Flooding analisys, using HEC-RAS modeling for Taquaraçu river, in the Ibiraçu city, Espírito Santo, Brazil

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1 Flooding analisys, using HEC-RAS modeling for Taquaraçu river, in the Ibiraçu city, Espírito Santo, Brazil D. Prata 1* ; M. Marins 1 ; B. Sobral 1 ; A. Conceição 1 ; F. Vissirini 1 1 Federal Fluminense University, Ag&San Engineering and Research Group, Rua Passos da Pátria, 156, , Niterói,RJ, Brazil *Corresponding author, darioprata@gmail.com ABSTRACT The rapid urban growth of recent decades has caused changes in engineering concepts as the approach to control storm water in cities. Over the past years flood events have occurred more frequently in several counties in the state of Espírito Santo. At Ibiraçu, the urban population has been suffering from constant flooding leading to socio-economic and environmental damage, caused by the irregular occupations along the Taquaraçu river basin. This study aimed to evaluate the behavior of the Taquaraçu river and tributary and propose helpful drainage interventions, using the mathematical model HEC-RAS for hydrodynamic modeling and analysis of rivers, simulating different levels of water and water flow for different return periods. KEYWORDS Computer modeling; HEC-RAS; Ibiraçu; rivers; urban flood. INTRODUCTION With the acceleration of urban development in recent decades, some engineering concepts regarding water control have been changing. Sanitary problems reflect on the population health, constant flooding and the deterioration of a rich and diverse environment in many regions. The transformation of a rural environment in an urban area increases the sanitary problems jeopardizing future generations. Flood events have been occurring more often in several counties on the state of Espírito Santo. Ibiraçu is one of the cities who have suffered from constant flooding in the last decades, causing serious damage to its population. The Taquaraçu river, which flows through the urban area, has suffered more often significant increases in water level as a result of the changes throughout its basin, in both rural and urban areas. The main changes observed in the basin are concentrated in the process of occupation and sealing of the margins as well as the inadequate disposal of urban solid waste. This project aimed to analyze the behavior of Taquaraçu river with the use of the hydrodynamic modeling software developed by the "Hidrologic Engineering Center, U.S. Army Corps of Engineers, to simulate different water levels, flow rates corresponding to different return periods and observe the conditions of the project from the available database provided by the Brazilian National Institute of Waterway Research (INPH). Prata et al. 1

2 The HEC-RAS system contains four one-dimensional river analysis components for: steady flow water surface profile computations; unsteady flow simulation; movable boundary sediment transport computations; and water quality analysis. A key element is that all four components use a common geometric data representation and common geometric and hydraulic computation routines. In addition to the four river analysis components, the system contains several hydraulic design features that can be used once the basic water surface profiles are computed (HEC-RAS, River Analysis System). With the computational modeling it was possible to analyze the behavior of the Taquaraçu river for different estimated peak flows related to return periods of 2, 10, 15, 20, 25, 50 and 100 years. The results from the modeling where then discussed with the INPH team and interventions were suggested focusing on the most useful interventions, the ones that presented a greater reduction on the water level. METHODOLOGY The methodology used to analyze the behavior of the Taquaraçu river on the city of Ibiraçu was composed of the following steps: Hydrographical characterization of the sub-basins in which the Taquaraçu river and tributary are inserted The river basin is the catchment area of natural water flows arising from the rainfall that makes the flows converge to a single exit point. The hydrographical characterization of a basin requires the definition of a watercourse, or reference section along the waterway and topographic information on the region. The sub-basin of Taquaraçu river belongs to the basin Piraque-Açu and to the Pólo Linhares Microregion, inserted in the state of Espírito Santo. It has an area of about 86 km² and englobes the city of Ibiraçu. The Taquaraçu river is a tributary of Piraque-Açu river, it borns near the Sauna stream at an elevation of 281 m, and has a length of approximately 16 km. The tributary has a length of approximately 5 km and is located next to roadway BR 101 (EVEREST, 2007). This tributary crosses an urbanized area, where a stretch is presented confined by an underground gallery. Taquaraçu river Tributary Figure 1. Basin of Taquaraçu river 2 Flooding analisys, using HEC-RAS modeling

