京都大学防災研究所年報 第 59 号 B 平成 28 年 6 月
Annuals of Disas. Prev. Res. Inst., Kyoto Univ., No. 59 B, 2016

Hydrological Modelling of Flash Flood at Wadi Samail, Oman
Mohammed ABDEL-FATTAH(1), Sameh KANTOUSH, Mohamed SABER, and Tetsuya
SUMI(1) Graduate School of Engineering, Kyoto University
Synopsis
In the arid regions, flash floods are among the most devastating hazards in terms ofhuman losses and economical damages. On the other hand, however, flash floodwater canbe an important source of water in arid environments. Unfortunately, there is often a lackof data on key hydrological processes in arid areas. This limits the ability to understandthe flash flood process and use this knowledge to minimize its threat. Oman is one of arid
countries that mainly faces flash flood in the coastal drainage systems. Wadi Samail(W. Samail) in the north coast of Oman is selected as a case study for flash floodhydrological modelling. Computation is carried out using sophisticated techniques where
remote sensing data used with Rainfall Runoff Inundation Model (RRI) to simulate
several flash flood events at W. Samail and compare between different mitigation
measures options. DEM data and satellite imagery have been used to propose suitablelocations of dams based on available reservoirs total volumes and delineated watershed.
Keywords: Flash Flood, Wadi, Oman, RRI

1. Introduction
The Arab region affected by many disasters thatare mainly flash floods and drought where flash
floods became more frequent in the last decade. Thisphenomenon should be discussed deeply, to findinterpretation for it. Climate change could be thedriving force for this amendment; or it is just due toincreasing media coverage and wide spreading ofadvanced monitoring tools. Oman is an arid country,
where the average annual rainfall, throughout mostof the country is less than 100mm varying from lessthan 20 mm in the internal desert regions to over 350
mm in the mountainous areas (MRMWR, 2008).
Recently, flash floods are frequently occurring in
the arid region as Oman, which counter with various
challenges to the management of wadi flash floods.
Records shows that major flash floods occurred in

Oman in 1989, 1997, 2002, 2003, 2007, 2010, 2015
and 2016.For instance, Gonu 2007 event caused 50
fatalities and 3.9 billion USD of economic losses inOman (Al Khatry and Helmi, 2008). Flash flood is aflood, which is characterized by its quick occurrence
resulting in a very limited opportunity for warnings
to be prepared and issued (Collier, 2007). Flashfloods causes could be excessive rainfall or an
accident like a dam or levee break. W.Aday, W.
Kabir and W. Samail are the main wadis that haveexperienced flash floods with major damage to
people and property (Al-Rawas and Caterina, 2006).
Rapid increase of population, urbanization andeconomic developments have pushed people forconstruction in high disaster risk zones such as on
wadi floods plain. The location of villages in arid
Arabic areas (including Oman) has been determinedby the water availability and therefore, many

― 533 ―

(b)

(a)

(c)

(e)

(d)

Fig. 1 W. Samail (a) location map (MRMWR, 2015), (b) main channel showing the wadi bed materials andits dry condition, (c) Al-Khoudh water flow measurement station and wadi week base flow at thedownstream part, (d) Al-Khoudh Dam and (e) Al-Khoudh Dam reservoir showing its dry condition andsiltation problem of it.
development activities exist mainly along the wadis.
Muscat (capital of Oman) is the fastest growing
city in Oman. Due to its geographic location, which
is surrounded by mountains, and the horizontal
expansion of the urban areas, most of the wadis andthe main channels have been occupied by urban
development (Al-Rawas and Caterina, 2006). Morehydrological studies are urgently needed to evaluate
flash floods risk in Oman and propose innovative
mitigation technologies.
Rainfall–runoff responses predictions in arid

