be directed into water butts installed on rain water down pipes or to permeable areas ... They stated that the curb-gutter-sewer systems complement the urban.
URBAN DRAINAGE INFRASTRUCTURE PLANNING AND DESIGN CONSIDERING CLIMATE CHANGE Hans Arisz Senior Associate, Hydro-Com Technologies, a division of R.V. Anderson Associates Limited, Fredericton, 445 Urquhart Crescent, Fredericton, New Brunswick E3B 8K4, Canada.
Brian C. Burrell Senior Engineer, Hydro-Com Technologies, a division of R.V. Anderson Associates Limited, Fredericton, 445 Urquhart Crescent, Fredericton, New Brunswick E3B 8K4, Canada. ABSTRACT: Climate change is a reality that planners and designers of drainage infrastructures must consider. The cumulative effects of gradual changes in hydrology due to climatic change are expected to alter the magnitude and frequency of peak flows over the service life of drainage infrastructure. Potential future changes in rainfall intensity are expected to alter the level of service of drainage infrastructure, with increased rainfall intensity likely resulting in more frequent flooding of storm sewers and surcharging of culverts. The expected effects of climate change necessitate a change in the approach used to plan for and design drainage infrastructure. New development should ideally be served by both a minor storm drainage system, such as a traditional storm sewer system, and a major overland storm drainage system designed to convey the excess runoff when the capacity of the minor system is exceeded. The planning and design of new drainage infrastructure should incorporate development features and sustainable urban drainage systems that provide multiple benefits (such as a reduction of localized urban flooding and harmful environmental impacts). Modifications to existing drainage infrastructure in existing development is complicated by the integration of the minor drainage system with other infrastructure and a lack of space for the construction of major drainage system components. KEYWORDS: urban drainage, climate change, hydrology, stormwater
INTRODUCTION The predominant scientific opinion based on the evidence currently available is that human activities have changed atmospheric composition with the result that the meteorological processes that define climate have been altered. The resulting gradual changes in weather patterns, increasing climate variability and anticipated increases in weather extremes are expected to affect hydrologic conditions and the hydrologic responses of watersheds. Engineers thus have no choice but to consider climate change in their practice in order to adapt and serve the public interest (Lapp, 2005). Even though the effects of climate change at the local level are not well understood and appear to be gradual, their potential cumulative impact over the service life of drainage infrastructure warrants a change in the basic philosophy of hydrotechnical design (Arisz and Burrell, 2005). This paper is an exploration of the effects of climate change on the hydrology that underlies the hydraulic design of drainage infrastructure in general, and urban drainage infrastructure in particular. In the following sections, the challenges imposed by a changing climate, possible adaptation, and the costs of adaptation are discussed.
A CHANGING CLIMATE The Earth's climate system has demonstrably changed on both global and regional scales since the pre-industrial era, with some of these changes attributable to human activities. Evidence exists that most of the warming observed over the last 50 years is attributable to human 1-4244-0218-2/06/$20.00 ©2006 IEEE.
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activities (IPCC W.G.1, 2001). An increasing body of observations gives a collective picture of a warming world and other changes in the climate system. Hengeveld (2000) stated the results of experiments into future climate change with advanced computer models indicated that the probability of extensive climate change was both real and imminent. The quantification of the effects of climate change are primarily based on the results of computer simulations of General Circulation Models (GCMs) for various scenarios developed based upon a number of assumptions regarding the future discharge of greenhouse gasses into the atmosphere. These computer simulation results are then applied directly or downscaled from a global level to a regional level, and although there are differences in the results of different GCMs, they generally predict increases in rainfall intensity over much of Canada. Globally averaged annual precipitation is projected to increase during the 21st century (IPCC W.G.1, 2001). Hengeveld (2000) stated that the output of the Canadian general circulation model (CGCM1) indicated an increase in average global precipitation over the next century, although there was considerable variation from region to region. By 2090, precipitation over most of Canada and northern Eurasia was predicted to increase by 10% to 20%, with most of these increases occurring during winter. Between the extreme positions of eliminating all greenhouse gas emissions immediately by banning the use of fossil fuels and doing nothing about a changing climate, are a wide range of possible courses of action. Effective action taken to mitigate climate change reduces the need to adapt to a changing climate, but given the
present global political-economy, the total impact of these measures are unlikely to substantially reduce the possibility of human-induced climatic change. Given that climate change appears to be inevitable, whether mitigation measures are successful or not, humanity needs to consider the potential impacts of climate change on society. Fowler (1999) identified four steps when assessing climate change impacts on society: projection of the future climate, transformation of the predicted climate change into biophysical impacts, transformation of the bio-physical impacts into societal impacts, and the identification of responses by society to these impacts. While limiting the primary focus of our discussions to urban drainage infrastructure, the first three steps listed above are discussed in the following section titled hydrotechnical design, while the last step listed above is discussed in the section titled adaptive options.
