Abstract
Considerable stretches of barren coastal areas of the Gulf of Suez and
the Red Sea have been covered by various anthropogenic activities during
the past few decades, thus assessment of the interaction of natural
occurring phenomena such as flash floods and the developed areas is a
necessity. Several dykes have been erected on the wadi beds constituting
the catchment of Wadi Ghuwaiba, which encloses the industrial zone of
Ain Al Soukhna on its large alluvial fan. The multitude of optical
remote sensing data, Digital Elevation Model (DEM) and thematic
geological data have been integrated into Geographic Information System
(GIS) to estimate the hydrological parameters of flash floods, pathways
and storage capabilities of the constructed dykes upstream of the
industrial zone. Due to the absence of rainfall/runoff data for these
ungagged wadis, a semi-distributed hydrological model was developed
using the extracted morphometric parameters from the DEM, and the
reoccurrence of flash flood events and the active channels from the
analyses of satellite images. Manning equation was used to compute the
open channel flow velocities, and the spatial functionalities into GIS
were used to estimate the cumulative flow times for each cell in the
catchment. Next, the catchments were subdivided into cascading time-area
zones using the derivatives of flow direction, accumulation and length
from the DEM. Then, the spatially distributed unit hydrograph was
computed using a hypothetical rainfall distribution and runoff
coefficients, and the obtained time-area zones. The dykes have been
visited in the field to measure the distribution of entrapped sediment
loads due to the accumulation of torrential flows in these specific
areas. The magnitudes of flash floods developed from an effective
rainfall of 20 mm will not be fully contained in the storage areas of
these dykes and overflows will reach the industrial zone via
uncontrolled pathways, thus the area is prone to flash flood
vulnerability and additional mitigation measures are still needed. The
slack deposits are accumulating with notable rates and have reached 60
cm in some areas, therefore the efficiency of these dykes are also being
decreased due to deposition of sediment loads associated with the flash
floods. It is strongly recommended to construct a conveying channel
downstream of the dykes to collect the surplus overflow and discharge it
into the Gulf of Suez via a well-defined channel-pathway.
KEYWORDS: Flash flood. Mitigation, Geographic Information
System. Ain Al Soukhna. Egypt
INTRODUCTION
The proposed sustainable development plans in the urban areas include
harvesting runoff from the catchment, the recharge rate of groundwater
aquifers and exploiting its resources (Megahed, 2020; Darwish et al.,
2020; Farrag et al. 2019 & Megahed and Farrag 2019). Flash flooding is
one of Egypt’s principal natural disasters (De Roo, 1999; Broadman et
al., 1994). Due to a lack of studies and the relatively infrequent
occurrence of runoff events and flooding and associated sediment
transport in urban areas and archaeological sites so receive recognition
as hydrological and environmental problems (Megahed, 2020; White, 1995;
Gheith and Sultan, 2002). Besides, a particular problem for hydrological
analyzes in dryland catchments is the lack of good quality rainfall and
discharge data (Megahed and El Bastawesy, 2020; McMahon, 1979). The
impact of flash floods in urban areas could vary from one catchment to
another and even within the same catchment at the different events of
flash floods depending on the attenuation of discharges and associated
sediment loads of the flash floods (El Bastawesy et al. 2009).
Geographical information system () provides several computerized
techniques enabling to automatically extract the hydrological parameters
from high quality digital elevation model data, such hydrological
parameters include watershed and flow path delineation (Wilson and
Gallant, 2000; Jenson and Domingue, 1988). Together with remotely sensed
data that can provide near-real-time information on the climate, the
atmosphere, the local environment and weather conditions (e.g.
precipitation). At the present time, integrating both techniques could
provide management of flash flood hazards (Walsh et al., 1998).
