...
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. 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.
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.
Most measures influence the quality of stormwater runoff by capturing, retaining and/or filtering flow. Moreover, many pollutants degrade while still in the water column, in solution or adsorbed on suspended sediment particles. Capturing of pollutants takes place at the inlet of the measure. For example: a sand catcher installed in the inlet of infiltration boxes captures pollutants even before the runoff reaches the measure. Also vegetation growing on/in a measure can capture pollution, for example by intercepting rainfall. Once the runoff enters a measure with storage capacity flow velocity is reduced and suspended sediment starts settling. Particle bound pollutants and pathogens are trapped. As many adaptation measures have a substantial storage capacity this treatment of stormwater by settling is quite effective. Some measures drain runoff through a soil filter. Such a filter effectively captures particles; dissolved pollutants are often adsorbed to the filter material, resulting in an effective removal of pollutants. In addition, all pollutants except heavy metals start being degraded by biological and chemical processes so that their concentration decreases. The longer the residence time the more degradation takes place.
Intensive green roofs are a slightly different case. Such roofs can only be sustained if the vegetation is fertilized. This generally leads to high nutrient levels in the runoff from such roofs, much more than from an extensive, unfertilized extensive green roof. Consequently, runoff from intensive green roofs is polluted with nutrients instead of being treated.
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)
...