THE CHALLENGE As the service life of drainage infrastructure (e.g., bridges, culverts and storm sewer systems) is measured in decades (generally 50 years to 100 years), the cumulative effect of the expected gradual changes in hydrology are likely significant. Semadeni-Davies et al. (2005) stated that there was a lack of both tools and guidelines in the technical literature for the assessment of climate change impacts in hydrology. Furthermore, for urban areas, attention has focussed generally on flood risk or water supply rather than stormwater drainage. The wide range of predictions from climate change scenarios based on different assumptions results in uncertainty, which limits the value of these predictions to stormwater management policy definition and planning. Despite this, SemadeniDavies et al. (2005) contended that studies using different climate change and urban drainage scenarios can be used to identify the direction of possible impacts and the thresholds of discontinuous responses with respect to urban drainage systems. Urban drainage infrastructure planning and design is further complicated by the hydrologic changes associated with urbanization. Morgan et al. (2004) investigated the hydrological consequences of urban growth and expansion of impervious surface area between 1950 and 2000 in the Laurel Creek Watershed, Waterloo, Ontario. Changes in the hydrological regime were assessed using census, climate (temperature and precipitation), land use and runoff data. The analyses found that, while water control infrastructure appeared to attenuate peak runoff, runoff volumes were noticeably increasing. Morgan et al. (2004) stated that changes in land cover, precipitation timing and magnitude, and water balance dynamics in the future may diverge from historical patterns, and should be considered by land use planners and others responsible for urban water issues. They found significant challenges in predicting the hydrological consequences of urban expansion in a changing climate using long-term data sets, and suggested the possibility of threshold-dominated watershed behaviour and the possibility that urbanization and indirect effects on the hydrologic regime (e.g. urban growth and higher evaporation rates) could result in a loss of vegetation that could alter runoff.
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The challenge posed to designers and planners by these complex, inter-related and imperfectly understood processes, is to make allowances for hydro-climatic changes during the design and planning of drainage infrastructure in a pragmatic way that protects both current and future public interests (financial as well as environmental and other societal interests).
DESIGN AND PLANNING The authors’ intent in this section of the paper is to review the approaches and methods that are commonly applied to the planning and design of urban drainage infrastructure, and (attempt to) quantify the effects of climate change on this approach and these methods. This review and quantification of effects will serve to focus the adaptive options that can be explored to ensure a pragmatic approach to protecting current and future public interests related to urban drainage.
