Floods and landslides are very common in many areas of the Mediterranean basin, inducing a high geo-hydrological hazard (Canuti et al., 2001; Guzzetti and Tonelli, 2004; Luino, 2005; Luino and Turconi, 2017) and causing many casualties and significant damages every year. The 2017 periodic CNR-IRPI report (CNR, 2018; Brunetti et al., 2015) on Italian population landslides and flood threats evidences 1789 casualties and 317 526 homeless in the period of 1967–2016, with all the regions affected. Liguria, despite its small surface, is located in the most affected region, scoring the third place in the mortality index calculated on both landslide and flood events.

Among the geo-hydrologic processes, flash floods are the most hazardous for the short development time that often does not allow the population to protect itself. Flash floods occur following very intense and localized rainfall events and their ground effects have been underlined by many authors (Roth et al., 1996; Massacand et al., 1998; Delrieu et al., 2006; Amengual et al., 2007; Gaume et al., 2009; Marchi et al., 2009; Barthlott and Kirshbaum, 2013; Faccini et al., 2015a, c; Faccini et al., 2018). Spreading of shallow landslides and debris flows and mudflow often occurs and their effects are superimposed and may locally magnify flooding, in particular in urban–suburban areas (Borga et al., 2014). Small catchments have a quick response to those events, reacting with a large discharge of water and debris to the usually densely urbanized floodplain (Pasche et al., 2008; Gaume et al., 2009). Many coastal Mediterranean areas are particularly liable to this kind of hazard: the general climatic context, with the interface between cold air masses and the sea; the steep territory; and a complex geologic and geomorphologic context are the main natural factors. In such a hazardous context the high vulnerability that characterizes most of the urbanization determines the elevated risk, while the intense anthropogenic modification of a large portion of catchments and of hydrographical networks tends to amplify the effects (Tropeano and Turconi, 2003; Nirupama et al., 2007; Audisio and Turconi, 2011; Petrea et al., 2011; Llasat et al., 2014; Faccini et al., 2018; Acquaotta et al., 2018b): impervious surfaces, induced by soil consumption and urban sprawl, increase the surface runoff and decrease the time of concentration (Shuster et al., 2007), while strictly constrained and often culverted riverbeds have frequently inadequate discharge capacity (Moramarco et al., 2005; Faccini et al., 2015b, 2016).

Furthermore, the modifications often affect even the hinterland: in addition to urban sprawl and fragmentation caused by infrastructures, in some areas the ancient artificial terraces realized for agricultural practice and largely abandoned constitute an increasing factor of geomorphological hazard (Brancucci and Paliaga, 2006; Tarolli et al., 2014; Paliaga, 2016). In recent years much evidence has been found in Italy: large areas of Liguria (Brandolini et al., 2018b; Cevasco et al., 2017) and Toscana (Bazzoffi and Gardin, 2011) are affected by terrace instability that may turn into a source of geomorphologic hazard. In the Mediterranean region many areas present a similar occurrence of terraces with analogous problems: the French Côte d'Azur, the Mediterranean, and insular Spain and Greece (Tarolli et al., 2014) are some examples. In the recent years some disastrous events involved terraced slopes: in 2011, during the Cinque Terre flood (Liguria, northern Italy) (Brandolini et al., 2018a; Luino and Turconi, 2017), many terraces collapsed and the subsequent debris filled villages at a height of about 3 m, and in 2014, in the Leivi village during the Chiavari flood (Liguria), a terraced slope collapsed, destroying a house and causing two fatalities (Luino and Turconi, 2017).

Within this framework risk mitigation strategies are more and more urgent but largely disregarded, unapplied or only partially pursued: few resources are allocated and, commonly, are used only for emergency actions while long-term planning and scheduling should be crucial to obtain significant results (Prenger-Berninghoff et al., 2014). In recent years, in Italy, some large structural works have been started to mitigate the worst flooding risk situations, but without following a broad approach at the catchment scale. The most important is the floodway channel for the Bisagno stream in Genoa (Liguria), but similar projects or culvert adjustment are ongoing in smaller neighboring streams. This approach allows the reduction of just a part of the risk, ignoring slope instability processes and related contributions to solid transport into a hydrographical network.

