0Research Article
Open Access
Nadhir Al-Ansari*, Ammar A. Ali, Qusay Al-Suhail, and Sven Knutsson
Flow of River Tigris and its Effect on the Bed Sediment within Baghdad, Iraq
DOI 10.1515/eng-2015-0054
Received Jan 22, 2015; accepted Oct 08, 2015
Abstract: River Tigris is a major river in Iraq. Sediment at the bed of the river within a reach of about 18 km from the center of Baghdad upstream was investigated. Sixty five cross sections were surveyed, and 46 sediment samples were collected and analyzed. It was noticed that fine sand was dominant in the bed (90.74%). The average median size within the reach was 2.49 phi (0.177 mm), while the mean size was 2.58 phi (0.16 mm). In addition, the sediments were moderately sorted, fine skewed and lep-tokurtic. The size of the bed sediment decreased relative to previous investigations due to the construction of the Adhaim dam on tributary, which used to be the main sediment supplier to the Tigris River before entering Baghdad. Furthermore, the discharge of the Tigris River for the period 1983-2013 (715 m3/s) decreased by about 40% and 30% since 1983 when compared with the periods 1931-1956 (1208 m3/s) and 1956-1980 (1015 m3/s), respectively, due to climate change and construction of dams upstream from Baghdad. This has decreased the capacity and the competence of the river. The bed elevation has increased compared to previous surveys. It was noticed that dredging operations and obstructions (e.g. fallen bridges and islands) have disturbed the flow of the river and sediment characteristics in several sites.
Keywords: Bed sediment; Regulated Flow; Tigris River; Baghdad; Iraq
Corresponding Author: Nadhir Al-Ansari: Department of Civil, Environmental and Natural Resources Engineering, Lulea University of Technology, Lulea, Sweden; Email: nadhir.alansari@ltu.se Ammar A. Ali: Department of Civil, Environmental and Natural Resources Engineering, Lulea University of Technology, Lulea, Sweden; Email: ammar.ali@ltu.se; Department of Water Resources, College of Engineering, Baghdad University, Baghdad, Iraq; Email: ammali_75@yahoo.com
Qusay Al-Suhail: Department of Earth Sciences, College of Science, Baghdad University, Baghdad, Iraq; Email: quab65@hotmail.com Sven Knutsson: Department of Civil, Environmental and Natural Resources Engineering, Lulea University of Technology, Lulea, Sweden; Email: sven.knutsson@ltu.se
1 Introduction
Iraq is part of the Middle East and North Africa (MENA) region. It covers an area of 433,970 km2 and is populated by about 32 million inhabitants (Figure 1). Baghdad City, the capital of Iraq, is bisected into two areas from the north to the southeast by the Tigris River for a distance of 60 km, 50 km of which are located within the urban areas, and the rest is in rural parts (Figure 2).
Within Baghdad, the Tigris River has a single channel characterized by compound meanders. Thirteen bridges have been installed along this reach to join the western and eastern parts of the city. A series of small meanders are noticed within the northern part of the river, which is located upstream from the center of Baghdad (Sarai Baghdad), and the banks of the river are protected by stones and cement mortar (Figure 2) [1, 2]. The southern part of the river is characterized by large meanders and about half of this river portion has it banks protected [1, 2].
Tigris River has 10 islands and 17 side and point bars along its reach inside Baghdad City (Figure 2) [1, 2]. These islands and bars are affecting the hydraulic performance of the river; this includes changing of the river cross sections, which reduces the flooding capacity of the river, and decreasing water depth at the intakes of water pumping stations, approaching the river bed from the intake mouths. In addition, there is the impossibility of navigation along the whole reach and limitation to discrete zones, and the threating of the banks' protection stability at some locations due to deep eroding incisions in the river bed. Furthermore, these obstacles are also affecting the environment and aesthetic characteristics, such as increasing the turbidity, growing of reeds, water hyacinth and ceratophyllum demersum at stagnant locations, as well as the disfiguration of aesthetic view over the river and its banks [1,2]. This has led Iraqi Ministry of Water Resources [3] to dredge parts of the river to attempt to overcome these impacts [2].
Three surveys have been conducted along Tigris River in the city of Baghdad. The first was conducted in 1976 by Geohydraulique [4], followed by University of Technology in 1991 [5], and finally by IMoWR in 2008 [3]. It is notewor-
© 2015 N. Al-Ansari etal., published by De Gruyter Open.
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License.