BASIC APPROACH Design of storm drainage infrastructure involves the determination of the size of the storm drainage system components required to convey a design flow; while the planning for storm drainage infrastructure focuses primarily on the allocation of land and easements to accommodate this infrastructure, and controlling or limiting the interaction between drainage infrastructures and surrounding development. The magnitude of this design flow is selected based on the level of service that a specific piece of drainage infrastructure should provide (i.e. what are the consequences of flooding and how often is flooding acceptable?), and is often defined in terms of the frequency of reoccurrence, either as a probability of flow exceedance or a recurrence interval between events of similar magnitude. Once the level of service is selected and the appropriate design frequency is chosen, hydrotechnical design involves the use of accepted design methodologies, considering hydrologic input and appropriate design parameters. Accepted methodologies for the calculation of design flow magnitudes consist of either empirical peak runoff methods, hydrologic simulation models, or statistical methods based on the analysis of hydrometric records. As the applicability of statistical methods is generally limited to the calculation of design flow magnitudes in natural watercourses, the design methods that are commonly used in urban drainage design generally consist of empirical peak runoff methods, and hydrologic simulation models. Both empirical peak runoff methods and hydrologic simulation models are based on parameters that describe the land use upstream of the infrastructure being designed, as well as values of rainfall and snowmelt that are appropriate for the local conditions. The land use parameters are a function of the percentage of the drainage area that is impervious (pavement or roof areas), the soil type, and the vegetation cover. To accommodate the variability in design parameters resulting from these site conditions (both in space and in time), technical and design literature generally present ranges of design parameters rather than single values.
The selection of the appropriate rainfall and snowmelt parameters for the local conditions is an important issue in stormwater system design, but is complicated by the high variability of these climatic variables in both space and time. Approaches commonly used to select appropriate values of rainfall and snowmelt in design of urban infrastructure are the intensity-duration-frequency (IDF) curves, historic design storms and synthetic design storms. IDF curves represent the statistical distributions of extreme precipitation data from a given location, historic design storm generally consist of the most severe historic storm event for a given location, while synthetic design storms are averaged precipitation patterns based on historical precipitation data for a larger area of interest. In the context of the effects of climate change on drainage design, it is important to note that all of the above approaches to the selection of rainfall and snowmelt design data are based on historic climate data and an assumption that there will be no change in climate over the project life. The validity of this assumption, which allowed the past environment to be used as an indication of future conditions (i.e. the magnitude of design rainfall and snowmelt is invariable with time), is reduced or negated by climate change. The reduction or negation of this fundamental assumption thus has the potential to affect the design of storm drainage infrastructure as well as the associated planning (as planning for drainage infrastructure is a function of its size and location).
MINOR AND MAJOR DRAINAGE SYSTEMS When applying a design philosophy to urban drainage, a distinction can be made between the minor drainage system on one hand, and the major drainage system and fluvial flood protection works on the other. Ideally, development should be served by both a minor storm drainage system (piped system such as a traditional storm sewer system) and a major storm drainage system (overland system). The minor storm drainage system is designed to convey stormwater runoff from more frequent storms (smaller, less severe events) thereby providing safe and convenient use of streets, parking lots, and other developed areas. The major storm drainage system is designed to convey stormwater runoff from less frequent storms (larger, more severe events) when the capacity of the minor storm drainage system is exceeded. The major storm drainage system generally consists of open channels, rivers and streams, roadways, and detention/retention ponds. It should be noted that the use of this type of “split” design philosophy for drainage infrastructure requires that the effects of surcharging the minor drainage system (such as flooding of property through the storm sewer system) be anticipated and addressed during design. For economic reasons the hydraulic capacity of the minor drainage system is limited (generally the hydraulic capacity is designed to convey a flow with a return period of between 5 years and 10 years), and during extreme events there will be overflow in the street and roadway system. The creation of a major drainage system and the potential upgrading of the hydraulic capacity of the major system components (drainage channels, retention/ detention ponds, stormwater storage areas) requires space. It is easiest to designate space for the major
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drainage system during the planning and layout stages of new development, as it is often very difficult to do so retroactively in existing development. Therefore, serious consideration should be given to the requirements of storm water management and the need for future upgrading of drainage infrastructure capacity (associated with both climate change as well as urbanization) during the initial planning and design stages of development. The requirements of the major drainage system during the planning and initial design stages of new development consist primarily of the allocation of space along both natural and proposed drainage paths as well as in lowlying areas. These requirements would include such things as: the strategic location of curb jumps to control the depth of flooding on streets and roadways, surface drainage channels and swales that are sufficiently large to be able to convey flow without inundating private property and critical public infrastructure, storm water management infrastructure (e.g. retention and detention ponds).