Liguria, and especially the Genoa metropolitan area, are paradigmatic of the mixing of high hazard, with heavy rainfall that appears to be increasing in intensity (Faccini et al., 2015b, d; Acquaotta et al., 2018a), elevated exposure of areas at risk and lack of long-time planning mitigation strategies at the catchment scale.

Apart from the structural interventions in the larger Bisagno catchment, even the smaller ones in the Genoa metropolitan area are considered to be at high risk by the local environmental agency (ARPAL, Agenzia Regionale per la Protezione dell'Ambiente Ligure – Ligurian Environment Protection Agency) and would request mitigation works to be planned and scheduled.

The aim of the research is to propose a quantitative support tool to decision makers in order to plan and schedule long-term interventions, identifying a priority scale among small catchments: their number and the different features that characterize them request a comparison tool in order to evaluate the ones that are more critical. A group of 21 small catchments in the middle of the zone more liable to heavy rainfall (Cassola et al., 2016) have been analyzed, comparing three sets of descriptive parameters. The comparison has been performed with spatial multicriteria analysis (S-MCA) using a total of 19 parameters and obtaining a priority scale among the 21 catchments. MCA procedures have been applied to address flood risk management options and cost–benefit analysis of mitigation measures in the UK (Penning-Rowsell et al., 2003; RPA, 2004), in the Netherlands (Brouwer and van Ek, 2004), in Germany (Socher et al., 2006), in Portugal (Bana and Costa, 2004) and in Canada (Akter and Simonovic, 2005). The S-MCA approach has been applied by many authors in flood risk and in natural hazard management (Gamper et al., 2006; de Brito et al., 2006), mostly to assess flood-prone areas, flood risk (Meyer et al., 2009; Fernaìndez and Lutz, 2010; Wang et al., 2011) and landslide susceptibility (Feizizadeh and Blaschke, 2013; Nsengiyumva et al., 2018) or to compare catchments through morphometric parameters (Benzougagh et al., 2017). S-MCA techniques are widely applied as a decision support system in planning and environmental sustainability decision making to compare different design choices or site selection (Jacek, 2006; Bagli, 2011). In the present work the authors applied S-MCA techniques considering a broad set of parameters and trying to address the peculiarity of highly modified small urban catchments in a mountainous territory where comparing different sets of parameters describing different and inhomogeneous features appears crucial. The rank obtained with the methodology could be used to evaluate the catchments that need more urgent actions in order to mitigate future eventual damage and casualties, considering that past extreme rainfall events hit bordering ones but, in the future, could replicate their effects. Then the necessary long-time planning could focus economic resources mainly on the more critical catchments, while the analysis of the descriptive parameters would be a support for pointing out the specific criticalities and then to designing the interventions.

The geodatabase, collected through the calculation of the 19 parameters and shown in Tables 4 and 5, evidences a certain variability in values. In Table 6 the times of concentration values obtained with the different formulae are shown; for the S-MCA calculation the mean value has been chosen.

The results of the parameter computation give a descriptive scheme of the small catchments; some have similar characteristics, and some have specific peculiarities. All the catchments share high slope and hypsometric index values. Time of concentration is always short while landslide surface (%) shows a large variability in the value of the Melton ratio and drainage density.

Flood events affected 15 on 21 catchments and some of them have been repeatedly hit. Flood hazard zones are quite extended in some cases and always involve densely populated areas.

Regarding catchment anthropogenic modifications, soil consumption is variable but always concentrated in the lowest part where at present even important infrastructures run along the coastline; in some cases, the value is particularly high. The highest quota slopes are usually in seminatural conditions, and in some catchments artificial terraces are widespread and mostly abandoned. The final kilometer culverted percentage for the main stream often assumes high values, in some cases 100 %. This modification represents one of the most critical as transport capacity is always inadequate in the case of intense rain events, causing flooding in the surrounding urban area. In addition, buildings have been built close or, more frequently, over the cover.

