Alteration of Catchments and Rivers, and the Effect on Floods: An Overview of Processes and Restoration Actions

Abstract

Flooding is a prevalent and growing problem involving significant economic losses worldwide. Traditional flood mitigation measures are based on the use of levees, dams, dredging, and river channelization, which can distort the perception of risk, leading to a false sense of security that can induce an increase in the occupation of flood-prone areas. An undisturbed watershed and its fluvial system provide regulating services that contribute to flood mitigation. However, anthropogenic activities can degrade and diminish such services, impacting the magnitude of floods by changing the runoff patterns, erosion, sedimentation, channel conveyance capacity, and floodplain connectivity. Restoration and natural flood management (NFM) seek to recover and improve their watershed regulation services. The bibliographic review performed here aimed to assess the degradation of the natural regulation services of watersheds, which allowed us to identify significant alterations to runoff and streamflow. Also, the review studies of NMF allowed us to identify the restoration actions oriented to recover or enhance the flow regulation capacity of catchments and their fluvial systems. A current challenge is to accumulate more empirical evidence for the effectiveness of such flood mitigation solutions. Currently, the results for large catchments have been obtained mainly by the application of hydrologic and hydraulic models. Also, the adequacy of the different NFM actions to catchments with different physiographic and climatological settings needs to be addressed.

1. Introduction

Adverse impacts caused by flooding are expected to increase progressively with time due to the steady growth of population, economic activities, and climate change [1,2]. Preisser et al. [3] indicated that flooding is the natural hazard with the largest economic impact in the United States. Bates et al. [4] estimated that, in that country, by 2050, the areas susceptible to flooding by 100-year events will increase by approximately 3%, compared to conditions in 2020, when considering the RCP4.5 climate scenario. Tanoue et al. [1] found, with an analysis of flood vulnerability, that low-income countries are the most affected, probably due to their lower levels of protection compared to higher-income countries. It is estimated that, between 1990 and 2013, economic losses globally exceeded USD 525 billion at 2005 prices [2]. In 2012 alone, the global economic losses exceeded USD 19 billion, and continue to increase [5]. In addition to the direct losses caused by damage to physical assets, inundations that last for long periods generate economic losses inside and outside of the affected area due to the interdependence of economic activities.

The traditional approach to flood mitigation has been based on the construction of infrastructure, such as levees and dams, and the dredging, rectification, and channeling of watercourses. These types of actions have distorted the perception of risk and have given rise to a false sense of security that has triggered a greater occupation of flood-prone zones. Some of these actions have led to an increase in the energy and velocity of flow, thus amplifying the downstream erosive power and destructive capacity [6,7]. Policies followed during the 20th century to minimize flood losses did not always have the expected results. For instance, the United States experienced 2.5 times more economic losses in the second half of the last century compared to the first half despite investing billions in protection projects [8,9]. In addition, there may be implications for the ecology of the sites. For example, in the 1960s, streams were channelized in the Kissimmee River basin in Florida, thereby reducing the flood risk, but concerns were raised about declining wetland storage and ecological conditions, and a restoration process was initiated in the 1990s [10].

Although floods are generated by natural causes, their magnitude and characteristics can be altered as a consequence of human activities. River management approaches aiming to mitigate the risks of flooding, water scarcity, or river bank instability have not been successful when based on the physical principles of hydraulics and sediment transport alone [11]. To improve the performance of flood mitigation actions, the nature-based solutions (NBSs) approach can be used (i.e., [12,13]), which may be economically effective and provide social and economic benefits, thus helping to develop resilience [14]. Moreover, a fundamental idea behind restoring rivers to a more natural state is not only for environmental reasons but also to mitigate flood and geomorphological risks [15]. In this direction, it is essential to analyze the alterations in the watersheds related to degradation and land cover change, the morphological changes in the channels, the interaction of rivers with their floodplains, and the modification of hydrological processes; these factors, among others, contribute to increasing runoff with the same precipitation conditions [16,17]. In the last few years, works have been developed that emphasize the importance of restoring rivers in order to recover their environmental and hydrological balance, and thus mitigate the impacts of floods in downstream areas that are of greater intensity due to the alterations that they have undergone [18,19]. For example, in the United States, at least USD 1 billion is invested annually in stream restoration [20]. The benefits provided by “soft engineering” for flood control have been identified, where restoration actions are developed in watersheds and their fluvial systems [21]. The hypothesis is that the restoration of river systems contributes to the reduction in vulnerabilities related to floods. To verify this hypothesis, it is necessary to evaluate the capacity of these actions to reduce the impact of floods [22,23]. Restoration requires knowledge of the hydrological, hydraulic, and sedimentological processes of the catchment and fluvial systems that have been altered, as well as the area to be recovered or enhanced through restoration activities [24,25].

