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The required storage capacity and normative runoff can be set based on the so-called Storage Discharge Frequency relations. These SDF-curves can be used to estimate the required gross storage demand in a district in view of the existing or future stormwater discharge capacity. The figure below shows an example of SDF curves.
Figure 1. Example of SDF curves for different return periods
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It must be noted that installing a larger pumping station is in general not sufficient as the hydraulic capacity of the stormwater drainage system that leads the runoff towards the pumping station must be increased as well. Thus, any plans made with the AST CRCTool should be considered approximations until engineers and planners can assess the effectiveness of conveyance to the pump station.
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Return time factor
The next issue is to control the peak flow of stormwater runoff to this discharge point. We can estimate the reduction of return time of a specific runoff volume as an estimator for the normative runoff; the return time factor. Setting a target for the normative runoff is by far not as critical as the storage capacity target value assessment. But green infrastructure detains or retains runoff and reduces peak flows so that flow velocities in canals are reduced. Hence this reduces bank erosion, sediment wash-off and sediment transport. Default value for the normative runoff is considered the event with the 2-year return interval. Depending on the erosion sensitivity of the area target values for the normative runoff could then be set events that occur every 5 or 10 years every five or ten years.
Evaporation (Evapotranspiration)
Evaporation means cooling. The more water evaporates, the less energy is available to heat the air and the less hot it gets. In hydrology we use the term evapotranspiration for the combination of (1) evaporation that occurs from surface water and intercepted water – that is water that is stored temporarily on surfaces - and (2) the water that is transpired by plants. So, the greener and bluer surface in our project area, the more water evapotranspires and the more cooling occurs.
Heatstress reduction
Heatstress reduction is expressed as cooling of the air temperature. One of the main causes of the urban heat island effect is reduced availability of vegetation in urban areas. Less vegetation in urban areas leads to a higher air temperature. A relation that has been determined for the Netherlands is shown below. This relationship is used to quantify the heatstress reduction in the CRCTool.
Heatstress reduction = -0.030 x Area_measure / Area_project
Steeneveld G.J., S. Koopmans, B. G. Heusinkveld,L. W. A. van Hove, and A. A. M. Holtslag (2011), Quantifying urban heat island effects and human comfort for cities of variable size and urban morphology in the Netherlands, Journal of Geophysical Research
Cool spots
Cool spots are areas where people like to be on hot days. Metric for heat stress reduction is the number of cool areas that is created in a project area by applying adaptation measures.
To qualify as a cool spot, adaptation measures should be over 200 m2 and have a significant cooling effect.
Measures that have a significant cooling effect are:
- Adding trees to streetscape
- Fountains, waterfalls, water facades
- Urban forests
- Tree pit bio retention
- Creating shadow
- Urban parks
Water quality
Water quality is extremely important for the functions and services that water can provide. Three groups of water quality parameters are considered: Nutrients, particle bound pollution and pathogenic organisms. Nutrients determine the eutrophication level; pathogenic organisms influence public health risk. Many relevant chemical pollutants tend to absorb on suspended organic particles, clay particles and iron-coated sand particles. Most heavy metals, poly-aromatic hydrocarbons, mineral oil, pesticides and other pollutants and the concentration of suspended sediment, BOD and COD can be seen as particle bound pollution.
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The remaining fractions of pollutant after each of these processes, that is 1.00 minus the treatment efficiency, is shown in Table 2.2. These figures are indicative estimates; results in practice will depend on many factors such as on quality of the rainwater and stormwater and presence of air and water pollution sources, on temperature, duration of dry spells, rainfall intensity and volume, on surface and subsurface characteristics and type of construction materials used.
Table Remaining fraction of pollutant after each of the treatment processes (i.e. capturing, settling, filtering and degradation) for nutrients, adsorbed pollutants and pathogenic organisms; Fertilization is an estimated multiplier for intensive green roof nutrient runoff concentration.
Not all measures perform all treatment processes. Pervious pavement, for example, filters runoff while a constructed wetland is settling and degrading pollution; adding trees to the streetscape helps capturing pollutants and so on. All measures were scored on relevant treatment processes; if a treatment process is irrelevant for a specific measure the ‘Remaining fraction’ is set to 1.00, as no pollutants are retained and/or degraded.
Evaporation (Evapotranspiration)
Evaporation means cooling. The more water evaporates, the less energy is available to heat the air and the less hot it gets. In hydrology we use the term evapotranspiration for the combination of (1) evaporation that occurs from surface water and intercepted water – that is water that is stored temporarily on surfaces - and (2) the water that is transpired by plants. So, the greener and bluer surface in our project area, the more water evapotranspires and the more cooling occurs.
Groundwater recharge
Groundwater recharge is relevant in areas sensitive to land subsidence because increased groundwater recharge enhances groundwater levels which in its turn can slow down land subsidence. Next to that, groundwater recharge enhances the availability of water for vegetation during dry periods.
Target values for groundwater recharge depend on the existing and the desired groundwater levels that result from overall groundwater recharge, groundwater abstraction and groundwater drainage next to local subsurface conditions and surface water levels. Setting a target value for groundwater recharge requires a groundwater system analysis using a detailed groundwater model would be required.
Cool spots
Cool spots are the number of cool areas that are created in a project area by applying adaptation measures.
To qualify as a cool spot adaptation measures should be over 200 m2 and have a significant cooling effect. Especially creating shade is an important criterion.
Measures that qualify are:
- Adding trees to streetscape
- Fountains, waterfalls, water facades
- Urban forests
- Tree pit bio retention
- Creating shadow
- Urban parks
Construction costs
If the available total reconstruction budget is known this figure can be used as a ‘target value’ in the sense that this budget should not be exceeded. Construction cost estimates of the AST CRCTool show considerable uncertainty because these estimates are based on generic unit cost prices and local conditions in the project area are not taken into consideration, if at all known. Moreover, costs often increase when making detailed and final designs due to the fact that extra functionalities are added to the first conceptual designs that are made during our workshops. In order to take this into consideration it is our recommendation to use only 60 – 75 % of the available reconstruction budget as target value in the ASTCRCTool.
Later on, in the planning session attention could be paid to the distribution of construction costs (and benefits) over the stakeholders, being both public authorities and private land and property owners.
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If the available annual maintenance budget is known this figure can be used as a ‘target value’ in the sense that this budget should not be exceeded. Maintenance cost estimates of the AST CRCTool show considerable uncertainty, because these estimates are based on generic unit cost prices; moreover, local conditions in the project area are not taken into consideration, if at all known. In order to take this uncertainty into consideration it is our recommendation to use only 60 – 75 % of the available maintenance budget as target value in the ASTCRCTool.
As with a fair distribution of construction costs, attention should be paid to the distribution of maintenance costs over the stakeholders, during the planning of adaptation measures.