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Product Details
| ISBN-13: | 9781843390572 |
|---|---|
| Publisher: | IWA Publishing |
| Publication date: | 09/30/2005 |
| Pages: | 238 |
| Product dimensions: | 6.12(w) x 9.25(h) x 0.75(d) |
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CHAPTER 1
Urbanisation and urban hydrology
This chapter provides the context for many of the issues that are discussed in the rest of the book. It gives an introduction to the process of urbanisation and the consequences for urban hydrology and related physical and environmental impacts of flooding. It highlights some of the pertinent issues within the context of urban stormwater management in cities in developing countries, and considers the institutional challenges in relation to land use, especially in cities that have a high proportion of informal developments and illegal settlements.
1.1 URBANISATION AND ITS IMPACTS ON URBAN HYDROLOGY
Urbanisation is one of the most important demographic trends of the twenty-first century. By 2030, it is estimated that the global urban population will reach 4.9 billion, an increase of 2 billion city people from 2000, which equates to a rise from 47% to 60% of the total population in the world (United Nations 2001). There are a number of important considerations in relation to this growth that should be taken into consideration:
(1) The majority of this growth is concentrated in towns and cities in developing countries and transitional economies.
(2) Although larger urban areas encompass an increasing amount of this population, the proportion of people living in mega-cities (urban agglomerations with more than 10 million inhabitants) will remain relatively small. In 2000, only 4.3% of the world's total population lived in mega-cities, whereas the proportion of the world population living in small cities is considerably larger – 28.5% lived in cities of less than 1 million inhabitants (United Nations 2001).
(3) Much of this urban growth is unplanned, with communities and private developers taking advantage of the weak regulatory capacity of local authorities, particularly in areas outside of municipal boundaries. In some cities, the proportion of the population living in these informal settlements can be as high as 20% (UN-Habitat 2003).
In many instances new buildings occupy floodplains and natural drainage pathways (see Figure 1.1) and the problems of stormwater drainage are frequently worsened by downstream flow constrictions caused by unregulated developments. Many cities lack effective storm drainage systems and ill-planned construction closes off natural watercourses. In some cities, urban wetlands are important physical features of the natural environment that provide essential hydrological functions for flood alleviation and maintain river/stream flows during the dry season. Unfortunately, these benefits are generally ignored as cities develop, during which natural watercourses are often destroyed or relined with concrete and wetlands drained to allow for developments.
As cities develop, the provision of urban infrastructure and services changes according to the level of economic development – both in terms of coverage and quality of the service. Table 1.1 summarises various urban environmental issues related to the brown (sanitation) and the green (environmental) agenda, and highlights the linkages between drainage, sanitation, water resources and solid waste management. As a result of these linkages, overflows from clogged storm drains and sewers during high rainfall are a major cause of flooding in urban areas.
The level of economic development also has implications to urban hydrology and stormwater management in other ways. For instance, the increasing use of the car and other forms of road transport results in a significant increase in impervious areas for the road surfaces and areas for parking. In heavily developed cities, roads and other transport-related impervious surfaces can constitute up to 70% of the total impervious urban areas (Wong et al. 2000). This trend is particularly apparent in industrialised countries such as the US where the average neighbourhood area per household devoted to streets and parking exceeds the area devoted to housing (Heaney et al. 1999). Although there are few other parts of the world where the level of car usage is so prominent, similar trends are observed in many cities due to economic growth and increasing demands for road transportation.
The densification of population living in urban areas and the associated construction of buildings results in dramatic increases in impermeable areas due to paving and roofs. Permanent physical changes to the catchment invariably result in changes to runoff patterns, which affect the magnitude and frequency of flooding. Based on data from Curitiba, Porto Alegre and São Paulo in Brazil, Figure 1.2 illustrates how the percentage of impervious areas increases with the population density. Although other cities follow similar patterns, this relationship should not be used directly to calculate runoff for design purposes in other locations as the curve will invariably be categorised by different development patterns.
In addition to the impermeability of the catchment, the discharge rate and volume of stormwater runoff from urban surfaces depends on other hydrological factors such as the surface depression storage and the antecedent rainfall conditions relating to the wetness of the catchment. The increase in impermeable areas caused by urbanisation has a number of important impacts on the hydrological response from a catchment related to:
(1) Reduced infiltration capacity of catchment surfaces caused by increasing impervious surfaces and compaction of soil, which reduces the capacity of the soil to absorb moisture.
(2) Reduced surface (depression) storage capacity because impervious urban surfaces are much 'smoother' than natural surfaces.
(3) Decreased evapo-transpiration due to the loss in the natural retention capacity of soil, reduced vegetation wetting and interception by plants.
