Design of the Rehabilitation of Cumbidanovu Dam (Orgosolo)

Cumbidanovu Dam construction was started in 2006; the execution was abruptly interrupted in 2013, due to a catastrophic runoff event, which caused the flooding of the site. The scope of work covers the design of completion works and the check of the existing ones. Rock excavation of dam abutments was already completed, and the upstream cofferdam too. The Project mainly concerned the design of the gravity concrete dam, the spillway and outlet works, in accordance with ITCOLD recommendations and Italian standards. 

The scope of works included the rehabilitation of 40,000 m3 laid concrete of the dam foundation (out of 300,000 m3 to complete the dam); the existing foundation was affected by widespread segregation of concrete, which compromised the hydraulic and static performances of the dam. A detailed investigation was therefore conducted to assess permeability and shear strength of existing concrete which results were used to plan activities required to reach standard performances (i.e. grout injections and reconstruction of joints between adjacent segments). The spillway was designed to manage the runoff flood with Return Time RT=1000 years, and the outlet works to ensure the drawing down of the reservoir in about one day. Due to the absence of temporary hydraulic by-pass, three culverts were conceived in the dam structure, in order to avoid the overflow of dam top crest during execution advancement.

The required services ranged from Hydrology studies to design of Hydraulic Works, including the management of irrigation supplying, the geological and geotechnical studies, the design of ancillary works, the handling of earthworks balancing and the detailing of building site and related operations. A Dam-Break modeling was required too.

Main features:

• Type of dam: gravity concrete dam
• Height dam at the crest: 67.17 m
• Height dam at the spillway crest: 65.15 m
• Total storage volume: 12.118 Mm3
• Surcharge storage: 2.20 Mm3
• Free board: 2.02 m
• Total volume of Dam structure: 290,000 m3
• Crest length: 263.50 m
• Max flow over spillway: 1,237 m3/s
• Max flow at outlet culvert: 185.36 m3/s

The geological site of dam and the afferent catchment is predominantly composed by granite rock: in case of flood event, the runoff coefficient increases rapidly, and reaches 0.6/0.7 values, causing a prominent peak flood in 3-4 hours, which corresponds to correlation time. These features were taken in account for adequate design of emergency spillway, and for transient events during construction, to arrange hydraulic devices to ensure safety conditions under dam execution.

The Italian Dams norm was compiled for designing the main works (DM 26/06/2014), and the ICOLD recommendations. For ancillary works, the technical norm about constructions was fulfilled.