Figure 2: River Tigris within Baghdad City (islands and sandbars border by red) [2].
thy to mention that none of these surveys studied the sediment characteristics of the river in details.
In this research, sediment samples from the bed of the River Tigris at a reach extending from the center of Baghdad at Sarai Baghdad gauging station to about Muthana Bridge in the north were been analyzed for their characteristics. The breadth of the river within this reach ranges from 150 to 360 m.
2 River Tigris
The catchment area of the river is 473,103 km2 and is distributed between Turkey, Syria, Iran and Iraq (Table 1) [811].
River Tigris enters Iraq four kilometers north of Fieshkhabur, near Zakho city (Figure 3). The Tigris is joined by its first tributary inside Iraq, which is known as Khabur River. This tributary is 100 km long; its catchment area is 6,268 km2 with an average discharge of 68 m3/s. The Tigris River runs southrfor about 188 km in a hilly area to reach Mosul city. At Mosul the average, maximum and minimum discharges of the river are 668 m3/s, 7,740 m3/s (on 2/5/1972) and 85 m3/s (in October, 1935), respectively. The elevation of the channel bed is 225 m above sea level [8,9,11].
ure 3). Its total length is 437 km with a mean discharge of 450 m3/s. It supplies 28.7% of the Tigris water [8, 9,11].
The Tigris River runs south and the Lesser Zab tributary joins the river Tigris (Figure 3). The total catchment area of this tributary is 22,250 km2. The total length of the tributary is 456 km with a mean discharge of 227 m3/s. About 30 km downstream of this confluence the Tigris River crosses Fatha gorge. The river Tigris mean, maximum and minimum discharges at Fatha gorge are 1,349 m3/s, 16,380 m3/s (on 3/4/1969) and 200 m3/s (in October 1930), respectively [8, 9,11].
The Adhaim tributary joins the Tigris 68 km south of Sammara Barrage (Figure 3). The tributary drains an area of 13,000 km2 lying within Iraq. Its length is 330 km. The mean daily discharge is 25.5 m3/s. The banks of the river Tigris south of its confluence with the Adhaim tributary are below the maximum flood peak level by 3 m from the left, and by 1.8 m from the right [8, 9,11].
Further to the south, the river reaches Baghdad. At Baghdad, the mean, maximum and minimum discharges are 1,140 m3/s, 7,640 m3/s (on 12/2/1941) and 163 m3/s (in October 1955), respectively. The slope of the channel is very low, i.e. 6.9 cm/km. It is noteworthy to mention that since 1990 the flow of the River Tigris at Baghdad is completely controlled by the regulating scheme at Sammara and the Adhaim dam.
About 31 km south of Baghdad, the last main tributary "Diyala" joins the Tigris (Figure 3). Diyala's drainage basin is 31,896 km2 with a mean daily discharge of 182 m3/s [8, 9,11].
Downstream of the confluence of the Tigris-Diyala rivers, the Tigris channel is characterized by its large number of meanders. In addition, the river discharge steadily decreases downstream due to losses. These losses include evaporation, infiltration, and mainly water withdrawal through irrigation canals. There are many small streams running from Iran toward Iraq where water is discharged in the marshes [8, 9,11].
The channel of River Tigris reaches its minimum width at Kasarah area south of Amarah city. At Qalaat Salih, the mean daily discharge of the river is 80 m3/s. Downstream this city the river joins the Euphrates River at Qurnah city forming Shatt Al-Arab River (Figure 3) [8,9,11].
3 Flow Characteristics of River Tigris at Baghdad
About 49 km south of Mosul toward Sharqat city, The average annual flow of the Tigris River is 21.2 km3 the Tigris joins its biggest tributary, the Greater Zab (Fig- when it enters Iraq. Its tributaries contribute 24.78 km3 of
Figure 3: Schematic diagram of Tigris River and its tributaries hydro-logical scheme [12].
Table 1: Catchment area of River Tigris.
Countries Tigris River
Catchment area (km2) Catchment area (%)
Turkey 57614 12.2
Syria 834 0.2
Iraq 253000 58
Iran 140180 29.6
Total 473103 100
water and there are about 7 km3 of water brought by small valleys from Iran that drains directly toward the marsh area in the south. This figure greatly fluctuates from year to year (Figure 4).
Records show that the flow of the Tigris has been decreasing with time, where the mean annual flow for the period 1931-1973 was 21.3 billion cubic meters (BCM) and it dropped to 19.1 BCM for the period 1974-2005 [10]. The maximum and minimum annual flow recorded have been 43.1 and 6.5 BCM, respectively [10].