Figure 1a shows the number of research documents related to land cover and inundations. They were obtained with search Q1, “‘land cover’ AND (flood OR flooding* OR floods OR inundation*)”, in Web of Science. The number of documents indicate a significant increase in the last decade. Figure 1b shows a word cloud of the abstracts of all the documents found in search Q1; note that river and wetland are among the most frequent words found. Regarding the actions of restoration oriented to the control and mitigation of inundations, the number of documents found with search Q2, “(flood OR flooding* OR floods OR inundation*) AND (mitigation OR control) AND restoration”, demonstrates a sustained growth in research documents since the mid-1990s (Figure 1a). Among the most recurring words found in the abstracts of these documents are wetland, river, and floodplain, highlighting the relevance of restoration actions to mitigate inundations. The documents that resulted from search Q3, “natural flood management”, are less frequent; the first documents appeared in 2015 and have shown an upward trend since then (Figure 1a). Catchment and river are among the most frequent words found in the abstracts; while less frequent, the specific actions of NFM are also found, such as leaky barriers, woodland, afforestation, and reforestation (Figure 1d). The United Kingdom (UK) appears frequently in the abstracts. Climate change is a pervasive element and appears as a common pattern in the abstracts of all searches (see Figure 1a,c,d). Based on a review of different research documents on the subject, Section 2 identifies some of the alterations to river systems and their watersheds that contribute to the increased adverse impacts of floods. Section 3 documents possible restoration actions and their potential effects or benefits. Section 4 discusses the procedures for evaluating the effectiveness of the identified restoration actions. Discussions and conclusions are found in Section 5 and Section 6.

2. Watershed Alteration and Its Effect on Flooding

2.1. Land Use and Land Cover

The transformation of the Earth’s surface has multiple consequences for its biophysical systems, among which is the alteration of stream flow [26]. Land Use and Land Cover (LULC) change has a relevant influence on the hydrological cycle within catchments [27]; it alters the interception, infiltration, runoff, and streamflow. Also, it modifies the erosion and sediment transport [28]; the transport of eroded material to downstream areas may give rise to issues related to the sedimentation of rivers, reservoirs, and harbors [29]. In vegetation-covered areas, the rainfall is divided into the interception loss (water evaporated from the surface of plants), throughfall, and stemflow that reaches the surface [30], as depicted in Figure 2. Vegetation also has the capacity to physically modify the structure of the soil, thereby defining the Preferential Flow Paths, which modulate the capacity of the infiltration [31]. Also, soil erosion and compaction, driven by agriculture, disrupt surface and subsurface water interactions by blocking interstitial spaces in the soil [32]; soil compaction from cropping and grazing activities is one of the potential drivers of increasing flood magnitudes [33]. The flow velocity of surface runoff is also modulated by vegetation, as the density of vegetation controls the surface roughness and the flow velocity [34]. Zell et al. [35] compared the water storage capacity in an agricultural zone and a mature forest zone with the help of a groundwater flow model that was calibrated with historical information; they found that the forest zone stored 75% more water than the agricultural zone and had 16% more evapotranspiration, consequently it had a greater influence on runoff. The left part of Figure 2 depicts the interactions of the vegetation, soil, and hydrological processes relevant to runoff modulation.

The pressure on catchments and their river systems is increasing due to activities related to agriculture and urbanization in riparian areas [25]. O’Connell et al. [36] documented a relationship between agricultural land management and flooding, considering information from the previous fifty years in the United Kingdom; they identified increments in the generation of surface runoff at a local scale, and pointed to soil compaction, the development of drainage systems, and the alteration of the interconnections of the fluvial system as being among the causal factors. On the other hand, Bradshaw et al. [37] collected evidence on a global scale in developing countries, and found that deforestation amplified the risk and severity of floods; with data collected between 1990 and 2000 from 56 developing countries in Africa, Asia, and Central and South America, they found that the frequency of floods increased as natural forest cover decreased, and also that the severity associated with floods (measured as the total flood duration, the number of people killed, and the damage to infrastructure) was higher. Several studies have been carried out to assess the effect of the LULC change to runoff and streamflow, and a list of some examples is shown in Table 1. Note that the studies fall into the following two categories: those based on an analysis of field data, and those based on estimations performed with hydrological models. Both confirm that the removal of natural vegetation cover leads to increments of runoff and streamflow.

