Study of Morphological Behavior of a Selected Reach of Gorai River in Bangladesh Md Arifur Rahman 1 , Shah Imran Kabir 2 , 1,2 University of Texas at Arlington, Civil Engineering, Arlington, TX, United States Abstract The Gorai River, one of the important distributaries of the mighty river Ganges recently facing severe problem of sedimentation, especially after the construction of Farakka barrage locatedat the upstream. Moreover, the river is morphologically active because of its dynamic fluvial processes. Therefore, the objective of this study is to estimate the hydrodynamic and morphological changes by using Delft 3D FM model. In this study the morphological behavior of Gorai River has been studied through historical cross-sectional data analysis, plan form analysis and morphological modeling. A reach of Gorai River from Gorai Railway Bridge to Kumarkhali is selected for this study as the discharge and water level data are available at these locations. From the hydrodynamic model simulation, it is found that the cross sectional mean velocity varies in between 0.35 m/s to 1.17 m/s. Moreover, the water depth during the dry period varies from 3.9 m to 7m and the level rises to 12 m during flood period. The historical data analysis reveals that the maximum reduction of cross-sectional area within the reach is 47 %. The geometric parameters such as thalweg, mean cross sectional depth and width/depth ratio also show deceasing trends, indicating that the river is being silted up gradually as a consequence of reduced flow. However, the planform analysis for 12-year period shows that the banks of the Gorai River are morphologically stable and there is no shifting of bank lines found for 12 years period. Moreover, the morphological modeling also reveals that the main channel at the u/s of the reach will be divided into two or three channels in 20 years if the present conditions remain the same as the morphological factor of 20 is used in the model. It is also discernible that erosion occurs in July when there are flood flows however, sediment is deposited in November during the beginning of dry period as the flow rate decreases resulting in reduced flow velocity. The overall morphological analysis as discussed in this study shows that the banks of Gorai River are morphologically stable. However, it has been silted up significantly in recent years due to lack of enough flow and the river will face more sedimentation problem in the near future if the prevailing flow conditions remain the same or decreases. Objectives Study Area The Gorai takes off from the Ganges at Talbaria, north of kushtia town and 19 km downstream from Hardinge Bridge. South of Kushtia its first offshoot, the Kaliganga branches off to join the Kumar near Shailkupa. The Gorai has a flood discharge of nearly 7,000 cumec but in winter its flow goes down to five cumec. The course of the Gorai is wide, long and meandering. In the downstream it is navigable throughout the year. Maximum recorded flow at Kamarkhali is 7,932 cumec. The width of the river increases as it flows down and at the end it is about 3 km. However, the study area has been selected from Gorai Railway Bridge to Kumarkhali where observed discharge and water level data are available. The Fig-1 shows the extent of the study area.  To analyze historical data in order to study the morphological behavior of the River Gorai.  To develop a 2D-hydrodynamic and morphological model for 10-15 km long reach of Gorai River.  To estimate the hydrodynamic parameters such as velocity and water level at different location of the reach.  To assess the morphological parameters including the sediment concentration and cumulative erosion-deposition at some selected locations of the river reach. Results Methodology Fig-1: Gorai River in Bangladesh Acknowledgement This work is supported by the authors themselves. The authors are grateful to Bangladesh Water Development Board (BWDB) providing the necessary data for this study and the Delft-Hydrulics for providing the license for the Delft-3D software From the historical cross-sectional data analysis, it is investigated that the maximum reduction in cross-sectional area within the reach is 47 % and this deposition occurs in RMGM 8 for the year 2006. However, no erosion phenomenon was found within the reach except a very little erosion for the year 2006 in the vicinity of RMGM 11. It is also obvious that deposition is dominant over erosion along the reach of Gorai River due to the reduced flow condition in the recent years. The line joining the deepest points of river cross sections known as thalweg that also shows a decreasing trend. Again, the analysis of geometrical parameters such as mean depth and width/Depth ratio depicts also decreasing trend which indicates that the river has deposited its sediments as a consequence of reduced flow. However, the planform analysis for 12-year period shows that the banks of the Gorai river are morphologically stable, that’s why there is no shifting of lines found in 12 years. The overall morphological analysis as discussed in this study shows that the banks of Gorai River are morphologically stable for 12 