Water in urban planning

“There is no life without water” is how H. Dreiseitl opens his article The Blue Treasure as Dynamic Infrastructures. [1]

This statement, even if it seems quite radical, happens to be absolutely true. However, when designing our buildings and cities, little attention is paid to water management, often set aside as an “engineers problem”.

This attitude has only lead to a current situation where water is neglected from our imaginary and never integrated in its surroundings. Not only architects, but people in general are nearly never aware of its existence and fundamental role in cities survival.

Nevertheless, its oblivion does not reduce its importance in urban centres. The capital of the Roman Empire depended completely on the aqueducts and great efforts were dedicated to their construction. On the other hand, their sewage system was already very complex. New York’s Central Park hides the construction of a big reservoir to ensure the cities supply, and the Catskill aqueduct was built to bring the water from the mountains with the same name.

The idea of water as an integrating element in city planning should be recovered and made a central issue for people in general and, specially, for architects and urbanists. In the following text, four main strategies for dealing with water management in urban centres are introduced and briefly discussed, with the aim of awakening some interest in the subject and promote further investigation.

ABOVE GROUND DRAINAGES

For starters, a seemingly simple and maybe dull intervention that can modify greatly the conception of water and its general treatment is the introduction of above ground drainages. Hiding the water in underground systems only leads to the misconception of its disappearance and its relation with a dirty and unpleasant world. However, if water is shown to people, a greater understanding of its relation with the city processes can be achieved, leading to a more respectful behaviour by users. [5]

On the other hand, above ground systems can become very sensible options economically and from a maintenance point of view. Poor connections can be prevented and problems of sealing or erosion easily solved. The installation cost is also much lower than underground systems and smaller intervention in the existing is needed, which makes them a sensible option to introduce separative systems in the city. On the other hand, they can be designed as a positive element in the urban landscape, making it dialogue with the surroundings and trusting them with a pedagogical role. [5]

The above ground water systems can be materialized in many different ways, from canals to open gutters, shallow roads or irrigation lanes. In any case, introducing these systems when developing a project in a city area should always be considered as possible and even positive. [5]

INFILTRATION AND DRAINAGE

The building and growth of cities has a clear consequence of extending impermeable areas and modifying, in this way, the natural hydrological cycle in the area. On top of that, the city’s water management has always been based on an immediate evacuation through the sewage system without allowing any infiltration on the way out. These two aspects combined induce an increase of the general runoff and a higher risk of floods. With the current Climate Change and the rise of the storms intensity, this situation will only aggravate. [2] [5]Therefore, it is necessary to focus on new ways of planning our cities that improve its current relation with water.

To be able to work on that aspect of water in urban planning it is necessary to implement new strategies based on infiltration and drainage systems. Their goal is to “prevent, reduce and delay rainwater discharges into the sewer systems or other water courses, trying to mimic natural catchment processes, reproducing as close as possible flow rates, volumes an water quality of the natural systems” [2]

To achieve this objective it is important to consider these aspects in the early stages of each project, being it architecture, urban design or urban planning. Each particular case should be carefully analysed to obtain its specific requirements and be able to propose an integrated solution. [2]Therefore, the form of infiltration and drainage applied to the project depends on different factors such as the groundwater level, the permeability of the ground, the quantity of the runoff, the space available, the climate… [5]

Generally, the intervention should be focused on the modification of the system in its most critical point: its contact with the terrestrial surface. [5]Therefore, two basic principles could be applied: on one hand minimizing the completely impermeable surfaces to the ones strictly necessary and, on the other, implementing the use of draining pavements. [5] At the same time, it is important to achieve a good control of the runoff though the design of urban elements, as grass areas, paths, trees and the relations between them.

The water balance in an unpaved surface shows more slow drainage, less runoff in the surface and more evaporation by plants. On the opposite end, the paved surfaces have less evaporation, more runoff through the surface and less seepage to groundwater. [VMM,2010] [5]

The final purpose of these systems is to approximate, as much as possible, the natural water balance. However, there is an option that can be also applied besides a good infiltration system: capturing rainwater and using it, which also has as a result a saving of soft water. [5]

RAIN WATER COLLECTINGAND SEPARATIVE SYSTEMS

Rainwater control and its reuse for watering has been a basic strategy for agriculture societies in history. However, this mentality of reusing has been lost through generations with a consequence of a higher expense of soft water and a bigger risk of floods.

The current system is based on mixing the different kinds of contaminated water and, afterwards, treating them through the same, extremely complex, method. When the water is depurated and soft water is achieved, it is also used for different kinds of needs, from watering the plants to flushing the toilets or drinking, without distinguishing the quality needed in each case. This arrangement is clearly imperfect and uses much more energy and resources than needed.

Therefore, two basic strategies should be developed. On one hand, working on the implantation of separative systems in an urban and building level combined. Besides, the collection of rain water and its posterior re-use close to the source reduces the risk of contamination of the water through the canalisations and improves the general management.

The implementation of these two strategies offers several advantages. Firstly, the reduction of subterranean water extraction by the reuse of rainwater. Secondly, it creates smaller water cycles, reducing the contamination risks. On the other hand, water buffering helps to reduce risks of floods. Finally, and probably the most important one, the soft water expense is greatly reduced. Moreover, these systems have proved to be of great effectivity and easy to integrate. [5]

NATURAL DEPURATION SYSTEMS

In general, architects or urbanists don’t know much about water treatments. However, ignoring techniques with such a relation with our profession has been proved of bringing good results for us. Soft water is a 0,1 % of the water resources in the world. The consumption of soft water is increasing in a worrying pattern; therefore a greater attention should be paid to its managing and treatment. [4]

The main sources of pollution are diverse, depending on the type of pollutants, which can be industrial, domestic or agricultural. Water treatments need to adapt to these differences and often find problems to deal with all the types of water pollution.

