Abstract

Key Words

Urban capability, urban feasibility, risk assessment

Introduction

Urban capability is the inherent physical potential of the land to effectively support urban development without degradation of land and water resources. Urban capability assessment is vital for planning and land use decision making—if ignored, it can result in profound damage to structures and the environment or even loss of human life.

Urban capability assessment has been practiced for several decades, e.g., Hannam and Hicks (1980), but in NSW its application has declined. There are many examples of high value homes being built to take advantage of water views on sites of very low capability. In such situations developers have been prepared to and can afford to pay for expensive mitigation measures such as erosion and sediment control works. Both planners and developers have identified difficulties in relating urban capability to planning and development decision making processes because it falls short of indicating what can be done or what is feasible.

This paper presents an extended approach to urban capability assessment. The developing approach embodies principles of risk assessment and is intended to assist in land use planning and decision-making processes within the NSW Coastal Comprehensive Assessment (CCA). The CCA initiative is to ensure population growth pressures along the NSW coast are properly planned. In the CCA process, urban capability is only one of a range of assessments including terrestrial and aquatic ecology, culture and heritage, visual sensitivity, demographic, economic and social studies.

Multi-criteria analysis

A major planning challenge faced by the CCA and similar planning projects is to interpret multiple disparate datasets. The CCA is adopting an additive form of multi-criteria analysis (MCA) based on the work of James (2001) known as Topdec.

Capability results need to be comparable against other impacting factors. Urban capability is just one of several other environmental factors (e.g., presence of endangered species, visual amenity), economic factors (e.g., market demand for new housing) and social factors (e.g., need for new infrastructure). Impacts between disparate themes need to be soundly ranked to ensure sound judgements are made. Most currently used capability systems do not allow these direct comparisons.

The four-stage approach begins with delineation of areas within scope of planning studies by excluding areas such as national parks and highly unstable building land. The second step takes into account demographics and planning considerations to define and select a number of development scenarios. The CCA scenarios include high rise, medium density residential, standard residential and rural residential developments. The third step relates to assessment of effects of the scenarios—impacts such as soil erosion, nutrient loads in waterbodies, visual impacts, alienation of agricultural land and changes in biodiversity. In the fourth stage, the impact of each parameter for each scenario is then given a positive or negative score. The scores for each impact are then summed by scenario. The scenario with the highest positive sum is the most desirable.

MCA provides a means of evaluating the relative impacts of multiple scenarios and addressing an increasingly common problem where multiple data themes require simultaneous assessment.

Meeting the needs of planners and developers

For urban capability information to be more widely used in planning and development processes, a number of issues need to be addressed. These include:

Information on the potential consequences of development of a site as well as the ease and cost of overcoming identified constraints.

• Outputs that are easily understood and interpreted. The significance of capability ratings needs to be made clear, for example, low capability land does not necessarily mean exclusion of a particular land use, but rather that comprehensive design and mitigation measures might be required.
• Quantification of results in terms of potential costs associated with different development scenarios to aid the decision making process. Financial costs are particularly meaningful to developers.
• Clear presentation of results. The reason for all low capability rankings should be readily apparent (e.g., due to potential mass movement hazard). This allows for the identified constraint to be dealt with from the outset of the planning and development process. Background supporting tables and documentation should be accessible and easy to interpret. Indication of variability within map units should be shown
• Clearly defined exclusion zones (not capable areas) based on standards and risks accepted by the society, e.g., flooding greater than a certain probability that creates a certain amount of damage.
• Scenarios ultimately ask for what is possible when limiting conditions have been met. There is a need for input of relative feasibility of actions to ameliorate limiting conditions.

Risk management framework

Capability assessment is essentially an assessment of risk, i.e., the likelihood of undesirable environmental, economic or social impacts resulting from prevailing biophysical conditions.

We adopted the risk assessment framework within the Australian and New Zealand Standards AN/NZS 4360:1999 to assess capability. To quote this standard: “Risk assessment is based on the chance of something happening that will have an impact upon objectives. It is measured in terms of consequences and likelihood”. Risk assessment works with many activities and allows disparate themes to be ranked in the same manner. Risk Management Frameworks can be used, for example, to assess and classify risks relating to financial, human, environmental and corporate concerns. The 1999 standard classifies consequences into categories such as insignificant, minor, moderate, major and catastrophic. Table 1 provides hypothetical examples of ranked consequences in an urban context and is an illustrative guide.

The likelihood or expected frequency of occurrence of impacts is also rated for each risk. Table 2 indicates how likelihood and frequency might interact in an urban context where development life is nominally considered to be 100 years for houses and associated infrastructure.

