Evaluation of Lesser Zab River Course Change between Dokan and Dibbs Dams (N-Iraq), Using GIS Techniques

Abstract


Introduction
The main objective of constructing dams on a river course is the water storage and electrical production besides regulating the flow to reduce the ward off flood risks. However, damming the river courses may affect negatively the ecosystem and the surroundings.
The lesser Zab River is one of the major tributaries of the Tigris River in Iraq. It is a transboundary river that originated in Iran and flows in an S-SE direction and enters the Iraqi region near Mawat town, joined with the Siwayl River, and flows then in an NW direction towards Rania town until it enters the Dokan Dam Reservoir. The river lesser Zab joins the Tigris River at (120 m) elevation in Al-Zwayiah village in Sherkat District (South Mosul Governate), and the total length of the river is about (400Km) with a catchment area of about 22,250 km 2 (Hassan et al., 2016). Two dams that have been constructed on the lesser Zab course in Iraq are Dukan and Dibbs Dams. Dukan Dam is a multi-purpose concrete arch dam, it was built between (1954 and 1959) to provide water storage, irrigation, hydroelectricity, and for regulating river flow during high rainfall seasons (Nurit, 1994). Whereas Dibbs Dam is a gravel-alluvial fill embankment dam, it was constructed between (1960 and 1965) to divert water from the river into the Kirkuk Irrigation Project (Adamo et al., 2020).
The impact of damming on river systems and geomorphology has been studied since the beginning of the global development of large reservoirs around the 1950s (Ronco et al., 2010). Morphological changes below the dams, river bed elevation, and channel cross-sections, besides the effect of the extensive change in water discharge transport capacity from the regulated natural runoff, are examined in the early works of Lane (1955) and Schumm (1960). Modifications of river morphology upstream and downstream of the dams have been studied by (Kondolf,1997;Brandt, 2000;Zhou et al., 2004;Zaghloul, 2006;Wang et al., 2007). Lewin (1977) recommended that the changes in floodplain hydrology following serial damming of a river may be natural, due to a change in precipitation or flood frequency, or anthropogenic, either direct (construction of a dam), or indirect (change in land use). Morphological changes in the downstream reaches of the dam have been reported using computer programs such as HEC-6 and GSTARS simulated by (Yang and Simoes, 2008). (Zuo and Liang, 2015) studied the effects of dams on river flow regimes based on IHA/RVA methods. (Langat et al., 2019) declares that the use of remote sensing data with GIS techniques and tools provides efficient and economical quantitative spatial and temporal analysis of river channel changes. (Roccati et al., 2019) used number of historical and recent maps and satellite images with GIS tools to calculate the morphological changes and human impact in the Entella River floodplain (Northern Italy) from the 17th century to the present.
Locally, previous studies on river morphology are rare. Most are on water quality and sedimentation of the river, and few are on numerical simulations or observations. Such as the Study of morphometric parameters of the lower part of lesser Zab using the GIS technique by (Abdulla, 2011). Ali (2012) studied the major and trace element distribution in stream sediments of the Lesser Zab River and found that the enrichment factor and the geo-accumulation index of the bed sediments are of natural sources and due to anthropogenic activities. (Al-Dabbas et al., 2020) studied the management of Bai Hassan unconfined aquifer within the lesser Zab River basin, in Kurdistan region/Iraq using a modeling approach, the resulting model predictions allow greater insight into the potential consequences of pumping the aquifer and provide science-based input to resource management decisions. (Sadeq and Alhamdany, 2021) studies the geometry of the Lesser Zab River channel between Altun-Kupri City and Dibbs Dam at Kirkuk (Northern Iraq) and investigate the morphological changes in the river channel. The results show that river discharge does have a direct linear relationship with the morphologic parameters of the river channel.
Therefore, the main aim of this study is to investigate the changes in the Lesser Zab River morphology at the downstream reach of the Dokan dam, using the information from the daily discharge data and the Satellite images of the river over time.