3 Hydrological Studies The hydrological study aims to forecast the peak flows of the project, based on rainfall data available on the Water National Agency (ANA) website. The methods for estimating peak flows from rain project are especially important in urban watersheds and in the process of urbanization. A project rainfall is an ideal event associated with a return time. By using a rainfall designed with 10 years of return period, for example, it was estimated the maximum flow rate using a model of transformation of rainfall into runoff, it is assumed that the maximum flow generated by this rain will be available in the basin at least once every 10 years. To estimate the maximum rainfall it was used Gumbel s statistical method. The maximum rainfall was estimated by applying the data available at the Aracruz Station (Station ) considering Gumbel s statistical distribution. The maximum values of rainfall were obtained from the National Water Agency (ANA) database for the range of years from 1970 to 2009, as shown in Figure 2. Monthly Average Rainfall (mm) January 143,3 February 110,2 March 127,3 April 98,5 May 52,0 June 45,5 July 65,4 August 52,3 September 81,6 October 125,3 November 206,5 December 208,9 Monthly Average Rainfall (mm) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Figure 2. Seasonal variability of rainfall in Ibiraçu, represented by the average monthly rainfall for the period 1970 to Methods for estimating peak flows from rainfall depend on the size of the basin. In very small basins the Rational Method can be used. In larger basins are commonly used models of rainfall-runoff transformation, as the Unit Hydrograph (UH). Modified Rational Method As the contribution area of the tributary river basin is smaller (<10 km ²), the flow rates were obtained by the Modified Rational Method (Environmental Standard VALEC, 2009). The method follows the following equation (1): Where: Q = discharge in m³ / s; Q = 0,278 C. I. A.σ (1) Prata et al. 3

4 C = dimensionless runoff coefficient (runoff); I = average intensity of precipitation over the basin in mm/h; A = drainage basin area in km ²; σ = retardation coefficient, expressed by A-0,10, dimensionless; and = Unit conversion factor. Unit Hydrograph To estimate the runoff of the Taquaraçu river (main river) it was used the Unit Hydrograph Method, developed by the Soil Conservation Service. The application of the Unit Hydrograph was based on the following assumptions: The intensity of effective rainfall is constant during the rain that produces the maximum UH; The effective rainfall is uniformly distributed throughout the drainage area of the basin; Time base or duration of the hydrograph of the direct surface runoff is constant, due to an effective rainfall of unit length; The direct ordered surface runoff hydrographs, with a common base time are directly proportional to the volume of hydrographs; The effects of all the characteristics of a given watershed, including shape, slope, detention, infiltration, drainage, storage capacity of the channel, etc.., are reflected in the shape of the unit hydrograph of the basin. Computational Modeling For the computational modeling it was used, as input data, the bathymetric data provided by INPH. The altimetric profiles of the sections were transported to the HEC-RAS model, aiming at the hydrodynamic simulation of the watercourses in question. Of the 99 sections entered, 89 are related to the Taquaraçu river (Stretch 1, S1 to S33 and stretch 2, S1- S56) and 10 sections refer to the tributary (Stretch 1A, S34 at the entrance of the gallery that runs through the city center and S44, upstream of the gallery that runs through the BR 101 roadway). In addition to the sections, three existing bridges were positioned along the Taquaraçu river, as well as the dam downstream of Ibiraçu city. With the computational modeling, it was possible to simulate the height of water level for different scenarios in order to estimate the values of quota reduction of flood along the river. RESULTS AND DISCUSSION The maximum flows obtained from the hydrological study for the different return periods, were applied to the hydrodynamic modeling software. It was then possible to analyze the height of water level along the Taquaraçu river and its tributary, for different flow conditions. Figure 3 represents the original height of water levels for return periods of 2, 10, 20 and 100 years, considering bridges along the river and the dam downstream of the city. The junction shown in the figure is the meeting point of the Taquaraçu river and its tributary. 4 Flooding analisys, using HEC-RAS modeling

5 30 Junction Legend WS 100 Anos WS 20 Anos Elevation (m) 25 Dam WS 10 Anos WS 2 Anos Ground Bridge 03 Bridge 02 Bridge Main Channel Distance (m) Figure 3. Longitudinal Profile - Taquaraçu river The influence of drainage devices along the Taquaraçu river was also analyzed by observing the backwater and the heights of water level in the present bridge and dam downstream of the city. Figure 4 shows the cross section of Bridge 2, where it is possible to see the different water level for several return periods. Elevation (m) Legend WS 100 Years WS 20 Years WS 10 Years WS 2 Years Ground Bank Sta Station (m) Figure 4. Water level for different return periods on Bridge 2 The water level analysis in the simulation for the current condition of the Taquaraçu river and its tributary shows that the most affected stretch, by the rains on Taquaraçu river, is the stretch that extends from the second bridge to about 400 m downstream of that point. Starting from that point and going downstream on the river, floods exceeding ground level quotas are shown for most of the return periods. The water level reaches up to 3.50 m for a rainfall of 100 years, corroborating the information obtained at the conducted field study that this region is a critical area in times of heavy rainfall. The outflow point of the tributary river is precisely in this critical region, however, according to local records and historical observations, the greatest water levels in this area are attributed to the contribution of Taquaraçu river basin. The large volume of water resulting from rainfall in the basin causes an increment on the flow of the main river, blocking the outflow of the tributary river and leading to raise water levels upstream of the tributary river, which runs below the city center. Prata et al. 5

