Infiltration well as compensatory measure of urban drainage: A financial analysis

: The growth of cities generates a waterproofing of soils, causing much of the rain to seep off superficially. To remove this flow as soon as possible from the urban perimeter, urban drainage works are carried out. As in Brazil, conventional drainage techniques still predominate, in which the flow is transferred to the downstream points of the basin, these over time become inefficient, thus generating the need for the adoption of compensatory measures, among which we can mention the infiltration well. In view of the above this article aims to size an infiltration well for different areas of capture and perform a financial analysis of the construction of these devices. Four catchment areas (100 m², 250 m², 500 m² and 1000 m²) were adopted in the city of Aracaju/SE and for the sizing of the well was used a method adapted from Carvalho Júnior (2020). Wells with total depth ranging from 1.15 m to 6.10 m were obtained, and construction costs ranged from R$ 2,465.95 to R$ 8,697.53. Based on the results obtained, it is concluded that the sizing method applied is easy to apply, but implies larger wells, and finally, the replacement of concrete rings by another type of coating, can reduce the total cost for execution.


INTRODUCTION
According to the National Information System on Basic Sanitation (SNIS, 2021), rainwater should drain superficially through natural paths to reach the lowest points of the watersheds.However, the cities alter this natural environment, so as to interfere with the natural cycle of water drainage, causing the need for intervention, seeking to minimize the impacts that hydrological events can cause due to human anthropic action, which leads to the adoption of control measures, composed of structural actions, which are physical interventions; and structuring actions, such as guidelines, legal standards, inspection and education.
The goal of urban drainage is to remove rainwater runoff as quickly as possible from the urban perimeter, avoiding damage to the city and ensuring that there are no impacts to water bodies, the recipients of stormwater (ANA, 2002).According to Silva et al. (2019), the concept of urban drainage is conditioned to the ancient practices of dealing with the problem of stormwater in cities, which results in the procedure of rainwater harvesting, seeking that this procedure occurs as quickly as possible, efficiently to avoid damage.
In Brazil, conventional drainage techniques still predominate, in which the runoff is transferred to the downstream points of the basin (CALIL, BERNARDI, RIGHES; 2016).Modernly, several actions have been adopted in order to contain or mitigate the generation of urban floods, which are called compensatory techniques.
Compensatory techniques present a concept of sustainable drainage, which seeks to reduce the impacts that the urbanization process causes on the hydrological cycle (GONZALEZ, 2014).If the drainage system fails, the city will suffer from flooding and this shows how important storage measures become to reorganize runoffs and mitigate problems (CANHOLI, 2014).According to Lisboa (2019), there are numerous compensatory techniques that can be used to help control floods, and the most widespread ones have a focus on storage measures (detention, retention, and in-lot reservoirs) and infiltration measures (green roof, infiltration trench, permeable sidewalk, infiltration trench, infiltration well, among others).
The infiltration well consists of a well, as a wall of perforated concrete pipes or solid brick masonry in screen coated externally with geotextile blanket and filled with gravel No. 1 on the sides (interface blanket -soil), and a layer of gravel of 30 cm also coated (CARVALHO JÚNIOR, 2020).Its function is to infiltrate rainwater, in order to dampen the peaks of flow that may reach the conventional drainage system and, consequently, reduce the volume of direct surface runoff by retaining the excess volume of rainfall in the soil (BARBASSA; SOBRINHA; MORUZZZI, 2014).The water table level also represents an important variant that influences the definition of the well geometry, and may even make its use unfeasible (SANTOS; CALVARIO, 2021).Martins (2017) apud Santos and Calvario (2021) points out minimum distances from the bottom of the reservoir to the water table, with values between 1.00m and 3.00m.A minimum distance of 1.5 m from the water table is the indication NBR 13969:199 (ABNT, 1997).It is recommended that the wells be built at least 1.50 meters away from other buildings, to avoid structural instability caused by infiltration or excavation of the well, and it is also necessary to conduct geotechnical surveys to identify the profile and height of the water level (SANTOS; CALVARIO, 2021).Given the above, this article aims to size an infiltration well for different catchment areas and perform a financial analysis for implementation of this device.