The industrial area of the northwest Gulf of Suez coastal zone is
located in the southern part of a large drainage system constituted by
several wadis, namely: Ghoweba, Bedaa, Hagoul, and Hamath. These wadis
collect their surface runoff during occasional flash floods from a
catchment area of 4300 km2 (Figure. 1). In 1999 the
National Centre for Water Research has recommended the construction of
several dykes on the main wadis to retain flow in their upstream to
prevent flash floods from reaching the coastal area (Figure. 2). The
locations and engineering designs for these dykes were based on amounts
of flash flood runoff estimated after meteorological (rainfall),
geomorphologic, geological, topographical, and field data available at
the time. Due to the economic importance of the area, the maximum
storage capacities for these dykes were designed to accommodate flows
generated from a high magnitude storm of 50 years return period with
rainfall depths ranging from 60 to 82 mm over the catchment. However,
observed annual repeatability of significant flash floods is usually
much smaller than the values considered for the selected storm design
that occurs once in 50 years. Also, variable amounts of sediment loads
are likely to accumulate behind these dykes and their storage capacity
will decrease with time. Because the catchments were initially un-gauged
with meteorological and surface runoff stations, the dyke’s construction
was based on a deterministic hydrological model that utilizes runoff
scenarios based on arbitrarily estimated runoff coefficients mainly
based on storms occurring only once each 50 years (El Bastawesy, 2005).
This is the main reason for the hydrological re-assessment of the
effectiveness of the erected dykes, with a particular emphasis on the
dykes deserving the Wadi El Abiad sub-catchment, which may endanger the
areas downstream of this wadi. The goal of the present study is to use
an adequate methodology to quantify and assess potential flash flood
hazards in the considered areas using remote sensing and GIS techniques
with fieldwork.
2. GEOLOGICAL SETTING AND CLIMATE
The rock units outcropping in the considered area of the drainage basin
are presented in the geological map (CONOCO, 1987) (Figure. 3). The hill
slopes of Wadi El Abiad are carved in the Middle Eocene limestone of the
Mokattam Formation, which caps the Northern Galala massif. This Massif
is limited to the north-west by several NE/SW step faults within the
steep slopes bringing the surrounding alluvial plain in abrupt contact
with the Galala cliffs. Most of Wadi El Abiad main channel is confined
to a deeply incised canyon running across the Galala massif. The
Mokattam exposed in the Northern Galala is capped in places (mainly in
down faulted blocks on the foothill of the faulted cliffs surrounding
the massif) by Oligocene clastic involving chert and some limestone
pebbles. By reaching the alluvial plain, Wadi El Abiad fans out forming
braided alluvial channels constituting large areas. The out-washed
Oligocene pebbles fill the bed of Wadi El Abiad channel and alluvial
fan. The abundant coarse clastic and pebbles imply a high infiltration
rate from occasional surface runoff in the drainage area. It should be
noticed that the planned facility will be constructed on the westernmost
distal part of the alluvial fan. This suggests that the local geological
environment has a marked influence on the nature of the flash floods
affecting the considered area, as the generated flows tend to decrease
significantly due to the actions of channel divergence and infiltration
by reaching the alluvial fan. Wadi El Abiad channel receives only less
than 20% at most of the total flow delivered to the alluvial fan by any
flash flood (Figure. 1). The remaining 80% or more of the flows are
routed through the other channels constituting the alluvial fan onto
which the industrial area of Ain Al Soukhna was constructed.
Regarding climate conditions, it is one of the main external driving
forces that trigger various hydrological and geomorphic processes.
Precipitation is the main and most influencing climatic parameter that
initiates and controls the occurrence of flash flood events in drainage
basins of all climatic zones. The collection of data about the frequency
and magnitude of precipitations is a fundamental step to describe,
understand, and model flash flood processes in the drainage basins of
arid environments. The acquisition of such data is normally obtained
from meteorological stations that are located in the neighborhood of the
drainage basins. These stations record among other climatic parameters
precipitations on a real-time basis. Although these data can provide
valuable information on a particular region, it is difficult to use them
for describing and modeling flash flood events that occur in drainage
basins in arid lands for two significant constraints:
These stations provide point measurement data that corresponds to a
limited geographic extension around the stations.
These stations are installed for helping navigation in seas and
similar water bodies and are accordingly located along with coastal
areas, as in the case of most stations in the Red Sea region. This
makes the obtained climatic records not representative of the remotely
located mountainous reaches, which receive the precipitations
responsible for flash floods.
