Urban Flood Evaluation in Maceió, Brazil: Definition of the Critical Flood Event Supported by a Mathematical Cell Model

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1 Urban Flood Evaluation in Maceió, Brazil: Definition of the Critical Flood Event Supported by a Mathematical Cell Model Vidal, D.H.F 1*, Barbosa, F.R 1, and Miguez, M.G 1 1 Federal University of Rio de Janeiro COPPE. Ilha do Fundão, Centro de Tecnologia, Bloco D, Sala I206, , Rio de Janeiro, RJ, Brazil *Corresponding author, davyd_faria@yahoo.com ABSTRACT Projects in urban drainage and flood control must necessarily consider the basin as a whole and it is important to work in the scale of the basin. However, sometimes it is difficult to define what is the scale of the basin, specially when focusing on tributaries that can be heavily affected by the main river. Gulandim River is one example of this situation. Located at the most downstream reach of Reginaldo River and suffering the effects of tides, it is notable that the main river plays a significant role in Gulandim flooding. This paper aims to evaluate which is the critical flood event for Gulandim River. Using the support of a mathematical model, MODCEL, which assumes that a basin can be subdivided into a set of homogeneous compartments, interconnected, called flow cells, which integrates the basin plane in an arrangement capable of reproducing the flow patterns that occur. The simulations with Reginaldo basin time of concentration were more critical. Gulandim River outlet location is the main reason for this conclusion, because the proximity to the Reginaldo River outlet and the Atlantic Ocean produce considerable backwater effects on the Gulandim River, influencing directly in urban floods in this watershed. KEYWORDS Hydrodynamic modeling, critical flood event, urban drainage, Reginaldo watershed. INTRODUCTION Urbanization around the world is remarkable over the last few decades. It has resulted in unplanned areas that may pose significant problems, such as regular flooding due to inadequate drainage facilities. Flood risks have been managed through structural measures that emphasize investment and constructed solutions or non-structural measures which aimed to reduce the exposure of the community and aware the population of the importance to prevent risks. Flood magnitudes and frequencies increase in urban areas due to the high impervious surfaces implemented in city development. Once there are no source controls, impacts are transferred to downstream in the major drainage. In many Brazilian cities, the economic losses due to floods are quite significant. The development of urban drainage projects consider the design rainfall duration equal to the time of concentration of the basin where the project will be implanted, showing the importance of this parameter. Runoff is associated to several structural problems mentioned above, however an important technical discussion by engineers hydrologists is associated with time of concentration (t c ) to be considered in the hydrological and hydrodynamics simulations because t c is closely Vidal et. al. 1

2 related to determination of critical rainfall events. This is considered fundamental parameter for the model to determine the position of the peak flow. The time of concentration of a watershed can be defined as the time required for the runoff generated on the hydraulically most distant point from the outlet of the main river to arrive at the basin outlet. In order to obtain t c almost all the engineering studies involves empirical equations. It is important to use this parameter as a hydraulic support in determining the critical rainfall event that should be considered in proposing solutions to the flood problems in the basins under study, as it directly influences the value of the maximum flows obtained. Floods are often aggravated due to the superposition of effects, for example, the combination of hydrographs of the tributaries to the main river, installation of new enterprises with high sealing rate generating higher flows, the combination of tidal effect with the rain event in specific areas located of the basin, among others. Thus, the solutions for urban flood problems should be evaluated in an integrated manner, so the combination of negative effects in time is avoided in the project. Projects in urban drainage and flood control must necessarily consider the basin as a whole and it is important to work in the scale of the basin. However, sometimes it is difficult to define what is the basin to be considered, specially when focusing on tributaries that can be heavily affected by the main river. Gulandim River is one example of this situation. Located at the most downstream reach of Reginaldo River and suffering the effects of tides, it is notable that the main river plays a significant role in Gulandim flooding. In this context, this paper aims to evaluate which is the critical flood event for Gulandim River using the support of a mathematical model, MODCEL (MASCARENHAS AND MIGUEZ, 2002). Flooding effects will be simulated for two different design rainfall events: one of them constructed to match the time of concentration of Gulandim River (90 min); the other related the time of concentration of Reginaldo River (230 min). METHODOLOGY Several methodological steps were combined to evaluate the effect of tidal influence and the Reginaldo stream influence into Gulandim River, among which it s important to mention the following: 1. Field visits campains for data acquisition and observation of the urban infrastructure and the riparian area throughout the basin; 2. Division of study area in flow-cells, contemplating the model to be simulated; 3. Formulation of the topological scheme used in MODCEL; 4. Definition of boundary conditions used in the model; 5. Hydrological-hydrodynamic simulation of the two scenarios studied, with different critical rainfall design events; 6. Preparation of charts and maps that represent concise and consistent acquired results. 2 Urban Flood Evaluation in Maceió, Brazil

