Riverscape Nature-Based Solutions and River Restoration: Common Points and Differences

Abstract

River restoration and nature-based solutions pertaining to the riverscape are measures frequently confused, but indeed they are not identical; they present both differences and common points, and only in some cases and following precise criteria, interventions can be considered both restoration and Nature-based Solution (NbS) projects. In other words, there is an intersection between the two concepts, both in a theoretical framework and in practical applications. The understanding of their distinctions and common points is important because it affects the objectives and implementation of measures, complying with a wide spectrum of relative importance of ecological goals and ecosystem services delivery, different critical issues for effective implementation, and different spatial scales. We provide a theoretical analysis of some simple criteria to identify interventions as riverscape NbS, river restoration measures, or both. We illustrate these ideas by means of three case studies of projects carried out in different European riverine environments: the real-world cases exemplify, respectively, pure river restoration projects, mere riverscape NbS, and finally, interventions representing both NbS and ecosystem restoration. These examples allow us to clearly show measures with a small number of goals, even a single one, and, on the other hand, multipurpose measures. We also illustrate the prioritization of objectives and their implications in planning and design, implementation phases, and stakeholders’ involvement. Particular attention is devoted to effective monitoring and assessment, considering that the quantitative evaluation of measures’ impacts is a difficult and resource-demanding task.

1. Introduction

Ecosystem restoration is defined as “the process of halting and reversing degradation of ecosystems worldwide, resulting in improved ecosystem services and recovered biodiversity” [1]. This definition was endorsed in 2021 within the United Nations (UN) General Assembly, declaring 2021–2030 the UN Decade on Ecosystem Restoration. It encompasses a large set of multifaceted actions addressed to impacted ecosystems with the twofold objective of preventing further ecological degradation and enhancing ecological recovery [2]. Ecosystem restoration aims at the regeneration of natural processes through engineered interventions whose main goal is the removal of anthropogenic stressors; indeed, this means implementing an “assisted regeneration” that is mainly driven by ecological goals [3].

Alongside ecological restoration, we find the concept and application of Nature-based Solutions, NbS, a different matter with respect to ecosystem restoration but with some common points, as we will illustrate in this perspective paper. Starting from the definition provided by the International Union for Conservation of Nature, IUCN [4], NbS are “actions to protect, sustainably manage, and restore natural and modified ecosystems that address societal challenges effectively and adaptively, simultaneously benefiting people and nature”. Thus, in NbS, with respect to restoration, Ecosystem Services (ES) are more central with closer attention paid to people and society’s needs [5], as it can be pointed out also in the formulation of the seven societal challenges that NbS can address according to IUCN, further analysed in the IUCN Global Standard for Nature-based Solutions [6,7]. It is therefore necessary to further clarify similarities and differences between NbS and ecosystem restoration, namely finding the relation between the term NbS, which recently gained more and more momentum, and pre-existing terms in the field of ecological engineering [8,9,10,11]. Actually, Nature-based Solutions and ecosystem restoration are frequently treated as the same, but they are not identical. Distinguishing between these two framings is crucial, as it shapes our goals for revitalizing natural systems, determines the stakeholders and resources involved, and influences every stage of planning, implementation, and evaluation of interventions.

Riverscapes can be profitably used as model systems to illustrate this analytical clarification, given that (i) streams are environments with a great potential for biodiversity improvement, representing ecological corridors and having a good structural heterogeneity of habitats [12,13,14]; (ii) riverine ecosystems, both aquatic and riparian, are among the ecosystems that have been most affected by stress conditions induced by climate change and anthropogenic stressors, being also among the most vulnerable ecosystems to biodiversity loss [15,16]; (iii) streams greatly provide ecosystem services, primarily regulating and provisioning ES, but also supporting and cultural ES [16].

River restoration includes diversified actions for the modification of river channels, banks and contiguous floodplain, as well as actions to control water, sediments and pollutant inputs to rivers. This whole set of actions aims at enhancing hydrologic, geomorphic and ecological processes of the altered riverine ecosystems, thus contrasting ecological damages and degradation [17]. River restoration went through a broadening of focuses and orientation, from channel morphology restoration aimed at regenerating river form (for example, with channel section reconfiguration, bank stabilization, river widening), to processed-based restoration oriented to the recovery of river function (mainly restoring connectivity in the longitudinal, lateral and vertical direction, and restoring water and sediment fluxes) [18,19,20,21]. Process-based restoration in terms of biotic response (for instance, in-stream habitat improvement and species management) is the most recent milestone [17,22,23]. Finally, vegetation plays a fundamental role in process-based restoration given its function in complex flow–vegetation–sediment feedbacks and habitat availability. This applies both for living plants [24] and for dead ones, in particular large woods [25,26,27]. Nature-based Solutions pertaining to the riverscape are those NbS that have a functional connection with the river. They are a subset of the NbS for water management such as the NbS related to fluvial flood mitigation and labeled Natural Water Retention Measures (NWRM) among which there are flood mitigation basins and floodplains, meanders, and abandoned channel restorations for flood management [28,29,30]; NbS for natural bank stabilization, namely the structural measures of riverbank soil bioengineering [10,11,31,32]; riparian vegetation strips for sediment and pollutant input control [33,34]; NbS for water quality control, among which are ecological management of Combined Sewer Overflow (CSO) outflows [35] and Constructed Wetlands (CWs) for CSO treatment [36].

