The model used in this figure is introduced by the ecologist Van Leeuwen (1981). He draws it this way to express not only the input and output but also the ability ...
Hydrological Processes and Water Management in Urban Areas (Proceedings of the Duisberg Symposium, April 1988). IAHS Publ. no. 198, 1990.
STRATEGIES IN URBAN WATER DESIGN S.P. Tjallingii Urban Design and Environment Group, Faculty of Architecture, Delft University of Technology, The Netherlands ABSTRACT Internal problems of water in the urban ecosystem are usually "solved" by increasing inflow and outflow. Thus problems of depletion may rise at the supply site and pollution and erosion may result at the discharge site. Landscape disturbance may occur on both sites. Water management in urban areas should therefore give priority to those sets of internal solutions that decrease both the input and output of the system. Thus internal and external water-management can be integrated. The purpose of this paper is to scan some technical-, design- and policy-implications of this priority statement. First the basic, problem-oriented concept of the approach is introduced. Then a model is given of urban waterflows demonstrating the role of technical principles in the general strategy.
INTRODUCTION Design strategies, as described in this paper, are interrelated sets of goals and means that may be useful to the architects, townplanners, landscape architects and civil engineers engaged in the process of designing or re-designing the urban landscape. In their present form the strategies are used and tested in environmental planning courses for students at the Delft University of Technology. The ultimate aim is to explore new ways of integrating urban water management in the design of buildings, cities and urbanized regions. Both the ecological background of the problem solving concepts and their use in the early stages of design determine the fundamental nature of the approach. Design is the stage of prevention. Therefore the designer should not start with compromises. Later, however, it may be necessary to choose second best solutions and for this purpose the strategies include priority schemes. WATER PROBLEMS A problem solving approach has to start with a careful analysis of the problems and their context in the particular situation of the plan. Here only a rather general description of urban water problems can be given, leading to a general formulation of strategic objectives. Figure 1 summarizes the basic items.
323
S. P. Tjaltingii a. the model
supply
\
resistance J
\
1
discharge
retention
b_. existing system and problems prcblei -sinking groundwater tables ••gJ^...... •zr^?,\ -landscape disturbance îî&ijjjÀ s&irw (supply-works, pipelines) j a p ^ -high consumptio -"flushing system"
"sink problems" -pollution of water and submerged soils -sewage effluent/sludge -peak runoff -landscape disturbance (treat^er.t works, erosion)
internai problems -shortage -floods, nuisance -pollution -health, risks -damage, costs -loss of plants/wildlife
c_. strategy : the self reliant system
design levels : building - town - region
Figure 1.
Water problems and their prevention, a general strategy for design.
The model used in this figure is introduced by the ecologist Van Leeuwen (1981). He draws it this way to express not only the input and output but also the ability for resistance and retention of the ecosystem. If we apply this model to water systems like a building, a city or a region, we may illustrate urban water problems in the following way: "Source-problems" include depletion of groundwater resources, sinking groundwater tables and resulting droughts, subsidence etc. On this side of the system, water supply works like pumping stations, reservoirs and pipelines cause landscape disturbance. "Sink-problems" include pollution of watercourses and submerged soils, but also floods and erosion caused by peak runoff. On this side of the system, sewage treatment plants reduce water pollution, but the effluent still causes eutrophication and the re-use of sludge has become hazardous because of toxic substances. Industry and agriculture discharge untreated water. Thus excess fertilizers, pesticides and other contaminants are polluting surface and groundwater. As a result of both pollution and pollution control, landscape disturbance occurs in many places. Increased floods and erosion also have their effects on downstream landscapes. "Internal problems" of the water systems include both drought through shortage and nuisance of too much water in the wrong places. In most cases both are due to the failure of the system to cope effectively with the irregularities of rainfall. The quality of surface water may also be threatened by internal pollution. Very often the problems of quality and 324
Strategies in urban water design