CLIMATIC CHANGE CONSIDERATIONS Whether event or continuous modelling is utilized, practitioners will need guidance on adapting input data to account for the anticipated effects of climate change (Bradford and Gharabaghi, 2004). Traditional approaches to designing and operating urban drainage systems have been based on past performance of natural systems and on the ability to extrapolate the performance of natural systems and engineered assets over the useable design life of the drainage infrastructure. Whether or not climate change significantly alters future weather patterns, designers and operators of storm drainage systems must prepare for grater uncertainty in the design of storm drainage systems (Ashley et al., 2005). The following information is an attempt at providing guidance on the quantification of the anticipated effects of climate change on hydrology and the design of urban drainage infrastructure. Guidance on quantifying the potential or anticipated effects of climate change on drainage design is most readily available from General Circulation Models. Although the results from different modelling scenarios and different models vary substantially, they do provide insight into the potential range of climate change effects. The primary driving factor of climate change as simulated by General Circulation Models is the concentration of greenhouse gasses in the atmosphere, and is most commonly characterized by atmospheric CO2 concentrations. One of the most common climate scenarios that are used by the models to predict future climate changes is based on an assumption of double the year 2000 atmospheric CO2 concentrations by the year 2050 (Bruce, 2002). The magnitude of climate change effects based on this assumed scenario is generally quantified by the models as either an increase of 15% to 20% in rainfall intensities or a halving of the return period of design storms (e.g., a storm event with a magnitude that would be classified as a 10-year return period event based on historic climate data would be classified as a 5-year return period event using 2050 climate data). With respect to the operation and the design of drainage infrastructure, changes in rainfall intensity have two
consequences. First, the flow to which a structure is designed is no longer constant over time. Second, the level of service provided by drainage infrastructure (once it is constructed) will gradually decrease over time (i.e. storm sewers will flood and culverts will surcharge more frequently).
change necessitate a change in the approach used to plan for and design drainage infrastructure. As the hydraulic capacity of the minor system is fixed once it is constructed, and as upgrading the capacity of the minor system is difficult and expensive (as discussed above), it would follow that the increases in flows due to the effects of climate change are most easily accommodated by the major drainage system. As the measures needed to address the hydrologic effects of climate change (increases in peak flows over time) and increasing urbanization are similar and are compatible with the environmental requirements of storm water management, these distinct processes could be addressed in an integrated manner. The adaptive options that could satisfy the climate change, urbanization and environmental requirements in an integrated manner (as presented and discussed in the following section), however, will require a comprehensive approach to the planning of new development.
The above consequences are illustrated graphically in Figure 1. Since the effects of climate change over the service life of infrastructure are likely to alter the hydrologic regime, caution must be taken in determining design flows based upon past flow measurements. The use of statistically determined design floods based on past hydrometric records by itself will not be adequate. Furthermore, continued reliance on the performance of existing drainage systems over its service life will be inadequate since future performance will depend upon future rather than past conditions. As the traditional approach related to drainage infrastructure design was based on past climate and operating levels of service that were fixed with respect to time, the effects of climate
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Figure 1: Expected Changes in Extreme Event Frequency Curves
ADAPTIVE OPTIONS Adaptation refers to activities that maximize the benefits and minimize the negative impacts of climate change. Two main concerns with respect to our ability to adapt to future climate change are the rate of change projected by climate models, and the projected increase in the frequency and intensity of extreme events (Warren et al., 2004).
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The adaptive options available to address the effects of climate change on urban drainage infrastructure are primarily limited to the prevention of an increase in flooding risk and the maintenance of the historic levels of service provided by drainage infrastructure. This section explores the adaptive options that are available to accommodate the effects of climate change, as well as the related effects of urbanization, on urban drainage infrastructure. As the effects of climate change and urbanization on urban
drainage are similar (increases in runoff and peak flows over time), the adaptive options available to address these two separate processes are likely similar and could be assessed and implemented jointly, rather than in isolation.
