*4.1. General Considerations on Stormwater Control Practices*

Although the effectiveness of LIDs on storm flow control has been demonstrated in a number of cases, barriers still exist to their broader implementation in new urban developments, due in part to the additional upfront costs for their implementation and long-term maintenance. Costing tools have been developed to allow designers to assess life-cycle costing of different LID practices and evaluate their efficiency [51,52]. These provide a framework to facilitate for capital, operation and maintenance costs estimation, and assess present life-cycle value. Their use, however, is limited by the availability of actual system components costs for specific areas, which sometimes cannot be easily estimated due to lack of previous installations.

The effect of LID practices should not be underestimated even in areas traditionally subject to high volume storms, since these practices successfully trap and filter a considerable portion of runoff, alleviating pressure on existing conveyance systems and reducing runoff side-effects such as downstream erosion, pollutant loadings, and damage to stream and riparian area habitats. Even in high-density urbanization areas, such as the center of the city of Athens (Greece), simulated introduction of LID practices showed potential peak flow reduction in the range of 13.4%–28.2%, and total runoff volume reduction in the range of 24.5%–29% [53]. A U.S. EPA review of 17 LID application case studies in the country showed that capital cost savings in infrastructure development following LID methods application ranged from 15% to 80% [54]. A model-based study concerning the selection of cost-effective LID strategies in Graz (Austria) considering the entire water balance and life-cycle-cost (including land costs) issues showed that there is not one specific optimal LID strategy, but that application of LID treatment trains, consisting of multiple interventions, shows high potential for cost-effective runoff reduction and control [55]. Cost-benefit analysis of LID for stormwater management in an urban catchment in Norway showed that these methods reduce combined sewer overflow (CSO) and that basin-wide optimized solutions in terms of maximum effects and minimum cost can be identified through the use of hydrological modelling [56]. Although no published studies have so far quantified the generalized impact of basin-wide LID practices in urban settings on storm sewers sizing requirements, it can be assumed that their wide-scale adoption could provide long-term benefits in terms of infrastructure design and investment costs.

A key factor in selecting the appropriate LID practice for a specific site lies in the understanding of the site specifics. For example, vegetated filter strips or rain gardens may be an ideal solution for small developments as in case 2 presented herein, but not for sites with large drainage areas. Some other limitations on potential LID installations include requirement of local codes' approval, possible increased pavement failures at LID/curb interfaces, liability and safety concerns, and reduced performance over time.

As interest in rainwater harvesting increases even in humid regions with well-developed water supply infrastructures, it is important to understand the functions and quantify the impacts of these systems. The most popular rainwater harvesting option for homeowners, the so-called "rain barrel" (or small buried storage, less than 1 m<sup>3</sup> ), often provides inadequate storage even for small irrigation demands in dry periods, and overflows frequently in response to intense storm events. Rain barrels, while providing a valuable demonstration and awareness function, do little to limit runoff, except in particular cases. Studies indicate that only larger rainwater harvesting systems, such as that described in case 1, may have substantial impact on both runoff volume capture and replacement of typical household irrigation demands [32].

In urban settings, regulations in most countries do not allow the use of harvested rainwater for domestic applications other than toilet flushing at the moment, but water utilities are increasingly confronted with customers aiming to decrease their household water footprint by treating rainwater onsite for drinking uses. According to literature, rainwater quality may be better than some surface waters, especially when these are mixed with treatment plant effluents that could still contain pharmaceutical residues or microbial contamination [57]. However, depending on local conditions, rainwater might also be susceptible to microbiological contamination from local pests or wildlife (e.g., avian or rodent species, and even large animals, such as dogs, boars or deer), hence precautionary methods or treatments should be adopted in such cases. A study of a new development district in the Amsterdam area, with total impervious area of about 93,600 m<sup>2</sup> , estimated that 64,000 m<sup>3</sup> of water could be harvested adopting current practices, covering about 51% of the drinking water demand of future residents [58]. A combined supply scheme (rainwater harvesting plus central drinking water production) was proposed; however, in order to maintain sufficient supply capacity to deliver drinking water at any time (including dry periods), treatment process and network design would be identical to a traditional system. Site specific economic and energy simulation would be needed in these cases to ascertain any advantage of such solutions. Several studies concluded that cost-efficiency of rainwater harvesting strategies for drinking water provision is strictly linked to local water prices, and that such systems should be preferably installed at the neighborhood level in new construction areas to be cost-effective [59–61].

In addition to cost factors, green infrastructure projects should include early community involvement and communication, and clear evaluation based on project motivation and outcomes. Public perception may be one of the greatest hurdles to overcome, since studies suggest that industrial and commercial users often choose to use municipal water over harvested rainwater, despite its availability [62]. Case study 2, where residents were actually involved in the planning and in the management of the rain garden, is a good example of such practices.
