Kentsel alanlarda yağmur suyu yönetiminin mekânsal bileşenlerinin Üsküdar meydanı havzası üzerinden değerlendirilmesi

Abstract

Giderek artan kentleşme ile sert zemin oranının yeşil alanlara baskın hale gelmesi doğal su döngüsünün bozulmasına neden olmaktadır. Bu durum yüzeysel akışa geçen su miktarında artışa sebep olarak yağmur suyu yönetiminde drenaj sorunları yaratmaktadır. Küresel ısınmanın da etkisiyle uzun yıllar projeksiyonlarında pik yağışlarda, yağış nedenli taşkınlarda ve birbirini takip eden yağışlar arasında artan süreye bağlı olarak kuraklıklarda artış olması öngörülmektedir. Bunlara rağmen, kentlerimizde hidrolojik döngüye olan müdahale biçimleri sürdürülebilirlikten giderek uzaklaşmaktadır ve sonucunda kentlerimiz yine olumsuz etkilenmektedir. Önerilen çözümler suyu statik bir maddeymiş gibi ele alan uygulamalardan ibaret olduğundan ve kentlerdeki geçirimsiz yüzeyler devamlı olarak arttığından dolayı sürdürülebilir bir yağmur suyu yönetimi sağlanamamaktadır. Bahsi geçen uygulamalar yağmur suyunu borular vasıtasıyla yerin altından metrelerce deşarj alanlarına taşıyan, gri altyapının bir parçası olan kentsel drenaj sistemleridir. Kentsel çevrenin saçaklanmasıyla paralel gelişen bu sistemlerin kapsadıkları alanların her geçen yıl biraz daha artması bakım ve onarım maliyetlerini de arttırmaktadır. Bununla birlikte, pik yağışlarda yetersiz kalmakta ve yağışa bağlı taşkınlara neden olmaktadırlar. Birçok farklı alt dalı olan hidroloji bilimi, yağış-akış ilişkilerini inceleyen genç ama hızla gelişen bir bilimdir. Hidrolojinin alt dallarından biri olan kent hidrolojisi, yüzey geçirgenliğine göre farklılık gösteren yağış, yüzeysel akış ve sızma farklılıkları nedeniyle kentte meydana gelen/gelebilecek debi miktarlarını konu edinmiştir. Yıllara bağlı maksimum yağış, topoğrafya, sızdırmazlık katsayısı gibi verilerin işlendiği, yağışla orantılı maksimum debi ve taşkın hacimlerinin olduğu bir dizi matematiksel hesaplamalarla hidrolojik yük hesabı yapılmaktadır. Hidrolojik hesaplamalar, gri kentsel altyapı olarak bilinen kentsel drenaj sistemlerinin yanı sıra bir kentsel havza için daha sürdürülebilir kentsel su yönetim sistemleri kurmak için kullanılmaktadır. Nüfusun her yıl biraz daha arttığı İstanbul'da su tüketimi ve nüfus projeksiyonları arasındaki ilişki incelendiğinde doğru orantılı bir artış görülmektedir. Bunun yanında, küresel ısınmanın da etkisiyle yağış rejimi değişmeye başlayan İstanbul, kuraklık tehdidi altındadır. Kentsel su yönetimi konusunda benzer sorunların yaşandığı çeşitli dünya kentlerinde hem mevcut kentsel drenaj sistemine destek olarak taşkınların önlenmesi hem de uzun kurak dönemlerde kullanılabilmesi için yağmur suyunun hasadı yöntemleri uygulanmaktadır. Kentsel Yeşil Altyapı Sistemleri olarak bilinen bu sistemlerde yağmur bahçesi, tutma havzaları, yağmur suyu tankları gibi farklı su tutma, toplama ve sızdırma teknikleri kullanılarak kentlerde yağmur suyu yönetimi konusunda başarılı sonuçlar elde edilmektedir. Ülkemizde, kentlerin karmaşık yapısı nedeniyle bahsi geçen yeni tekniklerin uygulanması karmaşık görülmektedir. Bu çalışmanın amacı, yağmur suyu toplama alanlarının kentsel tasarıma sayısal yöntemlerle nasıl dahil edilebileceğini ortaya koymaktır. Bu tezin çalışma alanı olarak belirlenmiş olan Üsküdar Meydanı ve yakın çevresinin uzun yıllara dayanan yağış analizleri üzerinden, topoğrafya analizleriyle bulunan havza alanına ait maksimum su hacimleri Rasyonel Yöntem kullanılarak bulunmuş ve hasat edilebilecek su miktarı hesaplanmıştır. Kent ölçeğinde yapılan analizlerde ise kentsel doku, kentsel gelişim, ulaşım, yeşil alan analizleri yapılmıştır. Bulgular ışığında, çalışma alanı için, kentsel tasarım ve peyzaj mimarlığı ölçeğinde yağmur suyu yönetimine katkı sağlayabilecek tasarım önerileri geliştirilmiştir. Her geçen gün biraz daha büyüyen kentlerimizde çok geç olmadan, sürdürülebilir, bütüncül, çağdaş bir kentsel yağmur suyu yönetimi sağlanması adına bilimsel verilerden yararlanarak planlamalarımızı bunlar üzerinden hayata geçirmek geleceğimiz için son derece önemlidir.