A combination of these factors results in a loss of natural attenuation capacity and runoff from urban catchments is characterised by increases in:
(1) runoff velocity (often measured as time of concentration);
(2) runoff volumes (i.e. the proportion of precipitation that becomes runoff);
(3) Discharge rates and flood peaks.
As shown by the runoff hydrographs in Figure 1.3, the loss of attenuation capacity caused by urbanisation gives flood events a 'flashing' appearance, which often causes hydraulic overload of stormwater drainage systems and consequently flooding. However, urban catchments in developing countries often have significantly different physical characteristics from those in industrialised countries, which cause wide variations in the rainfall-runoff response and the resultant volume of runoff and peak flows. Typically, these include lower percentages of impermeable areas and higher depression storage, which means that the runoff volumes and peak discharges are not always as high as would be expected in comparison with catchments of similar population densities in developed countries.
1.2 URBAN RUNOFF AND CLIMATIC FACTORS
In addition to the physical characteristics of the catchment, the other main factor that affects runoff is climate – in particular, rainfall intensities and duration. Many developing countries are located in tropical or subtropical climates where rainfall is characterised by large seasonal variations with a highly pronounced wet season in which the annual rainfall is concentrated during a few months only. The annual rainfall volume and the intensities are generally very high in the humid tropics compared with those in temperate climates. As shown in Figure 1.4, the highest rainfall is in the Andean region, West Africa and South-East Asia.
The rainfall in these regions is generally convective which is characterised by short, but very high rainfall intensities – sometimes exceeding 100 mm hr-1 under extreme storm conditions. This type of rainfall is critical for small urban catchments, which have short times of concentration, and has significant influences on the design of urban drainage systems as peak rainfall intensities are the frequent cause of flooding.
Figure 1.5 shows the average peak hourly rainfall intensities recorded in Brasilia, Porto Alegre and São Paulo for each month over a period from 1995 to 2001. These data illustrate the widespread seasonal differences in rainfall in different parts of Brazil and similar variations are observed in other parts of the world. The rainfall hyetograph shown in Figure 1.6 for the rain event in July 2000 resulted in severe flooding, causing widespread disruption, which brought the city of Mumbai to a standstill.
Other climatic factors, such as wind and temperature also affect the scale and nature of the problems related to urban drainage. Urbanisation on a big scale affects the micro-climate, which in return affects the rainfall distribution. Recent scientific studies suggest that climate change will cause shifts in the global rainfall patterns and subsequently increase the intensity of rainfall, and therefore the severity of flooding events. In cases where the global climate change is not taken into consideration, it is likely that there will be an increase in the flood risk for many human settlements and the impacts will fall disproportionately on the poor (McCarthy et al. 2001).
Potential climate changes may be taken into account as part of the planning, design and management of the urban stormwater system, and the first step is to modify design storms accordingly. However, a detailed description of global climate changes, how to deal with them and their impacts is beyond the scope of this book.
1.3 CAUSES, TYPES AND PHYSICAL IMPACTS OF URBAN FLOODING
The expansion of urban areas and the associated increase in impermeable areas, combined with the tropical rainfall conditions described above, are responsible for the increase in the frequency of urban floods. This situation is aggravated by the lack of planning and delays in the construction of drainage infrastructure that are common in cities in developing countries. However, although flooding is often associated with the disastrous consequences of large-scale storm events, there are also frequent minor flood events, resulting in more localised drainage problems caused by a deficiency in drainage infrastructure as shown in Figure 1.7.
Even though these smaller events are generally not considered to be of serious concern compared with large flood events, the problems associated with this type of flooding may be considered to be more of a problem by affected communities (see Chapter 2). The main types and causes of urban flooding are illustrated in Figure 1.8 and, as described in Table 1.2, these flood events are categorised according to the extent of flooding and the resultant impact.
While the terms flooding and inundation are often used interchangeably, flooding tends to be related to drainage-related problems where there is insufficient capacity in the drainage system whereas inundation refers to the rising of a body of water and its overflowing onto normally dry areas, such as is the case where a river flows over its banks and onto the floodplain.
1.4 ENVIRONMENTAL IMPACTS OF URBAN RUNOFF
As well as the increase in frequency and magnitude of urban flooding, urbanisation results in pollution problems in urban streams and other receiving waters. Much of this is caused by discharge of wastewaters during dry weather conditions, but wet weather has the effect of cleaning urban surfaces and drainage channels, resulting in significant pollution problems. Thus, the quality of runoff is influenced by many factors, including land use, waste disposal and sanitation practices. Figure 1.9 illustrates some of the pollution problems relating to waste discharges into urban drainage channels. In addition, high rainfall intensities have a particularly high-erosion capacity and suspended solids concentrations in runoff can be very high – particularly as a result of construction activities.
