Relief (Bh), Relief Ratio (Rr) and Ruggedness Number (Rn)
Basin relief (
Bh
) or total relief is the elevation difference between the highest and
lowest point in a given watershed [25].
Bh
parameter significantly controls
flooding patterns and sediment production by controlling stream gradient. Basin
relief is a measure of the potential energy of drainage basins. Thus, it is essential
to understand the denudational status of a catchment, drainage network devel-
opment, overland flow, through flow, and the fluvial-erosional conditions of a
catchment. Increase in
Bh
value caused high surface runoff, and low infiltration;
thus increasing surface erosion and sediment production. The basin relief value
of both W. Rajil and W. Wuheida is 604 m, and for the 21 sub-basins it varies
from 89 m to 843 m. High soil erosion loss and sediment production, and high
flooding potential is predictable for both watersheds. Schumm [25] elaborated
relief ration (
Rr
) as a dimensionless height-length ratio between the basin relief
(
Bh
) and basin length (
Lb
). The
Rr
variable allows comparison of the relative re-
lief of any catchment regardless of the differences in scale of topography.
Rr
val-
ues normally increase with decreasing catchment areas. For example in the W.
Wala catchment, southern Jordan, the
Rr
value is 15.1, whereas
Rr
values for 23
fourth-order sub-basins vary from 0.5 to 76.1. The basin relief values range from
96 m to 459 m [65]. Small relief ratio reveals the dominance of gentle slopes (3˚ -
5˚) and the subdued topography across the watershed.
Rr
ratio and other relief
morphometric parameters generally indicate the level of basin energy, erosion
potential of processes operating over a catchment, and sediment transport effi-
ciency [61]. The relief ratio for W. Rajil is 4.77, for W. Wuheida is 20.1, and for
the 21 sub-basins it ranges from 7.04 to 47.59. Ruggedness number (
Rn
) is a di-
mensionless parameter representing the product of basin relief (
Bh
) and the
drainage density [21] [24]. The
Rn
parameter has been elaborated to measure the
flash flood potential of a drainage basin [66] and to illustrate the geometric cha-
racteristics of drainage basins [67]. High values of
Rn
obtain when both relief
and drainage density are large. Present analysis shows that
Rn
for W. Rajil is
0.59, and for W. Wuheida is 0.95. Watersheds having high
Rn
values (>0.5) are
highly susceptible to an increase in peak discharge, high soil erosion rates, and
high sediment load production [55].
4.3. Shape Morphometric Parameters
4.3.1. Elongation Ratio (Re)
Elongation ratio (
Re
) is defined as the ratio between the diameter of a circle hav-
ing the same area as the basin (
A
), and the basin length (
Lb
) [25]. Strahler [24]
reported that
Re
ratio vary from 0.6 and 1.0 for a wide range of geomorphic en-
vironment. Values close to 1.0 are characteristic of watersheds with low relief.
Whereas values in the range 0.6 - 0.8 are typical for catchments with high relief
and steep slopes. Low values of
Re
(<0.5) imply that drainage basins are more
elongated, and at the youth-age stage of geomorphic evolution. When
Re
values
approach 1.0, the shape of the drainage basin approaches a circle [25]. A circular
basin suggests an early mature-age stage of geomorphic evolution [53], and is
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more efficient in runoff discharge than is an elongated one [68]. The
Re
value of
W. Rajil is 0.49 (Table 3), and for W. Wuheida is 0.59 (Table 2), which imply
that these watersheds are more elongated, and elongated, respectively.
4.3.2. Circularity Ratio (Rc)
Circularity ratio (
Rc
) parameter is indicative of basins shape, and the rate of in-
filtration including the time needed for excess water to reach the basin outlet.
Rc
refers to the ratio of the basin area (
A
) and the area of a circle with the same pe-
rimeter (
P
) as the basin [24]. Low, medium, and high values of
Rc
denote young,
mature, and old stages of the geomorphic cycle of the catchment. Drainage ba-
sins of different circularity ratio, ranging from 0.4 to 0.5, were considered by
Miller [46] as strongly elongated, with homogeneous bedrock and regolith mate-
rials, and a uniform rate of infiltration. Consequently, the excess runoff takes a
longer time to reach the basin outlet. More elongated or elongated shapes allow
drainage basins to be slow in disposing water, which resulted in a broad and
low-peaked hydrograph. Thus, reduction of water velocity can be attained
through the construction of water harvesting structures,
i.e.