Tigris River hygrograph at Sarai Baghdad gauging station (Figure 5) showed that the maximum flow takes place during April and May. Furthermore, floods and drought are themselves of variable magnitude. Such variations are due to changing meteorological conditions. The period extending from October to February is referred to as a variable flood period, where discharges in the river fluctuate depending on intensity and duration of rainfall at its basin. This period is usually followed by what is known as steady flood period extending from March to April. Furthermore, Figure 5 shows that the hydrograph is becoming flatter since 1990. This is due to the effect of the dams that were constructed upstream Baghdad gauging station, and due to the draught period that is affecting the area as a result of climate change [11].
Long term (1931-2013) monthly discharge records (Figure 4) indicate that there is a general decrease of the flow. It is noticed that the discharge of the river was relatively high during the period 1931-1960, when it reached 1,207 m3/s. During this period, there were no dams constructed on the river. Following that period (1961-2000) some dams were constructed that have caused a relative decrease of the discharge to 927 m3/s. More detailed information in this context indicates that annual water flow of the River Tigris decreased by 5.8% (2.07 km3) between the 1950s and '60s. This trend has continued with time due to, mainly, climate change and construction of dams. From 1980 onwards, the flow of the river has been at its lowest values, having reached 715 m3 /s. This is due to the construction of dams in Turkey and Iraq, mainly. In addition, water from all the valleys within in Iran that were supplying water to the Tigris has been diverted for Iranian use. Furthermore, for the years 2000-2013, the discharge dropped to 522 m3/s. This represent more than 50% reduction of the mean monthly discharge of the previous period, and is well below the flood discharges of 4,480, 3,050 and 1,315 m3/s recorded in 1971,1988 and 2005, respectively. The drop of the inclination of the trend line for the average monthly discharges is 22.5 m3/s per year for the last 24 years (Figure 6). Consequently, this indicates that the annual flow of the river was reduced by 59.3% during the last 60 years.
Years 1931 -1959 Average Monthly Discharge (1207
ears 2000- 2013 verage Monthly » Discharge (522 m'/s)
Figure 4: Mean monthly flow of the River Tiaris at Baghdad for the period 1931-2010 [13].
Figure 5: Decadal hydrographs of River Tigris at iiarai Bagholad for the period 1930-2013.
Figure 6: Trend line for the average monthly discharges at Sarai Baghdad for the period 1989-2013 (Modified after [14]).
The studied reach at Baghdad is about 18 km long, extending from the center of Baghdad at CS14 at Sarai gauging station upstream, until CS 1 near Al-Muthana Bridge (Figure 7). The sampling points labeled according to the cross section number (e.g. C#-BM) refer to term bed-
, , ,.• r,., . ,.• . .. Figure 8: Bed level of Tigris River at Baghdad (above
Figure 7: Lo cati on oft he studied c rosssect ions Eind sam pling
material (BM), while the location of the sampling point within the cross section is referred as: R: right, C: center, L: left of the cross section. The breadth of the river varies from 150 to 350 m. The depth of water within the reach varies from 0.05 to about 15 m (Figure 8). The depth generally increases at the outer edges of the meanders.
It should be mentioned, however, that the flow of the River Tigris at Baghdad is highly influenced by two factors. The first is due to the effect of climate change on the flow [15-17]. Rainfall data from the northern part of Iraq [18-21] as well as the central part of Iraq [22] shows that the trend is decreasing with time, which reduces future flow [23]. The second factor affecting the flow is the construction of dams and barrages upstream from Baghdad. Mosul dam on the river at the northern part of Iraq is the first dam constructed in 1986 on the Tigris once it enters Iraq from Turkey. Another dam (Dokan) was constructed in 1959 on the Lesser Zab River. Then at Sammara (about 100 km north Baghdad), Tharthar project controls
the flow of the Tigris River since 1956. In 1999 a dam was constructed on the Adhaim tributary. This implies that all sediments that are transported within the Tigris River are mainly transported from the area downstream Sammara City.
