ROVED CHANNELIZATION OF THE MITHI AMR FOR DRAIN. GE DECONGESTION litai

Abstract

Mumbai city experienced the unprecedented rainfall of 944.2mm in 24 hours with a peak intensity of 136 mm/hour, recorded by IMD raingauge station at Santacruz on 26`n July 2005. Extensive flooding and inundation were experienced in low lying areas adjoining Mithi River along with flooding in other regions upstream of Bandra Kurla Complex (BKC) area on 26`h and 27`n July 2005 causing loss of lives and extensive damage to public property and services. In order to suggest the means and ways to mitigate the floods of Mithi River the project authority, Mumbai Metropolitan Region Development Authority (MMRDA), Mumabi requested the Central Water and Power Research Station (CWPRS), Pune to undertake hydraulic model studies. CWPRS undertook one dimensional mathematical model studies based on CHARIMA model developed by the IIHR, University of Iowa, USA and submitted the recommendations through Technical Report (No. 4297) in January 2006. The CWPRS report recommended for deepening and widening of Mithi River and Vakola Nalla, throughout its length, with suitable bed gradient. While considering these recommendations MMRDA foresaw the problems that would arise as a consequences of implementation of these recommendations such as legal problems due to removal of authorized structures, social-political constraints in removing the slums along the banks, and phasing for widening of bridge structures. Taking into consideration these constraints, the MMRDA proposed alternative channelization proposal with restrictions imposed on the topwidth of channels at different locations of the Mithi River and Vakola Nalla. These two proposals were examined herein the context of mitigation of floods. In the background of the 25`x' July 2005 Mumbai floods, an attempt has been made in this study to estimate the flood water levels at different locations along the Mithi River and the Vakola Nalla using the HEC-RAS (version 4.0) software developed by the U.S. Army Crops of Engineers for the existing channel conditions and for the two channelization proposals mentioned above. The consequence implementing both these proposals were studied by simulating the maximum flood water levels at different locations along the Mithi River and Vakola Nalla for the monsoon and non-monsoon conditions and for the same rainfall events of 50 year and 100 year return periods, which were applied for simulating maximum flood water levels in the existing channel conditions in the presence of specified spring tidal conditions that would create possible worst interior flooding conditions. As no drainage network details were made available by the project authority, except the cross section details of the Mithi River that existed after the 26`h July, 2005 flood, the following study strategy was adopted: The entire Mithi river basin was segregated into six sub-basins including the two headwater sub-basins, the one located at the top of the basin consisting of the Vihar Lake and the Powai Lake, and the other being the Vakola Nalla sub-basin near the outlet of the Mithi River basin. It is assumed, in the absence of drainage network details, the basin area on either side of the river acts as overland flow planes of the basin contributing the runoff directly to the Mithi River instantaneously. The headwater sub-basins contribute runoff to the Mithi River in the form of triangular runoff hydrograph estimated by the rational method considering a runoff coefficient of unity. Using these proposed runoff v transformation mechanisms, the runoff was generated for the 50 year and 100 year return period rainfall inputs over the basin. Using the unsteady flow river routing option of the HEC-RAS, the upstream triangular hydrograph was routed along the river with runoff generated from other sub-basins entering the river as uniform lateral inflow. The highest tidal conditions observed in June 2007 were used as the downstream boundary conditions required for unsteady flow simulation. The maximum flood water level was simulated for the above flood formation scenario. Now the proposed channelization options were introduced to study their flood mitigation capability in the form of reducing maximum water level generated for the above flooding scenario under existing channel conditions. It was found that considerable lowering of flood water level along the Mithi River and Vakola Nalla would be achieved based on the CWPRS channelization proposal in comparison with that of the MMRDA proposal. Also the study revels that the tidal water exchange between the Mithi River and Arabian Sea and vice-a versa would be reduced considerably during non-monsoon when adopting the MMRDA proposal, which is undesirable from the environmental considerations. vi