, dams and surface
reservoirs [30]. The
Rc
value of W. Rajil is 0.10, and for W. Wuheida is 0.17, and
for the 21 sub-basins, it ranges from 0.07 to 0.32 (Table 4).
4.3.3. Form Factor Ratio (Rf)
The form factor parameter has been elaborated by Horton [20] to forecast the
flow intensity of a given watershed. It refers to the ratio between the area of ba-
sin (
A
) and the square of the basin length (
Lb
).
Rf
values vary from 0 for highly
elongated shape, to 1 for a perfect circular shape of the basin. Catchments with
low
Rf
value, tend to be elongated, which implies low peak flows for longer dura-
tion, and thus of less probability for the basin to flood. Furthermore, catchments
with high
Rf
values experience high peak flow of short duration, where the floods
will be stronger and have higher velocities associated with greater erosion and
transport capacities. The
Rf
value for W. Rajil is 0.19 (Table 3), and for W. Wu-
heida is 0.24 (Table 2). The
Rf
values for the 21 sub-basins range from 0.24 to
0.38. These values indicate that both watersheds and the related sub-basins are
more elongated and elongated in shape with low peak flows of longer duration.
5. Results and Discussion
Two approaches were employed to assess flash floods hazards and flooding risk for
two arid watersheds in Jordan. W. Rajil Watershed in the north which is developed
over the basalt “Harra”, and W. Wuheida in the south, developed in the “Hamada”
landscape. The potential hazard was analyzed, and sub-basins vulnerable to flood-
ing, and expected to cause heavy damages to the local inhabitants, their livelihood,
and infrastructure, were demarcated. Past experience reveals that Ma’an city and
the surroundings, the Amman-Aqaba highway, Azraq town and Qa’a Azraq were
exposed to recurrent severe floods of different magnitudes as a result of favorable
climatic conditions introduced by the Red Sea Trough (RST), or the frontal con-
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vective storms. In order to assess the hazardous effects of flash floods on the 16
forth-order sub-basins of W. Rajil, and five sub-basins related of W. Wuheida,
fourteen hydro-morphometric parameters were utilized in the analysis. The two
methods employed are: 1) El-Shamy’s approach [27] for flooding hazard assess-
ment (El-Shamy 1992) which is based on three morphometric parameters; and 2)
The morphometric hazard degree assessment method [28] [29] [30]. The second
method is considered a semi-quantitative measure determined on the bases of ele-
ven morphometric parameters.
5.1. Flash Flood Risk Assessment: El-Shamy’s Approach
Following El-Shamy’s approach, the relationship between the Bifurcation ratio
(
Rb
) versus Drainage density (
Dd
), and the bifurcation ratio versus Stream fre-
quency (
Fs
), morphometric data for the sub-basins of W. Rajil (termed 1 - 16)
and W. Wuheida (desighnated 1 - 5) were plotted in line with El-Shamy’s dia-
gram (Figure 6(a) and Figure 6(b)). With reference to W. Rajil, the estimation
(a)
(b)
Figure 6. Flooding susceptibility of W. Rajil based on El-Shamy’s approach,
Rb
vs.
Dd
(a)
and
Rb
vs.
Fs
(b).
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of flash flood risk based on the relation between
Rb
and
Dd
reveals that nine
sub-basins (4, 6, 7, 10, 11, 12, 14, 15, and 16) are located in Zone
(A
) and
represent high flooding susceptibility for flash floods. Three sub-basins (2, 8, and
13) are located in zone (B), which represent moderate susceptibility for flash
floods, and four sub-basins (1, 3, 5 and 9)are located in zone (
C
), which
represent low susceptibility for flash floods (Figure 6(a) and Table 6(a)). Addi-
tionally, and based on the relation between Bifurcation ratio versus Stream fre-
quency, seven sub-basins (7, 10, 11, 12, 14, 15 and 16) are located in zone (
A
),
which represents high susceptibility for flash floods, and five sub-basins (2, 4, 6,
8 and 13) are located in zone (B), which represents moderate susceptibility for
Table 6. Hazard degree analysis for W. Rajil (a) ad W. Wuheida (b) (based on El-Shamy’s
approach 1992).