4 Materials and Methods
The studied reach starts at the center of Baghdad (Sarai gauging station) and extends about 18 km upstream from the station. For the current work, field surveying was conducted recently between May-2012 and January-2013 and included:
1. Installing 15 benchmarks on the banks of the river along the study reach. The DGPS device TOPCON GNSS GR3 was used for determining the coordinates of these benchmarks based on UTM-WGS84 coordinates system; also a transformation to the Iraqi na-
Figure 9: Cumulative curves of the bed sediment of Tigris River at Baghdad for the sampling points CS2-L and CS7-C respectively.
tional triangulation network (known as "Polservice" according to the Polish firm that established these points) was done. All the banks were lined by limestone and cement mortar. Surveying was done in the upper river banks from the crest of the stony protection to the water surface at an average spacing of about 200m between the sections along the northern part of the river reach. Leica Builder 405 and TOPCON GTS 225 total stations were used in these measurements with the same coordinates system of the benchmarks.
2. Sixty five cross sections were surveyed at the same locations considered using EAGLE SeaChar-ter 480DF sonar with GPS and WAAS external antenna. Water surface elevations were measured at the beginning and the end of the reach segment, which was surveyed every working day, to transform the water depths to bed elevations as well as the locations coordinates. Extensive surveying of water depth were done around existing islands.
3. Forty six sediment samples were collected from the bed of the River Tigris at 15 cross sections (Figure 7) using van Veen grab. In each cross section, one sam-
ple was taken from the left side, another from the right side, and one from the middle. Additional samples were taken near the islands and in the meanders. These samples were dried in the lab and prepared for particle size analysis. Particle size was determined by sieving the dried sediment samples. The portion of the samples that were less than 0.0625 mm was tested using hydrometer test. During these tests the dispersion used was sodium hexameta phosphate, which was prepared according to the British Standard 1377 [24]. Details of the procedures are reported by Folk [25]. Cumulative curves (% coarser) were drawn (Figure 9), and the statistical parameters were calculated according to Folk [25]. The percent of sand, silt and clay were also calculated from the cumulative curves.
4. The water depths of the bathymetric survey of 2012 were transformed to bed levels using observed water levels at the nearest benchmarks. Triangulated irregular network (TIN) was created for the bed levels (Figure 8) along the reach using ArcGIS 10.1.
The same ArcGIS technique was used to create TIN maps for the distribution of bed composition percentages of sand, silt and clay (Figures 10(a), 10(b), and 10(c)) as well as for the distribution of computed statistical parameters such as median, mean, sorting, skewness and kurtosis (Figures 12(a)-12(c) and 13(a)-13(c)).
Table 2: Folk soil classification for Tigris River bed samples [25].
Folk class No. of samples Percentage %
Sand 33 71.74
Sandy silt 2 4.35
Silty sand 11 23.91
Table 3: USDAsoil classification for Tigris River bed samples [26].
5 Bed Sediment Characteristics
The bed of the river reach is mainly covered by sand (Figure 10(a)), while silt and clay cover small portions of the studied reach (Figures 10(b) and 10(c)). The percentage ratio of sand:silt:clay was 90.74:6.86:2.4. The size distribution curves of the sediment (Figure 8) showed deviation from straight line generally at two points. The first lies between 3.8 to 4 phi (0.074-0.0625 mm) while the second lies at 1.8 phi (0.3 mm). Using Folks classification [25], 72% of the sediments were sand and 24% were silty sand, while the remainder 4% was sandy silt (Table 2). When USDA [26] textural soil classification was used, then the majority of the sediment (72%) was sand, followed by loamy sand (20%), sandy loam (4%) and loam (4%) (Table 3). More recent and detailed classification of Blott and Pye [27] showed that 54.2% of the samples were very slightly silty very slightly clayey sand. This is followed by 15.2% very slightly clayey slightly silty sand, 11% very slightly silty sand, 6.5% slightly clayey silty sand, 6.5% very slightly clayey sand, 2.2% very slightly clayey silty sand, 2.2% slightly silty slightly clayey sand and 2.2% slightly clayey sandy silt (Table 4).
The sediment showed that the average median size within the reach was 2.49 phi (0.177 mm), while the mean size was 2.58 phi (0.16 mm). In addition, the sediments were moderately sorted. About 35% of the samples were well sorted, whereas 24% were poorly sorted (Table 5). The former samples were mainly located in places where the flow was not disturbed. The majority of the sediments were fine skewed (52.2%), while the remainders were strongly fine skewed or nearly symmetrical (Table 6). As far as the kurtosis of the sediments is concerned (Table 7), generally, more than half the samples (56.5%) were leptokurtic, while 17.4% were mesokurtic, and 13% were extremely leptokur-tic. Previous studies [28, 29] showed that the sediments were coarser on the bed of the Tigris River. This is believed to be due to the construction of Adhaim dam on Adhaim tributary in 1999, which has caused coarse sediments to become trapped within the Adhaim reservoir. That tributary is believed to be the main supplier of sediment in the
USDA class No. of samples Percentage %
Sand 33 71.74
Loamy sand 9 19.57
Sandy loam 2 4.35
Loam 2 4.35
Table 4: Blott and Pye soil classification for Tigris River bed sam-
ples [27].
