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Archive -Environment

Tackling flood and erosion on La Digue through mitigation and adaptation |31 August 2015

 

In late January 2013, La Digue recorded one of its worst flooding events in the history of this island during one of the heaviest rainfalls associated with the subtropical depression Felleng.

Further to that, the island was again hit with severe flooding in January 2014 and again many houses, properties, businesses and infrastructure were flooded and cost significant amount of losses to the inhabitants and businessmen on La Digue.

The Lanmar Soupap, the main storm water discharge channel and also acting as the cut-off point between the mountain and the plateau, overflowed its bank, flooding roads and compounds.

Government realised the urgent need to address the flooding issues on La Digue despite the works that were needed to address the flooding that had occurred on the south east coast also as a result of Felleng. The Drainage Task Force works had to be shifted to La Digue though not neglecting works that had already started at Au Cap and Anse Aux Pins.

Detailed site assessments were carried out on the island with the expert help of WSP, a private engineering consultancy agency, to investigate the causes and to come up the most appropriate methods to reduce flooding that severely affected the La Digue community.

The results of these assessments revealed some deficiencies that contributed to the flooding on La Digue in January 2013 and February 2014. Main findings informed the government of the inadequacy of the outlets for effective evacuation of storm water from La Digue plateau and Lanmar Soupap; inadequacy of the two main outlets at L’Union and La Passe in terms of capacity to discharge the heavy flows from the marsh; the inadequacy of the Lanmar Soupap in terms of water retention due to silting and in some areas construction has narrowed or divert the channel from original path; construction of bridges causing bottle neck effect due to their limited span allowing back flooding; most of the main and secondary roads are without storm water drains or the drains installed are not quite effective in the evacuation of surface flows from the roads and plateau and finally the lack of gradient, depressions and blocked channels in the plateau preventing ease of water flow and evacuation.

Once the major findings were concurred a series of consultations with the inhabitants and other stakeholders were carried out to assist in the planning of the interventions that would be recommended to mitigate the flooding. Topographical survey was carried out to define the levels and identify the appropriate locations and sizing required for the drains and outlets to be constructed. Such ground works started in March 2014 and this was followed by detailed design and tendering for several projects including: Major storm water outlet at La Passe and storm water outlet at L’Union and box culvert and outlet at the district administration’s office. These main projects have been managed by the Drainage Task Force and implemented by UCPS through tendering process at a cost of R8,117,436.01. 

De-silting of the Lanmar Soupap

In a bid to complement the efforts of the DTF the Coastal Adaptation and Management Section within the Ministry of Environment, Energy and Climate Change under its Capital Projects funding allocation had to prioritise the de-silting of the Lanmar Soupap to improve its retention capacity and flow regime. The section has as such carried out major desilting works with construction of new bridges crossing the marsh channel and rock armouring in required selected locations. This has been carried out by DM Construction at a cost of R785,714.50 of which the award came through formal tendering process.

Additionally CAMS is also undertaking this year two other drainage projects to further complement the flood mitigation works on the island including placement of receiving sump, box culverts and outlet at the police station and Nouvobanq to outlet into the sea. The latter two costing the section R1.178399 million all together and are due to commence in the coming week. Having successfully completed the phase 1 desilting the section has just recently awarded contract to DM Construction (similar to the box culverts) through formal bidding the phase 2 desilting of the Lanmar Soupap marsh at a cost of R894,472.30 and the works are to be started in the coming weeks.

It is to note that the section has ensured that the endemic terrapins within the marsh are protected from the disturbance of heavy machinery that worked and will work on the de-silting project. Prior to any works the section carried out a terrapin capture and release programme where these species were captured and relocated for the works. The importance of such conservation efforts were extended to school students of La Digue as well through an educational activity whereby they were allowed to learn about the capture and release techniques and the benefits of such programme.

To date DTF and CAMS have implemented the La Passe storm water outlet, the Lanmar Soupap phase 1 de-silting and the box culvert and outlet at the DA’s office. Earmarked for this year and to commence soon is the Lanmar Soupap de-silting phase 2, box culverts and outlet at police station and Nouvobanq as well as the major storm water outlet at L’Union which is to commence in September 2015. These major works are allowing for the major evacuation points to be dealt with as well as improving the retention capacity of the marsh to capture water first prior to further moving on with inland drain interconnections which will further mitigate flooding for the La Digue plateau. The mentioned works amount to a total of R10,676,021.81.

The CAMS furthermore in a bid to maximise financial resources have also taken advantage of the works for flood mitigation to carry out coastal erosion protection measure in the La Passe area. The small La Passe beach adjacent to Domaine de L’Orangeraie is now being rock armoured and a sand nourishment is to top up the armouring. Some of the materials to be used for this project have come from salvaging the rocks and sand used for the temporary cofferdam placed during the construction of the La Passe stormwater outlet reducing the cost of the project. Additionally the use of same machinery negating the cost for mobilisation has allowed for further saving in total cost of the project. The project is to allow this beach to be protected from further erosion and allow sand to be further deposited.

The works do not end here and much more is required to be done to be able to deal with the flooding issues of the small island. A phased approach however is the way forward to ensuring the implementation of all the required interventions for flood control management of the island. DTF and CAMS remains committed to future works not only for La Digue but also for other flooding hot spots on both Mahé and Praslin including Anse Aux Pins, Au Cap, Pointe Larue and Beau Vallon to mention the key areas.

 

 

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