The parameters describing the elements exposed to risk give an idea of the impact that a flood event may have on the urban area: both the percentage of the risk area, mainly residential, industrial, and hospital, and the number of punctual elements, including schools and cultural heritages, are variously present but reach the highest values in catchment no. 9.

The analysis of data in the geodatabase evidences how catchment no. 9, followed by no. 6, 8 and 17, often emerges for critical values. Particular attention must be paid even to no. 11, Polcevera's sub-catchment, and to 13 and 14, which are Bisagno's sub-catchments: in all these cases downward of the confluence with the main stream the urbanization degree is at the highest level with elevated population density and soil consumption. Recent flash flood events in 2011 and 2014 affected no. 13, 14 and 15, propagating the effects to the Bisagno catchment. Other peculiarities are present in no. 12: the largest of the small catchments, which constitutes the ancient Genoa amphitheater with the old harbor and the historical center. Finally, the western catchments show a lower soil consumption degree but larger widespread shallow areas of instability that during the recent intense rain events in 2011 and 2014 were activated.

Moving towards a quantitative approach through the application of the S-MCA techniques to compare the catchments' conditions, some more meaningful results may be obtained. The first application of the method has been performed without assuming different weights a priori for the describing parameters; even the same relative importance has been assumed for environmental factors (set 1 and 2) and for the elements of risk (set 3). The values obtained by the calculation have been ordered in five classes, with the number 1 being the most critical or the one that requests a higher level of attention for the risk reduction strategies. Results are shown in Fig. 8 while Table 7 provides the score values obtained using all the parameters (priority scale A), only the anthropogenic-origin ones (priority scale B) and only the natural-origin ones (priority scale C) for the environmental factors. A further calculation has been performed assuming proportional weights to the elements of risk factors, which gives major importance to the higher risk level with respect to the lower ones. The results are collected in Fig. 9 and in Table 7 and constitute the priority scale D.

The results of the application of the S-MCA technique to the 21 small catchments represent an attempt to create a decision support tool to plan and manage investments for works aimed at mitigating geo-hydrological risk in an area hit hard by floods, flash floods and landslides in the past, as addressed by many authors (De Brito et al., 2016). Ranking alternatives in flood and risk reduction strategies have been largely implemented and addressed by decision makers, using different S-MCA techniques (Andersson-Sköld et al., 2015; de Brito and Evers, 2016). The need for optimizing economic resources and reducing risk is essential in a critical situation with high inhabitant density, strong anthropogenic modifications and high hazard. In addition, flash flood events are strongly localized and in the recent years they prevalently hit some catchments (Table 4): no. 4, 10 and 16 present the highest numbers, even if the most critical events happened in no. 8, 9, 10 and 15. Considering that the whole studied area is characterized by high hazard for the possible hit of super-cell systems and presents high hazard even for the peculiar geomorphological features, the question is what would happen if a localized and intense event should hit every catchment. For this reason, and for the highly inadequate actual situation, it seems necessary to assess a priority scale considering both natural features of the catchments and the anthropogenic modifications that enhanced the risk level in order to obtain a priority scale on a quantitative base.

The priority scale A evidences the critical situation of catchment no. 9 that emerged even at a qualitative analysis level, with no. 3 and 8 in the second rank and no. 1 in the third and more difficult to recognize. These results suggest that, possibly, the highest attention in planning resources for risk reduction works at the catchment scale should be paid to these higher-level rank catchments. A detailed study for the punctual activities would be essential, considering the activities at the catchment scale.

Priority scales B and C have been obtained considering, respectively, only the anthropogenic parameters and only the natural ones in order to evidence the different eventual influence of the two sets. Considering scale C, the natural tendency of catchments to geo-hydrological risk emerges a bit differently and, examining scale A, a possible influence of anthropogenic modifications arises more clearly. Effectively, catchments no. 8 and 9 have been particularly affected by human activities: the soil consumption is high, as high as the percentage of the final kilometer of culverted riverbed. We can deduce that human interventions enhanced the most critical situations, while in other contexts the effect has been lower, even if always increasing.