URBAN DEVELOPMENT CONSIDERATIONS This section presents a review of the effects of urbanization on stormwater management and design of drainage infrastructure. This review is deemed to be relevant to the evaluation of adaptive options available to address the effects of climate change, as designers and planners have considerably more experience in dealing with the effects of urbanization on drainage infrastructure than they have in dealing with the effects of climate change, and as the effects of these two separate processes on drainage infrastructure are similar. Several stormwater management features can be incorporated in the planning and construction of new urban developments. These development features pertain both to subdivision layout and building design. Subdivision layouts should provide for drainage to low-lying areas that can be used to store or convey stormwater runoff. As these areas would thus be subject to repeated and frequent flooding, and would thus not be suitable for development, these provision are best satisfied through the use of Municipal Reserve and Environmental Reserve zoning designations during the panning stages of new development. Generally, the amount of impermeable area should be minimized, for example, permeable and porous pavements can be used to allow water infiltration over a larger area. Building design can incorporate features to reduce the rate and amount of stormwater runoff. Rainfall captured at the source (from impervious areas on building lots or within road right of ways) could be returned to the natural hydrologic cycle though infiltration and evaporation. The runoff from the roofs of larger buildings, such as apartments and office buildings can be reduced by incorporating rainwater harvesting devices, green (vegetated) roofs, and modular stormwater retention tanks. For smaller buildings, such as single-family residences, stormwater runoff from roofs can be directed into water butts installed on rain water down pipes or to permeable areas where water can infiltrate slowly into the ground. Sustainable urban drainage systems should provide multiple benefits, conserve water, and reduce localized urban flooding. New development should incorporate appropriate development features and urban drainage system to attain these benefits. Providing for sustainable urban drainage in areas already developed might be more difficult due to the difficulty in acquiring land for drainage facilities. Therefore, opportunities for improving urban drainage systems that arise from urban redevelopment should not be overlooked. The provision of sustainable drainage systems is again best realized during the planning for (re-)development. Research has shown that when impervious cover exceeds a threshold between 10 and 20 percent of the catchment area of a watershed, stream stability is reduced, habitat is lost, water quality degrades, and biological diversity decreases (Schueler 1994). Booth et al. (1997) stated that in western Washington, and likely in other humid regions as well, approximately 10 percent effective impervious area in
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a watershed typically yields demonstrable, and probably irreversible, loss of aquatic system function. Although degradation imposed on the natural system could begin at very low levels of urban development, they found that physical and biological effects can be consistently observed and measured once effective impervious area reached about 10 percent, which could be a basis for impact evaluation and management response (Booth et al. 1997). As stated previously, the environmental effects of climate change (when interpreted in the context of peak flow increases and channel destabilization) are similar to the effects of urbanization, and may be addressed using similar mitigative or adaptive measures. Engel-Yan et al. (2005) found from a review of literature that stormwater management is influenced by many components of neighbourhood form and design features such as lot and house size, and the amount of imperviousness. However, this information appears to be contradictory when evaluating the requirements of storm drainage and development cost, and comparing these against the requirements of environmental stewardship and water management. Large lot sizes result in: increased distances between development and transportation infrastructure, increased lot servicing costs, increased minor system cost, increased irrigation water usage, and a larger development “footprint”; while small lot sizes generally result in a reduction of the above factors. It should however be noted that the percentage of impervious surfaces associated with development (e.g. streets, driveways, roofs, walkways) is larger for small lot sizes than it is for large lot sizes (i.e. small lot sizes generally have a higher development density), which results in an amplification of the effects of urbanization (e.g. increased runoff and peak flows, and a reduction in groundwater recharge and reduced base flows), and an increase in the magnitude of associated environmental effects (e.g. stream destabilization, stream bank erosion, transport of contaminants) (Engel-Yan et al. (2005). Li et al. (1998) suggested that the efficiency of curb-guttersewer systems in conveying excess water to an end-of-pipe facility or to a water body can also contribute to environmental and economic problems. These problems include a lack of stormwater treatment capability, a reduction of groundwater recharge and a loss of stream baseflow, soil erosion at outlet points, operation problems, and high capital and maintenance costs. They stated that the curb-gutter-sewer systems complement the urban lifestyle. Curb-gutter-sewer systems are perceived as increasing property values, delineating streets, and providing relatively level land as drainage systems are largely underground. Therefore, curb-gutter-sewer systems are often specified by urban authorities and subsidized by governments, thus making it more difficult to implement alternatives. Li et al. (1998) identified alternatives to the curb-gutter-sewer system including designed grassed swales, which may be combined with infiltration trenches and pits, curbs and gutters without sewers, sewer systems without curbs and gutters, and curb-gutter-sewer systems, which include exfiltration-filtration trenches, and/or water quality inlets. Li et al (1998) stated that drainage officials should consider the environmental functions of alternative drainage systems. They contended urban standards could be established that recognize the value of “green” drainage systems while still utilizing the best aspects of the curb-
gutter-sewer systems where justified. Some of these approaches to sustainable urban drainage might also prove to be applicable to accommodating the effects of climate change on urban drainage infrastructure.