In this perspective paper, we clarify common points and differences between Nature-based Solutions pertaining to the riverine environment on one side, and river restoration on the other side, with particular attention devoted to different objectives. Riverscape NbS and river restoration have different criticalities and different benchmarks to assess their effectiveness. The paper is structured as follows: in Section 2 we first illustrate the differences between riverscape NbS and river restoration following several categorizations and criteria; then we analyse common points and we argue in which case projects and interventions can be considered both NbS and restoration measures (Section 3); in Section 4 we illustrate these ideas by means of three case studies; in the discussion we argue about criticalities and scale problems (Section 5.1) and effectiveness assessment (Section 5.2); finally, we draw conclusions in Section 6.

2. Differences Between Riverscape NbS and River Restoration

2.1. Objectives and Approaches: Anthropocentric vs. Ecocentric

The main distinction between river restoration and riverscape NbS is related to objectives. The first has the goal of ecosystem function restoration, thus reducing anthropogenic stressors and enhancing habitat complexity and biodiversity, in other words, the ecological goal is prioritized [9]; on the other hand, NbS aim at supporting sustainable development by addressing major societal challenges [8,9,37]; therefore, the goal is to provide ecosystem services to people. According to IUCN [6], the seven societal challenges addressed by NbS are climate change mitigation and adaptation, disaster risk reduction, economic and social development, human health, food security, water security, reversing environmental degradation and biodiversity loss; all of them are definitely ecosystem services, thereby anthropocentric scopes prevail and eco-health fades in the background. Dealing with NbS, even when particular care is devoted to actions to contrast environmental degradation and biodiversity loss, such interventions are often conceived to foster ES, which serve society following an anthropocentric perspective. For instance, healthier freshwater ecosystems represent improved resources for primary production [16]. Another example in this sense is considering enhanced biodiversity in order to get ecosystems more resilient to climate change and to perturbations in general [37]. On the other hand, the biodiversity gain achieved with river restoration is valorized for its intrinsic value besides ecosystem services [21,38]. Finally, with respect to restoration, NbS are generally conceived to provide multiple benefits (including biodiversity gain) [8].

A direct consequence of being a multipurpose solution rather than an intervention limited mainly to ecological goals is related to the planning, the design, the expertise involved in the measures’ implementation and monitoring. Usually, river restoration is conceived and planned by stakeholders of the public sector, such as local authorities and river managers; the remaining stakeholders are generally not numerous and mainly consist of environmental organizations. Scientists and practitioners designing the interventions have expertise almost exclusively in ecosystem function, especially with regard to hydromorphology and physical habitat restoration [9,17]. On the other hand, riverscape NbS are multipurpose measures that bring together several stakeholders beyond conservation and environmental interests. The expertise used includes, along with hydromorphology and ecology, knowledge and technical skills in urban planning and landscape architecture as well as in economic and social sciences [9]. Finally, the nonscientific community is generally more effectively involved through citizen science initiatives and community organizations’ engagement in activities for awareness enhancement as well as monitoring [21].

2.2. Typology: Actions to Restore vs. Designed Items

An important difference between restoration and Nature-based Solutions consists in the different types of ecosystems they are conceived for or represent. Restoration applies to natural, existing ecosystems that are altered and ecologically degraded; indeed, restored ecosystems are human-designed and managed, but they are not entirely new to a site, as their restored conditions are ideally similar to the original ecological state. Nature-based Solutions include a wider variety of envisaged ecosystems and consistent measures, following the classification by Eggermont et al. [39]: (i) NbS apply to natural and minimally altered ecosystems for which NbS consist in actions to protect the ecosystems and to improve the delivery of ES (type 1); (ii) NbS also apply to altered ecosystems, as in restoration interventions, and in this case NbS envisage “actions to restore”, namely the definition and implementation of management approaches resulting in engineered ecosystems (type 2); (iii) in ecological deadzones, where traditional restoration outcomes are not likely, as in severely altered ecosystems, NbS consist of artificial ecosystems and designed items with high level of engineering management (type 3). To this last class of NbS belong “synthetic ecosystems”, which are new ecosystems created from scratch, thus human-designed, but also human-assembled and -controlled, and characterized by a novel combination (with respect to natural ones) of species and abiotic features, including technological components [37,40]. For instance, natural, even degraded, ecosystems are not present in indoor spaces or in urban areas with a totally built environment. Still, it is possible to create items that are unprecedented ecosystems, totally new to the site [40]. Typical examples of such synthetic ecosystems are bio-solar green roofs and an engineered combination of bacterial and plant communities in constructed wetlands for gray-water treatment.

