Tulear, located on the south-western coast of Madagascar which has one of the major ports in the country, is subjected to periodic flooding during the fierce cyclonic season.
Tulear Flood Protection
This is largely 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 some 6000m² and pluviometric records in January 1999 indicated a daily precipitation from 105mm to 163mm in 24hrs, which corresponded to a flood return period of more than 50 years.
The design parameters for the protection works of Tulear included a discharge of 6000m³/s for a 1 in 100 return period and a riverbed width of 772m which resulted in a design average water velocity of 3,61m/s and water depth of 20.13m.
In January 2000, BCEOM with design assistance from France Gabions, proposed the use of double twist, PVC coated gabions and Reno® Mattresses for the protection works at Tulear.
Reasons for the use of these materials included:
· Nature of rock: the local limestone fissures and cracks naturally in fragments of a relatively small size and is not suitable for armour rocks but ideally suited 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 would have been required between the armour rocks and the riverbed than compared to a Reno® mattress lining. This is because of the smaller voids and the resulting lower residual velocity of the water at the interface with the riverbed.
· High flexibility: the structures making up the groynes and the dyke protection required high flexibility due to the dispersive nature of the riverbed material and the depth of the alluvial sand.