short-term. The downside of this approach is that the costs associated with upgrading the capacity of drainage infrastructure well before the end of its structural service life are likely to be substantial.
As stated previously, addressing the effects of climate change on drainage infrastructure in existing development (especially high density development) is difficult due to the integration of the minor drainage system with a host of other infrastructure, the potential absence of a major drainage system, and the general lack of space for the construction of remedial measures such as storm water management ponds. The adaptive strategy in these situations would be the designation of space for remedial measures when space becomes available through brownfield or urban redevelopment and the flood-proofing of existing development and infrastructure where possible.
The reason the above adaptive approaches work reasonably well for large drainage infrastructure is the fact that the hydraulic capacity of the drainage infrastructure is re-assessed and adjusted every time the infrastructure is replaced at the end of its structural service life. For example, the waterway opening of a bridge or major culvert can be adjusted when the concrete of the bridge piers or the structural integrity of the culvert barrel require that the bridge or culvert be refurbished or replaced. The ability to adjust the hydraulic capacity in these instances is facilitated by the fact that large drainage infrastructure (such as bridges and large culverts) generally is “stand-alone” and can be isolated for refurbishment or replacement. Urban drainage infrastructure, in particular the storm sewer system, is however integrated and “intermixed” with a large number of other infrastructure (water supply system, sanitary sewer system, gas lines, buried power utilities, communication systems, and transportation systems), and is surrounded by real estate that generally over time becomes more valuable and more densely developed. Both these factors complicate the periodic replacement of this infrastructure to accommodate the effects of climate change.
INFRASTRUCTURE PLANNING AND DESIGN The first approach to accommodate the effects of climate change is to design using historic design flows and accept a gradual decrease in the level of service provided by the drainage infrastructure. In situations where periodic flooding and the incurrence of minor damage associated with flooding are acceptable, this approach may be adequate. However, where serious damages are expected as the result of flooding or where the life and safety of people are threatened, this approach will be unacceptable. A second approach that can be taken to accommodate the effects of climate change is to design and construct structures with sufficient flow capacity to handle future, rather than current, flow conditions. Bruce (2002) suggested that the costs of expanding drainage capacities must be weighed against projected costs of more frequent flooding, with return periods of severe rainfall events projected to be cut in half (for example, a 10-year return period storm becomes a 5-year return period storm by the latter half of this century). Watt et al. (2003) recommended designing drainage infrastructure based on modelling a design storm determined using the available climatic records and then increasing the magnitude of the design storm by 15% to accommodate the effects of climate change. The implications are that infrastructure would be designed and built with hydraulic capacities appropriate for the end of their service life rather than hydraulic capacities appropriate for present-day requirements, with a financial burden that has to be borne before the increased hydraulic capacity of these facilities is fully needed. On the other hand, the over-sized infrastructure would provide greater capacity to handle extreme flood flows, whether or not these flood events are associated with climatic change. A third approach is to design and build infrastructure for shorter design lives, and then retrofit or replace the infrastructure in the future when conditions necessitate. The implications of climate change for drainage infrastructure can be managed through long-term planning that accounts for future increases in flows (Denault et. al., 2002). Using shorter design lives accommodates the uncertainty associated with determining design flows for longer periods based on the current methods of downscaling computer model predictions about climatic change. Drainage infrastructure would be designed and built that would be required to handle flows expected in the
