Experiencias internacionales con la introducción de cajeros corresponsales 18

barrage across the mouth of the Mithi river in 1988 and Shah Technical Consultants in the year 1997- 98 for the Postal Colony SWD diversion work.

After the deluge in July 05 studies have been carried out by various agencies at the instance of MMRDA. Highlights about their scope of work & their major recommendations are as below.

4.9 Interim Report Nov. 2005, by Maharashtra Engineering Research

Institute – Nasik on Flood Zone Mapping and flood control measures for Ulhas River and its tributaries in Thane District

4.9.1 The present interim report covers the hydrology of the basin, flood values, &

short term recommendation for flood mitigation. The total catchment area of Ulhas basin is 4900 Sq.Km. Two major storages Bhatsa & Barvi are situated in this Catchment. The rainfall varies from 2500 mm. to 3200 mm. from plain area to mountain. There are 83 rain gauge stations in the basin & in the nearby area. The basin recorded @ 1300 mm of rainfall in the spell of 25.7.2005 to 29.7.2005, maximum value being 765.8 mm. on 27.7.2005 at Nalde River gauge Station. The maximum hourly intensity however does not seem to have crossed 70 mm but the duration was long. The (24 hours) average rainfall for the basin has been worked out as 650 mm. and the maximum reported is 916 mm at Bendshil. It is such that studies on the Hydro meteorology of Ulhas basin are of general interest in the context of Mumbai’s hydrological situation also. They will be of direct interest for MMRDA but it will be useful for MCGM to keep track of those studies also.

4.9.2 The unit Hydrographs & design storms for the return periods of 25 years and

100 years as well as for the storm on 26-27/7 have been developed & maximum discharges for various sectors have been worked out in the report. From available data, flood levels for 25 years return period are worked out which in turn decide the floodable area. The spread of urban area in the river basin especially along Ulhas & Waldhuni River has been surveyed to study various obstructions to the flow & its impact on 26-27/7.

4.9.3 The report is based on the requirements of Flood Zone Mapping as per the

guidelines contained in the Govt. of Maharashtra circulars & their Dam Safety manual. It required defining of the prohibitive zone (required for passing 25 year’s return period flood or a flood equivalent to 1.5 times the capacity of established river channel), the restrictive zone (required to pass the maximum design outflow flood of 100 years return period) and the Caution Zone: (extending there beyond for floods of greater magnitude than 1 in 100 years return period) Along with other recommendations regarding removal of encroachment and better garbage management, the report also suggests that the automatic operations of the Barvi dam’s gates should be converted into manual operation for regulating the floods more systematically. Rivers in Brihan Mumbai are relatively small. But the experience of the flood zoning work being carried out in the Ulhas basin can provide useful guidelines for similar work to be carried out in Mumbai, once contour maps become available.

4.10 Study on Flood Mitigation Measures for Dahisar, Poisar & Oshiwara Rivers in North Mumbai – Interim Report – December 2005 by Water & Power Consultancy Services (India) Limited, Gurgaon.

4.10.1 Here the scope of work included topographic & hydrographic survey &

Mapping of rivers, marking land use pattern in 200 m. strip on either sides of all the three rivers, analysis of rainfall data for 50 years & 100 years return period, evolving hydrographs for different return periods, development of mathematical models for flood hydrographs with different tidal conditions & recommendations about channelisation. Remote sensing technology was used by WAPCOS to assess the alignment or the rivers and the land use patterns. Actual readings were taken for tidal variation by setting up tidal gauges in Malad & Manori Creek. Even current flows in the Creek were measured. Detailed analysis of rainfall data available from I.M.D. Colaba & Santacruz Stations was done to arrive at design storms for 50 & 100 years return period. Mathematical models of the rivers were processed for these storms and the events of 26-27/7 to ascertain peak discharges in various stretches & channelisation is suggested for flood mitigation by analyzing the above results.

4.10.2 The report points out that the developmental activity has resulted in reduction

of the river widths & depths have been reduced due to dumping of mud & siltation. Hydro-meteorological / Hydrological studies reveal that for 50 & 100 years return period 24 hours rainfall of 550 mm. & 667 mm. respectively be adopted for design hydrographs of the three rivers. Hourly rainfalls of 50 years, 100 years return period & hourly rainfall of 26/7 would be 119 mm, 134 mm. & 190.3 mm. respectively, which have been used by WAPCOS to determine the peak discharges of river. The channels sections for carrying peak discharges in the flood prone zone of the three rivers have been worked out are as under:-

Rivers Parameter

Dahisar Poisar Oshiwara

Existing Width (m) 30 10 20

Proposed Width (m) 45 20 25

Depth 4.5 4.0 4.5

Discharge (Cumecs) 1185 500 569

From their Detailed tables & the Cross Sections presented, in the report, it is clear that Widening & Deepening is essential in all the three rivers. This will have to be pursued further in greater details once the contour maps are in hand.

4.11 1-D Mathematical Model & Desk Studies for Mitigating floods of Mithi River in Mumbai. – Interim Technical report January 2006 by Central Water and Power Research Station.

4.11.1 In this case, the scope of work included examination of the tidal hydraulics of

Mithi river & Vakola nalla corresponding to return periods of 50 years, 100 years storms & 26/7 rainfall event for Bandra-Kurla Complex area and identifying the remedial measures for mitigating excess flood levels of discharges corresponding to 50 yrs. , 100 yrs. return period storms. Cross Sections of Mithi river up to the bank levels from Chainage 3 km. to outfall & available rainfall data from Santacruz & Colaba over a period of @ 50 yrs. & hourly records of July 2005 were used by

CWPRS as a base for developing the river model & the design hydrographs. The velocities observed in the year 2003 during monsoon & non-monsoon period were also taken into consideration. In order to reduce flooding, Channelisation requirements were studied, with specific attention to widening required at the Mahim Causeway.

4.11.2 As part of the data was still awaited, preliminary suggestions regarding

channelisation & dredging have been furnished for a storm of 100 yrs. return period. They which include widening of waterway from Mahim Causeway to Morarji Nagar (i.e. area below Vihar Lake) and providing modified bed gradients to increase the conveyance capacity of the river. Recommended waterway width at Mahim Causeway is 100 m., while that up to C.S.T. Bridge is 200 m. Further, upstream the recommended bed widths are 100 m., 60 m. & 40 m. Up to Mathurdas Vasanji Road, Jogeshwari Vikhroli Link Road & Morarji Nagar with adequate side slopes & deepening. In absence of ground level data and contour maps, flood plain width (spillage zone width) has been assumed by CWPRS as 350 m., 300 m., 200 m., on either side of the bank, in BKC, Air India Colony to M.V. Road and M.V. Road to Vihar Lake, respectively.

4.12 Development Action Plan for Environmental Improvement of Mithi River