Analysis of Flood Control Alternatives: The Anhangabau Watershed, Brazil

Flooding and overflows are common and recurring problems in several Brazilian cities, which usually undergo disorderly development, mainly affecting flow conditions and water infiltration into the soil. Their causes vary from increased impervious surface areas, deficiency / inefficiency of drainage structures and their maintenance, siltation of rivers, channel obstructions and climatic factors. This situation is even aggravated when occurred in large cities.

The Anhangabaú Watershed lies in the central portion of São Paulo – Brazil and covers a drainage area of 540 ha, emptying into the Tamanduateí River, which arises from other municipalities in the metropolitan region. The region is highly urbanized and is crossed by an important north-south road connection. During heavy rain period, portions of this interconnection passage become compromised, due to floods at the bus terminals, main streets and tunnels, completely disrupting the flow of vehicles, creating a chaotic situation for the population, as well as losses to the national economy.

In seeking solutions to the problems of flooding in the basin, which have been studied for decades, the São Paulo City Hall hired Hydraulics Technology Center Foundation (FCTH) to evaluate the performance of two traditional alternatives already proposed for the region system drainage and to offer a new alternative, based on modern concepts of water resources management. It was then elaborated a complex modeling network employing PCSWMM application, developed by CHI, seeking to represent all road and drainage system and their interconnections. The model represents, altogether, 110 km of roads, 50 km of drainage networks and 2,800 joint structures as curb inlets and drainage grates. Subsequently, the PCSWMM 2D module was applied to the entire watershed to represent the floods while taking DTM and buildings as restrictions to the water flow.

With the assessed model, 3 alternatives intended to mitigate the flooding problem in the lower valley were evaluated with different variations: (i) 2 flood detention reservoirs and reinforcement of main gallery system, designed to ensure safety against 25-year return period events; (ii) reinforcement of the main gallery and flow derivation tunnel leading to the Tamanduateí River for a 100-year return period; (iii) distributed linear retention spread over the watershed in stages of return periods of 10/25/100-years.

The alternatives were simulated for different design storms for a critical future urbanization condition. The comparison of the alternatives performance through various scenarios was deemed with the assessment of hazard indices (ratio between water depth and speed flow on the road), identified vulnerable assets and flow and water depth series on specific points of interest, such as tunnels and pathways.

As a result, the best solution was determined by a multi-criteria analysis technique, which takes into account not only the solution efficiency and avoided damage from performance indices generated by PCSWMM simulations, but also factors such as cost, permanent and temporary environmental impacts and public attention.