The cumulative effects of climate change on urban drainage infrastructure are amplified by the fact that the service life of urban drainage infrastructure is generally much longer than that of other infrastructure. For example, sections of the storm sewer system in some of our major urban centres are well over a hundred years old. In light of these aspects related to urban drainage infrastructure, a design approach that differs from the three adaptive approaches presented above appears to be warranted. A fourth approach to the planning and design of urban drainage infrastructure consists of a combination of the above approaches for different components of the overall drainage or stormwater management system. This combined approach consists of using historic design flows and the acceptance of a gradually decreasing level of service for the minor system, while accommodating the expected future effects of climate change during the planning and design of the major system. Care should be taken when employing this approach that surcharging of existing sections of the minor system above the levels allowed for during their original design does not result in unacceptable damages. During the design of new drainage infrastructure, the interaction between the minor and major system should be carefully assessed, and the hydraulic performance of the combined minor/major system should be designed in an integrated manner. The focus of the last approach presented above is on the major drainage system, and the ability to increase the discharge capacity of this drainage system component without incurring flood damages. It should again be noted that this ability hinges on the anticipation of this future need during the planning phase of development.
ADAPTATION COSTS AND BENEFITS
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After concerns related to human health/safety and protection of the environment, the selection of the preferred climate change adaptation strategies related to the design of drainage infrastructure is primarily a function of economics. The costs and benefits associated with adaptation strategies and options, is discussed in the following paragraphs. Adaptation projects provide direct benefits at the local level and perhaps regional levels, and may have little effect on the global environment (Fankhauser, 1998). Therefore, adaptation projects differ from many mitigation projects intended to reduce greenhouse gas emissions, which have an effect on the global environment. Fankhauser (1998) stated that the key adaptation benefit comes in the form of avoided climate change damages, which results from a lowering of the vulnerability of a local area or region to climatic change. Societies have already adapted to some extent to the present climate. Therefore, optimal climate change adaptation could be a mix of new and existing measures (Fankhauser, 1998). With respect to climatic change, the mix and strength of existing measures that need to be modified and the extent that climate change requires different adaptation standards also need to be considered. Fankhauser (1998) stated the assessment of adaptation measures requires a comparison of project costs and benefits. Basically if the cost savings obtained from maintaining current design standards is less than the present-day costs of future damages, implementation of adaptation options would be justified (Kije Sipi Ltd., 2001). Conversely, if the present-day costs of future damages are less than the costs associated with changing design standards, a case could be made for maintaining existing design standards. Therefore, drainage design criteria should ideally be reviewed and revised based on a costbenefit analysis and risk assessment considering the threat of climate change. Project costs include the costs of implementing adaptation measures (such as installing a larger sewer), ordinary climate damages associated with the current climate, remaining climate change damages associated with human activity induced climate change, and the costs of adaptation decisions on other infrastructure and society. The difference in total costs (the cost of adaptation plus damages plus indirect costs) between a climate change scenario and a reference scenarios can be calculated as the costs imposed due to climatic change. Benefits of an adaptation project occur in the form of avoided or reduced damages associated with the climate change and the current climate. A successful adaptation project is one in which the resulting benefits are greater than the imposed costs. Although the costs of climatic change will be substantial, there is no consensus on how the costs should be defined. Lomborg (1998) points out some of the difficulties in estimating the potential costs of climatic change: the uncertainty in climate change projections, the selection of baseline and projected climate scenarios, the discounting of future economic costs, the possibility of technological solutions, and the influence of social, economic, and political influences on decision-making.