(a)
Sub-basin no.
Rb Fs
HD1
Dd
HD2 FHD
1 5.47 0.64 L 1.18 L L
2 4.24 0.60 M 1.13 M H
3 4.88 0.68 L 1.07 L L
4 4.06 0.54 M 1.15 H H
5 4.92 0.67 L 1.15 L L
6 3.96 0.53 M 1.17 H H
7 3.44 0.58 H 1.29 H H
8 4.34 0.65 M 1.16 M M
9 4.70 0.57 L 1.11 L L
10 3.21 0.73 H 1.15 H H
11 3.39 0.73 H 1.01 H H
12 3.57 0.68 H 1.09 H H
13 4.21 0.86 M 1.14 M M
14 3.74 0.70 H 1.11 H H
15 2.61 0.61 H 1.77 H H
16 3.40 0.81 H 1.19 H H
(b)
Sub-basin no.
Rb Fs
HD1
Dd
HD2 FHD
1 3.48 1.93 H 1.81 H H
2 3.50 1.68 H 1.77 H H
3 3.87 1.74 M 1.85 M M
4 3.50 1.96 H 2.04 H H
5 4.74 2.28 M 1.83 M M
Rb
= Bifurcation Ratio, Fs = Stream Frequency,
Dd
= Drainage Density, HD1 = Hazard degree
Rb
vs.
Fs
, HD2
= Hazard degree
Rb
vs.
Dd
, and FHD = Final hazard degree from HD1 and HD2. L: low susceptibility for
flash floods; M: moderate susceptibility for flash floods; and H: high susceptibility for flash floods.
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flash floods. Finally, four sub-basins (1, 3, 5, and 9) are located in zone (C)
which represents low susceptibility foe flash floods (Figure 6(b)). Through inte-
gration of the results achieved based on the relation between
Rb
vs.
Dd
and
Rb
vs.
Fs
, showed that sub-basins 1, 3, 5, and 9 are categorized under low possibility of
flash floods in both relations (Figure 7). While sub-basins 2, 8, and 13 are
grouped under moderate possibility of flash floods. Likewise, sub-basins 7, 10,
11, 12, 14, 15 and 16 are classified under high susceptibility for flash floods based
on both relations, except for sub-basins 6 and 4.
With respect to W. Wuheida and based on the relation between Bifurcation
ratio (
Rb
) vs. Drainage density (
Dd
), it is found that two sub-basins (3 and 5) are
located in zone (
B
) which represents moderate susceptibility for flash floods, and
three sub-basins (1, 2, and 4) are located in zone (A) which represents high pos-
sibility for flash floods (Figure 8(a), Table 6(b)). None of the sub-basins were
categorized in zone (
C
) of low flooding susceptibility for flash floods. Based on
the relation between Bifurcation ratio vs. stream frequency, three sub-basins (2,
3 and 5) are located in zone (
B)
which represents moderate susceptibility for
flash floods, and sub-basins (1 and 4) are located in zone (
A
) which represents
high susceptibility for flash floods (Figure 8(b), Table 6(b)). Again through in-
tegration of the results based on the relation between
Rb
vs.
Dd
and
Rb
vs.
Fs
,
sub-basins 3 and 5 are grouped under the category of moderate susceptibility for
flash floods, based on both relations (Figure 9). In parallel, sub-basins 1, 2 and 4
are ranked under the category of high susceptibility for flash floods. None of the
sub-basins are ranked in the category of low susceptibility for flash floods
(Figure 9) It is evident that similar and consistent results were achieved regarding
Figure 7. Flooding susceptibility of W. Rajil based on El-Shamy’s approach (
Rb
vs.
Dd
layer superimposed on
Rb
vs.
Fs
layer).
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(a)
(b)
Figure 8. Flooding susceptibility of W. Wuheida based on El-Shamys approach,
Rb
vs.
Dd
(a) and
Rb
vs.
Fs
(b).
Figure 9. Flooding susceptibility of W. Wuheida based on El-Shamys approach (
Rb
vs.
Dd
) layer superimposed on (
Rb
vs.
Fs
) layer.
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flash flood susceptibility by use of El-Shamy’s approach as clarified through
sub-basins categorized under zone (
A
) and zone (
C
) for W. Rajil, and zone (
B
)
and zone (
A
) for W. Wuheida.