Continuing with the analysis based on the classification by Eggermont et al. [39], we can notice that while restoration deals with actions, practices and management “to restore”, NbS include both actions (types 1 and 2 according to [39]) and new artificial items and clearly identifiable objects and works (type 3). For instance, in the riverine environment, examples of NbS of the type “actions” are floodplain connectivity restoration, riparian vegetation restoration, and soil bioengineering for riverbank stabilization, while examples of NbS of the type “objects and works” are flood mitigation basins, constructed wetlands, live fascine, brush/branch-layering, brush-mattress (within the category soil bioengineering for riverbank stabilization). It is the opinion of the authors that the distinction between action and object is relevant because objects are concretely identifiable, and this makes them more easily understandable and thus more diffused both in the scientific and technical literature and in the applications. We think that for this reason among the NbS for water management, those pertaining the riverscape, the majority of which are actions, are far less present in the NbS literature with respect to NbS for the regulations of the urban water cycle and related to pluvial flood mitigation and water quality control [41,42]: this last class of NbS is represented by Sustainable Drainage Systems (SuDSs), among which we find almost exclusively items and works rather than actions, for instance rain gardens, bioswales, and vegetated permeable surfaces.

2.3. Presence of Living Organisms

A relevant factor distinguishing riverscape NbS from river restoration is the presence of living organisms, mainly vegetation, presence to be intended both starting from the implementation of the measure following an approach “building with nature vegetation-based”, and when, after the completion of the work, the emergence of new species is triggered by the improved conditions of physical, abiotic features of the ecosystem. In other words, the presence of living plants can take place in two different phases of the intervention:(i) either from the creation of the intervention, with plants as an integral part of the building phase, or (ii) when the building phase is over and plants colonize the site spontaneously. In both cases, further ecological successions, including animal species emergence, are possible and often envisaged, and this is indeed a relevant factor in the self-regulation of restored and synthetic ecosystems.

In the majority of studies on NbS, authors affirm that vegetation is an integral part of NbS from the beginning of the building phase: starting from design and implementation [37,43,44]. Plants indeed are the primary producers in natural ecosystems and the dominant living organisms on earth, and similarly, they underpin NbS, contributing to the regulation of the mutual relationships between abiotic and other biotic components of ecosystems [37]. Buckley et al. [37] illustrate how “learning from the implementation of NbS can enable the development of plant ecology theory, while plant ecology can inform the design and management of successful NbS”. Focusing on NbS for the improvement of water quality, Greksa et al. [43] illustrate that vegetation is a fundamental component given its role in processes like denitrification and phytoremediation, considering that vegetation is also the gatherer of bacterial and fungal communities with decontamination abilities [45]. Krauze and Wagner [44] analyse the environmental factors governing the implementation of NbS with a particular focus on water as the main limiting and driving factor to NbS, thereby arguing that plants are an integral part of NbS. According to the authors, NbS are actions within the green infrastructures, while according to Fink [46], among others, NbS are even a synonym of green infrastructures; both schools of thought, therefore, claim that vegetation is a necessary element for the definition of NbS.

On the other hand, in the design and implementation of river restoration projects, the presence of vegetation is not mandatory for the definition of such measures. Even if a few relevant exceptions are present, for instance, riparian vegetation restoration and riverbank stabilization through bioengineering, the majority of river restoration interventions addresses hydromorphological restoration such as measures for remeandering [47], meanders [48] and side channels reconnection [49], river daylighting [50,51]; and many interventions addressing sediment flux restoration, for instance coarse sediment augmentation [52,53] and management practices to avoid fine sediment colmation [54]. In these cases, vegetation and living organisms are not integrated in the projects, but when the building phase is over, ecological monitoring usually investigates the biotic response to restoration, with particular emphasis on streambed macroinvertebrates, one of the communities more vulnerable to altered ecosystem conditions [55,56].

