Three Dimensional Hydro-Morphological Modeling of Tigris River
Abstract: The River Tigris is a major river in Iraq. It divides Baghdad, the capital of Iraq, in two parts. The reach of the river within Baghdad is about 60 km long. The climate change within the region and the construction of hydraulic structures upstream of Baghdad has reduced the water discharge of the river by 44%. Despite the fact that huge volumes of sediment have been trapped in the constructed headwater reservoirs, substantial changes have occurred in the topography of the Tigris River within Baghdad City and the number of depositions is increasing. The debris of the destroyed bridges from the wars of 1991 and 2003 and their subsequent reconstruction have contributed to the development of these depositions. As a consequence, the ability of the river to carry the peak flood waters has been reduced. This has led to a potential increase of flooding in parts of the city. To predict the maximum flood capacity for the river, the bathymetric survey that was conducted for 50 km of the Tigris River by the Ministry of Water Resources in 2008 has been used with the one-dimensional flow model “HEC-RAS”. Calibration of the model was carried out using field measurements for water levels along the last 15 km of the reach, and the water level observations at the Sarai Baghdad gauging station for the last 10 years were used to validate the model. The model showed a significant reduction in the river’s capacity compared with what the river had carried during the floods of 1971 and 1988. This result agrees with previous surveys conducted on the same reach indicating that the ability of the river to convey high water has decreased. To overcome this problem, dredging operations started along most of the Tigris River inside Baghdad City to remove many islands and side bars, as well as cleaning water intakes. An examination for the dredging plan currently in progress and two additional proposed plans was conducted using the ‘HEC-RAS’ model for the 50 km long river reach to investigate whether the designed flooding capacity of the river can be recovered and how much it can be improved. Comparing the historical records of water level and discharge for the last three decades, some improvement of flood capacity was achieved. Cautions about the water intakes should be considered to maintain their functionalities with the expected drop in water levels due to dredging operations. Bathymetric and land surveys were conducted for the northern Tigris River reach (18 km length) in Baghdad, producing 180 cross sections. A riverbed topography map was established from these cross sections. Sediment transport rates and bed composition were investigated by collecting three different types of sediment samples at the quartiles of 16 cross sections along this reach. The Helley-Smith sampler was used to collect 288 bedload samples, a suction pump was used to collect 212 suspended load samples from different depths. The Van Veen grab was used to collect 46 bed material samples. The velocity profiles and the water discharges were measured using ADCP at the sampling sections. Bed sediment compositions were investigated by analysing the collected bed material samples. It was noticed that fine sand dominated the riverbed (90.74%). The average median size within the reach was 2.49 phi (0.177mm) whilst the mean size was 2.58 phi (0.16mm). In addition, the sediments were moderately sorted, fine skewed and leptokurtic. The size of the bed sediment relatively decreased compared to older investigations due to the decrease of the competence of the river. The bed elevation had increased compared to previous surveys. It was noticed that dredging operations and obstacles (e.g. fallen bridges and islands) disturbed the flow of the river and the sediment characteristics in several sites. Bedload rates were computed using the weights of the collected bedload samples. The spatial distribution of sampling cross sections took into consideration the variance of river topography where 7 meanders, 2 islands and several bank depositions characterize the geometry of the river reach. Twenty bedload predictors were applied to the same reach. The annual transported quantities of the bedload were estimated to be 36 and 50 thousand tons in 2009 and 2013 respectively. The total load discharge rate in the northern reach of the Tigris River was computed using the sediment concentrations of the collected suspended load samples after adding the bedload rate at each of the sampling cross sections. The results indicated that the suspended load is the dominant mode in the total load with a minimum percentage of 93.5%. The total load ranged from 29.1 to 190.3 kg/s. A total load rating curve of the power function was established. The associated errors from using the proposed rating curve are within reassuring levels and less than the errors produced from most of the other twenty-two total load formulas, which were applied to the same reach. The scattering of the results from the other formulas can be attributed to the spatial variance in the topography of the riverbed. According to the final results obtained, it is recommended to use the proposed procedure for establishing a spatial total load rating curve to estimate sediment rates for morphologically complicated rivers. The annual transported quantities of the total load were estimated at 2.47 and 4.23 million tons for 2009 and 2013 respectively. The three-dimensional morphodynamic model (Simulation of Sediment movements In water Intakes with Multiblock option - SSIIM) was used to simulate the velocity field and the water surface profile along the northern reach of the Tigris River using the findings of the current bathymetric survey of the river. The model was calibrated for the water levels, the velocity profiles and the sediment concentration profiles using different combinations of parameters and algorithms, those available in the model. The set of parameters that gave a minimum root mean square error (RMSE) was used for the validation process using another set of field measurements. The calibration and the validation results showed good agreement with field measurements, and the model was used to predict the future changes in river hydro-morphology for a period of 14 months. The results of the future predictions showed increases in depositions on the shallow part of the cross section having lower velocity and, on the other hand, the river deepens the incised route to fit its current hydrologic condition leaving the former wide section as a floodplain for the newer river. The net deposition/erosion rate was 67.44 kg/s in average and the total deposition quantity was 2.12 million tons annually. The locations of depositions are compatible with those of the river in the real world. An expansion in the size of current islands was predicted. An indication of the potential threats of the river banks’ collapse and the bridge piers’ instability was given by high erosion along the thalweg line.Keywords: Flood capacity, Dredging, HEC-RAS, Bathymetric survey, Bed sediment, Bedload, Total load, Helley-Smith sampler, Sediment transport, ADCP, Prediction formulas, 3-D morphodynamic model, Bed changes, SSIIM, underfit river, regulated river, Tigris River, Baghdad.
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