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However, engineers and planners have to make current decisions based on sound engineering principles and economics, considering both time and risk, to ensure the design and construction of appropriate drainage systems. In view of the uncertainties involved with the science and economics of climatic change, engineers and planners are faced with a tremendous challenge. The strategies that can be adopted to make these decisions range from invoking the precautionary principle and designing for the worse case scenario to reduce the risk of negative future outcomes, to ignoring the potential effects of climate change and maintaining the status-quo. In light of the fact that engineering economics is nearly always a matter of trade-offs, the best adaptation strategy is not likely to consist of either incurring drastic current costs to build infrastructure that has excessive capacity over much of its design life in an attempt to avoid all potential future damages, nor is it likely to consist of incurring potentially severe future damages by building infrastructure that has inadequate capacity near the end of its design life in an attempt to avoid current costs. A pragmatic approach could be to design for the present climate with a moderate allowance for change, but to include a sufficient degree of flexibility in the planning and design of drainage infrastructure to allow the incorporation of additional capacity for the predicted hydroclimatic conditions, if and when required. In terms of urban drainage infrastructure, this means that the design of the minor system (the traditional storm sewer system and associated underground infrastructure) can continue to be based on present-day methodologies and design parameters, with allowances made for the expected effects of urbanization, while the planning and design of the major system includes sufficient flexibility to easily accommodate increases in future flows that may result from climatic change. The advantages of this approach consist of: 1.
Deferral of changes required to accommodate the effects of climate change until these effects are better quantified and the costs and benefits of adaptation options are better defined.
2.
Minimization of the overall costs associated with accommodating the expected effects of climate change (by having these effects accommodated by the major system which consists primarily of surface infrastructure components which are relatively easy to access and modify, rather than by the minor system which consists primarily of buried infrastructure components which are more difficult to access and modify).
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Deferral of the costs associated with upgrading the capacity of the overall drainage system while maintaining the ability to do so at any time in the future without escalations in these costs.
CONCLUDING REMARKS Climate change is a reality that designers of drainage infrastructures must consider. Due to the difficulty
associated with upgrading the hydraulic capacity of urban drainage infrastructure, planners and designers of urban drainage infrastructure should pay particular attention to the effects of climate change. The primary effect of climate change on the design of drainage infrastructure is that the magnitude of rainfall events and the associated flows that are used as the basis of hydraulic design are no longer constant with respect to time, but are expected to increase gradually over time. The consequences of this effect are a decreasing level of service for drainage infrastructure, increased risk of flooding, and environmental damages resulting from channel destabilization. The adaptive strategies that are used to accommodate the effects of climate change on large drainage infrastructure are based on the review and adjustment of the hydraulic capacity of drainage infrastructure (either pro-actively or reactively) when it reaches the end of its service life. However, for a number of reasons discussed above, these strategies do not work well for urban drainage infrastructure. The following approaches are recommended in this paper to accommodate the expected effects of climate change on the design of new, and the upgrading of existing urban drainage infrastructure. To accommodate the expected effects of climate change on new infrastructure, an overall planning and design philosophy based on minor and major drainage systems should be used that incorporates the potential to upgrade the hydraulic capacity of the drainage system. The overall costs associated with these potential upgrades (by designing the minor system to be able to surcharge and having the major system accommodate the expected effects of climate change) should be minimized, and can be deferred without having these costs escalate. The accommodation of the effects of climate change on existing urban drainage infrastructure is more difficult and perhaps can be best achieved through the designation of space for remedial measures when space becomes available through brown-field or urban redevelopment, and the flood-proofing of existing development and infrastructure where possible. The planning and design of development in general and urban drainage infrastructure in particular should ideally be performed in a manner that integrates adaptive responses to climate change with sustainable environmental stewardship and minimization of the adverse effects of urbanization. To advance the optimization of adaptive climate change options with sustainability requirements that limit the effects of urbanization, future research is needed with respect to the effects of urban form and development density on the benefits and costs of urban drainage infrastructure. This comprehensive cost/benefit assessment should include consideration of development and service cost, storm water management (considering both the effects of climate change and urbanization), and sustainable environmental stewardship.
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