A matrix of consequences and likelihoods is then used to allocate the level of risk. Table 3 illustrates the matrix used to relate risks for urban capability assessments in comparison with other planning themes.

Table 1: Risk management framework – potential consequences of impacts in urban context (after AN/ANZ 4360:1999).

Consequence	Financial	Human	Environmental	Corporate	Infrastructure
Catastrophic	>$10 M	Multiple Deaths	Long-term loss of entire ecosystem	Judicial inquiry, criminal prosecution	Abandonment and loss of use
Major	$1–10 M	Death and/or permanent injury	Long-term serious damage to ecosystem	Long-term loss of political/public support	Regular major disruption and high cost repairs
Moderate	$100 000–1 M	Severe injury/hospitalisation—function regained	Moderate damage to ecosystem, many species adversely affected	Embarrassment/market loss	Moderate disruption and repairs
Minor	<$10 000–100 000	Significant number of work days lost	Delayed recovery of several species	Senior Management attention required	Episodic minor disruption and repairs
Insignificant	<$10 000	Minor injury. First aid or minimum number of work days lost	Minor perturbation to some species	Day to day management required	Minor disruption and repairs

Table 2: Risk management framework—likelihood and frequency of impacts in an urban context (after AN/ANZ 4360:1999).

Likelihood Level	Frequency
Rare	Much greater than development life
Unlikely	Greater than development life
Possible	Similar to development life
Likely	Decade
Almost Certain	Seasonal

Table 3. Risk management framework – risk and capability ranking matrix (risks after AN/ANZ 4360:1999).

Consequence/ Likelihood	Insignificant	Minor	Moderate	Major	Catastrophic
Almost Certain	M (3)	S (4)	H (5)	H (5)	H (5)
Likely	L (2)	M (3)	S (4)	H (5)	H (5)
Possible	L (2)	L (2)	M (3)	S (4)	H (5)
Unlikely	VL (1)	L (2)	L (2)	M (3)	S (4)
Rare	VL (1)	VL (1)	L (2)	L (2)	M (3)

Risk levels: VL = Very Low; L= Low; M = Moderate; S = Significant; H = High Risk

Capability class rankings: (1) = very high; (2) = high; (3) = moderate; (4) = low; (5) = very low

The risk classes may be directly equated with capability classes. For example, very low risk (VL) equates to class 1 (very high) capability; and high risk (VH) equates to class 5 (very low) capability.

As an example, mass movement such as hillside collapse where multiple buildings and lives are lost would be a high risk and Class 5 if there were a possibility of this occurring during the life of the development. Another example is where a house built on soils with minor shrink-swell in an area that experiences drought every decade would have moderate risk.

Residual risk management

The concept of residual risk is an important element of the urban capability and feasibility assessment process. Residual risk is defined by AS/ANZ 4360:1999 as “the remaining level of risk after risk treatment measures have been taken”. For example, large residual risk levels remain on very steep sites with erodible soils in areas subject to intense summer thunderstorms. In such areas, standard soil conservation efforts often prove ineffective. Site hardening (i.e., erosion and sediment control, landscaping, paving and terracing) to reduce risk is possible, but very expensive. Conversely, a low capability situation due to cracking clays is easily dealt with by simply choosing appropriate foundations at relatively little cost. These sites will have a low residual risk.

Areas with high residual risks are those that should generally be excluded from development and so are used in stage one of the Topdeck MCA process. All other areas may be capable of sustainable development if ameliorating actions are sufficient to lower residual risks to acceptable levels.

Proposed urban feasibility methodology

Urban capability is an indication of detrimental impact if a development goes ahead without site amelioration. In contrast, urban feasibility is the relative cost and reassessment of capability assuming ameliorative measures are put in place. It is essentially the cost of transforming land capability at a site to a point where acceptable development can go ahead. In other words urban feasibility is the relative cost of achieving acceptable residual risks.

The urban capability and feasibility assessment approach presented here is based on the general land capability assessment approach presented in Gray and Chapman (in prep.), which in turn was based on Chapman et al. (1992); and Wells and King (1989). It involves evaluating the combined effects of a number of key soil and landscape attributes. Broad definitions of each capability and feasibility class are given below:

Class 1: Very high capability; nil or minor constraints; very low risk; standard design.

Very high feasibility; very low residual risk; low treatment costs; straightforward or no maintenance; associated with negligible financial, environmental or social site costs; acceptable to society.

Class 2: High capability; minor constraints; low risks; standard design.