In general, the complete treatment of water involves several stages: a pre-treatment, which eliminates largest elements and is based on screening and grit removal; a primary treatment that separates and eliminates suspended solids through filtering or clarification; a secondary treatment with the main objective of eliminating organic matter using oxygen addition or filtering plants and, finally, a tertiary treatment that eliminates phosphorus and nitrogen- Sometimes, complementary treatment is needed to deal with heavy metals, nitrates and pathogenic germs. Conventional plants treatments are usually combined with ultraviolet rays, ozonisation, chlorination, lagooning or filtration. [4]

However, a standard treatment system has some important drawbacks. Mainly, its centralized nature implies that great distances need to be covered by water to reach the treatment point. This transportation requires an important amount of energy and the building of a large system of canalisations. On the other hand, large volumes of sludge are produced by the plant and they need a posterior processing phase. [5]

It has been known for a long time about nature’s self-purification capacity and lagoon systems had already been implemented in 1901 (Lake Mitchel in San Antonio, Texas). However, it was not until 1950’s that research was conducted on the subject and optimization of the phenomena was achieved. Nowadays, these techniques that require less space than the traditional rudimentary ones are being discovered and applied over the world. Many times, its implementation offers the possibility of treating water purification as not only a technical problem but also a project that involves social, urban and landscape issues. [4]

Natural depurating systems are those which treat the contaminant substances of the water using natural proceeding and components from the natural environment without using any chemical additive. Their use is limited to degradable polluted water to ensure the complete disappearance of the contamination. [6]

There are several different techniques of natural depurating systems that coexist and are sometimes used combined. They distinguish themselves mainly by the way the water circulates in the system, which can be vertical, horizontal, aerial, subsurface or combined. [4]

The lagoons with macrophytes are based on the circulation of wastewater through basins of decreasing depth, planted with floating or rooted plants. The treatment is carried on by bacteria placed on the roots of the plants, which also have the function of slowing down the water flow. [4]

Macrophyte beds or planted filters base their purifying performance on the bacteriological activity situated in the granular substrate. The plants offer oxygen and acid to the roots, which allows the bacteria colonization. On the other hand they also provide shading, insulation and maintenance of the gravel bed. A very positive point is the fact that wastewater is subsurface, with the consequence of reducing smell, mosquito colonies and contact with dirty water. [4]

Horizontal flow systems are built by a substrate of gravel and sands saturated with water. They achieve a very good performance on the purification of organic pollution, being much less effective with the reduction of mineral salts. Currently used as the secondary or tertiary treatment of domestic sewage. [4]

Vertical flow systems improve oxygenation of the filtering bed composed of gravel through the alternation of its basin’s supply. They have a good capacity for accepting large quantity of suspended solids, which makes them a good option for a primary treatment of domestic effluents or sludge from treatment plants. [4]

All in all, these alternative systems offer several advantages. On one side the amount of sludge they produce is insignificant compared to traditional systems. Moreover, they do not require the use of chemicals in the process. On top of that, the level of energy consumed is much lower, as well as maintenance costs. [4]

The main problem that can be found to apply on a large scale this type of water treatments is the difficulty to introduce them in urban environments, where there is an important lack of space to introduce changes and the inertia created by the use of current systems is more difficult to modify than in smaller scale interventions. However, there are also some ways they can be slowly introduced. [5]

Firstly, in the case of urban areas expansion or increase of density the problem of having to adjust existing system can offer an opportunity of introducing new solutions.

Secondly, the intervention can be applied to water containing a specific type of waste. As an example, the system can be applied to rainwater coming from public roads, without mixing it with wastewater. As it has been considered before, this systems offer the possibility of reducing water cycles.

Finally, the introduction of these systems can be linked to some urban interventions regarding leisure and green areas, which can incorporate some wetlands areas with a landscape design function and a depuration function.

All in all, these four strategies analysed are only some of the many that water management offers to those interested on working with it. They can be applied as a single approach or as a combination of methods, depending on the particular case at hand. Nevertheless, the most important aspect to bear in mind is to acquire water as an element more in the repertoire of our design toolbox.

To conclude, I would like to add a comment on the references. Everything written in the article is based on some people knowledge, much more extended and reliable than mine. Therefore, I understand this text as a mere introduction to a subject. If someone is interested in really learning something about water management in cities I deeply recommend taking a tour on the books and articles listed below.

 

References

[1]Herbert Dreiseitl, The Blue treasure as dynamic infrastructures, Waterscapes, Paisea 024

[2]Sara Perales Momparler, Gonzalo Valls Benavides, Sustainable Drainage Systems (SUDS), Waterscapes, Paisea 024

[3]Ignacio Pujol-Xicoy, Biological Purification of Ornamental and Bathing Water, Waterscapes, Paisea 024

[4]Hélene Izembart, Bertrand Le Boudec, Waterscapes, Editorial gustavo Gili, 2008

[5]Hiltrud Potz, Pierre Bleuzé, Urban green-blue grids for sustainable and dynamic cities, Coop for Life, Delft 2012

[6]Luis Moreno Merino, La depuración de aguas residuales urbanas de pequeñas poblaciones mediante infiltración directa en el terreno. Instituto geológico y minero de España. Ministerio de Ciencia y Tecnología de España

 

Author: Glòria Serra Coch