3. Common Points Between Riverscape NbS and River Restoration

In light of the analysis illustrated in Section 2, the common element between riverscape Nature-based Solutions and river restoration is ecology, namely the objective and resulting actions to restore natural processes. It should be recalled that restoring natural processes is the core benefit in river restoration, while it is a co-benefit in NbS for which the recovery of healthy ecosystems is mainly sought to benefit society (see Section 2.1). Moreover, it is worth noting that societal benefits arising from recovered ecosystems must not be assumed by default; rather, effective and equitable services for people need specific design and cannot be assumed to emerge as a secondary effect from mere ecological interventions. In many cases, however, a proper design and implementation of ecological restoration together with a specific design for the delivery of other ecosystem services is such that it is not obvious whether natural process recovery is a core benefit or a co-benefit. This leads to the question of whether there are projects and interventions that can be considered both Nature-based Solutions and restoration measures in the riverscape. The answer to the question is affirmative; those riverscape NbS for which ecological objectives and implementations are not marginal (namely ambitious, specific-designed goals and large scale) with respect to the fulfillment of other societal benefits belong to this category, as well as river restoration measures that are multipurpose and address not only ecological and environmental issues, but also social and economic ones [16]. Indeed, in the systematic review by Basak et al. [16], the authors argue that river restoration interventions with a comprehensive and consistent design of broad ecosystem services are the minority of river restoration projects; however, relevant examples can still be found in literature [34,57,58,59], and these can be thereby safely defined also as riverscape NbS. Finally, according to the argument set out in Section 2.3, interventions representing both NbS and restoration measures must include vegetation as an integral part of project design and implementation, rather than the effect of ecological successions taking place when the project building phase is over.

4. Exemplification Through Case Studies

In this section, we illustrate the ideas covered in Section 2 and Section 3 by analyzing three case studies of implemented projects in riverine environments across Europe. The three case studies have been selected in order to provide three different examples: the first representing an intervention identifiable strictly as a river restoration project (Section 4.1); the second case study represents a project that we classified as a Nature-based Solution solely (Section 4.2); the third and last case study (Section 4.3) consists in a large-scale intervention that can be labeled as an integrated set of either riverscape NbS or river restoration measures. The three real-world cases are synthetically reported in

Table 1. The selected three case studies. Second column: case documentation; third column: kind of intervention; fourth–eighth columns: criteria motivating the kind of intervention.

Case Study	Documents	Type	Relevant and Quantitatively Assessed Ecosystem Restoration	Relevant and Quantitatively Assessed Social and Economic Outcomes	Presence of Vegetation Starting from Design and Building Phase	Extensive Involvement of Multiple Stakeholders Including Inhabitants
1—Thur River Restoration	[60,61,62]	River restoration only	✔	✗	✗	✗
2—Lyon Confluence Regeneration	[63,64,65,66]	Riverscape NbS only	✗	✔	✔	✔
3—Emscher Catchment Restoration	[34,57,67]	Both river restoration and riverscape NbS	✔	✔	✔	✔

where the source documents are reported (at least three for each case, including relevant websites) as well as the type of intervention; this category is based on the criteria illustrated in Section 2 and Section 3, which are also reported in Table 1 (columns 4th–8th).

In the following, the case studies are presented in a complete but synthetic way. The objective is not to analyze and evaluate the case studies in depth, but rather we aim at classifying the interventions as river restoration or riverscape NbS or both. We also aim at motivating this classification, and we pay particular attention to the quantitative measurement of outcomes.

4.1. River Restoration Only: The Thur River Example

We illustrate an example of “pure” river restoration by means of the case study of the channel widening of the Thur River in Switzerland evaluated by Schirmer et al. [61] and Martín et al. [62]. River widening and levee setback/removal are indeed well-accepted and widespread actions also supported by experimental results attesting that increases in channel width have beneficial effects on bed complexity and sediment sorting [68,69], thereby improving river morphological quality and the distribution and heterogeneity of physical habitats. Indeed, stable and confined river reaches have poor lateral mobility resulting in low habitat turnover [70], whereas reaches with a wider channel bed, which promote the formation of sediment deposits, or reaches with larger sediment inputs promoting sediment heterogeneity, are characterized by a more dynamic and rich habitat mosaic [61,62].

The selected case study investigates restoration effects on the Thur River, a peri-alpine, gravel-bed river in Switzerland and one of the larger Swiss rivers without large retention structures (i.e., reservoirs) and thus characterized by a natural flow regime [61,62]. However, alongside preserved flow regime dynamics, the Thur River largely lacks lateral mobility due to the 1890s’ channelization. The restoration measures implemented consist, therefore, in dechannelization, and in particular, the intervention addressed in Schirmer et al. [61] and Martín et al. [62] is a dechannelized, 1 km-long reach located 12 km upstream of the confluence to the Rhine River. The study area also included a 1 km-long unrestored reach (width 50 m) downstream of the restored one (width 160 m) in order to benchmark restoration effects.

