Performance analysis of flood control facilities in São Paulo Metropolitan Region during extreme rainfall events, based on hydrological simulation

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1 Performance analysis of flood control facilities in São Paulo Metropolitan Region during extreme rainfall events, based on hydrological simulation Graciosa, M.C.P. 1* ; Coelho, G.A. 2 ; Vieira, J.R.S 3. e Canholi, A.P. 4 1 Hidrostudio Engineering, Rua Cardoso de Almeida, 167, cj. 72, , São Paulo, SP, Brazil. *Corresponding author: melissa@hidrostudio.com.br ABSTRACT The Metropolitan Region of São Paulo MRSP, comprises 19 million inhabitants spread over an area of km², which is completely within the Upper Tietê River Basin. Recurrent flooding problems on this watershed occur mainly because of the complexity of the system, which conducts the entire runoff of this urban area to the same watercourse, the Tietê River. The maximum available capacity of Tietê River is already achieved, considering the impossibility of enlarging the existing channel. Thus, flood control must be based on the reduction of contributions from the tributaries watersheds, by keeping or restoring its storage capability. The Upper Tietê Urban Drainage Master Plan PDMAT, developed by Hidrostudio Company for the Department of Water and Power of The State of São Paulo DAEE, has recommended many hydraulic flood control structures, mostly detention basins, to be implemented on the sub-catchments. Those facilities consist of the enlargement of river beds, detention basins and floodplain parks which are planned to be implemented in phases, according to its priority, defined by the graveness of each case. In this study, the effectiveness of the proposed structures was evaluated by using hydrologic modeling. Observed rainfall events occurred from 2009 to 2011 were analyzed. The current facilities provide protection against the most frequent rainfall events. After the implementation of all the planned detentions basins, the watershed is expected to have protection against floods until 100-years return period. KEYWORDS Hydrological Modeling, Metropolitan Region of São Paulo and Urban drainage. INTRODUCTION Flood control in the Metropolitan Region of São Paulo MRSP is a problem with many boundary conditions. One of the most important restraints for this problem is the limitation of capacity of water courses, notably Tietê River, to conduct the discharge originating from runoff during extreme rainfall events. Figure 1 shows the flow scheme of Upper Tietê River Basin, with the hydraulic capacity of the Tietê River and its main tributaries. The maximum available hydraulic capacity of Tietê River, downstream the junction with the Pinheiros River, that corresponds to the downstream limit of the River inside São Paulo city, is limited to m³/s. This limitation was settled considering the impossibility of enlarging the existing channel. Graciosa et al. 1

2 Thus, flood control must be based on the reduction of contributions from the tributaries watersheds, what can be achieved by keeping or restoring the storage capability of these watersheds. The Upper Tiete River Basin Urban Drainage Master Plan for flood control PDMAT, developed by Hidrostudio Company for the Department of Water and Power of The State of São Paulo DAEE, has recommended many flood control facilities to be implemented on the watershed, in order to control the runoff in the catchment areas where it is generated. Those facilities consist of enlargement of river beds, detention basins and floodplain parks which are planned to be implemented in phases, according to its priority, defined by the graveness of each case. Some of them were already implemented, and provide protection against the most frequent rainfall events. In this study, the effectiveness of the proposed structures was evaluated by using hydrologic modeling. Observed rainfall events occurred from 2009 to 2011 were analysed. Figure 1. Flow scheme of Upper Tiete River Basin. (Source: DAEE-PDMAT, ) METHODOLOGY In this study, the effectiveness of flood control facilities were evaluated by using hydrologic modeling. Rainfall records from stations operated by São Paulo s Inundation Warning System (SAISP), for the 2009/2010 hydrological year, were applied. The mainly watersheds comprised by the Upper Tietê Urban Drainage Master Plan (PDMAT) were studied. Three sceneries were evaluated: (1) Natural that considers the natural situation of the basin, without any flood control (2) Current that considers all the currently implemented facilities for flood control (3) Planned that evaluates the expected behavior of the basin after the implementation of all planned flood control facilities. 2 Performance analysis of flood control facilities in São Paulo Metropolitan Region