A significant amount of pollutants ranging from gross pollutants to particulates and soluble toxins are generated from urban catchments. There are a number of pollutants of principal concern in urban runoff and these affect organisms in receiving waters in various ways (see Table 1.3). It is important to note that these environmental stressors may interact to varying degrees in an antagonistic, additive or synergistic fashion (Porto 2001) – meaning that the cumulative effect of different pollutants is likely to be worse than the sum of individual effects of each one.
Runoff from roads and other paved areas is of concern as it harbours a vast array of particulates and chemicals arising from the activities that characterise the land use. According to Wong et al. (2000), roads and other transport-related impervious surfaces contribute a higher proportion of stormwater pollutants than other impervious surfaces (e.g. roof areas, pedestrian pathways, etc.). Runoff from transport-related surfaces consistently show elevated concentrations of suspended solids and associated contaminants (such as lead, zinc and copper), as well as other pollutants (such as hydrocarbons). In addition, as described in more detail in Chapter 2, problems related to microbiological pollution are caused by the flooding of sanitation systems and the discharge of pathogenic bacteria and other micro-organisms (viruses, protozoa, etc.) can cause intestinal infections.
Figure 1.10 illustrates how the nature of the pollutant influences the temporal and spatial extent of urban water quality problems. Typically, pollution problems related to discharge of stormwater occur within relatively short duration called acute impacts, although there may also be chronic impacts, depending on:
(1) The nature of the pollutant, for example impacts related to oxygen demand or toxicity from high ammonia concentrations will be short-term, but nutrient enrichment (eutrophication) will only occur in the long-term.
(2) The nature of the receiving water, the impacts will depend upon the flow conditions – for instance, stream, river or lake. In general, chronic pollution problems from storm runoff are rare in rivers – except where solids from deposited sediments smother the riverbed.
1.5 INSTITUTIONAL CHALLENGES
Many of the flood and pollution-related problems associated with urban runoff described above are common to cities in many different parts of the world (Tucci 2001). However, these problems present a particular challenge in developing countries due to various problems associated with the institutional arrangements for urban drainage. One of the key institutional issues relates to the fact that often drainage has no clear constituency until major problems occur (World Development Report 2003), and it is only after large-scale flood events that investments to improve the infrastructure are made.
One of the main problems in developing countries is that there is insufficient control over new developments due to deficiencies in the administrational systems for urban planning and control. A particular problem relates to the control of informal settlements, which may have a distinct set of drainage problems and a complete lack of infrastructure to drain stormwater. In these situations, buildings are constructed with no consideration for stormwater drainage and where these occupy floodplains or natural drainage pathways, the problems of stormwater drainage are increased due to the restricted flow capacity.
As described in further detail in Chapter 4, institutional problems affect broad areas of operational performance, which are qualitatively different from specific technical or procedural problems (Cullivan et al. 1998). Planning authorities and regulatory agencies often lack resources to develop and implement effective solutions for the control of runoff and mitigation of the flood events. Whilst the separation between city and disaster management continues, valuable opportunities for reducing urban risk are lost. Even where national disaster management systems have been formally created, good co-ordination between different government and other organisations does not necessarily exist, leading to confusion, contradictions, overlapping functions and gaps in responsibility (Sanderson 2000).
Other institutional problems relate to a lack of co-ordination between different agencies and organisations with interests in urban drainage. In addition, urban flooding is not bound by local administrative boundaries, because stormwater drainage and protection facilities are part of an environmental system that is larger than an incorporated city territory. The definition of the boundary areas also results in problems associated with the poor alignment between administration and hydrological boundaries (see Chapter 3). Lack of effective urban planning and management in developing countries is a widespread issue affecting urban drainage systems. The example described in Box 1.1 and illustrated in Figure 1.11 presents a particularly important issue related to the ownership of land. In this example, one private landowner managed to halt the construction of a large-scale drainage system in Dhaka for over a year.
(Continues…)
Excerpted from "Urban Stormwater Management in Developing Countries"
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Copyright © 2005 IWA Publishing.
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Table of Contents
Foreword, ix,
Preface, xiii,
Acknowledgements, xv,
About the authors, xvii,
1. Urbanisation and urban hydrology, 1,
2. Impacts of flooding on society, 18,
3. Integrated framework for stormwater management, 33,
4. Policies and institutional frameworks, 51,
5. Planning and assessment of improvement options, 66,
6. Configurations of urban drainage systems, 84,
7. Ecological approaches to urban drainage system design, 104,
8. Applications of computer models, 121,
9. Operational performance and maintenance, 140,
10. Non-structural flood mitigation strategies, 155,
11. Participation and partnerships, 172,
12. Economics and financing, 188,
Annex 1: Recommended reading, 202,
Annex 2: List of contributors, 208,
Index, 211,