5.2. The Morphometric Hazard Degree for Flash Floods
Assessment Method
The Morphometric Hazard Degree assessment method was employed to per-
form the required morphometric analysis, to estimate the flash flood hazard and
the degree of risk for the sub-basins of W. Rajil and W. Wuheida, Table 7 and
Table 8 display the results of this method for both watersheds, as expressed by
ranking score for the different eleven morphometric parameters based on the
relation with hazard whether proportional or inverse. To compile the flood risk
map using GIS, the summation of hazard degree values for the sub-basins was
grouped into five categories of susceptibility for flash floods as follows:
1) Low flooding susceptibility 19 - 23.9
2) Moderate flooding susceptibility 24 - 27.9
3) High flooding susceptibility 28 - 31.9
4) Very high flooding susceptibility 32 - 35.9
5) Extreme flooding susceptibility 36 - 39.9
Table 7. Hazard degrees of the effective parameters of the studied 16sub-basins of W. Rajil.
Relation
with
hazard
The
effective
morphometric
parameters
The
minimum
and
maximum
values
of the
parameter
in all
sub-basins
The value of the morphometric parameters to be assessed for each sub-basin
Hazard degrees of the effective parameters (
X
)
X
min
X
max
Sub
Basin
1
Sub
Basin
2
Sub
Basin
3
Sub
Basin
4
Sub
Basin
5
Sub
Basin
6
Sub
Basin
7
Sub
Basin
8
Sub
Basin
9
Sub
Basin
10
Sub
Basin
11
Sub
Basin
12
Sub
Basin
13
Sub
Basin
14
Sub
Basin
15
Sub
Basin
16
Proportional
Basin area (
A
) (Km2) 24.98
226.85
2.87 1.00 4.68 2.82 3.33 3.40 1.90 3.70 5.00 0.43 1.96 1.81 1.56 0.92 0.12 0.61
Drainage dens ity (
D
) 0.90 1.29 3.92 3.33 2.75 3.56 3.54 3.77 5.00 3.69 3.21 3.61 1.00 4.26 3.44 2.16 8.99 3.02
Stream frequency (
Fs
) 0.53 0.86 2.28 1.75 2.82 1.05 2.61 1.00 1.60 2.44 1.47 3.38 2.45 3.78 5.00 2.02 0.91 3.39
Basin shape index (Ish) 0.34 0.46 1.00 2.42 1.45 2.45 2.11 2.07 3.38 1.90 1.34 3.90 3.51 3.30 3.91 2.35 4.00 2.83
Relief ratio (
Rr
) 7.04 39.59 3.33 5.00 1.43 3.23 1.91 1.96 3.74 1.19 1.74 1.57 2.14 2.39 1.93 0.67 0.31 0.00
Ruggedness number (
Rn
) 0.11 1.00 5.00 4.51 1.91 3.08 2.16 2.26 3.03 1.58 2.40 1.28 1.51 1.94 1.46 0.44 0.22 0.00
Slope index (
SIn
%) 0.01 1.26 5.00 4.33 1.63 1.11 1.10 1.59 1.34 1.19 1.68 1.00 1.70 1.50 1.00 0.64 0.00 0.11
Drainage texture (
Dt
) 0.45 1.08 3.71 1.15 5.00 2.41 3.66 2.92 1.00 3.52 4.04 2.80 1.48 2.00 4.07 1.18 0.55 2.54
Inverse Weighted mean
bifurcation ratio (
WMRb
) 3.30 5.35 2.37 2.57 2.70 1.00 1.70 2.97 2.17 3.30 2.09 3.85 3.36 3.88 4.43 1.33 5.00 3.15
Length of
overland flow (
Lo
) 0.45 0.89 3.70 3.96 4.22 3.86 3.87 3.77 3.22 3.80 4.02 3.84 5.00 3.55 2.92 4.04 1.00 3.65
Sinuosity (
SI
) 0.22 0.76 5.00 2.94 4.72 3.96 2.38 4.28 3.07 4.40 4.79 2.48 3.16 2.94 1.48 2.80 1.00 2.57
Summation of haz ard degree 36.39 31.79 31.32 29.10 27.73 27.96 29.63 28.37 30.47 26.76 25.94 29.40 27.86 19.16 22.11 20.86
Classification of the
sub-basins hazard de gree Extreme
hazard
High
hazard
High
hazard
High
hazard
Medium
hazard
Medium
hazard
High
hazard
High
hazard
High
hazard
Medium
hazard
Medium
hazard
High
hazard
Medium
hazard
Low
hazard
Low
hazard
Low
hazard
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Table 8. Hazard degrees of the effective parameters of the studied 5 sub-basins of W. Wuhida.
