A past project off the south-western coast of Madagascar exemplifies how Gabion & Reno Mattresses can be used to successfully prevent flooding.
Maccaferri Middle East has actively participated in an array of past projects with this combination and has successfully managed to mitigate flooding and erosion. A past project that took place in Madagascar between 2001-2003 aptly exemplifies how a Gabion and Reno Mattress combination can prove beneficial to flooding affected areas.
Tulear is located on the south-western coast of Madagascar. It is still functioning today!
It has one of the major ports in the country and is subjected to periodic flooding during the tough cyclonic season.
The flooding mainly caused due to the overflowing of the River Fiherenana, which runs for approximately 150km and hence, presents a threat to the residents of Tulear and the adjacent agricultural land.
The catchment area is approximately 6000m² large. Pluviometrical records in January 1999 indicated a daily precipitation of 105mm to 163mm in 24hrs, which corresponded to a flood return period of more than 50 years.
Design parameters for the protection works of Tulea thus included a discharge of 6000m³/s for a 1 in 100 return period and river bed width of 772m that resulted in a designed average water velocity of 3,61m/s and water depth of 20,13m.
In January 2000, BCEOM with design assistance from France Gabions proposed that twisted double wire should be used. They also recommended that PVC coated gabions, and Reno mattresses should be utilized for the protection work at Tulear.
Reasons for the utilization of these materials included:
- The Nature of rock: the local limestone fissures and cracks naturally in fragments of relatively small size and are therefore not suitable for armor rocks but lend themselves ideally for gabion works.
- High water velocity: the average water velocity was 2m/s but could reach 4m/s around the head of the proposed groynes. Therefore, a thicker filter between the armor rocks and the river bed would have been required as opposed to a Reno mattress lining. This phenomenon occurs because of the smaller voids and the resulting lower residual velocity of the water at the interface with the river bed.
- High flexibility: the structures are making up the groynes, and the dike protection required high flexibility due to the dispersive nature of the river bed material and the depth of the alluvial sand.
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For more technical details please download our case history.