climate as wadi system always presents uniquechallenges (Abdel-Fattah, et al., 2015). One of themain challenges beside data limitation is thehydrological models themselves, where the majorityof models developed for catchments that havedifferent characteristic from wadi system features.
Hence, the need to evaluate the suitability ofalternative modelling approaches for wadi system
and its scarce dataset arises. In that regard, RainfallRunoff-Inundation (RRI) Model is selected in thisstudy. Another aspect of this paper is to focus on
both structural measures for flash flood retention as― 534 ―

dry dams, and water harvesting. The location ofsuch mitigation structures must be carefully
designed to avoid transferring the problems to thedeveloped downstream area of the wadi.
This paper objectives are to i) study wadi runoff
features and factors that affecting on it; ii) adopt
sophisticated methodology which face hydrologicalmodeling limitation in the arid environments; iii)
simulate and assess some flash floods events; iv) to
propose and evaluate flash floods mitigation
scenarios by a single concentrated and distributeddams.
2. Study Area
Oman’s ephemeral rivers (wadis) may beclassified into those that drain to the coast, and thosethat drain to the interior desert (Al-Qurashi, et al.,2008). Wadi Samail is one of the costal wadi,
situated in the north of Oman (Fig. 1-a). The
catchment can be subdivided into two areas; SamailSouth (upstream of the Al-Khoudh flow gauging
station and 8 km north Al-Khoudh Dam, withelevation above sea level (a.s.l) ranging from 74 m
to 2462 m, area 1669 km2) and Samail North (fromAl-Khoudh flow station to the coast, including AlKhoudh Dam and Samail Alluvial fan, ranging fromsea level to 109 m a.s.l., area 62 km2). Wadi Samail
total area is 1741 km2.
The main geological units in the target catchment
are: Ophiolite, Hajar Super group, Hawasinah,
Aruma Huqf and Hadhramaut group interspersedwith alluvial wadi deposits. The wadi channels bed
materials are mainly gravel and sands (Fig. 1-b) withweak base flow mainly at the downstream part
originated the subsurface water flow as depicted at
Fig. 1-c. Land cover is mainly bare rock and desert
with sparse vegetation cover. The terrain is
mountainous, with slopes reached to 76% at the hillslope (estimated from a 30 m resolution DEM).
Comparing to much of the Arabian Gulf area
(Nouh, 2006), (Al-Qurashi, et al., 2008), availabilityof rainfall and flow data for Wadi Samail isreasonable. There are nine recording rainfall gauges,
three telemetric stations and two flow measurements
stations. The annual average rainfall at these gauges
ranges from 90 mm at Samail Station to 212 mm at
Jabal Al-Hayl (MRMWR, 2015).

Al-Khoudh Dam is a recharge-flood protection
dam which was constructed in 1988 at thedownstream part of W. Samail (Fig. 1-d). The dam
is an earthen type with 5,100 m crest length, up to 11
m high, a spillway section of 3,000 m long (AlIsmaily, et al., 2015). The dam reservoir area is 3.2
km2 with a design capacity of approximately 11.5
million m3 (Fig. 1-d). The top soils of the dam
reservoir bed are experiencing the formation of a
relatively low-permeability cake or siltation—up to
3 m deep in some areas as shown in Fig. 1-e andstated by Al-Ismaily, et al. (2015). This siltation
process decrease the dam reservoir capacity,
decrease water infiltration to groundwater andincrease the evaporation losses. Due to limitation of
Al-Khoudh dam (in terms of its capacity andlocation) the Omani government start to constructmore dams in such important wadi.
The target area studied by many authors as
Al-Rawas and Caterina (2006) who explored themain challenges that are hindering progress on flash
flood research and used a physically-based GIS
supported modeling to assess and predict flash
floods. Saber (2010) simulate different flash flood
events at W. Samail using Hydro-BEAM model andapply the calibrated model to other wadi systems.
Al-Housni, et al. (2014) use a lumped tank model tosimulate flash floods in W. Samail. Al-Ismaily, et al.
(2015) make a hydropedological study to investigate
the impact of the of Al-Khoud dam construction on
soil development in W. Samail.
As result of increasing pressure of population,and unmanaged development, people started
cultivation of new lands even building their houses
at more prone areas for flash floods in W. Samail.
Therefore there are increasing risk of flash floods in
W. Samail and more hydrological studies to asses
this rising risk are highly recommended.
3.