The studies reveal that 12 years after embankment removal, new gravel bars formed, triggering complex spatial patterns of bank infiltration, affecting biogeochemical processes, and affecting habitat types and resulting biotic communities. Schirmer et al. [61] substantiate that restoration led to increased river-groundwater exchanges, a significant increase in soil functional diversity, and hyporheic zone activation with an overall degradation of organic pollutants and denitrification. The authors also document an increased taxonomic and functional diversity. Martín et al. [62] particularly pay attention to biotic aspects by measuring hyporheic sediment respiration, macroinvertebrate density and taxa richness, and they attest that ecosystem structure and function present a complex response to flow-restoration feedbacks, this being almost absent in the confined channel given the more stable conditions of water and sediment fluxes. The authors ultimately conclude that the restoration measure analyzed improved habitat heterogeneity and physical conditions of hyporheic sediments, resulting in enhanced biodiversity.

The Thur River case study here illustrated is definitely an example of “pure” river restoration since the intervention addresses several environmental and ecological goals but has no social and economic objectives and resulting actions. Moreover, vegetation was not an integral part of the project intervention, but rather it colonized the restored the wider morphology of the river reach. Finally, the Thur River case study does not present a sufficient co-design and active involvement of multiple stakeholders to be considered a Nature-based Solution (for instance, there is a questionnaire study long after the project implementation asking nearby villagers whether the restoration was perceived as reasonable and accepted [60], but evidence of previous local community involvement was not found).

4.2. Riverscape Nature-Based Solution Only: The Lyon Confluence Example

We illustrate an example of riverscape and cityscape Nature-based Solution through the case study of large rehabilitation measures in the district of the city of Lyon (France), located in the Perrache Peninsula and named Lyon Confluence [63,64,65]. The Perrache Peninsula is located between the Saone and Rhone Rivers. The Confluence district in the south of the peninsula is separated from the rest of the city by the A7/E15 highway skirting the right bank of the Rhone River and by the railway and Perrache train station cutting transversely through the peninsula. Moreover, in the second half of the 20th century, the Rambaud fluvial port was dismissed, and the industrial activities moved to the outskirts of Lyon, transforming the Confluence district into a degraded, post-industrial area completely disconnected from the city center [64]. The site was thus selected, starting from the adoption in 1991 of the “Blue Plan” by the Lyon City Council, for large rehabilitation interventions of urban and green spaces [63], with sustainable redevelopment of marginalized, post-industrial areas along urban river reaches being a typical set of NbS [64]. The adaptation project of the Confluence district began in 2003 and will end in 2030; it covers in total 150 hectares and is divided into two stages, with the progressive planning being an important and successful aspect of the planning process [64]. The reclamation of the abandoned industrial area consists of the creation of housing, commerce, cultural and recreational facilities, and the improvement of the public transport infrastructures; of utmost importance is the complete and radical transformation of the riverfront. The planning and implementation of the whole intervention is carried out through a close and extensive cooperation between the public and private sectors and with a systematic, step-by-step, and extensive involvement of the local community.

The project implementation achieves a diverse, inclusive and sustainable district by creating a built environment with high functional diversity: 5000 new houses by 2030 including social housing and providing energy-performing buildings; a rich and diverse economic system with a total of 1650 companies; a very large set of educational, cultural and recreational institutions and facilities [65]. The transformation of the waterfront is a major successful outcome of the intervention, making the banks accessible to residents and restoring natural features to the waterfront. In particular, the Saone embankments have been converted in the Quai Rambaud, a five-kilometer continuous promenade re-connected to the rest of the city and providing an integrated soft mobility infrastructure [63,64].

From an ecological point of view, large green spaces have been created for a total of 35 hectares and 4500 new trees; the blue-green spaces are distributed between the major Champ park, three aquatic gardens and several neighborhood gardens; tree species were selected in order to increase biodiversity, and in 2011 an inventory of animal species in blue-green spaces was carried, out resulting in the count of 32 bird species [65]. However, no analytical assessment of ecological enhancement is provided; for instance, a regular and quantitative monitoring is lacking, as well as a quantitative comparison pre-post intervention or restored-unrestored areas. Indeed, the available scientific literature assesses the value of green-blue spaces of the district just in terms of increased value of real estates [66]. Moreover, the actions undertaken to reduce anthropogenic stressors responsible for ecological degradation are neither sufficiently nor scientifically analysed (for instance, less than one-third of the district has a separated waste- and grey-water sewage system, and no documented enhancement of the receiving bodies’ water quality is provided).

