FLOOD & DRAINAGE MANAGEMENT

This component supports priority rehabilitation and improvement of flood and drainage management infrastructure identified as a priority by the inter-agency Flood Mitigation Task Force chaired by the Sri Lanka Land Reclamation and Development Corporation (SLLRDC). The investment packages under this component are essential and critical to reduce the impact of future floods in the catchment of the Colombo Water Basin. Most of the structural investments will be aimed at improving the system of primary and secondary canals, retention areas, and drainage of the Colombo Water Basin. The development of an integrated flood management system and interventions to enhance the economic value and aesthetic qualities of the water bodies will also be supported. The component comprises the following four subcomponents:

  • Enhancement of primary and secondary canals and lakes

  • Rehabilitation and upgrading of the micro-drainage system within the CMC

  • Capacity enhancement for flood and drainage management

  • Beira Lake Linear Park and Beddagana Park

ENHANCEMENT OF PRIMARY AND SECONDARY CANALS AND LAKES

Floods in Colombo are caused by heavy rainfall and a macro-drainage system with insufficient storage, conveyance, and outflow capacity. In the last decade the storage capacity in the Basin has reduced by about 30 percent, due to uncontrolled landfill and flood plain encroachments by illegal settlements. The conveyance capacities are restricted by solid waste, floating debris, and bottlenecks in the canals. The outflow capacity of the system is too small, particularly in the north through Mutwal Tunnel and the North Lock and South Lock in the St. Sebastian canal. During the monsoon season, the outflow is further limited as heavy rainfall coincides with high levels at Kelani Ganga, reducing the outflow through the North Lock to nil. Discharges at the Beira Lake are further enlarged by the pumped outflow from St. Sebastian Canal. Unauthorized sewerage discharges into canals have also led to low quality water and related diseases in inundated areas. All these impacts are likely to worsen in the future due to the effects of climate change; rainfall trends indicate larger loads on the system and sea level rise impedes gravity drainage.

Based on the above constraints to effective flood management, the following flood mitigation measures have been devised under this subcomponent:

Main Drain and Mutwal Tunnel: Rehabilitation of Main Drain bank protection (gabions)
reconstruction of Aluth Mawatha Culvert (reinforced concrete box culvert)
rehabilitation of Mutwal Box Drain, and tunnel inlet and outlet
Dehiwala Canal: Rehabilitation of Dehiwala Canal bank protection from Galle Road Bridge to outfall (gabions)
removal of rock outcrops in canal bed
St.Sebastian South and Dematagoda Canals: Rehabilitation of bank protection along St. Sebastian and Dematagoda Canals (gabions and sheet piles)
Secondary Canals (Sethsiripaya and Sarana Mawatha Canals): Rehabilitation of canal banks and improvement of culverts of Sethsiripaya Canal
rehabilitation of canal banks and improvement of culverts of Sarana Mawatha Canal
Madiwela East Diversion Scheme: Design of flood-control gate and spillway of Talangama Tank
rehabilitation of bank protection of Madiwela East Diversion Canal between Averihena Tank
Athurugiriya Road (gabions and turfing of earthen sections)
Wellawatta and Poorwarama Canals: Rehabilitation of Welawatta Canal bank protection from Galle Road Bridge to outfall (sheet piles)
rehabilitation of Poorwarama Canal bank protection
Formation of Lakes (Part I): Creation of Lake 06 with outflow control structure, and (ii) creation of Lake 07 with outflow control structure
St. Sebastian North and Sri Wickrama Canals: Rehabilitation of bank protection in St. Sebastian North Canal (gabions and sheetpiles)
rehabilitation of Sri Wickrama Canal bank protection
North Lock Pumping Station and North Lock Gates: Upgrading of set of North Lock Gates and downstream improvement with embankment protection
construction of pumping station at North Lock
St. Sebastian South Diversion Canal: Construction of St. Sebastian South Diversion Canal bypassing Beira Lake, including treatment at outlet
Construction of New Mutwal Tunnel: Construction of a new Mutwal tunnel with a capacity of 15 m3/s
Galle Road Bridge across Wellawatta Canal: Removal of bottleneck in Welawatta Canal at Galle Road Bridge by providing guide walls or additional openings on either side
Armagoda Culvert at Madiwela East Diversion Scheme: Provision of additional openings to Amaragoda Culvert to enhance the conveyance capacity
Restoration of Kolonnawa Retention Area: Construction of gate and pumping station and connecting outlet canal
Formation of Lakes (Part II): Formation of Lakes 01, 08, and 10
Beira Lake Water Quality Improvement: Interception of sewerage draining to storm water drains

REHABILITATION AND UPGRADING OF MICRO-DRAINAGE SYSTEM WITHIN THE CMC

Investments under this subcomponent will aim at improving the capacity and performance of the micro-drainage system in 15 selected flood-prone areas under the jurisdiction of the CMC. The CMC has identified 45 flood prone areas where flooding occurs regularly. The main problems causing recurrent flooding include (i) unauthorized constructions on and along drainages, (ii) dumping of waste in the drainages that obstructs/blocks free flow, (iii) drainage being impeded by backwater from the main canals system during major floods, and (iv) lack of regular maintenance and cleaning of the drainage system. Short duration rainfall amounts have also increased in the past decade.