Due to the hardscape dominating over green areas in increasing urban areas, natural hydrological system is disrupting. This situation causes an increase in the amount of surface runoff and creates drainage problems in stormwater management. With the effect of global warming, it is predicted in long range projections that there will be increase in peak precipitations, precipitation-based floods, and droughts due to the increasing time between successive precipitations. Despite all these facts, the methods of intervention to the hydrological cycle in our cities are gradually moving away from sustainability and as a result, our cities are adversely affected. Sustainable water management cannot be achieved because the proposed solutions consist of applications that treat water as a static substance.The mentioned applications are the systems that urban drainage systems which are part of the gray infrastructure systems, carry the runoff from underground to the discharge area via pipes. Because these systems develop in parallel with the sprawl of urban environment, their extent areas are also enlarging and these causing the increase in maintenance and repair costs. Moreover, they remain incapable in peak precipitations and causes precipitation related floods. The science of hydrology, which has many different branches, is a young but rapidly developing science that examines the precipitation-flow relationships. Urban hydrology, which is one of the sub-branches of hydrology, deals with the amount of flow rates in the city due to precipitation, surface runoff and infiltration that differ due to surface permeability. By using mathematical calculations based on data such as maximum precipitation depending on years, topography, and impermeability coefficient, hydrological load calculation can be found which includes flow rates and flood volumes proportional to precipitation in cities. These hydrological calculations can be used to install urban drainage systems, known as gray urban infrastructure, as well as for more sustainable urban water management systems for an urban watershed. There is a wide variety of methods by which hydrological calculation is done; These can be methods such as Rational Method, Superposes, Mockus based on a single formula, as well as methods using computer-based systems such as HEC-RAS, MIKE. Comprehending the operation of computer aided methods in hydrological calculations by those who are unfamiliar with the subject causes the process to take a little longer than formula-based methods, which creates a negativity. Which method should be used depends on the characteristics of the basin and its data. In various cities of the world, where same problems experienced in urban water management, various technologies have been developed in storm water management based on the idea of "imitating the ecosystem" in urban planning and development. Technological developments adopted to mitigate or eliminate the effects of negative consequences that may occur with rainwater have been developed over the years and detailed according to different situations. While a more singular and focused approach to flood mitigation was used in urban storm water management, over the years, it has been realized that the water in the city is included in a large ecosystem, as the problems continue. Thereupon, more holistic, basin-oriented, and sustainable approaches, in which ecological cycles and water cycles come to the fore, have begun to be adopted. Although Low Impact Development, Green Infrastructure, Urban Drainage System, Water Sensitive Urban Design, Low Impact Urban Design and Development, Water Sensitive Cities are the rainwater management technologies that have been developed under different names in different countries around the world, they are basically aims to minimize the discharge to streams by keeping rainwater where it falls. These systems, which are becoming increasingly widespread all over the world, not only provide rainwater management, but also contribute to the protection of water resources, prevent floods by supporting the urban drainage system and to use it long dry periods, the development of ecosystems and biodiversity, and the social aspect of the urban landscape with the sub-areas they create. In other words, it undertakes many functions and ensures that the urban landscape works like an infrastructure. By integrating green infrastructure systems to the city or to a watershed, successful results are obtained in stormwater management in cities by using different retention and collection techniques, such as rain gardens, dry basins, wetlands, green swales, drainage wells, rainwater tanks. Adaptation of all these green infrastructure elements to the city is achieved by constructing hydrology and landscape together. For this, a basin-based hydrological load calculation should be followed in the city. A holistic and functional urban drainage can be achieved with the green infrastructure proposals to be developed after the