quantity are closely linked. Combined sewage systems for example do not have the capacity to absorb peak runoff and thus their overflows pollute the urban surface water. A striking feature of current water policies in the Netherlands is the fact that they all increase supply and discharge. Thus there is a growing dependency on adjacent systems. In this way the external source- and sink problems of one system become the internal problems of other systems. Bearing in mind the simplified and general nature of this analysis there is but one conclusion: A problem solving strategy should focus more on the retention and resistance capacities of systems than on supply and discharge. The designer should aim at a "saving" system rather than at a "flushing" system. Within the general strategy a priority scheme of options can be given: a. Less use of water, not by frustrating reasonable demands of hygiene and comfort, but by designing efficient systems that do not waste precious water. Both limited use and reuse of water can be feasible. Even if there is plenty of clean and cheap water it seems wise to economize. Thus it may be possible to avoid the environmental effects and costs of supply. Besides it will not be necessary to oversize sewage treatment plants. For these reasons saving water is given first priority. Next comes: b. To cover the limited use one should first explore the possible contribution of rainwater, then consider surface water, and only think about groundwater use as a third choice. c. More storage of clean water to retain excess water for use in periods of shortage. The downstream problems of flooding after heavy rainstorms, are another important reason to retain rainwater both as clean and for as long as possible. Parallel to the supply and use strategies it will be necessary to concentrate on pollution control. The first option is: d. Re-use of dissolved substances. In many cases concentration of substances is a condition for re-use with important consequences for design. e. Purification of waste water by extracting toxins followed by concentrated and controlled disposal. Putting this at the end of the list does not mean this should have a low priority in general. In many cases this will be important and unavoidable. But in the process of design we should first consider all strategies for prevention before we turn to the curing of the disease. Some of these design objectives seem to be trivial but the systematic exploration of their possibilities is not common practice. The general analysis and strategy described here is in concert with and supported by extensive literature. Publications include: Barney (1980), Postel (1986), Bossel et al. (1982), Geiler & Hildebrandt (1985), Krusche et al. (1982) and Werkgroep Water (1979, 1980). Inspiring design approaches that have influenced the views described in this paper are among others: Hough (1984); Spirn (1984) and Kaenzlen (1985). Having outlined the general strategy, we now turn to the technical means. In some approaches (Schmalz, 1984; Minke, 1982) a "catalogue of technical means" is used as an instrument for design. Here it is possible to go one step further: not only feasible techniques can be listed but also a preference can be given for the sequence the designer should choose when he considers them. 325
S. P. Tjallingii
'JOAN
|air-pollution prevention/control [
SÏSTEM
a [urban green adjusted to climate J
retention and use - ponds
- onpaved surface - green roofs t-traps, oii-mterceptors w-form aeration i
% ^~"~--*-^ -flow •v^/^'U-i
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S[£
or.cwdter .-Mention | er.tiot; |
Jisposal(re-use) of sludge
Figure 2.
326
A strategy of technical means in urban water design.
Strategies in urban water design
INTEGRATION OF TECHNICAL MEANS In Figure 2 the main pathways of water through the urban system are indicated inside the channel that represents the urban surface water. Pollution increases from source to sink. On the outside the recommended technical means are arranged. a. Less use of water in the urban system in a direct sense may be achieved by adjusting the species of plants used in urban green-design to the regional climate. b. Thinking about the "less use" strategy one should be aware of the differences between the piped-water/sewer system inside buildings and the rainwater/surface-water system in cities. For the latter the "less use" strategy has to be interpreted as less through-flow. More storage of rainwater then is a condition for: Less use of piped water for watering lawns and trees. Less use of surface water supply from adjacent systems; In so doing the planning area is less dependent on external supply, which is often polluted. Besides, the design of more storage facilities within the system creates a structure of urban land use where areas around dwellings can be kept dry by natural drainage without troubling downstream systems. Within the urban system a number of technical means can be inserted by designers. Rainwater retention may be realised both by means of ponds with fluctuating water tables and by unpaved surfaces. A first priority it to reduce the impervious surface. The storage capacity of roofs can be enhanced by introducing: "grassy roofs". These are different from roof-gardens, which need water in dry periods. c. Once the rainwater has become runoff, it carries pollutants from roofs and the paved surface. Control of this pollution is a condition for further use such as watering gardens and public lawns, water as a place for childrens play and for all types of water-recreation. But also the quality of urban water as a habitat for plants and wildlife depends on runoff pollution-control. Among the technical means are silt-traps and oil-interceptors, aerating