The Diversion Works for
the Cua Dat CFRD in Vietnam
Prof. Pham Hong Giang,
M. Dinh Sy Quat, M. Michel Ho Ta Khanh
The Cua Dat CFRD is constructed on the Chu River in Vietnam. Its main characteristics are 1023 m of length, 119 m of maximum height and 10 hm3 of rockfill volume.
The adopted “Construction flood” is 1/20, with a peak inflow discharge of 5030 m3/s during the wet season. In the initial design, the diversion works were foreseen with 2 tunnels (L= 1250 m, D= 11 m) and an upstream cofferdam crest at El. 72.00 m. To minimize the cost and the delay of construction of the diversion works, it was later admitted to divert the flow during the most critical stage of the construction by only one tunnel (L=820 m, D=9 m) with a possible overtopping of the main partially constructed dam at El. 50.00 m. The downstream face of the dam was protected by several ranges of gabions, placed in steps of 1.5 m high and slightly anchored in the rockfill.
During the typhoon Lekima in 2008 centered on the site with a peak discharge estimated at 7300 m3/s, a portion of the dam collapsed with a maximum overflow depth of 4 m. The flood lasted 48 hours. The diversion tunnel was partially clogged during the flood and the initial cause of the dam failure was the destruction of the gabions.
Many interesting observations and lessons can be drawn from this experience:
- A “Construction Flood” of 1/50 or 1/100, for a large embankment dam, is generally more appropriated.
- The partially constructed dam would have probably resisted to the overflow if its downstream slope, toe and abutments were protected by a RCC lining in place of the gabions, as suggested by some experts following the good results of submerged RCC dams worldwide.
- The incremental damages were not very important, as the downstream maximum water level after the dam progressive collapse was not significantly higher than the natural water level for this flood. With the traditional method, the upstream cofferdam crest should be higher and, in case of its failure, the released flood and the river level would be higher, with probably much more damages downstream.
- The repair cost was 8.4 MUSD and the repair duration was 3 months, compared with the cost saving of 20 MUSD and a reduction of 2 years of construction permitted by the change of the diversion methods. This comparison shows that, even with this exceptional flood and failure, the diversion by means of submerged works can be often the best method taken into account the savings in the cost and the delay of construction.
Please find the full article enclosed!