hydrological calculations to be made by considering the amount of precipitation falling into the basin, the topography, and the orientation of the water in the land cover. There are many examples in the world where these methods have been applied and successful results have been obtained. In our country, the application of the new techniques seems complicated due the complex structure of the urbans. The aim of these study is to present how rainwater collection and infiltrating systems can be included in urban design with mathematical methods. When the relationship between water consumption and population projections is examined in İstanbul, a directly proportional increase is observed. In addition, İstanbul whose precipitation regime has started to change with the effect of global warming, is under treat of drought. It has been observed that the precipitation in Istanbul comes in the form of peak precipitation and the time between two precipitations has begun to lengthen. This situation shows that solutions should be produced, which disposes without flooding when peak precipitation arrives and where rainwater is stored for use in dry times. Üsküdar Square and its surroundings, which have been determined as the case area of this study, are located on the Anatolian side of Istanbul, on the shore of the Bosphorus. Üsküdar Square and its surroundings have been exposed to more and more frequent floods in recent years. Although İSKİ (Istanbul Water and Sewerage Administration) has taken a measure that can handle the maximum precipitation that may come in 35 years with the traditional gray infrastructure systems, a much longer-term planning is required for a sustainable and functional stormwater management. Considering the peak precipitation, which we have encountered more frequently with the effect of global warming in recent years, it is highly likely that we will encounter precipitation over 35 years. In such a case, the existing system will be insufficient and cause flooding. In this case, analyzes were made for the study area. The analyzes were carried out in two main branches, urban scale and hydrological. In the urban scale some analyses were made which are urban development, solid-void analysis, transportation, green areas, and land use. Hydrological analyzes aiming to find the maximum flow and flood volumes started with the determination of the basin boundaries from the topographic map. Rational Method was found to be the most appropriate method for the calculated size of the catchment area. The Rational Method is a practical method that is frequently preferred in hydrological calculations, based on the formula of the multiplication of precipitation density, size of the catchment area and the impermeability coefficient. As a result of the findings obtained from the hydrological analyzes, the maximum flow and flood volumes that may occur according to the years were found in the basin area. In the lighting of the findings, design proposals have been developed for the study area that can contribute to stormwater management at the scale of urban design and landscape design. While the findings obtained from the urban analyzes show in which areas rainwater can be collected and infiltrated to the soil with which green infrastructure model, the calculated hydrological load shows its amounts. Within the scope of the thesis, the suggestions developed in line with the findings show that rainwater management in the city can be achieved without taking very radical decisions. It is possible to provide a sustainable urban drainage and storm water management in our cities with analyzes, field studies and calculations to be made in the light of science. In our country, there is still a lack of data, the lack of a multidisciplinary environment in which the obtained data will be examined, and legal gaps and uncertainties encountered in putting the findings into practice. A successful example that will be put into practice result of a qualified scientific research will not only benefit urban stormwater management, but also contribute to the policy of sustainable urban drainage systems, the awareness of authorities and our people, the basis of multidisciplinary, holistic, and scientific work, and the economical use of our material and natural resources. It is vital important for our future to implement our plans by making use of scientific data to ensure a sustainable, holistic, and contemporary urban stormwater management in our cities which are enlarging each day before it is too late.