The situation changes a little, assuming a different weight to the elements of risk parameters; that is, it considers proportional major importance of the highest exposition to risk: the priority scale always sees catchments no. 3, 8 and 9 at the highest ranks, giving a further confirmation of how critical their situation is. At the opposite side of the priority scale, catchments no. 12, 18, 19, 20 and 21 are always stable in the lowest rank, meaning a possible lower level of attention, with respect to the other ones. For example, the Fereggiano catchment's (no. 15) critical situation is well known even at an international level: the heavy rainfall in 2011 caused six casualties and much damage. Despite that it ranks at the fourth level on the priority scale, its risk level is not high. Rather, it has been hit by heavy rainfall that caused devastating consequences. If such an event would hit one of the other studied catchments, like no. 9 for example, the effect could be similar or even worse. At the same time the D scale shows that catchments in the lower rank are almost half in number with respect to the ones at the same position on scale A.

Considering the high risk level of the whole area, the rank in the scale must be considered as additional information: this does not mean that no reduction work should be performed in catchments at the lowest rank position, but only that the other ones should be considered more urgent.

Another consideration regards limitations in the approach related to peculiar situations that do not emerge from the comparison: in the Geirato catchment (no. 14) there is a large landslide dam that is a potential source of high hazard (Paliaga et al., 2019), not limited to the catchment itself but also possibly affecting the main Bisagno catchment. This limitation could be overcome by adding a parameter for peculiar situations, but it has not been considered in the present work.

The prevention activity should include interventions on both streams and slopes, structural and nonstructural: the inadequate transport capacity of culverted streams is always seen as the only problem to be solved but considering the high solid transport and debris–mud that often add their effect during the intense rain events and that act locally, interrupting roads or impacting buildings and causing problems in the urbanized lower parts of the catchments, solutions should be studied holistically. The debate about using structural or nonstructural interventions for risk reduction has been discussed by many authors (Kundzewicz, 2002; Yazdi and Neyshabouri, 2012; Meyer et al., 2012) but in conditions like the studied one only the mutual concurrence of them may insure an acceptable result. Strong and continuous monitoring (Collins, 2008) and maintenance of the slopes, due to their straight closeness and relation with the urban area is crucial: from structural intervention in landslide stabilization to soil bioengineering techniques to reduce erosion and shallow landslide susceptibility and the recovery of abandoned terraces (Morgan and Rickson, 2003). The basic philosophy should be to act preventively on instability with even small and noninvasive interventions being widespread in the territory (Lateltin et al., 2005). These activities should be focused to reduce the potential debris and sediments that contribute substantially to saturation of culverts during intense rain events. Considering that the critical situation deriving from soil consumption cannot be modified, as re-naturalization is not an option considered acceptable by both decision makers and probably large parts of the population, other interventions may be addressed to reduce the negative effects of the anthropogenic modifications. Only in very limited situations would the eventual culvert elimination be possible without knocking down buildings, which is an option with a low acceptance level. In other cases the possible solutions are structural hydraulic interventions that may guarantee the reduction of the extension of flood hazard zones and then even of the elements of risk areas. This includes enlargement of embankments, restructuring of culverts and realization of diversion overflow channels. In the cases in which these high-cost interventions are crucial, like for catchments no. 8, 9, 10, 11, 12, 15 and 21, the reduction of solid transport in the streams, which is mainly reduction of erosion, shallow landslides and stabilization of abandoned terraces, would contribute significantly to risk mitigation. Cost of structural hydraulic interventions is usually high and of the order of millions of euro, while spread of small interventions on the slopes is usually less than an order of magnitude lower, but the integration of the two is essential in many situations, for example, in catchments no. 9, 10, 14 and 15 where landslides, abandoned terraces and high gradient slopes are close and coupled with densely populated areas and intensely modified riverbeds with inadequate capacity culverts. Conversely, catchments no. 3, 4, 5 and 6 are mostly affected by slope instability processes and present a lower level of soil consumption and, more in general, of anthropogenic modifications.