2.2. Sediment Dynamics

Sediment transport in rivers is able to influence the channel morphology and, consequently, to modulate the water stage during floods; therefore, it should be considered for flood hazard analysis, since river flooding occurs when the flow conveyance capacity of channels is exceeded [48]. The sediment particles along alluvial rivers experience several processes, such as production, erosion, transport, and storage [49]; changes in precipitation, temperature, and vegetation cover, among other factors, can adjust the erosion and deposition processes [17,28]. Based on information from global erosion studies, Montgomery [50] estimated that the highest erosion of geological origin occurs on tectonically active slopes, with magnitudes between 0.1 and 10 mm yr⁻¹; in contrast, the erosion generated in agricultural areas is within the same magnitudes, which highlights the impact of anthropogenic-driven erosion. Table 2 lists some studies where the sediment yield has been altered; in such cases, LULC change is the main driver. The increase in the sediment load can lead to a decrease in the hydraulic capacity of watercourses, favoring overbank flow and floods [51,52]; this is caused by sedimentation in the channels [53]. For example, disturbances to the Salt River watershed triggered by agriculture, mining, timber extraction, and road development led to significant rates of sedimentation in the river, with aggradation of up to three meters between 1965 and 2005 [52]. Table 3 lists some examples of studies where river channels are under morphological changes, modifying their conveyance capacity, and where one of the factors is the sediment yield to rivers. Furthermore, flow regulation in streams can modify the sediment transport regime [54,55], which can lead to alterations in the morphology of alluvial rivers [56,57] and changes in their discharge conveyance capacity. On the other hand, sediment accumulation in reservoirs is a common problem in many parts of the world and has important consequences for water management, flood control, and energy production [58]; the loss of the storage capacity of reservoirs has reduced their flood regulation capacity. An idea of the magnitude of sedimentation is, for instance, given by Syvitski et al. [59], who estimated a worldwide reduction in sediment reaching the coasts of approximately 1.4 billion tons per year due to retention in reservoirs.

2.3. River Morphology

The flow, sediment load, sediment size, and slope in a channel are interconnected; if one is altered, the others are modified to return to equilibrium [60]. Such processes of alluvial rivers are driven by their capacity to self-adjust so to dissipate excess energy and convey the water and sediment load provided from upstream reaches [61]. There is evidence that changes in channel cross-sections driven by LULC alter the local channel capacity, e.g., [62]. Another factor to consider is human intervention. The management and control of streamflow carried out in upstream reaches is one example of the anthropogenic alterations of river systems; such direct alterations change the flow and sediment load regimes, which shapes the morphology of alluvial streams [7]. For instance, across Europe, it has been identified that river training, the commissioning of hydraulic structures, and the removal of flood areas have all led to changes in the river morphology and water depth, thereby altering the flow wave propagation, and, therefore, modifying the peak, timing, and shape of the flood hydrograph [63]. The adjustment of one attribute of the channel triggers a change in other attributes, and ultimately regulates the flow conveyance and sediment transport capacities of the river. Rectification, dredging, and the widening of channels increases the hydraulic capacity and results in the faster drainage of upstream areas, thus concentrating the flow more rapidly toward the downstream areas [24]. The 20th century was a period of intense river engineering. Rivers were dredged and channelized, and these actions were accompanied by the construction of levees and dams to mitigate the impact of floods [64]. For instance, Ole [65] exposed that, during the 20th century, most of the streams in Denmark were channelized to drain to areas devoted to agriculture, industry, and urban zones. In Slovakia, this solution against floods was also common; it aimed to accelerate the drainage of affected areas [66]. Brookes [67] categorized channelization actions into the following four classes: (1) the widening, deepening, or straightening of the channel; (2) the removal of logs, vegetation, and obstacles from the streamflow, which modifies the channel roughness (CR in Figure 2); (3) the construction of levees to limit the overbank flow (see Figure 3); (4) bank stabilization. Modification types (1) and (2) involve increases in the hydraulic capacity due to the change in the channel geometry and roughness; consequently, this leads to a magnification of the shear stress acting on the perimeter [56]. The same authors exposed a case where channelization increased the stream slope, hydraulic radius, and velocity, resulting in an intensification to the sediment transport capacity by a factor of 15 to 50. Such conditions generate an imbalance in sediment transport, which propagates upstream and downstream, with implications for the flow conveyance capacity of the channels. Figure 3 depicts the input to rivers and the responses and interactions with the floodplain regarding the transference of water and sediment in response to runoff, sediment, and large wood entering the streams. Rivers can adjust their geometrical characteristics (the slope, width, and planform) and hydraulic characteristics, such as the roughness and water depth. Table 3 lists some of the cases related to river morphological change induced by alterations in the catchment and the effect on floods.

