Sustainable Rainwater Management Concept in a Housing Estate with a Financial Feasibility Assessment and Motivational Rainwater Fee System Efficiency Analysis

Abstract

Unresourceful usage of rainwater continues to be a serious problem as this type of natural resource is still treated as wastewater that most often ends up directly in the sewer system. The aim of the article is to present a concept of sustainable rainwater management within a completed housing estate in Cracow (Poland). Sustainable solutions are proposed, such as rain gardens, soakaways, permeable surfaces, and infiltration boxes, limiting the surface runoff, feasible for implementation in an existing development. Their efficacy was shown by estimating the runoff reduction and two financial assessments: (1) the required investment costs converted to costs per 1 m2 of residential area and as per 1 household, and (2) the assessment of financial profitability by the rate of return and the payback period, which are the basic tool for making decisions by potential investors: developers and residents. On the cost side, investment costs are included, and on the benefit side, savings resulting from the reduction of national and municipal rainwater fees. The analysis also aims to examine the incentive nature of these fees. National rainwater drainage fees, which have been introduced to encourage rainwater retention, have low non-incentive rates and the potential benefits of reducing them maybe around 1% of investment capital, which gives a payback period of almost 100 years. Higher rates of municipal fees are more motivating for investors; the proposed concept of rainwater management can potentially bring savings of around 9% (rate of return), and investment can be recovered within 12 years.

Assessment of the Impact of Loss-of-Retention Fees on Green Infrastructure Investments

“…The rational method of runoff calculation is based on rainfall intensity in the form of [60,[97][98][99]:…”

“…Stormwater fees are common in the U.S. and Canada, as shown by the latest survey from 2021 [55]. Research on stormwater fees has been the subject of many analyses of their impact on the development of green infrastructure, which proved their effectiveness; for example: (1) the assessment of stormwater fees in Germany (also noted as imperviousness fees) [56][57][58] showed a 10% decrease in the impervious area in Munich [56]; (2) Qiao et al, in two research papers [17,45], analyzed the governance factors of sustainable stormwater management and recommended the use of stormwater fees as an important incentive; and (3) analyses of stormwater fees in Poland were conducted in terms of their impact on the economic efficiency of green infrastructure [59][60][61], and they were also mentioned as important factors in the development of sustainable stormwater management [62,63]. Thurston et al analyzed auctions and fees as incentive methods, and concluded that relatively minimal financial incentives could encourage homeowners to retain stormwater runoff on their property [47].…”

“…The motivational role of financial instruments (tax reduction) depends on the rates and the system of rebates. The problem of low rates and, therefore, low benefits for investors and insufficient motivation to invest was noticed in the studies [49,58,60], indicating that this may require the inclusion of other additional incentives, such as grants and education and advisory programs [67][68][69].…”

“…The second fee discount level (reduction of 75% relative to the fee without retention equipment) can be achieved if the volume of the retention equipment is 10-30% of the annual runoff from impervious surfaces; in turn, this requirement seems very exaggerated, being approximately three times higher than the volume of equipment required when designing retention devices, e.g., a stormwater sewage system. For example, when designing retention devices within an urbanized area in accordance with standard PN-EN:752 2017 [118], the designed first rate of rainfall is the maximum rainfall (Pmax) with an occurrence probability of p = 20%, a return period of c = five years, and a duration of 15 min, which, according to Bogdanowicz-Stachy's method, is 211 dm 3 /s/ha [60,119]. The volume of this rainfall accounts for 3% of the annual runoff from the impervious area.…”

Green Infrastructures for Urban Water System: Balance between Cities and Nature

“…Godyń et al [10] discuss different rainwater management strategies for the retrofitting of housing estates. In Poland, investments in Green Infrastructure are still not common practice.…”