STATUS OF WORKS CARRIED OUT BY CWPRS FOR MMRDA INTRODUCTION: The association of CWPRS with MMRDA dates back to the year 1970 and the first study is related to the reclamation in the vicinity of Mithi river (Mahim Creek) in BKC area. Out of the original water spread area of 800 ha, as reported in 1930, about 400 ha area was reclaimed by the year 1973. To study the effects of reclamation of land on the existing flood levels, initially CIDCO and the then Mumbai Metropolitan Region Development Authority (MMRDA) referred studies to Central Water and Power Research Station (CWPRS) in 1975 for the first time for reclamation of 20 ha initially. The objective of the study was to determine the maximum permissible area of reclamation and its configuration without adversely affecting the flooding situation and also to ascertain the measures like channelisation and flood control. Accordingly these studies were undertaken at CWPRS on a physical model constructed to the scale of 1:300-H and 1:50-V. Model studies considered various scenarios of reclamation areas, channelisation of Mithi river in BKC area and bed levels at Mahim causeway. The results of these studies were reported vide CWPRS Technical Report No. 949, 1676 of May 1976 and Note of January 1978. The broad recommendations of CWPRS were as under: a) Channelisation of Mithi river in BKC area is essential prior to reclamation b) In order to reduce flood levels of Mumbai Agra road corresponding to 10 year return period of rainfall provision of sluice gate at Mahim causeway and channelisation is essential. c) Extent of reclamation possible is a function of the channelisation along with widening of existing waterways of bridges. Maximum reclamation of 220 ha is possible. d) Channelisation of Mithi river needs to be undertaken from downstream region to upstream region and the corresponding reclamation of pockets from upstream to downstream. e) The channelisation consists of: i. Widening of 5 km stretch of Mithi river in BKC area to 175 m at bed and 200 m at top between Dharavi Road Bridge and CST bridge. ii. Widening of 1 km stretch of Mithi river in BKC area to 60 m between Dharavi Road Bridge and Mahim causeway and further into Mahim Bay with suitable transition. iii. Widening of Vakola Nalla to 40 m at bed and 60 m at top for a stretch of 3 km. f) Deepening of Mithi river from CST bridge with bed RL (+) 2.0 m to Mahim causeway with bed RL (+) 0.60 m with respect to Chart Datum (CD) so as to provide uniform bed gradient of 1:4285. g) Deepening of Vakola Nalla with bed gradient of 1:1250. h) A bund of adequate height along the banks of Mithi river and Vakola Nalla to contain floods. i) The cross drains which drain into Mithi river should be provided with non-return valve and arrangements for pumping of waters into creek during floods. It was also indicated that in absence of a sluice gate, Mumbai Agra Road would be submerged by 0.6 m for 3.25 hour and 0.9 m for 4.5 hour for rainfall events of 2 year and 10 year return periods respectively. STUDIES FOR MITIGATION OF FLOODS IN MITHI RIVER: After the deluge of 26th July 2005, the Mumbai Metropolitan Region Development Authority (MMRDA) reviewed various options for avoiding the recurrence of the floods in the Mithi River. In this connection CWPRS carried out 1-D mathematical model studies and recommended channelisation of the Mithi River under Technical Report No. 4297 of January 2006. In order to mitigate floods, following remedial measures were recommended a) BKC Area i) Providing a dredged channel of 60 m width from (-) 2 m contour in the sea to Mahim causeway bed level dredged to (-) 1.0 m, and removing existing rock outcrops. ii) Widening the waterway from Mahim causeway to Dharavi Bridge to 100 m. iii) Widening of the bed width of Mithi river from existing 175 m to 200 m between Dharavi Bridge and CST Bridge. iv) Widening of the bed width of Vakola Nalla from earlier design width of 40 m to 60 m. v) Deepening of existing bed level at Mahim causeway from (-) 1.0 m, to (+) 0.67 m at CST Bridge. b) Upstream of BKC Area i) Widening of existing bed width of Mithi river from CST Bridge to MV Road to 100 m. ii) Widening of existing bed width of Mithi river from MV road to Jogewashwari- Vikroli Link Road to 60 m. iii) Widening of existing bed width of Mithi river from Jogeshwari-Vikroli Link Road to Morarji Nagar to 40 m. c) Deepening of existing bed levels CST Bridge (Ch 5.88 km) from (+) 2.0 m to (+) 0.67 m Air India Colony (Ch. 7.05 km) from (+) 3.11 m to (+) 1.0 m Air port (Ch. 9.38 km) from (+) 6.15 m to (+) 4.0 m. MV Road (Ch. 