Table 3

Analysis of channel change driven by alterations to the catchment and the effect on flooding.

Source	Method	Findings
James [68]	Analysis of historical and field data of the Bear and American catchments in the Sacramento Valley, California.	Hydraulic gold mining during the periods 1853–1884 and 1893–1954 in the Bear and American catchments produced aggradation and the stage–discharge relations.
Stover and Montgomery [62]	Analyzed field data of the Skokomish River, Washington.	Flooding at the end of the 20th century along the mainstem of the river was related to stage–discharge relation variations caused by aggradation; the upstream catchment was impacted by timber extraction and road construction.
Liébault and Piégay [69]	Utilized historical data (maps and air photos) to characterize the channel width of rivers in southeastern France.	The narrowing of channels accelerated in 1950–1970; the authors considered that this was driven by forest development on the river margins and the abandonment of intensive floodplain land use.
Lane et al. [70]	Utilized field data of the Wharfe River (UK) and a 1D–2D hydraulic model.	During the period from Dec-2001 to Mar-2004, the upper reach developed aggradation; the effect was a reduction of 6.1% in the bankfull discharge.
Dingle et al. [71]	Utilized a 2D hydrodynamic model of the Karnali River, Nepal, to simulate the effect of variable bed elevation.	Channel aggradation or incision in small magnitudes could significantly modify the extent of inundation across the low-relief landscape.

The natural regime of wood in rivers is an additional process along with the flow and sediment transport regimes, and one that is not traditionally given the importance it has; this is due to the history of the removal of wood from river corridors for navigation or flood mitigation purposes [72]. The removal of trees located near riverbanks suppresses the source of logs falling into the channel, thereby decreasing the enhanced channel roughness (CR in Figure 2) generated naturally by the logs (large wood), and also lessening the temporary storage capacity within the channels driven by the raised water level, which contributes to the flow regulation service within the channel, as well as reducing the flow velocity. In addition, the higher water level also contributes to the occurrence of overbank flow and the connectivity with the floodplain [21], which contributes to diminish the frequency and magnitude of downstream flooding.