years periods. However, from historical cross-sectional data analysis, thalweg profile analysis and geometric parameters analysis, and Morphological modeling, it has been found that the Gorai River has been silted up significantly in recent years due to lack of enough flow in the Gorai River. The river will be silted up more in the near future if the prevailing flow conditions are remaining the same or decreases. References [1] Banglapedia (2014). ‘Ganges-Padma River System’, in Banglapedia. [2] Rahman, A., & Yunus, A. (2016). Hydrodynamic and Morphological Response to Dredging: Analysis on Gorai River of Bangladesh. International Journal of Innovative Research in Science, Engineering and Technology, 15610-15618. [3] User Manual Delft3D-FLOW, 2011, WL | Delft Hydraulics, the Netherlands. H13K-1867 Conclusions 0 500 1000 1500 2000 2500 3000 3500 4000 01-Jan-10 01-May-10 29-Aug-10 27-Dec-10 Discharge (cms) Time (days) Discharge near Gorai Railway Bridge 0 2 4 6 8 10 12 14 01-Jan-10 01-May-10 29-Aug-10 27-Dec-10 Water level (mPWD) Time (days) Water level near Kumarkhali 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 01-Jun-10 11-Jun-10 21-Jun-10 01-Jul-10 11-Jul-10 21-Jul-10 31-Jul-10 Water Lvel (mPWD) Time (Days) Model Calibration at Kamarkhali Observed_WL Simulated_WL 3 4 5 6 7 8 9 01-Aug-10 10-Sep-10 20-Oct-10 29-Nov-10 Water Level (mPWD) Time (days) Model Validation at Kamarkhali Observed_WL Simulated_WL 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1-Jan-2010 1-May-2010 29-Aug-2010 27-Dec-2010 X-Sectional Mean Velocity (m/s) Time (Date) Fig-2:Image of study area Fig-3:2D mesh for delft 3D model Fig-5:U/S discharge boundary condition Fig-6:D/S water level boundary condition Fig-7:Model calibration at Kamarkhali Fig-8:Model validation at Kamarkhali Fig-9: X-Sectional Mean Velocity 3 4 5 6 7 8 9 10 11 12 13 1-Jan-2010 29-Aug-2010 Water Level (mPWD) Time (date) WL at U/S WL at 1st Bend WL at 2nd Bend WL at D/S 0 2 4 6 8 10 12 14 16 0 500 1,000 1,500 2,000 Elevation (mPWD) Distance from left bank (m) 1-Mar-01 30-Mar-06 25-Mar-10 30-Apr-14 -8 -6 -4 -2 0 2 4 6 0 2 4 6 8 10 12 Elevation (mPWD) Distance (km) Thalweg Profile 2000 2006 2010 2014 Fig-10: Variation of WL Fig11: Comparison of X-sections Fig 12: Comparison of thalweg profile Fig-13: Comparison of Plan form Types of data River Name Station ID Station Name Data Periods Discharge Gorai SW 99 Railway Bridge 1980-2016 Gorai SW 101 Kamarkhali Transit 1983-2016 Water level Gorai SW 99 (NT) Railway Bridge 1970-2016 Gorai SW 101 (NT) Kamarkhali Transit 1970-2016 X-Section Gorai RMGM(1-42) 200 Km reach 2000-2014 Sediment Ganges (SW-91) Hardinge bridge 2000-2014 Ganges SW-101.5 Kamarkhali transit 2000-2014 Image Gorai - Reach 2006-2018 Fig-4:Model bed level generation Fig-14: Spatial Velocity distribution Fig-15: Spatial Velocity distribution 0 5000 10000 15000 20000 25000 Area of Cross section (m2) BWDB Cross Sections Change of X-Section over 14 years 2000 2006 2010 2014 -50 -40 -30 -20 -10 0 10 20 30 % Change of X-Sectional Area BWDB X-Sections Erosion-Deposition within the model reach 2006 2010 2014 Deposition Erosion ID X-Sectional Area, A (m 2 ) % Change Year 00 & 06 00 & 10 00 & 14 00 06 10 14 6 24446 16026 20986 16436 -34 -14 -32 7 10565 7322 10028 8542 -30 -5 -19 8 9194 4817 7503 5578 -47 -18 -39 9 7358 5656 5686 5863 -23 -22 -20 10 5684 3788 4316 4223 -33 -24 -25 11 7040 7167 6077 5862 1 -13 -16 12 7531 6759 6295 5078 -10 -16 -32 2 3 4 5 6 7 8 1 2 3 4 5 6 7 Mean Depth (m) Distance (km) Mean Depth Profile within the reach 2000 2006 2010 2014 -6 -4 -2 0 2 4 6 8 10 12 14 16 0 200 400 600 800 1000 Bed Elevation (mPWD) Distance from left bank (m) Erosion-Deposition at U/S section January July November 0 2 4 6 8 10 12 0 300 600 900 1200 Bed Elevation (mPWD) Distance from left bend (m) Erosion-Deposition at 1st bend January July November From the simulation of morphological modeling where a morphological factor of 20 is used insinuating that the model has been simulated for 20 years period as the hydrodynamic run time is one year , it is evident that the main channel at the u/s of the reach will be divided into two or three channels in 20 years if the present conditions remains the same. It is also discernible that erosion occurs in July when there are flood flows however, sediment is deposited in November during the beginning of dry period as the flow rate decreases results in reduced flow velocity. This erosion deposition phenomena are supported by the sediment transport rate. The simulated sediment transport rates follow the decreasing trend along the reach. During the period of lean flow sediment sand transport rate varies from 0.01 kg/s to 0.1 kg/s and the transport rate increases to 1.85 kg/s during the flood period. Moreover, the mud transport rate also increases with the flow and decreases as the flow rate decreases and the rate varies from 0.001 kg/s in dry season to 0.027 kg/s in flood season. Fig-16: Comparison of X-sectional area Fig-17: X-sectional erosion-deposition Fig-18: Comparison of mean depth profile Fig-19: Time varying bottom change Fig-20: Erosion-Deposition at the u/s section Fig-21: Erosion-Deposition at 1 st bend