AST2.0 Ranking adaptation measures based on effectiveness
The AST2.0 scores the effectiveness of measures based on the local conditions. The score is than used to rank the measures. The method is a light version of the method that is described in (Voskamp and van de Ven, 2015).
To the score on effectiveness in assessed in two steps that accumulate in a final score:
- Technical feasibility assessment
- Site suitability assessment
Both steps use scores that have been obtained by expert judgement in which the effectiveness has been estimated relative to the effectiveness of the total set of measures. The table of effectiveness scores can be found here (LINK effectiveness score).
Technical feasibility assessment
The first part of the ranking method is the ‘technical feasibility assessment’, meant to generate a ranking of adaptation measures based on their technical feasibility at a project location. This assessment is a rapid and data-extensive step that utilizes basic field data on:
- Slope of the terrain (and expected groundwater depth and dynamics);
- Soil type;
- Scale level of implementation
Slope and groundwater are aggregated into one criterion.
Though climatic conditions were identified as relevant environmental characteristics, these are not included in the technical feasibility assessment. Climate characteristics rarely restrict the technical feasibility of implementation, but do affect construction requirements, performance and 8 effectiveness. The effect of climatic conditions on the applicability of blue-green adaptation measures will therefore be found in the results of the effectiveness assessment tool.
The user has to specify which level of spatial scale to consider for implementing blue-green measures. Adaptation measures at building scale or at city scale are in some cases infeasible. This input ensures that all scales of interest are included in the ranking of blue-green measures.
Accordingly, the list of blue-green measures is scored on three criteria:
- ‘dominant soil type’ has four categories: sand, peat, clay, and bed rock (score range 0-10);
- ‘level and slope’ is divided in three rather generic categories: sloping area, flat area on high ground with deeper groundwater levels, and flat area on low ground with shallow groundwater levels · (score range 0-10);
- ‘spatial scale of application’ is divided into the spatial scales of city, neighborhood, street, and building. The latter also includes the parcel –often private land - at which the building is situated. (score range 0-10).
The scores for three criteria are summed to determine technical feasibility score.
Site suitability assessment
After the technical feasibility assessment, the measures are scored on their site suitability. For this assessment three site suitability criteria are used:
- Existing space types: existing type of urban space;
- Subsurface availability: subsurface space requirements;
- Multi-functional land use.
The criterion ‘Existing space types’ is used to evaluate field characteristics that are vital to retrofitting a blue-green measure. For example: surface water has to be present to apply the measure ‘Storage by creating extra freeboard’ or, ‘adding trees in streetscape’ is only relevant when the site includes a streetscape. Hence, this criterion aggregates urban land use characteristics, utilization pressure and ownership in the following categories:
- Red space: buildings;
- Grey space: paved surfaces like streets, parking lots and squares; ·
- Green private space: parcels (gardens); ·
- Undeveloped land and green space without recreational use: ‘unused’ locations, abandoned grounds, empty lots, brownfields;
- Green space with recreational use and/or urban farming:
- Green space with high utilization pressure, e.g. parks and public gardens; ·
- Green/grey space like sports and playgrounds: green space with high utilization pressure, though with a larger share of impervious surface than the previous category;
- Blue space: surface water.
The scores for the applicability given the available existing space types range between 0 and 10. The maximum value for the applicability for a measure given the available existing space types is used in the suitability score.
All adaptation measures receive a score on multi-functionality. Multi-functionality means that the space that is required for the measure can still be used for other spatial functions as well. As blue-green measures have to compete with other spatial demands in urban areas, in particular in complex, high density environments, multi-functionality is an important characteristic to evaluate in a retrofitting process. The more multifunctional a measure is, the higher the score. The scores range between 0 and 10 and are multiplied by the importance of multifunctional land use that is given in by the user which ranges between 0 and 1. For user friendliness the following categories are presented to the user:
- Not important: 0.3
- Very important: 1.0
The multi-functionality and the existing space type scores are multiplied to obtain the first part of the suitability score.
Suitability of a site also depends on the depth or type of space that some measures require underneath it. Based on the average sub-surface depth required by blue-green measures to operate, four categories of sub-surface space availability are distinguished ranging from very low to high. Data on present subsurface infrastructure and contamination can be used to identify availability of subsurface space. If the actual subsurface availability is larger than the subsurface availability requirement of the measure a value of 1 is given to the measure, else a value of 0.
Measures applied on roofs - intensive and extensive green roof and a water roof - require flat roofs or roofs with a slope of less than 35 degrees. So, the fifth criterion in this category is related to the type of roof that is present in the project area. If no flat roofs or roofs with a slope of less than 35 degrees are present in the project area a value of 0 is given to those measures that require flat roofs or roofs with a slope of less than 35 degrees, other measures get a value 1. If roofs with a slope of less than 35 degrees but no flat roofs are present, measures that require a flat roof get a value 0, all other measures get a value 1.
The second part of the suitability score is calculated as the product of the sub-surface depth and roofs score.
The total suitability score is 100% of the first part of the suitability score in case that suitability score based on the sub-surface depth and roofs score is 1 and it is 40% in case that suitability score based on the sub-surface depth and roofs score is 0.
The final step of the selection assistant is the combinability assessment. The technical feasibility is adjusted by multiplication with the system capacity score. The total scores of the blue-green measures on adjusted technical feasibility and site suitability are than summed to evaluate their complementarity to the current system capacities.
The system capacity score is determined by the adaptation targets for pluvial flooding, water safety, drought and heat stress resilience can be set for the project area. To simplify the usage of the tool no distinction is made between threshold and coping capacity. All adaptation targets have an equal weight of 1 and are summed based on whether the adaptation target has been selected. The score of the adaptation target is converted to the system capacity score based on the following conversion:
0 => 1
1 => 1.35
2 => 1.50
3 => 1.60
4 => 1.75