2.4. Connectivity Channel–Floodplain

Another factor that can contribute to increased flood risk in downstream regions is the modification of the lateral connectivity of rivers. Corridors where rivers and wetlands develop and interact were a common feature globally that have been diminished by anthropogenic activities, and which today are commonly unknown to river managers [73]. Levee construction next to river banks to maximize land use has incentivized development in flood-prone areas [74]. For instance, Ding et al. [75] assessed how both levee construction and urban expansion are linked in the contiguous United States; they found that levee construction was associated with a 62% acceleration in floodplain urban expansion, while the average in the country was 29%. Rivers and their floodplains are components of a single dynamic system [76,77], in which floods are the mechanism that trigger interaction through overbank flows. However, prevailing river management approaches consider that the flow occurs mainly within the channel, where occasional flooding of its floodplains takes place, although there is increasing evidence that these types of conditions are a historical legacy, that is, a consequence of anthropogenic alterations [78]. Since the second half of the last century, it has been recognized that natural rivers are actively connected to off-stream environments [79]. The lateral dimension of alluvial rivers (near overbank zones) experiences a cyclic expansion and contraction, caused by seasonal variation in floods, that generates an interaction between the channels and their floodplains. For instance, ref. [80] estimated that, in the Amazon River, there is an exchange with its floodplains of approximately 25% of the mean annual flow. Modi et al. [18] highlighted the importance of the river space provided by floodplains by considering the hydraulic, geomorphological, and biological interactions generated by flood pulses. Vertical accretion resulting from sediment deposition during channel overbank flow generated by floods is one of the processes related to floodplain formation [81]. Knox et al. [6] described that artificial levees built on riverbanks to protect infrastructure disconnect floodplains and compromise the biodiversity of the riparian region; it also has implications for flooding in the short- and mid-term. When a river is connected to its floodplains, the latter functions as temporary storage for various materials [82]. Temporary storage in the floodplains during floods helps to reduce peak flows in downstream reaches and diminishes the probability of overbank flows. For instance, Farrag et al. [83] analyzed the interactions of floodplain storage and flood risk in the German part of the Rhine basin. Their results indicated that floodplain storage lowers water levels and discharges, reducing risks by over 50%. In addition, during overbank flow, there may also be sediment transfer to the floodplains. For instance, Rodríguez et al. [84] studied the flood-driven overbank sedimentation in the lower Orinoco floodplain, and estimated deposition rates in the floodplain ranging from 0 to 225 kg m⁻² yr⁻¹, depending on factors such as the distance from the river banks and vegetation cover. When the connection of rivers with their floodplains is interrupted by levees, the transference of sediment to the floodplains is interrupted, and the sediment is transported downstream and deposited, thereby altering the hydraulic capacity of the rivers. Floodplains provide the service of sediment storage that otherwise would be transported downstream (see Figure 2), with the risk of the sedimentation of rivers.

3. Restoration Techniques for Flood Mitigation

3.1. Perspectives of Flood Mitigation

The previous section described how the alteration of catchments and fluvial systems contributed to the degradation of their temporary storage capacity of water (storage service, indicated in Figure 2), thus increasing the magnitude of floods; natural infrastructure in upland landscapes can help to reduce the flood risk for downstream communities [85]. This section describes actions to restore or enhance such services. Wohl et al. [86] define restoration as “support for the establishment of improved hydrological, geomorphological and ecological processes in degraded rivers and watersheds, and the replacement of natural elements in the system that have been lost, damaged or compromised”. Early restoration actions were focused on improving fish habitats and the landscape of rivers; later, they were extended to a broader range of actions to improve river processes [87]. Modal and Pravin Patel [88] highlighted that restoration techniques can include enhancing the ecological quality of the river reach as well as the riparian buffer zone to store runoff and to reduce downstream flood peaks. Currently, initiatives involving the restoration to more natural states do not exclusively involve environmental aspects, but also to reduce flood and geomorphological risks, with the advantage that they contribute to diminishing the costs of operations, as well as the maintenance and construction of traditional protection infrastructure (“gray” infrastructure) [89]. In addition, the accelerated pace of hydrological and land-use changes in recent years has produced conditions characterized by emergent, potentially large, and uncertain adjustments to watersheds, which should be considered in restoration projects [90]. Also, the assessment of anthropogenic-caused impacts on rivers must consider the entire watershed as the basic functional unit [55].

Nature based solutions (NBSs) are actions inspired by, supported by, or copied from nature [91]. Natural Flood Management (NFM), a subset of NBSs, consists in protecting, restoring, and emulating the functions of catchments so to reduce flood risk [92]; it considers strategies such as restoring natural land cover and the interactions of streams with their floodplains [93]. NFM contemplates approaches to increase retention, for example, with reforestation, the creation of wetlands and ponds, the modulation of the drainage capacity with the placement of large wood in channels, floodplain restoration [94], enhanced storage in rivers with leaky dams, the blocking of ditches and gullies, and land cover management [95]. Actions based on the restoration of vegetation aim at both preventing flooding by increasing the interception and retention, and by improving infiltration [96] and to control erosion. However, reconciling erosion control and flood prevention with biodiversity restoration remains difficult because solutions must be performed at the watershed scale. Investing in reforestation has shown in some regions to have economic viability to decrease the flood risk; however, this is a measure that show results in the long term [97]. In addition, social aspects should also be considered. Poledniková and Galia [23] evaluated the relationships between public perception and ecosystem services in the restoration of a river based on the following four categories: ecological and hydro-morphological, aesthetic and landscape, flood protection, and recreational. Peters et al. [98] indicated that the DNA of a river helps to design river plain restoration projects; the DNA of rivers refers to the intrinsic characteristics that define their structure, such as their hydro-morphology, geomorphology, and ecological processes. The following subsections present the specific actions reported in the literature to restore or enhance the services provided by catchments and fluvial systems to mitigate floods.