Study Area
The study area is part of lesser Zab River (NE Iraq), lies between latitude 35° 41′19″N to 35°57′15″N and longitude 44°06′33″E to 44°57′10″E (Fig. 1). Starting from Dokan Dam (about (60 km) NW of Suleimani city) and passing through several towns and villages in the Kurdistan Region of Iraq, and ends at the mouth of Dibbs Dam (about 45 Km to the NE of Kirkuk city). The climate is humid to sub-humid, with an average annual rainfall of (710-384 mm/year) at the upper and lower reach of the study area, respectively (Jirjees et al., 2020). Geologically; the main formations that are exposed at the surface of the northeastern part of the study area ( Dokan Dam Reservoir area) are of the Late to Middle Cretaceous age, which are: (Qamchuqa, Kometan, Shiranish, Tanjero, Kolosh, and Sinjar formations with recent alluvial deposits), While the rock units in the middle and southern part of the study area belong to the Tertiary age ( Fig. 2), where the main exposed formations are: Fatha (middle Miocene), Injana (Late Miocene), Mukdadiya (Pliocene), Bai Hassan formations (Pleistocene to Pliocene), and Quaternary deposits (Jassim and Goff, 2006). Structurally, the studied area lies in the high Folded and low folded zones of the platform foreland of Iraq (Numan, 2001), where the anticlines are a tight double plunging anticline and their northeastern limb are steeper than southwestern one, thus they have been formed by Fault propagation (Khanaqa and Karim, 2015). Meanwhile, the Dukan Dam was constructed across the Lesser Zab River on the axis of the asymmetrical doubly plunging Sara anticline through a transversal narrow deep valley (Jassim and Buday, 2006).
Topographically Lesser Zab in the study area flows from the mountainous region with an elevation of about (560 m) a.m.s.l. then extends to the southwest through a series of hills and plains till it joins the Tigris River at an elevation of (120 m) a.m.s.l. (Fig.3).

Data Acquisition
The change in the channel pattern of Lesser Zab River is measured using ArcGIS (10.1) software, based on the Digital Elevation Model (DEM 30x30 m) adopted from Shuttle Radar Topography Mission (STRM) that (4) Landsat images taken from the United States Geological Survey for the years (1980,2002,2012, and 2020). Each image was georeferenced to the UTM coordinate system. The Lesser Zab River is extracted by editing Spatial Analysis Tool-Hydrology -Arc Tool Box in ArcGIS using the following steps: Arc Tool Box > Conversion Tools > To Raster and open polyline to Raster Tool-stream raster streamline calculator), overlapped bank line of different periods of Lesser Zab River image.
The daily discharge of the river are measured at different stations (upstream and downstream of Dokan dam) and (Dibbs dam) gauging Stations by (The Iraqi Ministry Of water resources, General Authority for Dams and Reservoirs, Dokan Dam). The annual and mean monthly discharge records are measured from Dokan Dam downstream gauging station for the years (1980-2020) and used to study its impact on the river channel regime. River length has been measured from tools in ArcGIS by converting polygons to polylines so as to measure the shortest distance of the midline of the river channel and then evaluate the river's meandering characteristics following (Leopold and Wolman, 1957), These parameters were used to find out the main factors that controls the development of the rivers (Khan et al., 2018). sinuosity index is measured according to a formula based on the length of the midline of the channel (Lcmax) and the total length (LP) of the stream (Morisawa, 1985).

Results
Over tens of years, the alignment of the Lesser Zab River has altered. The changes within the river course from downstream of Dokan Dam up to the mouth of Dibbs Dam have been studied for the years (1980) up to (2020), taking the year (1980) as the corresponding counterpart base year (Fig. 4). To calculate the changes in the length of the river, the stream channel is divided into three segments (Part 1, Part 2, and Part 3) according to the change in the river gradient (Fig. 5).  Table 1 declares that the length change in segment (Part 1) in 40 years was (325m) with an average of (8m/year), as the minimum change in the period  was (43m), and the rest was in the period (2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018)(2019)(2020). The middle segment (Part 2) shows a total change in length of (1214m) through the period , with an average of (31m/year). While the third segment (Part 3) has a total change in length of (3060m) through the study period, with a mean of (75 m/year), and the contrariwise of segment Part (1), the most changes were in the period . The upper reaches of the river at the study area constitute a sector starting from the downstream of the Dokan dam at an elevation (444 m.a.s.l.) to a point at Goptatapa village (located at latitude 350 55' 56" N and Longitude 440 25' 52" E) at an elevation of (378 m a.s.l.) just after the river bends to the southwest Fig (6). This sector has a total length of about (30 km). Along this length, the elevation of the Lesser Zab River drops about (65) meters Fig (7), with an average gradient of 0.21%.
The length change in segment (Part 1) within 40 years was (325m) with a mean change of (8m/year) and the minimum change (43m) was in the period , and the rest was in the period (2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018)(2019)(2020), indicating that the river channel is well defined and has stable bend channel with narrow, rough bed (boulders, pebble, and gravel) as the composition of the bed and bank material is from the hard rocks of Pilaspi and Fatha formations, also it is sinuous as the sinuosity index value is (>1.5) (Morisawa, 1985), and it is a single-phase, equiwidth channel according to Brice (Lagasse et al., 2004), and the curve is of increased extension simple type according to (Hooke, 1977). The middle section (Part 2) of the river is between the Goptapa and Taq Taq cities which are about (42 Km.) with a total elevation drop of (48m) and an average gradient of (0.117%). The river between the two cities meanders irregularly. The segment (Part 2) shows a total change in length of (1214m) through the period of (1980-2020) with an average of (31m/year), this change is may be due to the rate of downstream migration relative to the rate of growth of amplitude of bends which probably depends on the distribution of shear against banks (Leopold and Wolman, 1960). Characterized as single-phase irregular width variation, wider at the bent, and with some point bars according to Modified Brice classification of meandering channels (Lagasse et al., 2004).
The lower reaches of the river extend from Taq Taq city to the mouth of the Dibbs Dam, with a distance of about (80 km) with a total elevation drop of (85 m.), and a channel gradient of (0.14806%), the river channel in this part of the study area is characterized by moderately sinuous with several branching reaches, single phase, irregular width variation gentle slope, having point bar at the river meander, the bed features slightly consistent riffle/pool reaches, with wide alluvial channels, and distinctive floodplains (EPA, 2018). The sinuosity index was (>1.49). Thus, the LZR river meander in the studied area is of a single channel type with a well-developed bend, maximum bend sharpness. The third segment (Part 3) has a total change in length of (3060m) through the study period, with an average of (75 m/year), but on the contrariwise of segment (Part 1) the most change was at the period .