Overall, this case study is highly multipurpose, addressing several societal needs; has a continuous and extensive involvement of multiple stakeholders with particular attention to inhabitants; is characterized by an extensive use of vegetation; and is characterized by overlooked environmental and ecological actions. Thereby, this case study can be identified as an NbS but not as an ecological restoration.

4.3. Both River Restoration and Riverscape Nature-Based Solution: The Emscher Catchment Example

In the current subsection, we analyze a case study that belongs to both categories of river restoration and riverscape NbS; we selected the example of the Emscher catchment restoration, a basin in the “Ruhr Metropolitan Area” in North Rhine-Westphalia (Germany) [34,57]. The Emscher River is a tributary of the Rhine River, and it has a catchment area of 865 km² characterized by urban and peri-urban areas with a dense population (average 2775 inhabitants per square kilometer) and a post-mining and post-industrial landscape [34]. In 1990, the catchment was featured by abandoned mines and factories, as well as large impermeable, artificial land cover, and the Emscher and its tributaries were not only channelized and paved but also directly conveyed wastewater. In 1990, a catchment-scale 30-year restoration project was undertaken in order to reconvert the Emscher and its tributaries to natural streams, create a separate sewage system, reconnect and reconvert the floodplain and enhance consequent ecosystem services [34,57].

The restoration project began with the construction of an underground sewer network of 423 km in length in order to separate waste and river water; fitodepuration basins were constructed as well. Then the streams’ concrete shell was removed and cross-sections widened, including the reclamation of terraces and floodplain. In addition to vertical and lateral connectivity restoration, stream renaturation also included riparian vegetation establishment. The resulting nitrogen, phosphorus and carbon retention due to extensively vegetated streams and a reconnected floodplain was quantified in tonnes per year, comparing pre- and ex-post intervention conditions [34]. The same quantitative analysis has been carried out for habitat quality and species in terms of the increased number of streams’ kilometers reaching the good ecological status (according to the Water Framework Directive [71,72]) and increased taxa richness [34,57]. The restoration project also includes the transformation of a post-industrial area into a permanent lake: the Lake Phoenix (Dortmund) stores 700,000 m³ in normal weather and can retain an additional 240,000 m³ for flood prevention. The economic benefit resulting from the flood risk reduction is evaluated in terms of avoided cost of flood damage, real estate’s increased value (because of the natural amenities provided by the lakeshore), and the opportunity of recreational and cultural ecosystem services [34].

Overall, all the interventions carried out in this case study, both measures of ecological restoration and measures of ecosystem services enhancement, are designed and assessed during and after the project implementation according to both biophysical and economic evaluation. Indeed, the Emscher catchment restoration is planned and evaluated following the DESSIN ESS Evaluation Framework [67], a procedure developed within the European research project DESSIN (Demonstrate Ecosystem Services Enabling Innovation in the Water Sector, 2014-2017) providing guidance for analyzing the difference in value of Ecosystem ServiceS (ESS) in a system before and after restoration and management measures. Moreover, the DESSIN ESS Evaluation Framework associates each case-relevant ES to local stakeholders [67], with particular attention to residents and the water board [34], these being informed, engaged and at times actively involved in the co-design from the start of the planning processes [57]. Finally, thanks to the approach based on ES, the objectives and actions also addressing social and economic needs, the extensive involvement of multiple stakeholders, and the restoration project of the Emscher catchment can also be labeled as a set of multipurpose riverscape Nature-based Solutions.

5. Discussion

5.1. Monitoring and Scale Problems

The research community has emphasized a persistent critical issue in NbS and river restoration intervention: limited project monitoring [17,73]. An effective monitoring implies applying the conditions’ assessment of the ecosystem before, immediately after and following recovery from project implementation in order to catch condition changes and their cause–effect relationships. In particular, in river restoration, there are cases where natural processes at least partially shape the environment. Here, conditions’ assessment can be very demanding and detailed [74]. Furthermore, specific criticalities in monitoring ecological restoration are (i) the choice of biological communities, being some species more sensitive than others to physical conditions of the environment [56], (ii) the very long times, decades, needed to obtain the establishment of mature complex communities [57] and (iii) the cost of aquatic organisms monitoring, with both traditional methods and environmental DNA (eDNA) and metabarcoding [75]. On the other hand, the major criticality for NbS implementation and monitoring is the need for a holistic approach that simultaneously addresses societal challenges and provides benefits for biodiversity and ecosystem health; this means a highly multipurpose set of solutions requiring a multifaceted monitoring. This large demand for collected data can represent a critical issue, and given that it is often necessary to provide interventions’ evaluation ex-post. In this case, proxies can be effectively used in absence of direct measurements: for instance, in streambeds and floodplains, land cover is a proxy for soil types, each of which is characterized by a specific nitrogen, phosphorous, and carbon retention rate [34]; an example of socio-economic proxy is the rental price of apartments as a proxy of the increased demand for residential properties near restored sites [34].

