Solution to Colombo floods

Published on February 8, 2011   ·   No Comments

by Archt Harshan De Silva

According to the findings of recent substantial study carried out for the Greater Colombo Basin by Nippon Koei Co Ltd together with SLLRDC, 380 ha of retention areas should be reserved together with Madiwela South Diversion and New Mutwal tunnel in order to maintain a safety flood level of 1.75m MSL corresponding to 50-year return period rainfall event.

In addition to the above 3,000,000 m3 of retention volume has to be preserved in Parliament retention areas. However, the extent of currently available retention areas around Parliament Lake is 268 ha which is inadequate. Therefore, retention ponds which are located above normal water level are being dredged in identified feasible locations in each sub basin around Parliament Lake in order to maximize the retention capacity of the Colombo Basin. It is expected to drop the high flood level corresponding to 50-year return period flood event by one foot having dredged all lakes. Retention ponds are being dredged at Rampalawatta, Talawatugoda Junction, Waters Edge, Upper Parliament Lake and some more lakes have been identified to be dredged with immediate effect.

Raising spill level of causeway at Awerihena tank

Spill level of the causeway at Awerihena tank was raised by 0.5 m, with the objective of diverting more water via Madivela diversion canal in order to minimize storm water runoff draining to Parliament Lake during heavy rainfalls. It is expected to divert additional volume of floodwaters at a flow rate of 4m3/s. The current capacity of East Diversion canal is 20 m3/s corresponding to 25-year return period.

Proposed short term solutions

In addition to the measures taken for ongoing projects mentioned above, following solutions are recommended for mitigation of floods within the Colombo Basin:

Implementation of drainage proposals

Non implementation of following drainage proposals forwarded by SLLRDC, where locations are at the upper catchment caused a significant impact on elevated flood level at Parliament Lake. Urban Development Authority was the implementation agency for the projects.

Floods dampen daily activities of people. File photo

* Proposed Drainage Management Plan under Battaramulla Town Centre Development Project

* Proposed Drainage Management Plan under Talapathpitiya Town Centre Development project

* Proposed Drainage Management Plan for HSBC at Rajagiriya

* Madiwela Government Project in the vicinity of Parliament (Retentation ponds of extent 2.5 ha out of 5.75 ha recommended has been dredged whereas reclamation work completed)

Construction of pumping station

North lock is one of the main outfalls for draining out the storm water collected in the Greater Colombo basin. There was a discharge of 90 m 3/s during last flood occurred in November. However, the width of opening at this location is confined to 5.4 m whereas width of canal in the upstream is 20.0 m. Therefore, widening of gate subjected to a minimum width of 20.0m is of prime importance for quick recession of flood together with construction of a pumping station with a capacity of 10m3/s in the event of higher water level in Kelani river than the water level in Colombo Basin.

Rehabilitation of pumping station at South Lock

The capacity of existing pumping station at South Lock maintained by SLPA is 3.5m3/s, which is not functioning at present. Rehabilitation of the pumping station and enhance the pumping capacity up to 5 m3/s will significantly contribute to lower the flood levels in Colombo Basin.

Desilting of upper part of Talangama tank up to Hokandara Road

The upper catchment of the Talangama tank up to Hokandara Road should be desilted in order to obtain the maximum retention capacity of Talangama tank which is located in the upper catchment of Parliament Lake which will minimize the volume of water discharged to the Parliament Lake.

Long-term solutions for mitigation of floods

Madiwela South Diversion scheme

This is the proposed main long term remedial measure proposed aiming at reducing storm water runoff to the Parliament Lake catchment. The Madiwela South diversion canal envisages diverting storm water runoff from the upstream basin to the Bolgoda Basin.

A closing bund is proposed upstream of Parliament Lake to lead storm water runoff in the reverse direction, the stream in the upstream basin is canalized and a crossing culvert is constructed at the railway and High Level Road.

The maximum water level in the lowland upstream of Parliament Lake is set at 2.4 m above MSL and the design runoff of the crossing culvert is estimated at 40 m3/sec for the probable 50-year rainstorm event. The downstream side in the Bolgoda Basin is also canalized to cope with the diverted runoff.

The proposed Madiwela South Diversion Channel mostly passes through abandoned paddy and vacant land. The diversion of storm water from the Parliament Lake Basin into Weras Ganga will result in water level rise in the Bolgoda Lake. There won’t be a significant harmful impact since the water level rise is estimated as 10cm for the Weras Ganga and 3 cm for the Bolgoda Lake North.

Since the water level is estimated to raise a few cm, resettlement by expansion of the inundated area with water level rise will not be caused. However, implementation of the storm water drainage plan in the Greater Colombo Basin together with the required additional measures in the Weras Ganga Basin will result in a satisfactory drainage mitigatory solution. Construction of the Madiwela South Division should be implemented as soon as possible in order to avoid difficulty related to relocation and land acquisition in the future due to urbanization, even though the project cost of the Madiwela South Diversion is the largest among other measures proposed.

Construction of new Mutwal tunnel

New Mutwal tunnel was also proposed by the JICA study in 2003 for enhancing the draining out capacity of the Greater Colombo basin having left other outfalls for gravity drainage. An inlet channel branches from the Main Drain around 100 m upstream of the crossing of the Port Access Road is proposed. A tunnel section with a 4.0 m diameter and a length of 740 m starts near Walls Lane and runs to the North. An outfall to the sea is located on the seaside of the Muthwella Mawata. The flow capacity of the tunnel is estimated at 15 m3/s.

Construction of Gotatuwa pumping station

Construction of Gotatuwa pumping station having a capacity of 30 m3/s is also identified as a vital solution for recession of flood in the Colombo Basin.

This will reduce the water level of the downstream of Parliament Lake so that flow rate at the outlet of Parliament Lake will significantly increase giving rise to quick recession of floods in the Parliament catchment.

Widening of bridges in Wellawatta canal at High Level Road and Galle Road

It is proposed to widen the bridges in Wellawatta canal at High Level Road and Galle Road subjected to a minimum width of 24.0 m whereas width of canal and bridge are 22.0m 14.0m respectively. This will significantly contribute for quick recession of floods in Colombo Basin which is a great hindrance to flow at these locations especially during floods.

The writer is the chairman Sri Lanka Land Reclamation and Development Corporation

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