Impact of Growing Islands on the Flood Capacity of Tigris River in Baghdad City Ammar A. Ali, Nadhir A. Al-Ansari, Sven Knutsson Department of Civil, Environmental and Natural Resources Engineering, Lulea University of Technology, Sweden Emails: [email protected], [email protected], [email protected] INTRODUCTION In recent years, substantial changes have occurred in the morphology of the Tigris River within Baghdad City. Although huge volumes of sediment are being trapped in recently constructed headwater reservoirs, the number of islands in the Tigris at Baghdad is increasing. The debris of bridges destroyed in the wars of 1991 and 2003 and their subsequent reconstruction have enhanced the development of these islands. As a consequence the ability of the river to carry the peaks of flood waters has been reduced. This has led to potential increase of flooding in parts of the city. Map of Iraq showing the Tigris and Euphrates Rivers. Al-Muthana Bridge Diyala River Tigris River Confluence of Tigris with Diyala Rivers Aerial photo of Baghdad City DISCHARGES OF TIGRIS RIVER FOR THE PERIOD 1960-2010 DISCHARGE OF TIGRIS RIVER FOR THE PERIOD 1990-2010 1800 1600 1400 Average Monthly Discharge for 1200 years 1990~2010 19% the slope of the (674 cumecs) 1000 trend line 800 600 400 Average Monthly Discharge Average Monthly (cumecs) Discharge Monthly Average for years 2000~2010 (531 cumecs) 200 Months CONTROL STRUCTURES UPSTREAM BAGHDAD CITY Mosul Dam Dokan Dam Adhaim Dam Samara Barrage Hemrin Dam Baghdad City BRIDGES ON Muthana Bridge TIGRIS RIVER WITHIN BAGHDAD CITY Al-A’ameh Bridge Adamiyah Bridge Sarafia Bridge Bab Muadam Bridge Al-Ahrar Bridge Al-Shuhada Bridge Jumhuriya Bridge Sinak Bridge Suspension Bridge Jadiriyah Bridge Twin Bridge Al-Rasheed Bridge Tigris – Diyala confluence CHANGES IN TIGRIS RIVER BED ELEVATIONS DURING 1976, 1991 AND 2008 28 27 26 25 24 23 22 21 20 19 Bed Level (m) Bed Level 18 17 16 15 1976 14 1991 13 2008 12 25000 20000 15000 10000 5000 0 Distance (m) CHANGES IN GEOMETRY SHAPE OF SARAI BAGHDAD GAUGING STATION 39 37 35 33 31 29 27 25 Elevation Elevation (m) 23 21 19 1976 1991 17 2008 15 10 30 50 70 90 -10 110 130 150 170 190 210 Station (m) HYDRAULIC MODEL • A one-dimensional steady-flow hydraulic model using HEC-RAS was used to predict the maximum flood capacity for the river. • The upstream condition was the average daily discharge in Baghdad for the past ten years. • The downstream condition was the updated rating curve at Diyala River confluence. METHODOLOGY River geometry • The survey conducted in 2008 • 105 cross sections at average intervals of 250m. • The length of the reach 26 km, starts from Suspension Bridge at u/s till the confluence with Diyala River at d/s. METHODOLOGY Model calibration • 59 observation points for water levels along last 15 km of the reach were used to calibrate the model. These observations were recorded at the same day for a discharge of 400 m3/s. • Modification for Manning’s ‘n’ for the main channel and the floodplain was included to achieve coincidences between computed water surfaces and observed one. • Minimum R.M.S.E. (0.026m) was achieved for Manning’s ‘n’ as 0.0285 for the main channel and 0.042 for floodplains. • 5 m3/s discharge for Diyala River was considered as lateral inflow. • No precise data available for water consumption along the reach. METHODOLOGY Model application • To find out the critical discharge that can cause inundation, a set of discharges starting from the average daily discharge for the past ten years were used in model as u/s condition with the updated rating curve at d/s. • Most of these discharges were examined for the same area in previous studies (Geohydraulique in 1977 and University of Technology-Baghdad in 1992). Observed water surface elevations with the computed for different discharges of Tigris River and 5m3/s discharge of Diyala River. 37 36 35 34 33 32 31 30 29 28 27 Water Water Surface Elevation (m) 26 25 400 500 800 1100 1300 1500 24 2500 2700 3500 4000 Banks Observed (R.M.S.E. = 0.026m) 23 25000 20000 15000 10000 5000 0 Distance (m) RESULTS AND CONCLUSIONS • The critical elevation for inundation is 35m at station 22+000m • The inundation could take place along approximately 3 km reach of the Tigris River for discharges greater than 4000 m3/s. • No significant effect found for more three lateral inflow values (25, 50 and 100 m3/s) for Diyala River on the water surface elevations in Tigris River with highest discharges. RESULTS AND CONCLUSIONS • Since the water is eroding below the protected banks now, this will lead to the collapse of parts of these protecting banks in future. • The bed levels fluctuation was the minimum according to the 2008 survey relative to those of 1976 and 1991. • The average slope of the river bed became higher in 2008 compared with previous surveys. .