High feasibility; associated with minor financial, environmental or social site costs; straightforward or low maintenance; low residual risk; acceptable to society.

Class 3: Moderate capability; moderate constraints; moderate risks; site-specific actions, investigations and designs are required; risks are marginally acceptable.

Moderate feasibility; moderate financial, environmental or social costs beyond the standard (equivalent to 20% of total development costs over the life of the development); frequent maintenance required; moderate residual risk; marginally acceptable to society—other factors may intervene.

Class 4: Low capability; high constraints; significant risk; site-specific actions, investigations and designs are required.

Low feasibility; high financial, environmental or social costs beyond the standard (approximately 20–50% of total development costs over the life of the development); special mitigating measures are required; regular specialist maintenance; moderate to high residual risks and costs; not usually acceptable to society.

Class 5: Very low capability; severe constraints; high risks; site-specific actions, investigations and designs are required.

Very low feasibility; risks very difficult to control even with site-specific investigation and design; very high financial, environmental or social costs beyond the standard (potential costs equivalent to >50% of total development costs over the life of the development); regular specialist maintenance may be mandatory; there is a risk that costs will be incurred even if highly specialised and costly mitigating measures are applied; residual risk is high; not acceptable to society.

An overview of the process is given below.

i. Constraint analysis—the ratings for individual constraints such as erosion, flood and foundation hazards are determined for the subject parcel of land by the use of separate constraint assessment tables. This involves weighting attributes from various datasets according to likelihood and consequence. These tables are presented in Gray and Chapman (in prep).

ii. Capability analysis—the ratings for the above constraints are applied to the various urban capability tables. We use the example of standard residential housing (see Table 4). The overall capability of the site is that of the lowest ranked constraint, i.e., the limiting factor.

iii. Treatment cost rating—an indicative costing is applied to the ameliorative actions or treatments in terms of the percentage cost of the development. It is assumed that ameliorative action is an almost certain likelihood. Additional tables (not presented here) will assist in this process. The costing example is provided in bold on Table 4. This is the rating of most interest to the developer.

iv. Residual risk rating—the potential costs of possible impacts arising from the development including ameliorative actions is estimated. This includes costs of required repairs and an estimation of environmental and social costs converted into dollar terms (further discussed in section 7 below). Additional tables (not presented here) will assist in this process. An example of residual risk rating is given in italics in Table 4.

v. Overall feasibility—this is determined by selecting the limiting factor out of either the cost of treatment or the residual risk ratings. Planners and society are concerned with overall feasibility ratings.

The process is illustrated by the following examples of two sites being considered for standard suburban residential development.

Site A: current bushland, steep slopes (25%), very high erosion hazard, all other attributes have nil or minor constraints.

Standard residential

• overall capability: 4
• treatment cost: (to harden site): 4 approx. 30% of initial construction costs.
• residual risk: 2 (after site hardening)
• overall feasibility: 4 (highest of treatment cost and residual risk assessments)

Site B: current pasture land, gently sloping site (8%), moderate erosion hazard, medium heavy clay with subsoils having high shrink-swell potential (15% COLE) resulting in high foundation hazard and high underground infrastructure hazard, all other attributes–nil or minor constraints.

Standard residential

• overall capability: 4 (shrink-swell)
• treatment cost: 2 (with estimated 10% increase in construction costs)
• residual risk: 2 (treated with raft foundations and deeply buried underground services)
• overall feasibility: 2 (highest of treatment cost and residual risk assessments)

Table 4: Standard residential capability table with worked example.

Constraint	Very high cap’y	High cap’y	Mod. Cap’y	Low cap’y	Very low cap’y	Rationale/ consequence	Treat-ment
Erosion Hazard	1	2 BA	3	4 AA	5	loss of soil, degrades water quality, sedimentation of waterways	Detailed erosion control plan & implementation
Flood Hazard	1 AAA BBB	-	2	-	3, 4, 5	threat to property and life
Acid sulfate Soil Risk	1 AAA BBB	-	2	3	4, 5	threat to aquatic ecosystems, corrosion of structures
Foundation Hazard (small structures)	1 AAA	2 B B	3	4 B	5	damage or failure of structures	Shrink-swell ameliorated by appropriate foundations
Underground Infrastructure Hazard	1 AAA	2 B B	3	4 B	5	damage to services	Shrink-swell ameliorated by burial
Capability (limiting factor)				BA
Cost of Treatment	B			A
Residual Risk (after treatment)		B A
Urban feasibility rating		B		A

Letters A and B represent sites A and B. Normal capitals indicate capability rankings. Letters A,B (in bold) represent cost of treatment and letters A, B (in underlined italics) represent residual risk assessments following treatment for sites A and B respectively.