3.2. Restoration and Enhancement of Flow Retention

Runoff Attenuation Features (RAFs) are a type of NFM that seek to reduce runoff rates by slowing the flow velocities and creating a temporary storage of water during storms. They target runoff flow pathways and create temporary flow storage [99]. The storage within the catchment categorized as “green” includes forests, wetlands, and soil; those categorized as “blue” include water bodies and floodplains [93]. The green infrastructure approach aims to reduce flood peaks by introducing natural elements into existing impervious areas to reduce and slow runoff in conjunction with gray infrastructure so to mitigate the effect of flooding. Some ecological restoration actions in some ecosystems can greatly change the runoff patterns, thus providing flood regulation [100]; the expansion of vegetation cover resulting from land-use change plays a positive role in restoring ecosystem services related to floods [101]. For instance, Goudarzi et al. [102] observed that restoration through the revegetation of eroded peatlands provides natural flood management benefits by reducing and delaying flood peaks. At the urban scale, Song et al. [103] proposed to apply green infrastructure, such as green roofs, infiltration storage facilities, and porous pavement, in flood-prone areas. The restoration of wetlands provides storage and helps to enhance flood resiliency [104]; according to the modeling results, Wu et al. [105] found that riparian wetlands helped to reduce peak flows downstream. Similar results were obtained with the partial restoration of the lower reaches of the Yangze River, China [106].

Based on the analysis on the Pacific coast of the United States, Collins et al. [107] identified a process that they refer to as the “floodplain large wood cycle”, a mechanism that generates features in the floodplains that influence river dynamics. The physical processes of this cycle include log jams in the channel, which extend the area exposed to river flooding by the backwater flow that the jams generate, mainly during floods, and the consequent deposition of sediment and wood, as well as to the creation and sustaining of wetlands [107]. Floodplain forest restoration allows for a gradual increase in the amount of wood entering river channels and contributes to a reduction in the downstream flood risk with a relatively easy but long-term implementation. In addition, wood increases the morphological complexity of the near-stream floodplain, with a consequent impact on the runoff velocity [108]. They also evaluated the effect of riparian forest growth and deadwood captured by streams using heuristic numerical models; they found that this process contributes to the delay response time of floods. In the same direction, Thomas and Nisbet [109] indicated that the presence of trees and wood debris increases the surface roughness, thereby decreasing the runoff velocity during floods and increasing the storage capacity within the floodplain. Placing large wood in streams can modulate the river hydro-morphology, thereby locally increasing the flow resistance and water level [110], and increase the lateral connectivity [22], thus creating the conditions for water storage in the floodplain. Wood features based on large wood can be placed in gullies to slow and store overland flow [111]. Recently, rock retention structures (RDSs) have been placed in the south of Arizona, USA, and north of Sonora, Mexico, to reduce the severity of flooding events and to increase the water availability [112]. RDSs slow the flow and enhance the infiltration.

Leaky dams aim to provide temporal storage during rainfall events. An example of this kind of implementation is presented, for instance, by Chappel and Beven [113], with an outline of the criteria for the design of NBSs for flood mitigation. van Leeuween et al. [114] report that, after installing eight leaky dams in a catchment of 1.1 km², the peak flow reduction was highly variable, with an average of 10% for events and with a return period of up to one year. The ecological restoration and conservation of river hydraulic conditions are two aspects that can mitigate the magnitude of floods [115]. Among restoration techniques, Smith et al. [116] mentioned modifying the geometry of streams. The case of the Rhine River was discussed by Peters et al. [98], where the lateral channels of the main channel were rehabilitated (by removing deposited clay), and the sediment transport processes, water flow, and riparian vegetation were re-established. Smith et al. [116] identified associations between restoration techniques and watershed characteristics; for example, in low-energy environments, direct morphological interventions are required. Sholtes and Doyle [24] investigated restoration actions that increase both the temporary storage of flow and the dissipation of flow energy based, for example, on re-meandering, which reduces the slope and increases the length of the river relative to the valley, thereby reconnecting the channels with their floodplains. There are also beneficial side effects. Anderson [7] discussed the restoration of the Kissimmee River channel geometry in the USA, which allowed for the formation of an organic depositional layer on the riverbed; this reduced the active transport of sand so to maintain the river bars near the meander bends. Mendel et al. [52] proposed restoring the multi-channel form of a river, where secondary channels allowed for sediment transfer to the floodplain during floods, while the main channel was efficient at transporting water at all times. Dixon et al. [21] found that in-channel log jams (performed as part of restoration actions) in a UK catchment, along a reach of the channel of approximately 4 km, increased the time of the concentration. Sholtes and Doyle [24] carried out similar numerical simulations, where they considered higher roughness; they found that, in short reaches (1 km), flood attenuation downstream was reduced, and, to have significant effects, longer reaches (5 to 10 km) should be considered to observe effects at the basin scale.