10.47 km) from (+) 8.12 m to (+) 6.35 m. Aarey Dairy Foot Over Bridge (Ch. 12.18 km) from (+) 12.75 m to (+) 10.0 m Jogewashwari-Vikroli Link Road (Ch. 14 km) from (+) 20.25 m to (+) 18.0 m All the suggested cross sections of Mithi river upto Ch. 10.5 km need to be provided side slopes of 1:5. Further upstream, upto Morarji Nagar, the cross sections need to be provided with side slopes of 1:2. The MMRDA and the Municipal Corporation of Greater Mumbai (MCGM) have undertaken the desilting and channelisation of Mithi River in a phased manner after 2005, in their respective area of jurisdiction. However, limitations were encountered in the implementing of the channelisation proposal due to slums and legal structures coming in the way of the widening of the river. The revised development plan was prepared by MMRDA in which rectangular sections were adopted for the river channelization with vertical retaining walls on the banks. It is reported that the work of channelization, to a large extent, has been taken up and the widening of old bridges and construction of new bridges is also being planned. The pertinent studies on revised channelisation and widening of bridges were also referred to CWPRS by MMRDA / MCGM on case to case basis and necessary model studies are being carried out at CWPRS using 1 – D mathematical model. The recent hydraulic model studies undertaken by CWPRS for Mithi River are as follows: 1. 1 – D mathematical model studies for assessing the effect of revised channelisation proposal in Mithi river as per site conditions (Interim report sent and Final Report under preparation) 2. 1 – D mathematical model studies for deciding the HFL and waterway under the proposed bridges across Mithi River at MTNL Road and Lathiya Rubber Road (Interim report sent and Final Report under preparation) 3. Site inspection and model studies for assessing the tidal water exchange for mangrove area near Mini Confluence adjacent to Vakola Nala (Report submitted) The review of the revised development plan for Mithi River Channelization using mathematical model study indicates that there could be increase in the flood levels at the existing bridge cross sections, for extreme flood event of 1 in 100 return periods. Therefore, the widening of the bridges may be taken up on priority. REVIEW OF SLUICE GATE PROPOSAL AFTER THE FLOOD EVENT OF 2005: MMRDA referred to CWPRS to examine the feasibility of providing Sluice Gates upstream of Mahim Causeway considering 100 year return flood events and higher value of runoff coefficient due to increased urbanization and in the light of the deluge of 26 th July 2005. The studies were carried out using the 1-D mathematical model by reproducing the major bridge structure and the sluice gate. The preliminary findings of the study are as follows: 1. The model simulations indicated that the Mahim Causeway bridge is a major bottleneck for flood discharge and causes considerable afflux conditions in the upstream river reaches especially for extreme flood conditions. The five bridge structures from Mahim Causeway to Dharavi Bridge would have further adverse effect due to additional afflux conditions especially for extreme events of flooding. 2. The low water (LW) and high water (HW) levels in Mithi River, immediately upstream of Mahim causeway, corresponding to 0.3 m LW and 4.81 m HW in Mahim Bay are 2.0 m LW and 4.6 m HW respectively. Thus the effective basin capacity for accommodating flood waters with provision of sluice gate would lie between the above tidal levels in the Mithi River and is of very limited role in accommodating the flood discharge due to very rapid filling of the basin during extreme flood events. 3. It is inferred from the model simulations that, at the location upstream of Air India colony where there is abrupt change in the gradient of the river bed, the instantaneous surge conditions (amplification of flood wave) occur for peak flood discharge conditions and the backwater effect is predominant in this reach (i.e. upstream of Mahim causeway) even before the flood wave reaches the sluice gate location. 4. In the event that the high tide level coincides with the peak flood discharge condition, the model simulations indicate that the sluice gate would be ineffective even for flood event corresponding to 2 year return period and 2 hours storm duration. 5. Preliminary simulations for provision of sluice gate with pumping arrangement to reduce the flood levels indicate that pumping would be effective in lowering the flood levels only in the river reach downstream of the pump intake location.