Table 5 shows that the limiting factor of soil erosion for site A can be treated to achieve high capability, but that the cost of doing so is very high and so, is not feasible. The limiting shrink-swell attribute for site B affects underground infrastructure and foundation hazard qualities, but in both cases can be treated relatively simply and cheaply.

Assessment of costs and residual risks

The potential costs given in the feasibility definitions are proportional to the total cost of development/construction over the life of a development (nominally 100 years from start of construction). Costs may be attributed to:

• direct financial expenses for detailed site investigations, additional design work, special construction and impact mitigating measures, ongoing maintenance and especially, for major repair work that may be required if the special design and mitigating measures are not properly applied, and/or
• indirect environmental and social costs converted to an equivalent financial cost, which may be required if the special design and mitigating measures are not properly applied or fail. This may be based on the costs necessary to return conditions to pre-impact state, e.g., the cost of neutralising the effect of acid conditions following disturbance of acid sulfate soils, or an estimate of the loss of public amenity.

Whilst some cost estimates for treatments are relatively easy to obtain (such as those that relate directly to commonly used foundation types), there are others that are problematic (e.g., treatment of disturbed acid sulfate soils) due to the large number of variables, wide range of treatment methods and uncertainty of treatment success.

A good example of costs in relation to urban soil erosion constraints is provided by DLWC (1999). The project examined seven case studies and collected both actual and relative costs of implementing erosion and sediment control measures over a variety of sites with varying soil and landscape conditions. In the case of a steep site with high potential for erosion, the cost of implementing erosion and sediment control measures was $11 000 per block or 26% of total development infrastructure costs. Flatter sites on more stable clay soils incurred soil erosion and sediment control costs of $700–2000 per block or 5–10% of infrastructure costs.

Studies on the costs associated with mitigating other soil and landscape hazards are of benefit in the further development of capability costings, but unfortunately, these tend to be rare.

Conclusion

The system of urban capability and urban feasibility assessment presented here provides a useful approach to evaluating the influence of soil and landscape constraints in urban planning and development. The approach is still at a preliminary stage and is to be developed further. The system provides an indication of the nature of the constraining factors giving rise to the low capability. It gives an indicative cost of the required ameliorative measures and residual risks remaining after this treatment. The method allows for the comparative estimation of "go ahead with best-practice" costs and residual risks.

The feasibility method attempts to equate the assessment of environmental hazards to potential dollar costs via risk assessment. It discriminates limiting factors that can be economically and successfully ameliorated against those which cannot. It provides a rating system that allows direct comparison with other criteria that to be evaluated in urban planning and development processes, e.g., ecological, economic and social factors. It can, therefore, be easily incorporated into multi-criteria analysis for scenario evaluation. By giving urban capability information in terms of approximate cost, the information becomes more meaningful to developers. This makes it easier to use and compare different development scenarios, meaning that the capability assessment information is more likely to be used.

References

Australian and New Zealand Standards (1999) Risk Management. AN/NZS 4360:1999, (Australian Standards, Homebush Sydney, NSW).

Chapman GA, Morse RJ and Hird C (1992) A Framework for Assessment of Urban Land Suitability for New South Wales, Proceedings of 7th International Soil Conservation Organisation Conference: People Protecting their Land, Sydney, Australia 27–30 September, 1992.

Gray JM and Chapman GA (in prep.) Specific Land Use Capability Assessment, Tech. Report, (Dept of Infrastructure, Planning & Natural Resources, Sydney, NSW).

Department of Land and Water Conservation (DLWC) (1999) Report into Soil Erosion and Sediment Control Costs in Residential Urban Development, prepared by Mark Sabolch, (Soil Services Division, Dept of Land & Water Conservation, Sydney, NSW).

FAO (1976) A Framework for Land Evaluation, Soils Bulletin 32, (Food and Agricultural Organisation of the United Nations, Rome, Italy).

Hannam ID and Hicks RW (1980) Soil conservation and land use planning. Journal of Soil Conservation NSW, 36: 3.

James D (2001), Topdec: Multi-criteria analysis for the NSW Coastal Comprehensive Assessment project, unpubl. report, (Dept of Land & Water Conservation, Sydney).

Wells MR and King PD (1989), Land Capability Assessment Methodology–for rural residential development and associated agricultural land uses. (Dept of Agriculture, Land Resource Series No. 1, Perth, WA).