**1. Introduction**

Sustainable stormwater management has been and still is a long-time issue in urban drainage systems. In addition to potentially causing adverse impacts on wastewater treatment operations in traditional combined systems [1], flooding during storm events has always been a common problem in urban areas: in fact, almost every city, regardless of the type of sewer network, is potentially vulnerable to this phenomenon, whose frequency has exacerbated due to the increased intensity and recurrence of extreme hydro-meteorological events linked to long term climate variability. Increases in impervious urban surfaces, poor resiliency of urban drainage system design and increased frequency of downpours in urban areas can increase peak storm runoff with corresponding impact on human life and health, property and water security. The Intergovernmental Panel on Climate Change (IPCC) reported that the number of heavy precipitation events has significantly increased in inland areas worldwide [2]. Climate indicators for the last decades show generalized statistical increase of event-specific maximum precipitation in many cities [3,4]. In addition, extreme hydro-meteorological events impact on the physical, chemical and biological parameters of water in urban water bodies, both through direct runoff, and separate or combined sewer overflows (CSOs) [5]. Pollutants conveyed by storm flows in addition to organic matter include pathogens (Fecal Indicator Bacteria, FIB), nutrients, metals and emerging contaminants [6,7], the latter often at low level concentrations, which are difficult to monitor by traditional means [8].

Safe urban stormwater management is becoming a major concern: the conventional approach based on piped drainage is currently criticized as poorly efficient as, only partly effective during meteorological extremes, it does not eliminate environmental problems [9]. In the 1980s, structurally intensive approaches were proposed such as the "deep tunnel" concept: large, underground collectors designed to relieve urban sewer systems from excess stormwater flow and curtail overflow frequency. These systems were built in large cities in the U.S. (e.g., Milwaukee, Chicago, Boston, Atlanta) and around the world (e.g., Hong Kong, Guangzhou, Singapore) [10,11]. In addition to the high cost involved (the deep tunnel project in Milwaukee, one of the first of this kind, required 14 years, at a cost in excess of US\$2.3 billion, to complete [12]), the features of these systems may raise unexpected management challenges [13]. Furthermore, their operation requires high energy inputs for pumping and subsequent treatment of dilute sewage, increasing the already high greenhouse gases (GHG) emission footprint of water systems [14].

While these may have proven use in large urban areas, this approach may not be fully resolutive for the targeted impacts. Low-tech, more sustainable methods in accordance with the EU Water Framework Directive (WFD) [15] may be effective in many cases, especially in smaller urbanizations [16]. Sustainable storm water management should promote source control methods at, or nearby, the source. The most effective approach, which could successfully complement technologically-intensive approaches, consists of trapping stormwater and storing it into temporary impoundments for evaporation or ground infiltration. Rain and roof gardens, grassy swales or ditches (bioswales and bioretention basins) and permeable pavements could be highly beneficial [17]. Many mitigation measures are also being proposed to increase urban systems' resilience against floods [5,18]. These include discharge separation at source [19], local water reuse [20] and implementation of decentralized water management [21]. Modern stormwater management requires separation of rainwater from sewage, providing a higher level of service and benefits such as: elimination of CSOs, pollution prevention and possible use of stormwater as an alternative resource.

Sustainable storm water management is connected with so-called blue-green infrastructure (BGI). BGI foresees the implementation of either natural or man-provided solutions to enhance management of water resources and water infrastructure and services risk resilience. Innovative fiscal and non-fiscal tools, which may include payment for ecosystem services schemes [22], may be introduced to encourage their implementation on public and private property [23]. These practices are described in the literature under various labels, such as Low Impact Development (LID) [24], nature-based solutions (NBS) [25] or Sustainable Urban Drainage Systems (SUDS) [26]. All reduce the impact of constructed impervious surface areas (ISA) and of their hydrological and ecological disturbances. A generally accepted scale to assess ISA impact on urban watersheds indicates stress conditions, with ISA between 1% and 10%, impacted if between 10–25% and degraded if greater than 25% [27]. The global ISA average is estimated at 93 m<sup>2</sup> per person. As a comparison, ISA in Poland is about 110 m<sup>2</sup> /person, similar to other European countries (Germany, Italy, the United Kingdom, the Czech Republic, Hungary, Austria, Switzerland, Bulgaria, Slovakia, and Denmark) with ISAs around 100-150 m<sup>2</sup> /person. In France, Portugal, Belgium, Ireland, Sweden and Spain, ISA is between 150–220 m<sup>2</sup> /person, in the USA, close to 300 m<sup>2</sup> /person, and in China around 68 m<sup>2</sup> /person [28].

This article presents two case studies of sustainable stormwater management practices in small urban developments in Polish catchments, respectively concerning: (1) on-site retention and reuse, and (2) rainwater infiltration gardens. The aim of the analysis of the two approaches is to highlight

their application's sustainability, considering also related legal and economic aspects. With this objective, a simple cost-benefit analysis and a general discussion on urban stormwater management are also presented. *Sustainability* **2020**, *12*, x FOR PEER REVIEW 3 of 19