The spatial scale is a major difference between riverscape NbS and river restoration, with a direct consequence on intervention efficacy. While riverscape NbS can be effectively implemented also at small scales, successful river restoration needs large scales most of the time, particularly catchment scale measures. Indeed, there is a high proportion of restoration projects that do not achieve significant improvement in river functions and biological communities recolonization [17], and the prime cause is the lack of interventions on large-scale stressors, which typically are in altered basins (i.e., agricultural and urban basins) with the input of chemicals, pollutants and contaminated fine sediments into streams [56]. Actually, these catchment-scale stressors are responsible for poor water quality (high levels of total organic carbon, phosphorus, nitrogen; presence of hazardous substances; low level of dissolved oxygen) [56] and streambed colmation of fine sediments [54], both hindering ecological efficacy of restoration measures, especially for aquatic and benthic organisms, such as macroinvertebrate communities [56]. Indeed, for the example of the Emscher catchment restoration illustrated in Section 4.3, the ecological success in terms of vulnerable species recolonization and richness [57] followed a radical improvement of streams’ water quality [34]. On the other side, riverscape NbS can achieve environmental improvement and biodiversity net gain also at smaller spatial scales, but they do not achieve a proper ecological restoration above all of aquatic ecosystems (if so, the solutions belongs to both categories, as illustrated in Section 3 and Section 4.3). We illustrate this idea by discussing the case study of the Lyon Confluence transformation presented in Section 4.2: in this case, those measures such as Champ park, aquatic gardens, neighborhood gardens and SUDs improved the vegetation cover and vegetation biodiversity and represent ecological niches for some non-aquatic animal species [64,65], but the surrounding, larger abiotic conditions of rivers and floodplain remained unchanged; in other words, anthropogenic stressors determining ecological degradation persist. Continuing to illustrate the NbS question of scale by means of the Lyon Confluence example, we argue that other (different from ecological goals) societal challenges addressed, such as social inclusion, energy-performing buildings, urban fabric livability, social housing supply, and recreational and cultural opportunities, are achieved almost irrespective of scale, so much that all the challenges just listed were attained for subareas of the entire district following a multi-phase plan and implementation with consistent and measured outcomes for each step [65]. This applies, in any case, taking into account the landscape surrounding the intervention area in order to connect this latter to the environment in which is embedded [76] (see for instance, new infrastructures and public transport services connecting the Lyon Confluence district to the rest of the city [64,65]).

5.2. Effectiveness Assessment

To date, much research and project design is treated in accordance with the concept of ecosystem services and the related multiple aspects concerning its potential as a support for policy- and decision- making [67]; however, two major obstacles to an effective ecosystem services approach persist [67]:~(i) practical application of the ecosystem services approach continues to be hindered by a diffused highly theoretical treatise and (ii) many established assessment frameworks provide aggregate accounts of ES, instead of an integrated evaluation of multiple ES with feedbacks and cause–effect relationships. This criticality in assessment efficacy holds true for both river restoration measures and riverscape Nature-based Solutions. In the current subsection, we evaluate two different assessment frameworks, related, respectively, to two illustrative practical examples; the first assessment framework effectively provides for an integrated evaluation of multiple ES with feedback and cause–effect relationships, and the second one is defective in this respect.

The DESSIN ESS Evaluation Framework was developed within the European project DESSIN (Demonstrate Ecosystem Services Enabling Innovation in the Water Sector, 2014–2017), it allows its users to evaluate changes in Ecosystem ServiceS (ESS) related to the measures implemented in a given freshwater ecosystem. In other words, it basically compares the ecosystem state and services before and after measures’ implementation [67]. This means to simply and effectively analyze the causal relationships, and this is possible because the DESSIN framework is based on the Driver–Pressure–State–Impact–Response (DPSIR) adaptive management cycle, as illustrated in Figure 1.

