Santiago is located in a semi-arid zone; however, historically the city has been the victim of intense rains concentrated in short time periods, the largest recent record being 150 mm that fell in a 24-hour period in 2002. This type of rainfall causes huge damage due to floods, with the most affected sectors located in the communities of Maipú, Cerro Navia, El Bosque, Lo Espejo, La Cisterna, and Conchalí. This week’s column will address the importance of green areas as a complement to the system of rainwater drainage by contributing to the prevention of floods and also generating public spaces that improve the quality of life for community inhabitants.
Santiago’s urban expansion has had a direct consequence on making the constructed ground impermeable to water, preventing the filtration of rainwater into aquifers. This has resulted in the increase of water running along the surface or runoff, which affects downstream sectors, especially those located in the most depressed parts of the city, particularly the communities of Maipú, Cerro Navia, El Bosque, Lo Espejo, La Cisterna and Conchalí.
The situation described is accentuated by the discontinuity in the city’s network of roads, which prevents the continuous flow of runoff, causing the accumulation of water and thus the flooding of some sectors of Santiago. Another impact factor when confronted with runoff problems is the shortage of green areas and the lack of connection between those green areas that do exist. According to the Centro de Investigación Periodística CIPER Chile (Center for Journalistic Research) and the data provided by the Metropolitan Regional Government of Santiago, Santiago’s surface area is 133,506 hectares (1). Prior to this measurement, the area considered as urban was 123,272 ha. There were 25,147 ha corresponding to undeveloped land, of which only 12,298 ha were effectively destined to be green areas, and of which only 6,001 ha were built (2), corresponding to 5% of the total urban area of that time and less than 4.5% of the current one.
(1) See P. Trivelli, La Disponibilidad de Suelo Urbano en el Gran Santiago y la Región Metropolitana (The Availability of Urban Soil in Greater Santiago and the Metropolitan Region), Centro de Investigación Periodística (2014) <http://ciperchile.cl/2014/09/03/la-disponibilidad-de-suelo-urbano-en-el-gran-santiago-y-la-region-metropolitana/>.
(2) Gobierno Regional Metropolitano de Santiago,(Regional Metropolitan Government of Santiago, Regional Policy on Green Areas) Política Regional de Áreas Verdes, Gobierno Regional Metropolitana de Santiago (2014).
To make up for the lack of filtration and avoid flooding in the low points of the city, systems that capture and redirect runoff caused by rainfall are usually used. These are located underground and are known as rainwater collectors. The primary collectors are designed, built and maintained by the Hydraulic Works Directorate (DOH), while the Housing and Urbanism Service is responsible for the secondary collectors, along with the design of the streets. Due to the lack of coordination between these two government offices, the design of the streets does not necessarily favor the optimal functioning of the primary collectors (3). Other factors that cause problems in the operation of the collectors are the lack of adequate maintenance, the lack of leaf and garbage removal that accumulate daily and the presence of neighborhood open-air fruit and vegetable markets, which contribute to the accumulation of waste.
(3) Information obtained from the Hydraulic Works Directorate.
Countries such as the United States, Australia and Scotland, among others, have implemented rainwater filtration and drainage techniques known as Sustainable Urban Drainage Systems (SUDS). These systems have a double utility. First, they provide all the virtues of green areas and recreational spaces, shade, and climate regulation. Second, and depending on the design established, they can filter and store large quantities of rainwater that can then be reused for watering the same green areas.
It is important to emphasize that for the use of these systems it is necessary to have a design that prevents the entry of contaminating materials into aquifers. In some cases, the depth of the aquifer decreases the likelihood that it will be adversely affected. For this reason, it is recommended that it be located at least 1.2 meters below the surface to avoid aquifer saturation (4). In Santiago, some aquifers are located below 90 m as in the La Pintana district, while in other places the depth barely exceeds 14 m as in Renca (5). In the latter case, it is important to filter the waters through SUDS to prevent contamination from entering the groundwater.
(4) See Ministry of Public Works, Directorate of Hydraulic Works, Manual de Drenaje Urbano: Guía para el Diseño, Construcción, Operación y Conservación de Obras de Drenaje Urbano (Manual for Urban Drainage: Guide for the Design, Construction, Operation and Conservation of Urban Drainage Works (2013).
(5) Information from the website of the General Directorate of Water (2016) <http://snia.dga.cl/BNAConsultas/reportes>.
The Ministry of Housing and Urban Planning of Chile presented the SUDS in 1996 as an alternative measure to prevent urban floods, but it did not reach extended usage and thus did not acquire the necessary importance to incorporate the system into the national reality. Despite this lack of widespread usage, in October 2013, the DOH designed a manual for its implementation (6).
(6) See n.4
According to the information gathered and developed for my thesis in landscape architecture, Trama Urbana y Drenaje : Factores Claves para Mejorar la Calidad de Vida en Sectores Vulnerables (Urban Design and Drainage: Key Factors to Improve the Quality of Life in Vulnerable Sectors), floods are the result of uncontrolled urbanization, inefficient planning and unpredictable behavior, which is characteristic of all natural elements. Consequently, the use of systems like those described is a way to recover part of the soil and its natural filtration characteristics ideally with autochthonous vegetation. These systems do not seek the elimination of the filtration collectors, but rather the construction of a complementary system that used together could offer an approach to achieve the ideal integration of urban processes with the flows of natural systems.
Ignacio Rivera Muñoz is a student in the Master’s degree program of Landscape Architecture at the Pontifical Catholic University of Chile. The contents of this column and the accompanying images form part of his thesis in process Trama Urbana y Drenaje: Factores Claves para Mejorar la Calidad de Vida en Sectores Vulnerables, directed by academic Danilo Martic Vukasovic.