**2. An Overview of Sustainable Stormwater Management Practices 2. An Overview of Sustainable Stormwater Management Practices**  Sustainable urban stormwater management solutions face increasing complexity from

Sustainable urban stormwater management solutions face increasing complexity from conflicting demands resulting from increasing urbanization, influence of expected climate variability and financial and budgetary constraints of cities. To address these, the current approach has evolved to accommodate increasing use of LID techniques: these aim to restore urban watersheds functions to pre-development stage hydrology, increasing resilience to external stresses, without compromising the requirements of modern urbanization. Figure 1 summarizes the effects of urban development on flow volumes and frequency. LID can be optimally applied in new urbanization planning, but also as retrofit of existing infrastructure. In addition to purely hydrologic issues, recent stormwater quality regulations are a major factor in LID adoption. While traditional urban stormwater management relies on fast conveyance of excess water away from affected areas, LID relies mainly on infiltration, evapotranspiration and the incorporation of natural hydrologic features extending water retention and reducing runoff, peak flows and pollutant loads. A review of implementation and performance of low impact development approaches was recently published [29]. conflicting demands resulting from increasing urbanization, influence of expected climate variability and financial and budgetary constraints of cities. To address these, the current approach has evolved to accommodate increasing use of LID techniques: these aim to restore urban watersheds functions to pre-development stage hydrology, increasing resilience to external stresses, without compromising the requirements of modern urbanization. Figure 1 summarizes the effects of urban development on flow volumes and frequency. LID can be optimally applied in new urbanization planning, but also as retrofit of existing infrastructure. In addition to purely hydrologic issues, recent stormwater quality regulations are a major factor in LID adoption. While traditional urban stormwater management relies on fast conveyance of excess water away from affected areas, LID relies mainly on infiltration, evapotranspiration and the incorporation of natural hydrologic features extending water retention and reducing runoff, peak flows and pollutant loads. A review of implementation and performance of low impact development approaches was recently published [29].

**Figure 1.** Effect of urban development on stormwater flow (**A**) and return frequency (**B**). With increasing urbanization, the recurrence interval of a 100-year flood can be reduced by one or more orders of magnitude, increasing the probability of risk to life and property. **Figure 1.** Effect of urban development on stormwater flow (**A**) and return frequency (**B**). With increasing urbanization, the recurrence interval of a 100-year flood can be reduced by one or more orders of magnitude, increasing the probability of risk to life and property.

LID stormwater management occurs mainly by means of two approaches: infiltration-based and retention-based. Both reduce an urban basin's effective impervious surface [30]; however, neither of the two individual approaches is generally sufficient to successfully restore a natural flow regime. Management of rainwater may also include installations for collection and reuse of precipitation [21], which can help decrease consumption of water, treated to drinking quality. Several combined solutions to limit runoff and promote use of collected rainwater were recommended by the European Commission [31]. Possible uses that do not require potable water are car washing, garden and lawn watering, laundry making, or toilet flushing. These have been already implemented around the world. Depending on climatic conditions, type of building and use, the reduction of demand for mains water may be as high as 60% [32]. LID stormwater management occurs mainly by means of two approaches: infiltration-based and retention-based. Both reduce an urban basin's effective impervious surface [30]; however, neither of the two individual approaches is generally sufficient to successfully restore a natural flow regime. Management of rainwater may also include installations for collection and reuse of precipitation [21], which can help decrease consumption of water, treated to drinking quality. Several combined solutions to limit runoff and promote use of collected rainwater were recommended by the European Commission [31]. Possible uses that do not require potable water are car washing, garden and lawn watering, laundry making, or toilet flushing. These have been already implemented around the world. Depending on climatic conditions, type of building and use, the reduction of demand for mains water may be as high as 60% [32].

Infiltration-based approaches assist in baseflow restoration by recharging subsurface and groundwater flow [31]. They include swales, infiltration trenches and basins, unlined bioretention systems (e.g., rain-gardens), and porous pavements. Their effectiveness is highly affected by site conditions, hence the wide reported range of performances. Infiltration-based approaches assist in baseflow restoration by recharging subsurface and groundwater flow [31]. They include swales, infiltration trenches and basins, unlined bioretention systems (e.g., rain-gardens), and porous pavements. Their effectiveness is highly affected by site conditions, hence the wide reported range of performances.