Blott and Pye class No. of Percentage
samples %
very slightly silty very 25 54.2
slightly clayey sand
very slightly clayey 7 15.2
slightly silty sand
very slightly clayey silty 1 2.2
slightly clayey silty sand 3 6.5
very slightly silty sand 5 11
very slightly clayey sand 3 6.5
slightly silty slightly 1 2.2
clayey sand
slightly clayey sandy silt 1 2.2
river above Baghdad. For this reason, the grain size of the bed has decreased in size, and the bed load now transported are of the size ranging from about 0.0625 to 1.0 mm in diameter (4-0 phi). Part of the load is expected to be transported by dragging or rolling on the bed (about 1.00.25 mm in diameter), while the other part (0.25-0.07 mm) by saltation [30].
A TIN of flow depths (see Figure 11) was extracted from bed levels along the studied reach at 500 m3/s discharge. This value represents the flow of the river during the field work and is very close to the average monthly discharge (Figure 4). A value of 6.9 cm/km was used for water surface slope. The water mean depth at CS1 ranged between 2-6 meters, with the deep part of the channel close to the right side. This area is characterized by sediments with a median and mean size range between 2.22.6 phi (0.22-0.178 mm) and 1.5-3.5 phi (0.35-0.088 mm),
Figure 11: Water depths (500 m3/s discharge and 6.9 cm/km water surface slope).
Table 5: Sorting of sediment samples from the bed of River Tigris within Baghdad.
Type of sorting No. of Percentage
samples %
Very well sorted 3 6.5
Well sorted 16 34.8
Moderately well sorted 10 21.7
Moderately sorted 3 6.5
Poorly sorted 11 23.9
Very poorly sorted 3 6.5
Extremely poorly sorted 0 0
respectively (Figures 12(a) and 12(b)). They are also moderately well sorted, fine skewed and leptokurtic to mesokur-tic (Figures 13(a), 13(b) and 13(c)). Midway to CS2, water depth begins to decrease to 2-4 m and sometimes to 0.052.0 m (Figure 11). This is due to accumulation of sediment forming an island close to CS2 [1, 2]. In view of this, the
Table 6: Skewness of sediment samples from the bed of River Tigris within Baghdad.
Type of skewness No. of Percentage
samples %
Strongly fine skewed 17 37.0
Fine skewed 24 52.2
Nearly symmetrical 5 10.8
Coarse skewed 0 0
Strongly coarse skewed 0 0
Table 7: Kurtosis of sediment samples from the bed of River Tigris
within Baghdad.
Type of kurtosis No. of Percentage
samples %
Very platykurtic 0 0
Platykurtic 2 4.4
Mesokurtic 8 17.4
Leptokurtic 26 56.5
Very leptokurtic 4 8.7
Extremely leptokurtic 6 13.0
deep part of the channel shifted toward the left, and this change of flow pattern caused disturbances for the sediments, whereby the median and mean changed from 2.2 to 3 (0.22-0.125 mm) and 1.5 to 4.4 phi (0.35-0.046 mm), respectively (Figures 12(a) and 12(b)). The depth of water from CS2 until CS5a was between 2-6 m (Figure 11). Further downstream, between CS5b, CS5a and CS5, the deep channel changes from right to the left and again to the right (Figure 8). The median and mean size of the sediments is within the range of 2.2-2.4 phi (0.22-0.17 mm) and 1.5-3.5 phi (0.35-0.088 mm), respectively (Figures 12(a) and 12(b)). The sorting of the sediments in this section of the river were poorly to well sorted, although the majority were well sorted (Figure 13(a)). The skewness of the sediment shows that they were either strongly fine skewed to fine skewed (Figure 13(b)). The kurtosis of the sediment was mainly very leptokurtic to leptokurtic (Figure 13(c)). It seems that the flow through the meander of the river at this part did not disturb the sediment.