Relation
with hazard
The
effective The minimum
and maximum
values of the
parameter in all
sub-basins
The value of the morphometric parameters
to be assessed for each sub-basin
Morphometric parameters
Hazard degrees of the effective parameters (
X
)
X
min
X
max Sub
Basin 1
Sub
Basin 2
Sub
Basin 3
Sub
Basin 4
Sub
Basin 5
Proportional Basin area (
A
) (Km2)
21.43 59.22 1.64 1.38 2.76 1.00 5.00
Drainage density (
D
)
1.77 2.04 1.64 1.00 2.13 5.00 1.87
Stream frequency (
Fs
)
1.68 2.28 2.65 1.01 1.37 2.86 5.00
Basin shape index (
Ish
)
0.42 0.48 3.97 4.36 2.68 5.00 1.00
Relief ratio (
Rr
)
25.04 47.59 5.00 4.87 2.44 4.07 1.00
Ruggedness number (
Rn
)
0.61 0.75 5.00 3.01 1.72 2.03 1.00
Slope index (
SIn
%)
0.14 0.27 2.72 2.15 5.00 1.00 1.18
Drainage texture (
Dt
)
0.93 2.47 2.18 1.00 2.83 1.39 5.00
Inverse Weighted mean
bifurcation ratio (
WMRb
) 4.09 5.23 5.00 2.20 1.00 2.20 3.14
Length of overland flow (
Lo
)
0.89 1.02 4.36 5.00 3.87 1.00 4.13
Sinuosity (
SI
)
0.26 1.95 3.68 5.00 4.20 1.00 3.62
Summation of hazard degree
37.85 30.97 29.99 26.55 31.94
Classification of the sub-basins hazard degree
Extreme
hazard
High
hazard
High
hazard
Moderate
hazard
High
hazard
With respect to the sixteen sub-basins of W. Rajil watershed, sub-basins nos.
14, 15, and 16 (18.75% of the total) have the lowest overall values, therefore
representing the category of low flooding susceptibility (Table 7 and Figure 10).
Sub-basins nos. 13, 11, 10, 6, and 5 (37.5% of the total) have intermediate score
values, thus characterized by moderate flooding susceptibility. Conversely,
sub-basins nos. 12, 9, 8, 7, 4, 3, and 2 (43.75% of the total) have high overall
score values, and thus represent relatively dangerous sub-basins with high
flooding susceptibility. Additionally, sub-basin no. 1 has the highest overall score
value, thus, representing the most dangerous sub-basins with extreme flooding
susceptibility (Figure 10). None of the sub-basins of W. Rajil is classified as of
very high flooding susceptibility. It can be concluded that 50% of W. Rajil
sub-basins are expected to suffer high and extreme flooding susceptibility. It can
also be deduced that 81.25% of W. Rajil sub-basins are expected to experience
moderate, high, and extreme susceptibility to flooding. Such results reveal that
the main hazardous sub-basins are located on the northwest and eastern parts
of the watershed, and directly threaten the lower part leading to Qa’a Azraq,
Azraq town, wetland reserve, Azraq-Safawi and the Azraq-Qrayyat (to Saudi
Arabia), major roads seriously threatened by expected flooding. Thus, the
protection of the town, agricultural areas, and, the major roads from repetitive
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Figure 10. Sub-basins flooding hazard degree of W. Rajil based on the morphometric
hazard degree assessment method.
flooding is essential to maintain future development of the Azraq area as a tour-
ism and environmental conservation center.