Applicable mitigation measures present a good level of ecological compatibility, in particular the bioengineering ones along the slopes, for their low environmental impact, while structural hydraulic interventions would be carried out in urban areas producing only temporarily impacts on population due to the construction site setup. Regarding the potential acceptance of the population, the interventions along the slopes should not be problematic for their usually modest dimensions, while the structural hydraulic interventions would have higher impact, even if limited in time, and elevated cost could be a little more problematic. Actually, some important works are ongoing along the Bisagno stream, with traffic disturbance and influences on economic activities lasting for some years, but the population risk awareness has risen after the last devastating flash flood in 2011 and 2014.

The actual basin master plans adopted by the local authority – Regione Liguria – point out some structural and nonstructural interventions (Autorità di Bacino Regionale, 2017a, b, c) but no ranking has been carried out in order to compare the small catchments or to improve the overall functionality in a holistic way. The performed comparison would help in supporting the decision process, including interventions that actually are not considered erosion reduction works or slope instability preventive measures on terraces or on dormant landslides.

Finally, risk reduction works would have a direct influence in the priority scale method: in addition to the stabilization of landslides, the structural interventions on streams would have the effect of modifying and reducing the extension of flood hazard zones and then even of the areas exposed at risk. In this way the methodology could be used even to simulate the effects of some structurally important and expensive works on the overall rank on the priority scale. This information could be included in the cost–benefit analysis of the planned structural interventions.

Mitigation strategies for geo-hydrological risk request a catchment-scale approach that results particularly crucially in a composite context in which hazards related to natural features occur together with high anthropogenic modification of the territory and high vulnerability (Pasche et al., 2008). More in general, prevention of geo-hydrological risk requires a decision-making process that is complex, affected by uncertainty (Akter and Simonovic, 2005; Kenyon, 2007) and often with limited economic resources at disposition.

In addition, an area characterized by many small urban catchments is complex to manage and strong programming and planning is essential. The proposed method for prioritizing planning for risk mitigation works among catchments could be used as a support tool to quantitatively address economic resources that usually are limited and require a strong optimization (Gamper et al., 2006). The approach could be used even in different contexts at a sub-catchment scale to point out the more critical sub-catchment and basing the comparison on different sets of parameters depending on the active processes in the area. The procedure may be adapted and modified with weighting of selected parameters in order to give major importance to the ones considered more important. Another adjustment of the method is possible considering the relative importance to the environmental set of parameters with respect to the elements of risk: depending on the value that we would assign to the different aspects of the evaluation, different weight may be assumed.

The application of this methodology in a high-risk area allowed us to obtain a priority scale that is actually partially confirmed by the structural intervention that local authorities are operating: some are in the design phase and some are in construction. The critical situation of catchment no. 9 is actually being approached and the solution has been found in some important design for the adjustment of the culvert and of stream embankments; in addition an overflow channel is going to be realized in the Bisagno catchment, involving even the Fereggiano catchment (no. 15). These works are largely expensive but are now essential to reduce risk in a situation in which anthropogenic modification almost saturated all the available spaces in the floodplain, as has occurred in all the small urban catchments examined in the present study. Risk reduction would require a holistic approach at the catchment scale, considering all the processes acting on the catchment and their mutual relationships and trying to address all the problems, considering that what happens along the slopes influences even the lowest portion of the catchment itself (Samuels et al., 2006; Blöschl et al., 2013). Moreover, the cost of interventions along the slopes usually impacts the economy significantly less than structural works do.

The cost of interventions has not been considered in the present study as the aim of the work was to compare the small catchments and realize a priority scale of attention to address planning on a risk basis but could be included in the methodology and perhaps developed in a subsequent phase. Its role would be at the same level of environmental and elements of risk factors and a weight could be assigned to find a balance among the three. Such evaluation could be carried out after a preliminary assessment of the interventions in all the comparing catchments; the application of the method in such a case could more precisely address the investment of economic resources.