3 Hydrologic Modeling The hydrological modeling was performed by using the SCS rainfall-runoff model (Natural Resources Conservation Soil, 1976). The simulations were done with the software HEC-HMS (U.S. Army Corps of Engineers). The main watersheds studied in the Upper Tietê Urban Drainage Master Plan were evaluated. This article presents the results for Tamanduateí River Basin, one of the main tributaries of Tietê River in the south part of the MRSP, that encompasses, partial or totally, areas of the municipalities of Mauá, Santo André, São Bernardo do Campo, São Caetano do Sul, Diadema and São Paulo City. For this study, the watershed was divided in sub-basins, according to the catchment areas of its tributaries. Four precipitation events, which registered significant total precipitation amount, were simulated: September/2009, December/2009, January/2010 and January/2011. The hyetographs are shown in Figure 2. September 8th, :00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 2,0 4,0 6,0 8,0 12,0 14,0 16,0 18,0 22,0 5,0 15,0 25,0 3 35,0 4 45,0 5 55,0 January 21th, :00 23:00 00:00 01:00 02:00 03:00 04:00 05:00 06:00 07:00 2,0 4,0 6,0 8,0 12,0 14,0 16,0 18,0 5,0 15,0 25,0 3 35,0 4 45,0 5 December 8th, :00 20:00 22:00 00:00 02:00 04:00 06:00 January 18th, :00 18:00 20:00 22:00 00:00 02:00 04:00 06:00 PROSPERIDADE CÓR. ORATÓRIO TAMANDUATEÍ - MAUÁ COR. SARACATAN CÓR. CHRYSLER RIB. MENINOS - VOLKS RUDGE RAMOS RIB. MENINOS RIB. COUROS VILA MARIANA CÓR. IPIRANGA R. TAMANDUATEÍ - VD PACHECO CHAVES Figure 2. Simulated hyetographs. RESULTS Simulations for the event of January/2011resulted on the higher discharge peaks. For this event, simulated hydrographs obtained for Tamanduateí River watershed are shown in Figure 3. The hydrographs on Figure 3 corresponds to the main modelling junctions: 79 exutory of upper Tamanduateí River basin; 99 Meninos River basin; 133 exutory of upper Meninos River basin and 173 mouth of Tamanduateí River, on Tietê River. Figure 3 also shows the location of the precipitation gages. Table 1 shows the peak discharge simulated for the nodes presented on Figure 3. Attenuation of the peaks for each scenario is presented. Graciosa et al. 3

4 Figure 3. Simulated hydrographs for rainfall-runoff event of January/2011. Table 1. Simulated discharge peaks. JUNCTION DRAINAGE AREA (km²) DESCRIPTION Tamanduateí River basin NATURAL DISCHARGE (m³/s) EVENT: JAN/18/2011 CURRENT ATTENUATION DISCHARGE (m³/s) PLANNED ATTENUATION % % Meninos River basin % % Meninos River basin % % mouth of Tamanduateí River, on Tietê River % % 4 Performance analysis of flood control facilities in São Paulo Metropolitan Region

5 The hydrographs compares the three evaluated sceneries: natural, current and planned. The red line represents the natural scenario, that corresponds to the state of the watershed before the implementation of any facilities. The blue line corresponds to the hydrograph that represents the simulations with all structures currently implemented on the watershed. The green line corresponds to the planned scenario. The orange lines represent the discharge capacity of the channel cross sections. CONCLUSIONS Results have shown that the detention pounds can attenuate discharge peaks and restore the storage capability of the watershed. The concept of those structures are based on the best management practices in urban drainage, once they control floods within the watersheds they are generated. The discharge attenuation provided for the basins has effects along large river reaches downstream the structure. The outflow functions of the reservoirs were adopted considering the optimized operation for the project storm of 25 years return period. Those functions remained constant for all the simulated sceneries. Current studies have shown that the efficiency of detention pounds can be optimized, if the inflow structures are planned to be controlled according to the rainfall forecasting. This procedure consists of design the reservoir operation system in order to allow the detention volume to be optimized for the predicted volume or precipitation. Once the peak of reservation is as close as possible to the peak of the hydrograph, the operation of the detention basin is optimized. That goal can be achieved by controlled that operates according to the precipitation forecasting. This type of system ensures that the water levels in the detention basin will be close to its capacity. Simulated hydrographs based on observed rainfall events have shown that the existent facilities provide flood peak reduction when compared with natural peaks. The planned detention basins are expected to decrease flood risk in a range of 10 to 100-year return periods. The continuation of the structures implementation is recommended in order to prevent against floods with increasable higher return periods. REFERENCES Canholi A.P. (2005) Drenagem Urbana e Controle de Enchentes. Oficina de Textos, 302p, São Paulo. Hidrostudio / DAEE Departamento de Água e Energia Elétrica Plano Diretor de Macrodrenagem da Bacia do Alto Tietê. Relatório Síntese (2009). United States Army Corps of Engineers (2008) Hydrologic Modelling System user s manual. Institute of Water Resources, Hydrologic Engineering Center. Davis, CA, 278p. Natural Resources Conservation Service, Conservation Engineering Division (1986). Urban Hydrology for Small Watersheds. Technical Release 55. SAISP Sistema de Alerta a Inundações do Estado de São Paulo. Fundação Centro Tecnológico de hidráulica FCTH. Access: September 8-09, December 8-09 and January Graciosa et al. 5