Out of the 45 localized flood-prone areas, the following 15 sub-projects have been identified for implementation by the CMC:

  • Prevention of flooding at Garden No. 175 and No. 211, Nagalagam Street and Garden No. 75, Ferguson Road

  • Prevention of flooding along K.Cyril. C. Perera Mawatha from George R. De Silva Mawatha up to Arthur De Silva Mawatha Junction

  • Prevention of flooding at Kimbula Ela Housing Scheme

  • Prevention of flooding at Amour Street, Sangharaja Mawatha, Jethawana Road opposite Diesel and Motor Engineering (PLC), and Prince of Wales Avenue

  • Prevention of flooding at Green Lane, George R de Silva Mawatha, and Ratnam Play Ground Area

  • Prevention of flooding at Saunders Place

  • Prevention of flooding at Maligawatta Housing Scheme

  • Prevention of flooding at Norris Canal

  • Storm water improvements at Wijerama–Horton Place junction and surrounding areas

  • Storm water improvements at Siridhamma Mawatha and surrounding areas

  • Prevention of flooding at High-Level Road, Kirullapone Junction, and Robert Gunewardhana Mawatha

  • Prevention of flooding at Poorvarama Road and Kandewaththa Road

  • Prevention of flooding at Park Road

  • Upper catchment of Torrington North Canal (Thummulla Junction)

  • Marine Drive development from Dehiwala Bridge up to Bambalapitiya Station Road (7 sub-catchments)

CAPACITY ENHANCEMENT FOR FLOOD AND DRAINAGE MANAGEMENT

This subcomponent will aim at improving the overall drainage system management for the Greater Colombo Water Basin to ensure the sustainability of project investments. It will comprise (i) investments for selected landscaping along canals and supporting a pilot water-based transport system to demonstrate the viability of using the main canals for public transport, (ii) purchase of canal maintenance machinery, and (iii) development of an integrated flood management system for the Colombo Water Basin. The integrated flood management system will include monitoring and management of the retention/wetland areas, operation and maintenance systems for wetlands, canals and drainage systems, monitoring and enforcement of land-use planning to safeguard the capacity of the basin to cope with floods, and interagency coordination protocols and mechanisms.

BEIRA LAKE LINEAR PARK AND BEDDAGANA PARK

This subcomponent will aim at improving the public fruition of the east and southwest Beira Lake through the development of a continuous promenade (linear park) and green areas (nodal parks) along its shores, and establishing a natural park on a wetland branch of the Parliament Lake. The following two key sub-projects will be implemented:

Beira Lake Linear Park:

The objective is to complement the flood-reduction measures under Component 1 by providing opportunities for urban regeneration along the east and west Beira Lake waterfronts. Currently, the Lake is not accessible to the public due to inadequate bank protection and multiple uses along the shoreline. A phased approach will be adopted to address these issues. In the first phase, protection walls will be constructed along 2.5 km of shoreline on the west and east Beira Lakes and steps will be taken to restore the McCallum Lock Gates, which have historic value. In the second phase, a 4-kilometer pedestrian path/promenade, with an average width of 6 meters, will be constructed on top of the protection walls. This will provide a natural connection between the east and west Beira Lake waterfronts and the walkways connected to the southwest Beira lake waterfront. Ultimately, a contiguous recreation space within the core area of the city, which has potential for development by the private sector, will be created.

Beddagana Park:

The objective is to ensure the protection of the Beddagana Wetland Sanctuary and Kotte Ramparts from future encroachments and enable it to function as a flood-retention area for the city. The target area is a 32-hectare marshland and wetland habitat in the SJ-KMC and a part of the Parliament Lake. In a context of rapid urbanization, this area provides locals and visitors a rare glimpse of a wetland habitat that is home to several species of flora and fauna. Interventions will include (i) enhancing the hydrology of the wetland area by cleaning existing canals, (ii) reforestation of areas that have been deforested to enhance bird habitats, (iii) provision of bird-watching hides, towers, board walks, nature trails, and orientation centre to create awareness about the sanctuary to visitors, (iv) provision of play areas at the boundary of the wetland sanctuary and a 2.2 km jogging track for recreational use, and (v) minimal interventions to improve the existing bund road. Interventions will have minimal impact and will use resources that are environmentally friendly. Overall, the outcomes of these interventions are expected to improve livability for local people, facilitate protection of a historic area, and increase ecotourism opportunities.