Swale systems (open channels filled with vegetation) can be used to replace traditional curbs and for erosion control in peri-urban areas. They are designed to induce infiltration, sedimentation, Swale systems (open channels filled with vegetation) can be used to replace traditional curbs and for erosion control in peri-urban areas. They are designed to induce infiltration, sedimentation,

and filtration during flow conveyance, resulting in some degree of water quality improvement. Infiltration trenches usually consist of gravel-filled channels covered with soil and vegetation.

covered with bark mulch. Finally, permeable pavements (including paving blocks, plastic grids,

and filtration during flow conveyance, resulting in some degree of water quality improvement. Infiltration trenches usually consist of gravel-filled channels covered with soil and vegetation. Bioretention ponds (also called rain gardens) are landscaped low areas where onsite reduction and treatment of runoff occurs. They are generally vegetated with shrubs, perennials, or trees, and covered with bark mulch. Finally, permeable pavements (including paving blocks, plastic grids, porous asphalts and concretes) allow slow runoff infiltration, promoting pollutant removal by entrapment, adsorption or biological degradation.

Retention-based approaches are designed to hold collected stormflow and reduce outflow from a catchment. They have substantial influences on local flow regime, reducing peak flow, but may result in increased outflow persistence. They include wetlands, ponds, green roofs, and decentralized rainwater harvesting; some have been used extensively for years. Although they can be quite effective for pollutant removal, they have limited effect in reducing overall runoff volumes, since this occurs mainly by evapotranspiration.

Green roofs have proven beneficial for stormwater control in many studies. It has been claimed that green roofs, in addition to runoff control, may induce other additional environmental benefits, such as air quality improvement, urban heat-island effect mitigation and urban aesthetics amelioration. According to existing experiences, there are few disadvantages to this solution, the cost of installation being the main one. In addition to the cost of the vegetated component, which may vary according to execution and aesthetic requirements, the added expense to install a green roof, compared to a traditional flat roof, consists mainly in higher structural costs, as the underlying structure may have to be strengthened to cope with the extra structural load. Green roofs show effectiveness in stormwater retention, with ability to attenuate runoff peaks from events with 2–100 years recurrence intervals, reducing the need for detention basins, with beneficial environmental impact. Green roofs can effectively retain 100% of rainfall in events with precipitation less than 12 mm, and significantly delay hydrologic responses, slowing onset of runoff by an average of 5.7 h, and peak runoff response by an average of 2 h. Annual runoff reduction between 38% to 54% and peak flow reductions up to 90%, were reported [33]. Green roofs have an effect on buildings' energy requirements, raising winter roof temperatures by up to 6 ◦C, and lowering it by up to 19 ◦C in the summer, with much narrower ranges of diurnal fluctuations [34]. Several municipalities in Europe provide economic incentives for this practice. Over the last 25 years, many such projects have been completed in the USA and Northern European countries, including Germany and Switzerland.

Decentralized stormwater harvesting may significantly improve water retention within a catchment, reducing annual runoff volumes. Stormwater harvesting is more efficient in terms of runoff reduction if designed to supply water on a daily (short-term), rather than seasonal, basis. Harvested water may be readily used onsite, e.g., for irrigation, or can be further treated for high-quality uses, becoming a significant component of urban hydrology. It is increasingly seen as a valuable resource, especially in water-scarce areas. The potential range of rainwater use as public water substitute is limited by quality and cost of any necessary treatment [20], but it can also imply significant energy requirements and emissions reductions for supply systems [14]. In some areas, due to particularly favorable environmental quality, rainwater could be directly used as a drinking water supply source [35].

In Poland, although the University of Warsaw Library is considered one of the most beautiful and the largest roof garden in Europe, with surface of 1 hectare [36], issues concerning sustainable, low impact stormwater management have largely remained outside the mainstream of research and application interest, with few significant examples.