The DPSIR structure is a guide to evaluating the impacts of each possible choice of action undertaken to address a specific environmental or societal challenge. In order to illustrate this analytical model, it is useful to start from the “Responses” element: this is the set of single or integrated actions in terms of technology, management, and policy that the decision-maker can adopt. Responses can directly act on (i) drivers, (ii) pressures and/or (iii) state, which are, respectively, (i) anthropogenic activities affecting the environment (e.g., artificial land cover and use in a urban catchment); (ii) direct environmental effects of drivers (e.g., pollutant inputs of CSOs in streams); (iii) the ecosystem condition (e.g., nutrient excess in streams’ water and resulting eutrophication). Direct actions on drivers indirectly affect pressures, in turn affecting the ecosystem state (similarly, direct actions on pressures indirectly affect the ecosystem state). Depending on its state, the ecosystem can provide specific ecosystem services that represent the ultimate effect (impact) of responses. With respect to the classic DPSIR model, the specific one here reported (Figure 1) integrates the ESS-cascade concept: impacts are distinguished between Intermediate and Final ES, indeed an important distinction allowing to capture different concepts of ES’s economic value (i.e., use and non-use values associated to Final ES and Intermediate ES, respectively) [67]. Overall, we find this structure highly analytical and at the same time essential for an effective assessment of ES changes due to measures’ implementation; this efficacy is proved in the evaluation of the Emscher catchment restoration [34] illustrated in Section 4.3.

The second assessment framework selected for the current discussion is the framework for the verification, design and scaling up of NbS provided by the IUCN [76], which is basically a guidance to evaluate if implemented measures fulfill the global standards for NbS [6]. This guidance is actually a theoretical, long publication providing 8 criteria and 28 sub-criteria that an NbS should meet; these requirements turn out lengthy, repetitive descriptions, and above all, the total number of criteria makes the framework dispersive with unclear feedback and causal relationships. Overall, this assessment framework turns out to be ineffective and redundant, and we even found a case study evaluated through this guidance by an IUCN team in which the criteria are mistaken for indicators [73]. Either way, the IUCN guidance [76] provides a self-assessment spreadsheet to enable standard users to identify the extent to which their intervention fulfills IUCN criteria and global standards; the procedure provided by this tool is illustrated in Figure 2, while for a deeper analysis the reader is directed to Berg et al. [77,78]. We find this tool useful, mostly for planning rather than for evaluation, since it sums up in a tabular form the large number of IUCN criteria and also provides a few targeted questions for each criterion. Our opinion on the IUCN assessment framework finds confirmation in the analysis by Berg et al. [77,78], which identifies the IUCN assessment framework as a process-oriented framework, namely a tool providing an overview of the extent to which a project incorporates processes that are of relevance to achieve NbS effectiveness. The authors argue that the IUCN framework is not able to evaluate the outcomes of an NbS (process-oriented vs results-oriented framework).

6. Conclusions

In this perspective paper, we address interventions and management measures in the riverine environment, and we distinguish such measures between river restoration and riverscape Nature-based Solutions. The distinction between the two concepts is a relevant clarification because (i) they are often confused and treated as the same even if they cannot be assumed to be identical, and (ii) distinguishing the two framings has a direct consequence on shaping objectives, each phase of project design and implementation and stakeholders involvement.

In our paper, we analytically explain differences and common points between riverscape Nature-based Solutions and river restoration measures, providing some clear criteria to classify projects into one of the two categories. This allows us to illustrate in which cases the set of riverscape NbS and the set of river restoration measures intersect, i.e., in which cases an intervention can be considered both an NbS and a river restoration. We substantiate these ideas by means of the critical analysis of three real-world cases providing examples of river restoration only, riverscape NbS only, and both river restoration and NbS.

Our analysis provides a clear and motivated classification of riverscape NbS and river restoration, but we definitely do not suggest that one measure is better than the other. Indeed, the choice of implementing an NbS or an ecological restoration is case and site-specific. Generally, in very densely populated areas where the demand for ecosystem services is higher and often the population experiences inequalities in access to nature and healthy environments, Nature-based Solutions are preferred in order to sustain social and ethical needs. In other settings, generally more natural, the priority can be the safeguarding of nature and the mitigation of anthropogenic stressors; therefore, ecosystem restoration is preferred over NbS.

From our analysis, two major, intertwined issues are manifest for both riverscape NbS and river restoration measures. (i) Practical application of the ecosystem services approach to date is still hampered by a diffused, highly theoretical mindset, and (ii) many established assessment frameworks provide aggregate accounts of ecosystem services, rather than an integrated evaluation of multiple ecosystem services with feedbacks and cause–effect relationships. As a consequence, data acquired during and after interventions are often insufficient or of low scientific value. Therefore, future improvements shall focus on enhancing monitoring through a quantitative assessment of intervention outcomes.