With respect to the five sub-basins of W. Wuheida, the summation of the ha-
zard degree scores for the sub-basins of the watershed, was grouped into five
categories of susceptibility for flash floods such as the following:
1) Low flooding susceptibility 20 - 23.9
2) Moderate flooding susceptibility 24 - 27.9
3) High flooding susceptibility 28 - 31.9
4) Very high flooding susceptibility 32 - 35.9
5) Extreme flooding susceptibility 36 - 39.9
Accordingly, sub-basin no. 1 of W. Wuheida (20% of the total) has the highest
overall score values: it thus, represents extreme flooding susceptibility (Table 8
and Figure 11). Whereas sub-basins nos. 2, 3, and 5 (60% of the total) have high
score values and are thus, characterized by high flooding susceptibility. Alterna-
tively, sub-basin no. 4 (20% of the total) has an intermediate score value, hence it
represents moderate flooding susceptibility. It is obvious that 80% of Wadi Wu-
heida sub-basins are of extreme and high flooding susceptibility. Sub-basins nos.
1, 2, 3, and 5 are the most hazardous sub-basins, and are located within the
headwaters of W. Wuheida in a mushroom-like shape (Figure 11). This zone is
directly affected by the track of depressions which approach the catchment ei-
ther from the west to east (frontal storms) or form southwest to northeast (Red
Sea Trough storms). Furthermore, the mushroom shape provides a larger
headwater area (56% of the total watershed area), which comprises the highest
terrain in Ras En Naqb/Sharah highlands, with an average annual rainfall
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Figure 11. Sub-basins flooding hazard degree of W. Wuheida based on the morphome-
tric hazard degree assessment method.
approaching 160 mm, since this area receives the highest precipitation, and rec-
orded repetitive destructive floods associated with high slopes yielding high ru-
noff potential [17]. The extreme flood of W. Wuheida on 11 March 1966 (or
Ma’an flood), and the major flood of 2 - 3 February 2006 resulted in high peak
discharge, estimated at 540 m3/s for WadiYutum south of Ras En Naqb [7] [8],
and 270 to 320 m3/s for W. Wuheida, north of Ras En Naqb/Sharah, during the
2006 flood [17].
Consequently, Ma’an city, El-Jafr playa, local inhabitants in the upper reaches
of the watershed, and the Amman-Aqaba highway are seriously threatened by
destructive flash floods. For this reason, flood control in the upper catchment is
highly requisite to protect Ma’an city and other infrastructure facilities against
flooding in order to maintain future development of the Ma’an-Ras En Naqb
area.
6. Conclusions
In the present study, hydro-morphometric analysis, and GIS-based flood hazard
and flood susceptibility mapping were carried out to display flood-prone areas in
W. Rajil and W. Wuheida catchments. Flood risk analysis was conducted using
two morphometric analysis methods within a GIS environment. These methods
are 1): El-Shamy’s approach; and 2): the morphometric hazard degree for flash
flood assessment method. Sub-basins affected by floods of low, moderate, high,
very high, or extreme flooding susceptibility can be demarcated. Consistent re-
sults were achieved on flash flood susceptibility utilizing El-Shamy’s approach as
illustrated through sub-basins categorized under zone (
A
) and zone (
C
) for W.
Rajil; and zone (
A
) and zone (
B
) for W. Wuheida. In this regard, sub-basins nos.
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1, 3, 5 and 9 of W. Rajil are ranked under low susceptibility for flash floods based
on the relation between
Rb
vs.
Dd
, and
Rb
vs.
Fs
. Similarly, sub-basins nos. 7, 10,
11, 12, 14, 15, and 16 are categorized under high susceptibility for flash floods
based on both previous relations (except sub-basins no.6 and 4). Reasonable re-
sults were obtained for W. Wuheida. Sub-basin no. 1 was ranked under an ex-
treme susceptibility for flash floods, and sub-basin 4 was grouped under the class
of moderate susceptibility for flash floods based on both relations between
Rb
vs.
Dd
, and
Rb
vs.
Fs
. The superimposition of the two thematic layers of El-Shamy’s
approach,
Rb
vs.
Fs
layer, and
Rb
vs.
Fs
layer for both W. Rajil showed that
sub-basins 7, 10, 11, 12, 14, 15, and 16 (44% of the total) are grouped under high
possibility of flash flood on both relations. With reference to W. Wuheida,
sub-basins 1, 2, and 4 (75% of the total) are classified under the category of high
susceptibility for flash floods. None of the sub-basins are ranked in the category
of low susceptibility for flash floods. The results of the morphometric hazard
degree for flash floods assessment method indicate that 50% of W. Rajil
sub-basins are expected to suffer high and extreme flooding susceptibility. Thus,