Methodology

Hydrological modelling was used to determine
the most vulnerable areas in wadi Samail, leading to
the development of a mitigation plan aiming to
reduce flood damages. Remote sensing data as well
as GIS technique have been utilized for data input,
preprocessing and visualization of the results.
Rainfall Runoff Inundation model (RRI) has been

― 535 ―

Fig. 2 Schematic diagram of RRI model (Sayama,
et al., 2012)
utilized to simulate several flash floods events andevaluate different management scenarios of flash
floods at the target basin. RRI model is ahydrological model (Sayama, et al., 2012), whichdeveloped for humid areas and it is first time to be
used for flash floods evaluation at Oman.
This model has been calibrated using Al-Khoudh
water measurement station ((Fig. 1-c). Due to data
availability the measured and the simulated
discharge are daily average so further calibration for
more fine temporal resolution is required. A
Sensitivity analysis is implemented to evaluate theweight of different parameters within the model. The
parameter setting is based on the field investigationand literature review.
RRI simple dam operation rule has been used tocompare between the different mitigation scenarios.
Reservoir water evaporation and infiltration havebeen ignored in this version of RRI. The hydraulicdesign of dam bottom outlets to retard flood wave,
cut the peak flow discharges and safely passing thefloodwater only through the bottom outlet.
The criteria for the proposed dam locations andtheir characteristics are based on the following: 1)
flow directions of each cell and watershed boundary
have been estimated using GIS and DEM data; 2)
rainfall-runoff hydrological modelling using HydroBEAM and synthetically design storm to make ageographical distribution of the surface runoff andallocate the potential prone area of flash floods; 3)
estimation of cross sections capacity of the wadi
drainage channels by using DEM and Google Earthimages to detect the safe passing flow capacity ofeach channel; 4) propose several dams locations
according to the surrounding topography; 5)establish height and volume curve (H-V) for eachdam using in-house developed code, DEM and flow
directions information of each mesh; 6) calculate theexpected design flood volume and the required damheight to mitigate this flood; 7) outlet setting in eachdam compatible with the downstream channel
capacity and 8) comparing between the different
dams locations and decide the best option that canretain the flood volume with the lowest dam heightand don’t affect the surrounding developed areas asagriculture land and houses.
3.1 RRI Model
The used model is 2D Rainfall-RunoffInundation (RRI) model, which is capable ofsimulating rainfall–runoff and flood inundation
simultaneously (Sayama, et al., 2012). Fig. 2 showsa schematic diagram of the RRI model. The model
deals with slopes and river channels separately. At agrid cell on which a river channel is located, themodel assumes that both slope and river are
positioned within the same grid cell.
A channel is discretized as a single vector along
the centre line of the overlying slope grid cell. The
channel represents an extra flow path between gridcells lying over the actual river course (Sayama, etal., 2012). Lateral flows are simulated on slope cells
on a 2D basis. Slope grid cells on the river channel
have two water depths: one for the channel and theother for the slope (or floodplain) itself. The inflow–
outflow interaction between the slope and river iscalculated based on different overflowing formulae
depending on water-level and levee-height
conditions (Sayama, et al., 2012).
3.2 Input Data
Watershed delineation
This study primarily used HydroSHEDS data
because they include flow-direction and flowaccumulation data sets, that are useful for identifying
the locations of rivers (Lehner, et al., 2008). They
are also considered one of the best topographic data
sets that are globally available for flood inundation
simulation (Inomata, et al., 2010). The used
HydroSHEDS data is 30-s resolution.