The northwest Gulf of Suez catchment is located in an arid region, where
the rates of evaporation exceed the rates of precipitations by many
times. There are only two climatic stations in the catchment, namely:
Jebel Attaqa and Adebah stations which are installed near the coastal
area. As aforementioned, the poorly present precipitation data at few
points cannot confidently be interpolated and/or extrapolated over the
catchment area. The magnitude and intensity of the storms are often
variable over the dry-land catchments within short distances.
Consequently, an alternative source of the remotely sensed climatic data
of the Terrain Rainfall Monitoring Mission (TRMM) (NASA, 2004) was
considered.
Three hourly accumulated of rainfall in the last 13 years (2006-2020)
has been acquired for the catchment from TRMM data (Figures. 4 – 7) and
analyzing these data showed the following:
- Annual rainfall patterns are almost similar to each other considering
the spatial coverage and areas of high precipitation, magnitude, and
intensity. The number of rainy days over the catchment area is larger
than that of other catchments on the Red Sea coast.
- Annual rainfall is between 17 mm and 101 mm, the highest precipitation
amounts are found to be over areas of high topography in the south and
north-west of the catchment. Coastal areas (where meteorological
stations are located) receive the lowest amount of rainfall (Figure.
4).
- Most of the rainfall events occurred in winter months (November to
March).
- Wadi El Abiad sub- catchment receives a modest amount of precipitation
in the range of 25mm to 65 mm (Figure. 5).
- The total accumulated rainfall in a month might be high. Nevertheless,
if no significant accumulated rainfall occurs through any single day
of that month, no flash flood should occur.
- For El Abiad sub-catchment, the maximum monthly precipitation occurred
during February 2020 with a total amount of 60 mm that nearly equal
the annual average rainfall over this sub-catchment (Figure. 6). 10 mm
of precipitation was the maximum daily rainfall recorded during 10th
February 2020 (Figure. 7).
From this analysis, it is possible to determine the months of the year
which are the most prone to receive precipitations sufficient to trigger
significant flash flood events. It has been found that the total annual
rainfall over the north-western Gulf of Suez catchment is the highest
recorded for any Red Sea catchment in Egypt. It should also be noted
that the deviation in precipitation recorded in the watershed north-west
Gulf of Suez is not very large, and the precipitation behavior on the
studied catchment does not change much in terms of spatial coverage and
intensity from one year to another. This indicates that odd storms of
high magnitudes that are recorded over other catchments every decade or
so are less likely to develop in the considered area, in which a rather
persistent rainfall pattern prevails.
MATERIALS AND METHODS
Digital Elevation Model (DEM)
The digital elevation model (DEM) is remarkably important for extracting
various topographic and hydrological parameters remotely (Wolock and
Price, 1994; Zhang and Montgomery, 1994). It enabled extracting
topographic features including “topography slopes (magnitudes and
aspects), slope length and shape” besides, Hydrological parameters
including “flow direction, flow accumulation, watershed delineation,
flow networks, and flow length”. Both parameters of flow accumulation
and length were used to calculate the hypothetical hydrograph of the
considered sub-catchment area (Singh, 1997).
From the DEM, the elevation of the north-west Gulf of Suez drainage
basin ranges from 3 to 1274 meters above sea level, while the elevation
of Wadi El Abiad sub-catchment ranges from 39 to 1274 m. (Figure. 8).
3.2 Flow direction and
accumulation
Hydrologically, the flow direction represents the D-8 algorithm of the
direction of the flow within each pixel in relation to the eight
neighborhood pixels. The flow direction has been extracted from the SRTM
digital elevation model, which was a key input parameter to extract the
flow length. Figure 9 shows the flow direction of Wadi El Abiad drainage
basin, which are dimensionless values that range from 1 to 128 (see
figure 10 – D-8 algorithm) (Jenson and Dominique 1988). The flow
accumulation represents the total number of the upslope cells that flow
to each down-slope cell that accumulates the flow to the main streams.
Significant use of the flow accumulation is extracting the stream
networks and sub-catchment areas. ArcGIS and the watershed function used
in combination with the flow accumulation grid to compute the active
upslope contributing area.