The area between CS5 to CS6-5 showed that the deep part of the channel shifted from the right to the left at CS6-1; eventually an island appeared on the left side (Figure 11). The water depth in this part was about 4-6 m, but it should be mentioned that in the outer part of the meander it reached 8 m in some parts, while on the inner part of the meander it decreased to less than 2m. The disturbance of flow due to the presence of the island within
(a) (b)
Figure 12: Distribution of the sediment (a) median grain size; (b) mean grain size of the bed of the Tigris River in Baghdad.
the meander disturbed the characteristics of the sediment. The median and mean size was of the order of 2.2-3.6 phi (0.22-0.08 mm) and 1.5-3.5 phi (0.35-0.088 mm), respectively (Figures 12(a), 12(b)). The sorting of the sediments was mainly poorly to moderately well sorted, while they were strongly fine skewed to fine skewed, and very lep-tokurtic to mesokurtic (Figures 13(a)-13(c)). Between CS6-5 and CS7, the channel of the river is relatively straight. The deep channel is confined to the left side. The water depth in this part of the river was varying between 2-6 m. It seems that the flow was relatively not disturbed due to the effect the distribution of the sediment that had median and mean size of 2.2-2.4 phi (0.22-0.18 mm) and 1.5-2.5 phi (0.35-0.177 mm) respectively. In addition, the sediments were mainly well sorted to moderately well sorted, while they were fine skewed, and mainly leptokurtic.
Downstream CS7 the deep channel shifts from the left side toward the right at CS9. A meander exists in this area. At the outer part of the meander the depth reaches up to
12-14 m, while it reaches less than 2 m in the inner part of the meander (Figure 12). It should be mentioned, however, that the area on the left side of the river between CS 7 and CS8a was dredged, and the left bank of CS8 was scraped by excavators [1,2]. Furthermore, a bridge exists downstream from CS8. The median size of the sediment grains in this area starts at 2.2 phi (0.22 mm) and gradually increases up to 3 phi (0.125 mm); mean starts at 2.3 up to 3.5 phi (0.180.088 mm). The sediments within the vicinity of CS7 are well sorted, and subsequently the sorting gradually deteriorates to poorly sorted at CS8a, then returns to well sorted when reaching CS8 and deteriorates again to poorly sorted at CS9 (Figure 13(a)). The skewness and kurtosis of the sediment in this part of the river were strongly fine skewed and leptokurtic to very leptokurtic, respectively (Figures 13(b)-13(c)). It seems that the variations in the characteristics of the sediment are due to the dredging operations and the existence of the bridge.
Figure 13: Distribution of the sediment (a) sorting; (b) skewness; (c) kurtosis of the bed of the Tigris River in Baghdad.
The last section of the reach extends from CS9 to CS14 at Sarai Baghdad gauging station (Figures 8 and 11). Two meanders exist within in this section. It can be noticed that the deep part of the channel generally follows the outer side of the meanders always and reaches up to 14 m in depth. Midway between CS9 to CS11 dredging operations took place on the right bank side, then on the left side until CS13. It is noteworthy to mention that a bridge, just 200 m upstream from CS11, was knocked down in 2003 [1,2]. The sediments in this part of the river have a median diameter of 2.4-3 phi (0.18-0.125 mm) in the first meander, which then drops to 2.2 phi (0.22 mm) along the downstream meander (Figure 12(a)). The mean sediment size is entirely between 2.5 to 3.5 phi (0.177-0.088 mm) in the first meander between CS9 and CS11, while it ranges from 1.5 to 3.5 phi (0.35-0.088 mm) in the second meander (CS11-CS14) (Figure 12(b)). Moderately to moderately well sorted sediments are noticed in the first meander, and the sorting decreases to poorly sorted in the second meander (Figure 13(a)). As far as the skewness of the sediments is concerned, they were mainly strongly fine skewed in the first meander, changing to fine skewed in the second meander (Figure 13(b)). Finally, the kurtosis of the sediment within the first meander was mainly leptokurtic, while it changed to mesokurtic downstream from the second meander (Figure 13(c)). In this reach, it is believed that dredging operations and the collapse of the iron bridge caused diversions of the flow that had their effect on the distribution of sediment and their characteristics.
At Sarai gauging station, the cross section of the river changed with time (Figure 14). It seems that the cross sectional area has been decreasing with time since 1971. It is believed that the decrease of the quantity of flow (Figure 4) is causing a decrease in the capacity and competence of the river. Sections taken at Sarai gauging station since 1976 indicate that 34% of the area of the cross section was reduced when a 2012 survey is considered.