― 536 ―

Topographic Data
Topographic data were obtained by the Shuttle
Radar Topography Mission (SRTM) data (USGS,
2008). The used data set is void filled 1arc-second
elevation data that are result of additional processing
to address areas of missing data or voids. The study
area is characterized by high range of relief withelevation ranges from 2462 a.s.l. at the upstream, tosea level at the downstream, and generally slopes
towards the North discharge the flow into the
Arabian Gulf as shown in Fig. 4.
Rainfall Data
As mentioned before we have nine rainfall
stations at the study area and we use Thiessen
method to interpolate this rainfall measurements allover the watershed as shown in Fig. 3 for Gonu 2007
event. For this flash flood which was generated fromsevere cyclone, only four stations form the ninestations have measured this event.
4. RRI Model Application for Flash Floods
Estimation
RRI model has been applied for flash flood
simulation in W. Samail after its calibration andvalidation as indicated in Fig. 5. The simulatedhydrographs show good fitting with the measured
one and indicating the wadi flash floods uniquefeatures as shown in Fig. 5-a. The wadi flow gauging
station is 7 km upstream of Al-Khoudh dam (Fig. 1-c
and Fig. 5-a).

Flash floods events between 2005 a 2010 havebeen simulated as shown in Fig. 5-a. The most severe
event is June-2007 events, which generated from
Gonu Cyclone which have high impact on many
wadis at Oman. Using the merit of the distributedmodels discharge of each mesh all over the basin iscomputed. Variation of hydrograph peak can benoticed from one event to another. This is due todifferent rainfall intensity and pattern for each event.
The simulated hydrographs show the flow featuresof flash floods at wadi system, where it takes a few
hours to reach to the peak discharge and thengradually decreasing until end of the event.
Surface runoff geographic distribution maps (Fig.
5-b&c) confirm that for the same rainfall event,some parts of the watershed have flash floods andinundation and on the other hand, some locations
have no flow. But more surface runoff rate existed at
the downstream part. It is can be noticed that theinundation map is highly affected by the topographic
and land use condition.
These discharge and inundation distribution
maps can be helpful in flash floods, water resources,
urban and land use management. It can be useful indetecting flash floods prone areas and consequently
mitigate and mange flash floods at those areas. In
addition, it can be valuable for wadi development
and land use management to identify the best
location for residential, touristic, industrial,
agricultural activities. Surface runoff zones can givesigns of the potential groundwater locations, whereÜ

Samail_DEM
Elevation (m)0 - 299
300 - 415
416 - 531
532 - 676
677 - 908
909 - 2,462

0

4

8

16

24

Km
32

Fig. 4 W. Samail Digital Elevation Model (DEM)― 537 ―

Fig. 3 Measured rainfall for Gonu 2007 event in
W. Samailg400

Wadi flow measurement station

350

Discharge (cms)300

Simulated Discharge
Measured Dischrage

300

Cyclone Gonu Event

250

200

200

100

150
100

0
6/4/2007

50
0Dec, 2005

(a)

400

Jul, 2006

Jan, 2007

Aug, 2007

Feb, 2008

6/10/2007

6/16/2007

6/22/2007

Sep, 2008

Mar, 2009

Oct, 2009

May, 2010

Date

(b)

(c)

Fig. 5 Flash flood simulation at W. Samail using RRI model: a) long term measured and estimatedhydrograph (average daily discharge m3/s) from 2005 to 2010 highlighting Gonu 2007 flash flood event,b) peak estimated discharge distribution map, c) maximum estimated inundation distribution map
the transmission losses and groundwater recharge
are linearly related to surface runoff (Saber, 2010).
Once sustainable groundwater resource is detected,
especially at arid environment like wadi system,other development activities as land reclamation for
agriculture can be constructed.
5.