Discussion
The geometry of a river channel is the result of hydraulic flow (discharge, velocity, roughness, and shear stress), sediment load, the channel shape at the different parts of the river, and the bank material and composition (Morisawa, 1985). The cause of change in river geometry may be natural, due to a change in precipitation or flood frequency (Pradhan et al., 2021), or its due to anthropogenic causes, such as the construction of a dam/ or change in land use (Lewin, 1977). Although the variation in the daily discharge upstream depends on the precipitation in the river basin over the years (Choi et al., 2005), it is significantly reduced downstream due to the presence of the impoundments that regulate the pattern of the monthly downstream discharge. The Data examined for the river from 1980 until 2020 ( fig.8) shows that 65 % of maximum yearly discharges are in the years  with an average of (207m3/sec.), decreases in the years (2001-2020) (Average (142) m3/sec,). Fig.9 shows a maximum value (in October-January) and decreasing (in June-September). The results highlight the fact that variability of the discharge released from the Dam downstream (58-445 m 3 /Sec.), and the differences over longer time intervals cause a change in water level downstream of the dam, this will affect the distribution of erosion and deposition along the river channel. The geometry of the channel at the upper segment did not significantly change, as the comprehensive channel resistance increased under the riverbed coarsening. The middle and lower reaches of the Lesser Zab River changed due to erosion at both banks of the channel (more at the east bank of the channel especially at the lower reach of the river), and deposition at bankfulls. Moreover, the formation of different types of meandering at each segment of the river (the sinuosity index ranges between sinuosity and meandering behavior) may be due to the fluctuations in the velocity of flow at the bent of the river ( The extreme meandering was in the eastern part of the lower reach of the river, close to Altun Kopri city), where a secondary circular current would be formed in a plane perpendicular to the direction of the flow, or the Coriolis force in areas of no bend, producing dissimilarity in the velocity distribution on the two banks of the river, causing the deposition of sediments on one bank, and erosion at the other (Maurya and Agnihotri, 2017). Likewise, the channel curvature at low flow will bring about the formation of point bars at the middle and lower segments of the studied area (Nagata et al., 2013). For all the above, it was found that due to the migration of the river course, the length of the channel had been gradually increasing for about 40 years.

Conclusions
For the present study, Satellite images and GIS software were, achieved to analyze the possible morphological changes in the Lesser Zab River course at the study site. The studied data can provide a synoptic view of the changes in a river course that occurred within the area, especially when combined with other surface variables. It is an undebatable fact that the damming of the Dokan dam has a significant effect on the geometry of the lower Zab besides the natural causes such as climate change that persist for a longer period, and maybe due to change in land use.
The presence of the impoundments regulates the pattern of the monthly downstream discharge. The variability of the discharge released from the Dokan Dam, besides the differences over longer time intervals causes a change in water level downstream of the dam, and this will affect the distribution of erosion and deposition along the Lesser Zab River channel, and emphasize the changes in the main channel and reaches; either in the stable river banks as in the upper reach of the river channel that shows, well-defined, stable bend channel, with a change in the channel length over the time of the study, or not stable as in the middle and the lower reaches of the river where there is a graduality increase in the channel length, increasing the meandering index and the formation of alluvium islands at the river curves.