Previous work at Sarai gauging station in Baghdad [31] indicates that the average annual sediment discharge at the station was 4.6 million tonnes during the period 1969/70-1974/75. Later, Al-Ansari and Toma [29] calculated the annual sediment discharge for the period 1958-1985; an average of about 2.36 million tonnes was transported. In addition, during March, April and May 66% of the load is usually transported [28, 29, 32, 33]. It should be mentioned, however, that the average mean daily discharge at that period was 1,140 m3/s and it has dropped to 522 m3/s recently. This implies that the load being currently transported is less than it used to be, especially before 1999, when the construction of the Adhaim dam took place.
Figure 14: Cross section of River Tigris at Sari gauging station ait different periods.
6 Conclusions
Sediments of the bed of the River Tigris were studied through a reach about 18 km long starting from the center of Baghdad at Sarai gauging station upstream to Al-Muthna Bridge. It was noticed that fine sand covers the bed of the river and the sand:silt:clay ratio was 90.74:6.86:2.4. The average median size within the reach was 2.49 phi (0.18 mm) while the mean size was 2.58 phi (0.15 mm). In addition, the sediments were moderately sorted, fine skewed and leptokurtic. The size of the bed sediment decreased relative to previous studies. This is due to reduction of flow and the construction of a dam on Adhaim tributary in 1999, which used to be the main sediment supplier to the Tigris River before entering Baghdad. The effect of climate change and the construction of dams is reflected in the flow of the river, where the discharge of the Tigris River for the period 2000-2013 (522 m3/s) has decreased by about 57% and 44% since 2000 compared to the period 1931-1959 (1,207 m3/s) and 1960-1999 (927 m3/s ), respectively. This suggests that the reduction of annual flow from the 1940s to 2000s reached 59.3%. The bed level has increased compared to previous surveys. This is believed to be due to the decrease in the capacity and competence of the river to transport sediment; the cross sectional area at Sari gauging station, as an example, has been reduced by 34% since 1976.
It was noticed that dredging operations and obstructions (e.g. fallen bridge and islands) have disturbed the flow of the river in several sites. This has disturbed the characteristics of the sediment in the vicinity of such areas.
Acknowledgement: The research presented has been financially supported by Lulea University of Technology, Sweden and by "Swedish Hydropower Cen- tre—SVC" established by the Swedish Energy Agency, Elforsk and Sven-ska Kraftnat, together with Lulea University of Technology, The Royal Institute of Technology, Chalmers University of Technology and Uppsala University. Their support is highly appreciated. The Iraqi Ministry of Water Resources gratefully helped the authors to conduct field works. Baghdad University gratefully granted a scholarship for the second author.
References
[1] Ali A.A., Al-Ansari N.A., Knutsson S., Morphology of Tigris River within Baghdad City, J. Hydro. Earth Syst. Sci. 2012,16,1-8.
[2] Ali A.A., Al-Suhail Q., Al-Ansari N.A., Knutsson S., Evaluation of Dredging Operations for Tigris River within Baghdad, Iraq, J. Water Resources and Protection 2014, 6(4), 202-213.
[3] IMoWR (Iraqi Ministry of Water Resources), The aims of the dredging of Tigris River, http://www.IMoWR.gov.iq/?q=node/ 571 (accessed on 20-8-2013), 2013, (in Arabic).
[4] Geohydraulique, Tigris River Training Project within Baghdad City, Report Submitted to the Iraqi Ministry of Irrigation, Paris, 1977.
[5] University of Technology, Training of Tigris River inside Baghdad City, Report Submitted to the Iraqi Ministry, Baghdad, 1992.
[6] United Nations Iraq, Iraq provinces, available online: http:// www.uniraq.org/images/GEOs_Iraq.jpg
[7] Map of Baghdad City, available online: http://www. globalsecurity.org/military/world/iraq/images/baghdad_ nima_2003.jpg
[8] Al-Ansari N.A., Knutsson S., Toward Prudent management of Water Resources in Iraq, J. Advanced Science and Engineering Research 2011, 1, 53-67.
[9] Al-Ansari N.A., Management of Water Resources in Iraq: Perspectives and Prognoses, J. Engineering 2013, 5(8), 667-684.
[10] ESCWA (Economic and Social Commission for Western Asia), Inventory of Shared Water Resources in Western Asia, Salim Dab-bous Printing Co., Beirut, 2013.
[11] Al-Ansari N.A., Ali A.A., Knutsson S., Present conditions and Future Challenges of Water Resources Problems in Iraq, J. Water Resources and Protection 2014, 6(12), 1066-1098.