6 In order to minimize the effects of the floods caused by rains in the Ibiraçu city, interventions proposed by the INPH were applied to the model. The interventions applied were the use of a regular section and slope, deviation and plumbing of the tributary river to downstream of the city, and expansion of the bridge span of the Ibiraçu-Aracruz bridge. The first intervention was to adopt a regular section and determine an average slope for the river, on the stretch that goes from the first bridge to 900m downstream of the Bridge 3, suggesting a profile for dredging silt from the river channel and, consequently, reducing water levels on the river. It was used a single regular section to analyze both the Taquaraçu and tributary rivers and its dimensions can be seen in Figure 5. Figure 5. Used regular section The slope used was 1.60 m / km, it was obtained through quota values and horizontal distance on the stretch mentioned above. The same methodology was applied to the tributary, and the slope used was 5.45 m / km. The dredging of the Taquaraçu river and its tributary proved to be very efficient when regarding reductions on the water level along the river, with reductions ranging from around 0.5 to 1.5 m in the main river and up to 1.40 m on the tributary river. The plumbing and deviation of the tributary river, second intervention applied to the model, was based on the use of the Multidimensional Channel type 04, whose section is shown in Figure 6. It is an efficient, high-tech innovation with applicability in urban macrodrainage and registered international patent. The positive points of this technology are the proven efficiency, the ease of deployment, and the allowance of vehicles flowing directly on the surface. Figure 6. Multidimensional Channel - Type 04 Section 6 Flooding analisys, using HEC-RAS modeling

7 Thus, the project considered the design of the channel along the Avenues Count D'Eu and Getulio Vargas, parts considered critical and more susceptible to flooding during rainy season. With the implementation of the Multidimensional Channel Type 4, the simulation showed that there would be no more water flow in the affluent region. Moreover, the original outflow region for the tributary on the main river (critical region) showed reductions of up to 1.5 m in the water level, because of thereducing contribution of water by the affluent. The last intervention applied to the model was the opening of the span on the Bridge 3 in order to allow better flow of the Taquaraçu river. To do so, it was introduced a support pillar and removed the abutment on both sides of the bridge as shown in Figure RS=1199 Upstream (Bridge) (a) (b) Figure 7. Interventions on Bridge 3 (a) Bridge with abutments; (b) Bridge with support Pillar This intervention considered Taquaraçu river dredged and the plumbing and deviation of the tributary with the Multidimensional Channel type 04, influencing the Taquaraçu river. It starts to demonstrate significant reductions on the flood quota from the junction between the tributary and main river. In the third section of the bridge were observed reductions in water levels for all the return periods, with the largest reduction (0.35 m) occurring for the return period of 100 years. Through these analyses, it was concluded that the interventions are necessary and were effective as measures of flood reduction. The dredging of the Taquaraçu river is the main intervention, and can be taken as first aid measure, once it showed satisfactory results in reducing the quota for the Taquaraçu river, especially in dense urban areas directly affected by the seasonal floods. A second measure to be adopted would be the deviation and pumbling of the tributary river, with Taquaraçu river previously dredged. That would remove the water volume of the critical stretch and the greater flow in the sections downstream of the outflow (deviated channel) did not show considerable increases in their water level. Moreover, these sections are on less urbanized areas far from the city center, providing no direct risks to the population in case of occasional floods. Therefore, it was possible to conclude that interventions generally provide a considerable reduction in quota for return periods of 10, 15 and 20 years (used as reference periods for urban drainage). That is, in most sections, it was noticed that the water level reduced to below city level quotas after the interventions were applied, reducing most of the drainage problems in parts of Ibiraçu city. Prata et al. 7

8 REFERENCES HEC-RAS, River Analysis System. User s Manual. Version 4.1, US Army Corps of Engineers. Hydrologic Engineering Center, 2010, Davis, EUA ( visited 15 December EVEREST, Hydrologic and hydrographic studies and basic design of engineering for dredging of the drainage channel of the river Taquaraçu - Step 1. Volume 1. Municipality of Ibiraçu ES. ANA, National Water Agency. Hydrological information system - HIDROWEB. ( Visited 20 August ENVIRONMENTAL STANDARD VALEC, Superficial drainage protection against erosion. ( Visited 12 June INPH, Brazilian National Institute of Waterway Research. Top - hydrographic and sedimentological measurements at Taquaraçu river and tributary, at Ibiraçu ES. 97 pages. INPH, Brazilian National Institute of Waterway Research. Top - hydrographic complementary measurements at Taquaraçu river and Jucu river Barra do Jucu and Ibiraçu ES. 24 pages. 8 Flooding analisys, using HEC-RAS modeling