3 In this work it was used two computer models to simulate the downstream reaches of Reginaldo River and Gulandim basin. A hydrological model for flood hydrographs calculation that will represent the boundary conditions in the main simulation and a hydrodynamic model capable of simulate the flow dynamics of the canals and the surface flow on the urban environment. The hydrologic modeling was done through HydroFlu, a system, which, among several applications, include a concentrate rainfall-runoff hydrologic model. The hydrodynamic model MODCEL will be briefly in next topic. Booth programs were developed by the Federal University of Rio de Janeiro - UFRJ MODCEL MODCEL (MIGUEZ, 2001; MASCARENHAS & MIGUEZ, 2002) is a computational modeling tool, which help to identify the urban flooding patterns in large inundated areas. The model is able to assess the watershed in a systemic way, taking into account hydrological and hydraulic characteristics. This mathematical model can be considered quasi-bidimensional, once it uses classical one-dimensional hydraulics equations; in a looped flow net developed to represent space in two dimensions. This approach allows the interpretation of the watershed in an integrated way, simulating channels, galleries and flood plains. In general, the flow cell concept proposes to represent a basin through homogeneous compartments, which, are able to represent the urban landscape and to simulate a complex integrated flow net that interacts with urban structures. Therefore, this concept joins storm-water drainage system and urban landscape in the same modeling process, allowing to map interdependences between storage and conveyance patterns. A set of different kinds of cells improves model capacity of representing rivers, channels, stormwater galleries, natural or urbanized floodplains, and reservoirs. At each open compartment, there are hydrological processes modeled to transform rainfall into runoff. All compartments are integrated and subjected to the mass conservation principle. The definition of a large set of different types of hydraulic links between cells allows the reproduction of multiple urban flow possibilities. The main list of hydraulic links modeled for urban areas involve: Saint Venant dynamic equation, with or without inertia terms, broad crested weirs, orifices, gullies, reservoir discharge (combining weir and orifices), pumping stations, flap gates, and galleries (acting as open channels or drowned). The main cell types are defined during the study area division. The two types of cells used in this work are described as following: river or channel cells which represent the main drainage free surface flow, and; urbanized plain cells, used to represent the surface flow in floodplains, as well as storage areas, associated to a certain pattern of urbanization, linked each other by streets. MODCEL, as well as most hydrodynamic models, contemplates the execution of three basic steps, consisting in pre-processing, analysis and post-processing. The pre-processing prepares the model input files, for example, files with the characteristics of cells and its connections, with boundary conditions and the representation of rainfall. The second step consists in implementing Vidal et. al. 3

4 all the numerical calculations represented by the model departing from the use of various hydraulic laws. The last step is to produce flood maps and hydrographs, based on the result of the model. Representation of the area into the cell model The study area was divided in 61 cells. In this study the modeled portion of Gulandim River watershed was divided into 57 flow cells, with 6 channel cells representing Gulandim River and 51 urban superficial cells representing the pattern of urbanization in the basin. It was also necessary to add part of the Reginaldo River to evaluate the effects of backwater and as well as the necessary boundary conditions. Reginaldo River was divided into 4 channel cells. Figure 1 shows part of the study area divided into cells and the center of each one. Connection among centers is responsible for defining a flow net. Figure 1. Example of cell division in an urbanized area at Reginaldo River watershed. Three types of hydraulic links were used: river link (Saint Venant dynamic equation), weir link (simulating a broad crested weir) and urban plain link (Saint Venant dynamic equation without inertia terms). Figure 2 shows the hydraulic links proposed for the same area shown in Figure 1. 4 Urban Flood Evaluation in Maceió, Brazil

5 Figure 2. Representation of hydraulic links between cells in an urban landscape. CASE OF STUDY Study Area The basin of Reginaldo River became focus of development of various research studies and projects. It is of the major urban watersheds of Maceio/AL, Brazil, and presents problems of urban floods, comprises various socioeconomic population classes, and shows a great variability in its topography (plain areas, main valley well fitted, flat coastal areas) and present a considerable state of environmental degradation at several determined areas. This basin encompasses total or partially 18 of the major neighborhoods in the capital of Alagoas. Gulandim watershed is one of the principal basin inserted into the lower part of Reginaldo River. Gulandim River presents a drainage area of approximately 27.1 km² and its elevations ranges from 11 m at its spring to 9 m at its outfall. Gulandim River travels about 1.9 km through channels and galleries. Being an extremely plain area, near beaches and downtown, the region was subjected to an accelerated urbanization process. Nowadays this basin is one of the most urbanized areas of Maceió city, which led to the strangulation of Gulandim River at several points as well as the construction of houses at its margins, confining and transfering the flow with major speeds to downstream. The change in the infrastructure of the urban environment in the proximity of Gulandim River causes, in extreme precipitation events, occasional flooding distributed along the basin as well as increasing upstream floods, due to the occurrence of backwater. It is noteworthy that besides the several singularities that exist along the main course of Gulandim River, it suffers in its outfall the influence from both Reginaldo River and the tide. Vidal et. al. 5