3.3 Drainage networks and Sub-catchment
delineation
To delineate a drainage network or catchment area, the flow accumulation
number thresholds have to be defined and selected by the user
(Hutchinson 1989). This means that setting a threshold of 500 cells for
the drainage network delineation will initiate the first fingertips of
streams whenever the flow accumulation values exceed 500. These first
fingertip streams are adjoined and connected in a downstream direction
to form the drainage network map of the catchment. The threshold of 100
cells for the north-west Gulf of Suez catchment is appropriate to
portray a drainage network with comparable spatial accuracy and density
as those shown on the satellite images. On the contrary, the automatic
catchment delineation requires selecting the flow accumulation of the
most downstream of main channels (i.e. a point that will capture flow
from whole the surface drainage network of its upstream). Therefore, the
flow accumulation number at the outlet channels of El Abiad
sub-catchment was defined and used as the minimum threshold required to
delineate the sub-catchment watershed divides. The delineated
sub-catchment of Wadi El Abiad has an area of 330 km2(Figure. 11).
3.4 Runoff velocities and hydrograph
estimation
For estimating the overland flow velocity, it is required to calculate
the magnitude and direction of the flow. Considering the use of Manning
equation (equation 1) (Mays 1999) we managed to estimate the magnitudes.
The hydraulic radius was assumed to be as minimal of (0.05), the slope
was obtained from the DEM of the area and Manning’s coefficient value
(n) was retrieved from standard tables (Arcement and Schneider, 1989 &
El Bastawesy, 2006).
\(V=R\left(\frac{2}{3}\right)*s\left(\frac{0.5}{n}\right)\)……………………………………………….
(1)
Where;
V = Velocity m/sec; R = Hydraulic radius (i.e. = depth); S = Slope
percentage; n = Manning coefficient
The flow direction network represents the direction of flow within each
cell in the studied area to one of the eight adjacent cells (D-8
algorithm) (i.e, diagonal-red, or orthogonal-blue) (Figure. 10). Because
the resolution of the DEM is known, it is possible to calculate the flow
length within each cell which will be equal either to the grid cell
length (L) in case of orthogonal or L√2 in case of diagonal (Figure.
10).
On this basis, the flow direction grid was reclassified to get the flow
length within each cell. Once the flow velocities and flow lengths of
each cell are estimated, the flow time of each cell is obtained by
dividing the flow length by the velocity. The flow time network, which
is the time required for the flow in each cell to reach the output, is
estimated by using the flow length function in the Arc Info software,
which uses the following convolution equation (Equation 2) (ESRI, 2000).
\(Vij=\sum_{p}{cp*dp}\)……………………………………………..
(2)
where,
Vij = the output result of the convolution for cell (I, j); d = the
slope distance between the centers of two adjacent cells along the
minimum-cost path; c = the unit-distance cost value; p = the
minimum-cost path.
Consequently, the grid representing the cell-travel time was regarded as
the unit-distance cost value and was implemented in the calculation of a
weighted flow-length. The time of flow grid represents the time in
seconds required for the flow of each cell to reach the outlet. Finally,
by dividing the time of flow grid by 3600, then reclassifying the
resulting map-enabled to express the estimated travel time in hourly
intervals and then aggregated for the different zones in the time-area
zones map (Figure. 12).
RESULTS AND DISCUSSION
4.1 GIS spatially distributed unit
hydrograph
Each cell in this cumulative time grid has a value that indicates the
time (in seconds) necessary for the runoff that is delivered to each
cell for reaching the catchment outlet. Thus the catchment can be
subdivided into different time-area zones (in hours) (Maidment, 1993).
It is necessary to investigate the effect of the distribution of local
rainfall and the consequent patterns of loss of transmission on the
propagation of the runoff (Cooke et al., 1982; Beven, 2002). The
calculated amount of runoff for the time area zones in Figure 12 using
effective infiltration-surplus precipitation of 10 mm over the entire
catchment is around 3,300,000 m3 (Figure. 13). The
high permeability of wadi alluvium which is filled by gravels and chert
boulders promotes a very high transmission loss rate from the surface
runoff. The permeability was estimated to 50 mm per hour on average. The
main active channels areas of the sub-catchment were measured from the
satellite images, and it was found that the alluvial fan active area is
nearly 13.2 sq km. But the main channel feeding the alluvial fan has a
very small active area (2.6 sq km) when compared with the alluvial fan.