THEORETICAL FOUNDATION
The occupation of urban areas increases the sealing of built areas generating increased surface runoff, which causes a higher frequency of urban flooding (REIS; ILHA, 2014).According to these same RIMA, v.5, n.1, 2023, e211.authors, one way to reduce the impact caused by occupation are the compensatory technical solutions, based on stormwater infiltration techniques, which have been widely proposed for building systems projects.
Urban drainage systems modernly designed with environmental protection concepts prioritize the maintenance of the water balance existing during the pre-urbanization period; one of the solutions adopted is the introduction of drainage systems at the source that aim to restore the water balance balance in the area to be occupied by inducing infiltration of rainwater, in addition to the reduction and retention of surface runoff (REIS; OLIVEIRA; SALES, 2008).
One of the ways to induce rainwater infiltration is through infiltration wells.According to Barbassa; Sobrinha and Moruzzi (2014) these wells are systems widely employed in Europe and the USA, being a micro-scale technological alternative whose function is to retain rainwater at the expense of the runoff parcels.It is a compensatory technique called punctual (small area in plant), which aims to mitigate the effects of direct runoff in its generation and can also be designated as a control technique of direct runoff at the source, given that, by allowing the infiltration of water into the ground dampens the peaks of flow and, consequently, reduces the volume of direct runoff (BARBASSA; SOBRINHA; MORUZZZI, 2014).
The design of compensatory techniques, such as the infiltration well, should consider the characteristics of the soil, as this is who will receive the rainwater being its outlet, evaluate the possibility of contamination of the groundwater aquifer and avoid sanitary problems; the constructive aspects, such as support material of the walls and its permeability, the existence or not of pre-treatment upstream of the well and the distribution of water through the walls that can affect the performance of the well The infiltration wells can be built with perforated concrete rings, surrounded on the sides and base by gravel and geotextile blanket that acts as a filter that retains particulate material, pipe to bring the water from the impermeable areas and overflow connected to the public drainage system or gutter (SILVIERO, 2008).Reinforcing this need for the use of gravel on the side Ferreira; Barbassa; Moruzzi (2018) report that studies show that rainwater infiltration occurs better when using a percolation area on the walls and base having gravel on the sides and bottom and geotextile blanket surrounding the entire well.
Among the advantages in the use of this device, we mention the reduction of pressure in the conventional drainage system by reducing the volumes drained by the conventional drainage system, financial gain by reducing the conventional drainage system, low value for design and deployment, RIMA, v.5, n.1, 2023, e211.groundwater recharge, contribution to the development of vegetation around the well and the ability to be deployed in the most varied topographies (CASTRO, 2002 apud ANTOS; CALVÁRIO; SOUZA, 2021).
Among the disadvantages are: the possibility of clogging the walls and bottom of the wells that can hinder infiltration; the risk of groundwater contamination, especially in sandy soils and high water table; and the saturation of hydraulic conductivity around the well, interfering with its ability to infiltrate water into the soil (ARAUJO, 2010).(CASTRO, 2002; MARTINS, 2017; BARBASSA; FERREIRA; MORUZZZI, 2018 apud SANTOS; CALVÁRIO; SOUZA, 2021).

METHODOLOGY Study area
The city of Aracaju/SE will be used.For financial analysis of the well construction costs four areas will be adopted (100 m², 250 m², 500 m² and 1000 m²), dimensioned one well for each, and finally, a budget estimate will be done.

Sizing of the infiltration well
To size the well, a method adapted from Carvalho Júnior (2020) described below will be adopted:

i)
Initially the design flow rate will be calculated determined by the equation below: Where: C = runoff coefficient (adopted equal to 1), i = rainfall intensity (mm/h) for a return period of T=1, adopted as 116mm/h for Aracaju according to NBR 10844 (ABNT, 1989); A = catchment area (m²) and Q = flow (L/min).
ii) Calculate the total volume precipitated over the contribution area according to equation 2 below: Where: V prec = volume (Liters); Q = design flow rate (L/min) and t = time (minutes).The time of 5 minutes will be adopted, because according to Carvalho Júnior (2020) this value is able to reduce the flood peak; iii) Calculate the useful depth according to equation 3, below: RIMA, v.5, n.1, 2023, e211.

Dimensions of the infiltration wells
Following the method presented above, and considering the different coverage areas, the dimensions presented in Table 01, below, were obtained: A direct relationship is observed between the catchment area, the volume of the infiltration well and its dimensions (diameter and depth).In our study, with we chose to keep the same diameter, the larger the catchment area, the larger the well volume and consequently the greater the depth.When the water table is a limiting agent, there are two possible solutions, increase the diameter and decrease the depth, or execute more than one well.

Quantities of services and budget estimate
In this stage of the work, based on the well dimensions, the quantities of excavation volume, concrete rings, gravel volume, geotextile blanket and concrete cover will be determined.Finally, the prices will be researched at SINAPI -Sistema Nacional de Pesquisa de Custos e Índices da Construção Civil.The results obtained are presented in Tables 2, 3, 4 and 5, below:   perform tests to determine the soil infiltration coefficient; b) Considering the soil as impermeable, in the design process, results in a well with larger dimensions and, consequently, higher execution costs; and c) Since the lining of the well (precast concrete rings) is the item that accounts for most of the costs, an alternative to cheapen the devices would be to use other linings, whenever possible.
) and, d = diameter of the concrete rings (in this article we will adopt concrete rings with d = 1.50 m).iv) Determine total depth, which the sum of the useful depth plus 30 cm of the gravel layer, and an upper clearance of 30 cm; v) In possession of the dimensions of the people, carry out the quantitative survey of the construction services of the device (excavation, concrete rings, gravel layer and concrete cover); vi) Based on the prices indicated by SINAPI, calculate the construction costs of the dimensioned wells.
Aiming at a more detailed analysis, Graph 01 was made for the different infiltration wells, indicating the percentage of each construction item in the final cost, presented below.CHART 01: Percentage of contribution of services in the construction of an infiltration well.SOURCE: Survey data (2022).RIMA, v.5, n.1, 2023, e211.

TABLE 01 :
Dimensions of the infiltration wells.

TABLE 02 :
Cost estimate for infiltration well with catchment area 100 m².

TABLE 03 :
Cost estimate for infiltration well with catchment area 250 m²

TABLE 05 :
Cost estimate for infiltration well with catchment area 1.000 m² SOURCE: Survey data (2022).