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Archive -Environment

Tackling flooding issues in the face of climate change |23 May 2016

More than three years ago, La Digue Island was hit by one of the worst rainfall events in its history which severely disrupted the daily activities on the island.

This happened precisely in late January 2013 and it associated with the tropical depression Feleng which also affected districts on Mahe’s southeast cost.

More than 225.9mm of rainfall battered the island on January 22, 2013 and this caused severe damages to houses, businesses, government buildings and other infrastructures and eventually inflicted heavy losses to homes and businesses.

The government moved in with an emergency taskforce comprising personnel from several ministries and agencies to address the situation on the ground and alleviate flooding that had almost crippled the island.

In the process, the two main outlets at La Passe and L’Union Estate were demolished to allow the rapid evacuation of storm water flow from Lamare Soupap that had overflowed its bank and flooded almost the entire community of La Digue. Several earthly drains were also excavated for the purpose of evacuating storm water from the roads and the plateau below the Lamare Soupap.

These two main bridges were to be re-constructed much wider to increase their discharge capacity in the event of heavy rainfall as they were found to be inadequate to evacuate storm water flow from the Lamare Soupap and other areas and also a few other major outlets along the main road to sea outfalls and some lateral drains further inside the plateau of La Digue were proposed by the National Drainage Taskforce based on the assessments and surveys conducted by government officials and some private consultants.

 

La Passe bridge

 

The previous bridge had a dimension of 2.2m and fitted with a weir to minimise salt water intrusion from further inland. A 6m span bridge has now been constructed, capable of discharging 3m3/s with the connecting drain and outlet facing northwards to minimise sand accretion in front of the outlet.

The construction period took about six and a half months at the cost of R4 million. The discharging capacity of the bridges caters for a storm event of one-in-ten years return period, though the Feleng event was one-in-seven years return period.

To complement the new bridge, the first phase of Lamare Soupap was also de-silted and widened so as to increase its retention capacity, some bridges were also re-constructed to eliminate the bottle-necks for the flow during heavy rainfall.

L’Union bridge

 

L’Union Bridge is almost a replica of the La Passe bridge. It has been built with the same concept, except for few minor changes for protection and aesthetical purposes. Construction period was slightly more than six months and it cost the government R4.4 million.

Also to complement the construction of the new bridge, flood mitigation works on Lamare Soupap phase 2 was implemented and it comprised de-silting works and widening of the marsh creating sedimentation pond just before the bridge.

Landfill bridge

 

With the construction of the L’Union Bridge, the bridge to the landfill area crossing the L’Union Estate marsh is crucial for the flow from further inland bordering the agricultural land of L’Union Farm.

It was found to be inadequate in capacity and a new bridge is under construction at a cost of R0.9 million.

The project started in late December 2015 after some delays with rainfall and other events and it will take another four months to complete.  Its dimension: 4m wide at 1.8m high from the previous 1.8m wide and 0.8m high. 

 

Biodiversity protection

 

Though these flood mitigation works necessitate the use of heavy machinery, the methodology took into consideration the biodiversity aspect of the area being impacted by the civil works. Protecting the biodiversity in the surrounding environment was a priority and one of the iconic species of the La Digue, the ‘terrapins’ (black and yellow bellied mud turtles) had to be protected at all cost.

The project management team with local consultant implemented a capture and release project for the ‘terrapins’ for the first and second phase of the Lamare Soupap de-silting works. This involved the trapping of these small mud turtles by using baited fish traps and relocating them to a safe environment away from the disturbed area while the works were ongoing and after the works had been completed, they were returned to their previous habitat.

For the de-silting works in Lamare Soupap phase II, the school children of La Digue and some people in the community took part in the capture and release activities. This was part of the awareness and educational activities involving the community in the protection of the biodiversity of the Lamare Soupap. The key message was to impart the knowledge to the community on how flood mitigation and protection of the biodiversity can be made possible through good planning and involvement of all stakeholders.

 

Other flood mitigation projects

 

The other flood mitigation projects comprise the construction of a series of outlets from the main road to sea outfalls, namely police station box culvert and a box culvert near the Fish Trap Restaurant, tendered as one project, costing the government R1.2 million.

The police station culvert and drain is ongoing and is to be completed in four months’ time and will then be connected with internal lateral drains from the housing estate and alongside access roads.

Construction of the other outlets will be implemented in at different stages and they are scheduled until 2019 according the NDTF drainage master plan for La Digue. A total of nine new outlets will be constructed along the main road from La Passe to L’Union and then flood mitigation works will move to the areas inland to the housing estate.

The NDTF schedule of works is still on track despite some delays with some projects, mainly attributed to securing way leaves, weather conditions and other activities that have hampered the progress of the works on site.

 

 

Compiled by J. C. Labrosse for NDTF 

 

 

 

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