cascades like "flow-forms", and wetlands with marsh vegetation (Van der Aart, 1985); Mônninghoff, 1986). These represent mechanical, chemical and biological processes of purification that may fit in the urban design for residential areas, small parks etc. d. The same can be said of "first-flush" devices, that could be assembled in the street gully-pots. The polluted "first-flush" of a rainstorm is led to the foul-sewage network. Thus it is separated from the relatively clean runoff water that falls afterwards. e. Once an effective control of runoff pollution has been realized, there can be no more criticism on the functioning of separate sewage systems. So an old discussion about the advantages of combined and separate systems, as mentioned by Hengeveld & de Vocht (1982), can be settled. On the other hand, separating and concentrating polluting substances seems to offer the best perspectives. Therefore the separate sewage system is a better starting point to abate both point and nonpoint pollution. f. Provided the runoff water is kept relatively clean or treated to become so again, it can be used for groundwater recharge. The new town of Woodlands (Texas) is a good example of the role runoff retention and groundwater recharge may play in urban design (Spirn, 1984). g. Contrary to the techniques mentioned above the retention or detention of stormwater in special ponds or lakes is regular practice in many countries. Both detention and retention facilities slow runoff from developed sites, but only the retention ponds maintain permanent waterpools. Hough (1984) describes interesting examples in Ottawa and 327
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Winnipeg (Canada). Spirn (1984) mentions the examples of Boston and Denver (U.S.A.). New Towns and new extensions in many European countries have also adopted the idea. A successful example is the case of the "balancing lakes" in Milton Keynes (U.K.) described by Hengveld & de Vocht (1982). Success or failure of these stormwater facilities can be evaluated in terms of flood prevention, but in the urban environment their multi-functional qualities are a major factor. According to Adams et al. (1986) retention ponds are preferred by the public and are inhabited by more fish and wildlife. Besides they require less maintenance. Surely the combination of retention ponds with marshes is most promising for both quantity and quality control in a multi-functional setting. The technical means described above may be feasible in many urban areas. A design example of a residential area is described in Tjallingii (1988). Agricultural water systems, however, are different, and yet they dominate water management in the Netherlands (Ministerie van Verkeer en Waterstaat, 1983). The separation of agricultural areas from water systems for nature conservation, recreation, urban parks and residential areas may open new doors for the integrated design of urbanized regions.
REFERENCES Aart, PJ.M. van der (red.), 1985: Wetlands for the Purification of Wastewater. Utrecht Plant Ecology News Report, Dept. Plant Ecology, Utrecht university, 260 pp. Adams, L.W., T.M. Franklin, L.E. Dove & J.M. Duffield, 1986: Design Considerations for Wilflife in Urban Stormwater Management. Trans. 51 st. N.A. Wildl. d. Nat.Res.Cont. p.250-259. Barney, G.O., 1980: The Global 2000, Report to the President of the U.S. Pergamon Press, New York. Voll, p. 155-167, II, pp. 137-161, 333-345. Bossel, H., H.J. Grommelst & K. Oeser, 1982: Wasser. Fischer Taschenbuch Verlag, Frankfurt a.M., 294 pp. Geiler, N. und R. Hildebrandt, 1985: Wasserkonkret, Schritte zur Neuorientierung Hessischer Wasserpolitik. Die Grunen im hessischen Landtang, Wiesbaden. 199 pp. Hengeveld, H. & C. de Vocht (éd.), 1982: Role of Water in Urban Ecology. Elsevier Scientific Publ. Co. Amsterdam 362 pp. Hough, M., 1984: City form and natural process. Croom Helm, London & Sydney. 281 pp. Krusche, P., D. Althaus, I. Gabriel & M. Weig-Krusche, 1982: ôkologisches Bauen. Hrsg. Umweltbundesamt, Bauverlag, Wiesbaden. 360 pp. Kuenzlen, M. & Oekotop Autorenkollektiv, 1985: ôkologische Stadterneuerung. 2nd ed. Verlag C F . Muller, Karlsruhe. 274 pp. Leeuwen, C.G. van, 1981: From Ecosystem to Ecodevice. In: Tjallingii & De Veen (eds.), Perspectieven in Landscape Ecology, Pudoc, Wageningen. Ministerie van Verkeer en Waterstaat, 1983: De Waterhuishouding van Nederland. Staatsuitgeverij, Den Haag. Minke, G., 1982; ôkologisches versus industrialisiertes Bauen. Werk, Bauen und Wohnen, sept. 1982. pp.35-43. Menninghof, H. (éd.), 1986: Natur nahe Abwasserreinigung. ôkobuch Verlag, Freiburg. 91pp.
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Postel, S., 1986: Increasing Water Efficiency. In: Brown, L.R. (ed.) State of the World, 1986. Worldwatch Institute. W.W. Norton & Cp. New York/London, pp.40-62. Schmalz, J., 1984: Das Stadtklima. Verlag C F . Mailer, Karlruhe. 137 pp. Spirn, A.W., 1984: The Granite Garden, Basic Books, New York. 334 pp. Tjallingii, S.P., 1988: Water in urban areas, an environmental design approach, (in prep.). Werkgroep Water, 1979: Schoonwaterbœk. Uitg. Ver. Milieudefensie, Amsterdam. 160 pp. Werkgroep Water, 1980: Vuilwaterbœk. Uitg. Ver. Milieudefensie, Amsterdam. 159 pp.
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