[12] MWR (Ministry of Water Resources - Iraq), Schematic diagram of main control structures in Iraq, General directorate of water resources management, hydrological studies centre, Baghdad, 2005.
[13] Al-Shahrabaly Q.M., River discharges for Tigris and Euphrates gauging stations, Ministry of Water Resources, Baghdad, 2008, (in Arabic).
[14] Ali A.A., Morphology of Tigris River inside Baghdad City, Licentiate Thesis, Lulea University of Technology, Sweden, 2013.
[15] Chen X.J., YangX.H., He J., Xia X.H., Mujere N., Analysis of Rainfall Variations Under Climate Change in Miyun Reservoir, Thermal Science 2014, 18, 5, 1481-1485.
[16] Li Di C., YangX.H., Xia X. H., ChenX.J., Li J.Q., Multi-Scale Modeling of the Response of Runoff to Climate Change, Thermal Science 2014, 18, 5,1511-1518.
[17] YangX.H., He J., Li DiC., Li J.Q., Vulnerability of Assessing Water Resources by the improved Set Pair Analysis, Thermal Science 2014,18, 5,1531-1535.
[18] Al-Ansari N.A., Abdellatif M., Ali S., Knutsson S., Long Term Effect of Climate Change on Rainfall in Northwest Iraq, Central European Journal of Engineering 2014, V.4, No.3, 250-263.
[19] Al-Ansari N.A., Abdellatif M., Ezeelden M., Ali S., Knutsson S., Climate Change and Future Long Term Trends of Rainfall at North-eastern Part of Iraq J. Civil Engineering and Architecture 2014, V.8, No.6, 790-805.
[20] Al-Ansari N.A., Abdellatif M., Zakaria S., Mustafa Y., Knutsson S., Future Prospects for Macro Rainwater Harvesting (RWH) technique in north east Iraq, J. Water Resource and Protection 2014, V.6, No.5, 403-420.
[21] Al-Ansari N.A., Abdellatif M., Al-Khateeb M., Knutsson S., 2014, Desertification and Future rainfall trends north Iraq, 8th edition of the international scientific Congress of GIS and geospace applications Geotunis 2014 April 02 to April 06, 2014 Tunis/ Ham-mamet.
[22] Yassin O., Al-Ansari N.A., Abdellatif M., Aljawad S.B., Knutsson S., Expected Future Precipitation in Central Iraq Using LARS-WG Stochastic Weather Generator, Engineering 2014, V.6, No.12, 948-959.
[23] Issa I.E., Al-Ansari N.A., Sherwany G., Knutsson S., Expected Future of Water resources within Tigris-Euphrates Rivers basin, Iraq, J. Water Resource and Protection 2014, V.6, No.5,421-432.
[24] British Standards 1377. Methods of testing soils for civil engineering purposes. British Standards Institution, 1967.
[25] Folk R., Petrology of Sedimentary Rocks, Hemphill Publishing Company, Austin, Texas, 1980.
[26] USDA, Textural soil classification, Module 3, Soil Mechanics Level 1. ftp://ftp.wcc.nrcs.usda.gov/wntsc/HSH/training/ soilsOther/soil-USDA-textural-class.pdf, 1987.
[27] Blott S.J., Pye K., Particle size scales and classification of sediment types based on particle size distributions: Review and recommended procedures, J. Sedimentology 2012,59(7), 20712096.
[28] Al-Ansari N.A., Toma A., Bed characteristics of the River Tigris within Baghdad, J. Water Res. 1984, 3(1), 1-23.
[29] Al-Ansari N.A., Toma A., Suspended Sediment waves in the River Tigris at Sari station for the year 1982, Inter. Symp. on Erosion, Debris flow and Disaster Prevention Keynote, Japan, 1985,157162.
[30] Visher G.S., Grain size distribution and depositional processes, J. Sedimentary Petrology 1969, 39(3), 1074-1106.
[31] Al-Ansari N.A., Ali S.H., Taqa A., Sediment discharge in the River Tigris at Baghdad (Iraq), International Congress in Hydrological Science, Canberra, Australia, 1979, IAHS Pub. No. 128,399-407.
[32] Al-Ansari N.A., Ali J.L., Suspended load and solute discharge in River Tigris within Baghdad, J. Water Res. 1986, 5, 35-50.
[33] Al-Ansari N.A., Toma A., Al-Sinawi G.T., Continuous monitoring of suspended sediment and solute loads of the River Tigris at Baghdad, J. Geol. Soc. Iraq 1986,19(1), 1-23.
