Design of Alternative Option for the Mitigation Measures for Flood and Hydrogelogical Risk of the City of Olbia

The urban area of Olbia is historically subject to flooding due to rainfall events of a certain intensity. Following the severe flooding event of November 18, 2013, the Basin Authority of the Region of Sardinia undertook a study for Hydrogeological Structure Plan (PAI), in order to identify interventions for the safety of the hydraulic risk of Olbia. In 2014 the Feasibility Study was drawn up and, based on the results of this Study, the Municipality of Olbia developed the Final Project of the interventions, divided into 4 lots, and the related Environmental Impact Study.

The Region, approved the Project as "Framework of hydraulic risk mitigation works", already approved by the Institutional Committee of the Basin Authority. This Project was subsequently challenged and criticized through numerous observations, which also emerged in the public presentation of the Project and the related SIA, so much so that counterproposals with different intervention scenarios emerged.

In the light of the critical issues that emerged for the previous solution and the strong opposition to the implementation of the interventions, the Municipality of Olbia intended to develop, in a direct form, an "alternative" design solution to what the Region has already done.

The tasks of assignment included a detailed hydrological and hydraulic study for the definition and analysis of alternative solutions of intervention able to combine the objective of hydraulic safety with the critical issues related to intervention in an urban context such as that of the city of Olbia.

The aim of the service was the identification, through a detailed hydrological and hydraulic analysis, of solutions for the safety from hydraulic risk through interventions that reduce the flow rate at the end sections of the watercourses that cross the city, i.e. an optimized combination of several types of intervention: spillway, rolling tanks, recalibration and renaturation of the valley sections of the watercourses. The proposed solutions have been verified through modelling and maximizing, for the same overall costs, the objectives of hydraulic risk reduction by minimizing the environmental impact on the territory.

Hydrological Study.

In order to analyze the hydrology of the different sub-basins of the city of Olbia, covering an area of 72.5 km², a single model was implemented in HEC HMS which includes all of them, so that the effects of the proposed interventions can be assessed overall. 

The hydrological schematization of the study basins in their current state has been subsequently adapted to include also the different design hypotheses analyzed (expansion tanks, flood spillways, diversions) in order to evaluate in a dynamic and comparative way their effect on the modification of the flood hydrographs with respect to the actual state.

Hydraulic Modelling.

The hydraulic analysis of the present situation and intervention options was conducted using two software: HEC RAS, in 1D configuration, and InfoWorks ICM, in 1D-2D configuration, in order to simulate the evolution of flooding in the riverbed and floodable areas.

For each watercourse studied was reconstructed in its actual state on the basis of cross-sections of the riverbed extrapolated from the LiDAR survey of the Ministry of the Environment, verified and integrated with the topographic survey carried out by the Writers, data and surveys made available by the Customer, data collected during the inspections carried out on purpose, etc..).

Design od interventions.

The project has foreseen interventions to limit the water entering the urban area through:

• construction of a flood control basin on the Rio Seligheddu with 600,000 m³ of reservoir.
• construction of a flood diversion tunnel (length 11.4 km, diameter from 5 m to 8 m) capable of intercepting the watercourses surrounding the city of Olbia, draining the flood water in the Rio Padrongianu.
• construction of secondary tunnel to connect the several rivers intercepted to the main diversion tunnel.

Other interventions were included in the design, in particular regarding the upgrade of existing watercourses through the urban area of the city, described as follow:

• upgrading of the existing river courses in the city area, for a total length of 15 km;
• demolition and reconstruction of 20 road bridges.
• construction of diversion channels upstream of the existing burrier reaches, in order to exclude them from their function as receptors of watercourses, since it has been verified that it is not economically, environmentally and socially convenient to adapt them. Therefore, only sewer water receptors have been provided for graves.