This means that the flows produced in the catchment different time zones
will suffer huge transmission loss as soon as they enter the alluvial
fan area. The flow generated onto Wadi El Abiad catchment 6 zones was
linearly routed in a cascading way from zone 6 successively up to zone 1
(i.e. alluvial fan and dykes area).
Flows generated within the zone number 6 are subjected to transmission
loss into the alluvium channel along with all the way downstream (i.e.
from zones 6 to 1) until they reach the alluvial fan. But the flow
generated within zone number 2, for example, will only suffer
transmission loss into the alluvium of zones 1 and 2. Accordingly, only
the flow generated within the 5th time zone (due to its large area) was
sufficient to exceed the transmission loss into the downstream and to
reach the outlet of the main wadi, but the surface runoff produced in
remaining zones is completely consumed through transmission loss. We
estimate that out of 834, produced from an excess rainfall event of
(under a constant transmission loss rate of 50 mm/hour into the
alluvium), only will reach the outlet. Therefore, the geological setting
of this catchment (i.e. alluvium with large particle sizes) plays an
important role in its flash floods, where most of the flow is lost
through transmission loss.
Runoff and sediment load
validation
The developed hydrograph of Wadi El Abiad suggests that small amounts of
flow can be delivered to the outlet even under whole storm coverage with
considerable intensity. As aforementioned, this is attributed to the
geological setting and the large scale transmission loss. The preserved
high water stands and their deposited layers of slack-fine sediments
were used to validate the hydrological model output. It was found that
only four events were able to deliver runoff and sediment to the dyke
areas since construction in 2000. The maximum high stand observed was 80
cm in average behind the first dyke. Tape measurements along with GIS
analysis suggest that the stored water volume of that event was nearly
21000 m3. The total thickness of clogged fine
sediments reached 60 cm over an area extended upstream behind the dyke
for a nearly 40 m on average (Figures. 16& 17). It is worth to mention
that the fine sediment load indicates that all the delivered flow behind
the dykes were of low magnitude as the cobles and gravels spreading the
alluvial fan were not triggered as a flow-bed load to reach the dyke.
The slack deposits thickness is 15 cm and spread over a semi elongated
area (probably scooped by bulldozers during construction) measuring 80 m
in length and 11 m in width (Figures. 14&15). The observed water high
stand on that area was nearly 60 cm on average, and therefore, we can
estimate that the maximum amount of water delivered to that dyke was 530
m3. It is evident that most of Wadi El Abiad flash
floods are delivered to the other dykes and very little amounts of
floods reach the dykes erected further downstream in the main wadi.
Under the prevailing climatological and hydrological conditions, it is
safe to say that complete silting up behind the dyke will not occur
within the forthcoming 50 years. The silting up rate behind the other
dyke in Wadi El Abiad catchment is much higher and it may be fully
clogged within the next 50 years as nearly 15 cm of fine sediments are
added up behind this dyke once every two years.
Field measurements
The field work and observations aims at validating the remote sensing
results for flash floods and to assess the effectiveness of the
precautions and remedial measures taken to protect the study area from
flash flood hazards.
All the constructed protection dykes investigated were traced from
satellite images and visited in the field to determine the following:
- The construction design of the dykes (i.e. building material, height,
etc).
- Evidences of past flash flood records; this includes trash marks
defined by traces of high stands of water on the walls of the dyke or
surrounding channel banks. The traces are expressed by drifted shrubs,
wood or mud drapes.
- The layers of slack mud deposits accumulated behind the dykes have
been investigated by determining the number of successive mud layers
in the dyke pool area, as it is directly related to the number of flow
delivered to this area. Only one of the visited dykes displayed three
trash marks expressing water high stands of different flood events
(Figure. 18). These have been correlated with the thicknesses of the
different sequences of slack deposits accumulated behind the dyke. The
thicknesses being taken as an expression of the amounts of a suspended
load of the flood and the height of the trash mark can be used to
estimate the total volume of the delivered water. In Wadi El Abiad
three main dykes were erected in the year 2000 to control flash flood.
The other two consecutive dykes, to the east of the previously
mentioned dyke, are controlling most of the flow delivered to the Wadi
El Abiad alluvial fan. The dyke located upstream showed evidence for
four flash floods, while the downstream dam displays no records of
flash floods.