6 These larger water bodies aggravate problems of flooding due to overflowing of the River near its outfall. To assist in the definition of the types of cells and its modeled connections, it was necessary to conduct a field visit to understand the behavior of the river and its basin, generating a photo relatory showing the appropriate types of connections and the singularities along the Gulandim River. Figure 3 shows the delimitation of the watershed of Gulandim River, the route performed in the field visit and some pictures of the main areas that affect the flow dynamics. According to Pimentel (2009), the basin is included in a 0.16 km² area, constituted of 45.7% of paved roadways, 51.9% of cobblestone and 2.4% of unpaved roads. About 90% of its drainage area is served by the sewage collection and transportation system, which often helps to minimize the flooding problem consequences. Figure 3. Delineation of watershed Gulandim River and field visit within the basin. RESULTS The results of this paper elucidate the question about the critical duration time for the design rainfall that should be considered to determine in the critical situation for Gulandim River. Various evaluation scenarios were constructed to understand the real contribution of the Reginaldo watershed at the outlet of Gulandim River. This approach was necessary to understand where to focus interventions to remedy the flooding of that watershed. 6 Urban Flood Evaluation in Maceió, Brazil

7 The design flood adopted had a time of recurrence of 25 years and durations of 230 and 90 min, representing Reginaldo and Gulandim time of concentration, respectively. These times of concentration were estimated by hydrology equations such as Kirpich and California Practice Culverts. The results show that both the tidal effect at the outfall of Reginaldo River and its awn floods, together or isolated, causes backwater on Gulandim River outfall. The hydrodynamic model allows the representation of this phenomenon with great efficiency. Figure 4 exhibit the behavior of the modeled reach of Gulandim River in the first simulated scenario attributing Gulandim River time of concentration as the project rainfall duration. Figure 4. Water level in Gulandim River for rain project of 90 minutes. In the presented scenario it s possible to realize that the water level in Gulandim River reached around 4.3 m in the upstream modeled reach (cell 6) while water levels near Gulandim outfall reached approximately 3.7 m ins a posterior time. The peak of the hydrographs produced by the local design flood occurred approximately a little bit less than 2 hours after the simulation started. It is also noticed that past Gulandim River water high levels, which correspond to the hydrograph concentration time, the recession does not happen even with the gap in the level of Reginaldo stream. Thus, the maintainance of a water high level in Gulandim occurs because of the tide influence, and it could be verified during the field visit, where the outfall of this stream in dry weather was observed. Figure 5 presents the modeled reach of Gulandim River behavior during the second scenario simulated, adopting the time of concentration of Reginaldo River as project rainfall duration. It shows that the drained water level in Gulandim River reached about 4.6 m at the upper reaches cell 1, near the outfall, reached 4.5 m, almost at the same time. All the simulated hydrographs are highly influenced by downstream effects and the peak of the hydrographs occurred approximately four hours after the simulation started. Vidal et. al. 7

8 In this scenario it s observed that since the rainfall started, the influence of Reginaldo watershed in conjunction with tide events impose the water level of Gulandim River. These simulation permits to realize that the backwater is propagate along Gulandim River, which presents higher water levels during than that obtained in the preceding case. Figure 5. Water level in Gulandim River for rain project of 230 minutes. CONCLUSIONS Analyzing the simulated scenarios it is possible to understand that an intervention intending to solve drainage problems in Gulandim basin should consider the time of concentration equal to or near the Reginaldo watershed s. The simulations with Reginaldo watershed time of concentration were more critical. Gulandim River outlet location is the main reason for this conclusion, because the proximity to the Reginaldo River outlet and the Atlantic Ocean produce considerable backwater effects on the Gulandim River, influencing directly in urban floods in this basin. Probably, interventions on Reginaldo River, aiming to minimize flood levels in this basin will indirectly help, in a significant way, to solve flood problem in Gulandim river. REFERENCES MASCARENHAS, F. C. B.; MIGUEZ, M. G Urban flood control throug a mathematical cell modell. Water International, United States, v. 27, n. 2, p MIGUEZ, M. G Modelo matemático de células de escoamento para bacias urbanas. Tese de D.Sc. COPPE/UFRJ, Rio de Janeiro, Brasil. MIGUEZ, M. G., MASCARENHAS, F. C. B., MAGALHÃES, L. P. C., D ÁLTERIO, C. F. V Planning and design of urban flood control measures: assessing effects combination. Journal of Urban Planning and Development,Vol. 135, Nº 3, p PIMENTEL, I. M. C., Avaliação quali-quantitativa das águas do Riacho Reginaldo e seus afluentes. Dissertação de M.Sc. em Rec. Hídricos e Saneamento, UFAL, Maceió, Brazil. 8 Urban Flood Evaluation in Maceió, Brazil