Tulear Flood Protection
Problem
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.
Solution
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.