Flash Floods Mitigation Scenarios
Comparison
In the arid region, flash floods are in factcommon, but their occurrence and processes arepoorly understood. There is an urgent need tomitigate and utilize floods water as a new supply tosustain a minimum water resources base in ruraldesert areas. The management approach should
involve developing a strategic methodology forevaluating wadi flash floods potential, mitigation,and floods water resource management as well as arainfall-runoff simulation model (Kantoush, et al.,
2011& Sumi, et al., 2013). This research flash floods
management approach based on the hard structure― 538 ―

Table 1

Characteristics of the proposed dams

Concentrated Dam

Distributed Dam 1

Distributed Dam 2

Distributed Dam 3

23.554344
58.103665

23.554344
58.103665

23.371503
58.090958

23.368906
58.157186

Reservoir
Capacity (MCM)

75

40

9.6

25.8

Height (m)

50

40

22

22

Location: Y:X

Fig. 6 The proposed flash floods mitigation structures and the simulated hydrographs before
and after mitigation scenarios application
measures concept and comparison between applying
single concentrated or several distributed dams.
Mitigation structures should be designed in acoordinated manner, to assess their overall effect(Abdel-Fattah, et al., 2016). This study analyzes thewadi flash flood mitigation for three cases: 1) nodams, 2) three distributed small dams allover overthe catchment in the upstream, and 3) proposed large
dam in the middle or downstream area of the wadi asshown in Table 1 and Fig. 6. The effect must be
quantified through a comparison of theconsequences with and without mitigation structures― 539 ―

over the whole wadi. Various factors are considered
to study and improve the assessment methodologies.
The design storm is selected as Gonu 2007
cyclone which was the most severe rainfall event in
the recent history of Oman and it has a return periodof 1000 year at that time. Table 1 shows the different
characteristics of each dam. The dam height is basedon the calculated reservoir volumes that estimatedusing the DEM data. Distributed dams application
decrease the concentrated dam height and its
reservoir volume.
From the simulated hydrographs (Fig. 6) and thegeographical distribution maps of the surface runoff,
it can be noticed that both strategies could be
effective in floods mitigation at the downstream partof W. Samail. Where mitigation dams application
decrease the daily average surface runoff rate from350 cubic meter per second (cms) to 150 cms at thedownstream reach. There are some difficulties fordam construction in the upstream areas, whereusually these locations are remoted with noconvenient accessibility. However, the distributed
dams will have better mitigation for the upstream
part. Other merits of the upstream dams that it could
manage the water resources and sediment all over thewatershed for water use and development for the
upstream area beside the downstream area. The
distributed dams could be used for local groundwater
recharge.
6. Conclusion
With increasing stress due to flash floods disasterand water resources limitation in Oman, applicationof rainfall-runoff models can be a part of the solution
to mitigate flash floods, and at the same time manage
and sustain the water resource. The main problem atflash floods studying is obviously the lack ofobservations in most of the flash floods prone basins,
so there is urgent necessity to adopt a new
methodology to simulate and forecast flash floods inarid regions. This study aims to discuss flash floods
simulation and management in wadi system andevaluate different concepts of flash floods mitigation.
W. Samail at the Oman Coast is selected as a casestudy for this target.
A trail to simulate several flash floods events at
our target study case was implemented, where

remote sensing and measured data utilized beside
RRI hydrological model as a main tool. This flash
floods simulation methodology, which presented inthis manuscript, should be further calibrated andvalidated for more fine temporal resolution and morewadis in Oman. The simulation results highlight themain feature of wadi flash floods as its hydrographs
have extreme steep and rapid rising to peak flow
after a few hours, which increase the degree of riskof flash floods comparing to normal floods.
The distributed map of simulated discharge forflash floods events, indicating high spatial
variability of runoff. Not all flash floods have ahomogenous geographic distribution, due to rainfall,geomorphologic and land use variation. The
distributed dams have advantage of water resourcesmanagement all over the watershed for water use anddevelopment for the upstream area beside thedownstream area. Other comparison aspects should
be taken into account in future as economicalassessment, sediment management, and water
evaporation for better evaluation of the different
management scenarios.
Acknowledgements
The authors are grateful to Ministry of Regional
Municipalities and Water Resources (MRMWR),
Oman especially En. Ahmed Al-Barwani and
En. Tariq Helmi for their help in providing data andinformation about Wadi Samail.
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(Received June 13, 2016)