The creation of flood areas helps to dampen flooding [25]. Floodplain restoration, where lateral reconnection with streams is considered, helps to mitigate flooding [117]. Setting back sleeves is a process where levees are relocated away from river banks to recover part of the floodplain for floodwater conveyance [74]. This type of restoration is limited for rivers crossing through urban areas or other densely used landscapes [82]. Secondary effects of floodplain reconnection should be evaluated. For example, dos Santos et al. [118] analyzed the effect of restoring flooding and drying processes in floodplains in Austria. They argued that such processes can generate greenhouse gases. Artificial wetlands upstream of the areas targeted for flood protection can contribute to the regulation of runoff [66]. However, wetlands require certain conditions for development. Dawson et al. [119] investigated how modulating the flow, regardless of the land-use history, could initiate a restoration response in wetland vegetation; the restoration efforts also considered removing levees to bring back connections between the river and its floodplain. Riparian vegetation depends on the occurrence of floods. Liu et al. [120] evaluated the eco-hydrological restoration of the Tarim River, China. They analyzed the temporal variability and correlations between the groundwater level and the vegetation growth; they highlighted how floods influenced the riparian forest development. Also, the restored vegetation on streambanks showed the capacity to modulate coarse sediment transport [121].

3.3. Sediment Management

Previous studies [122] have shown that vegetation plays a positive role in alleviating soil erosion. Xu and Pan [123] assessed the soil conservation services in the Jinghe River basin, China, and determined that grasslands were the main provider of soil conservation there. Soil erosion depends on the shear stress that the runoff generates and the resistance of the soil to erosion [124]. Restored vegetation on slopes helps to control erosion by reducing runoff velocities and to trap sediment coming from the higher parts of the slopes [125], which otherwise would be deposited in river beds, ponds, and reservoirs. Consequently, vegetation restoration can contribute, through indirect effects, to flood mitigation [126]. Near streams, riparian vegetation provides protection to riverbanks by reducing erosion processes and the contribution of the sediment load to downstream reaches; it also helps to reduce velocities during overbank flow [127]. Between 1970 and 1989, soil conservation works were undertaken in the Moldavan Plateau of eastern Romania; they included the construction of dams and reservoirs, check-dams to control gully erosion, and large afforestation on landslides and gullies [128]. Maetens et al. [129] assessed 353 runoff plots in Europe and the Mediterranean, and discussed how the soil and water conservation techniques reduced the exceedance probability of annual soil loss by approximately 20%. The conservation techniques identified there included cropland and vegetation management (e.g., grass buffer strips, and mulching), soil management (e.g., no-tilling and the reduced tilling of contour tillage), and mechanical methods (e.g., terraces and geotextiles).

The mitigation of erosion requires identifying the sediment sources, rates, and connectivity along streams. For instance, the identification of sediment storage locations within a catchment is an important indicator of where to locate NFM measures [130], since it is an indicator of low-velocity flow conditions. Pulley and Collins [131] proposed a technique based on the analysis of the color of sediments deposited during floods to identify their source and to guide mitigation actions. Land management practices are widely used to minimize runoff and sediment load at the watershed scale; however, their effectiveness is not easily assessed [16]. For instance, Sun et al. [132] determined that the grain size distribution changed over time in response to the change in the sediment transport regime, and attributed this to the ecological restoration performed in the middle basin of the Yellow River, China.

More complex actions involve the elimination of the bank protection of rivers to allow erosion, and the free adjustment of the stream geometry to variations in the flow and sediment load [133]. Restoration and NFM actions can trigger a change in the geometry of the channel, sediment transport conditions, roughness, and even the sediment grain size, which will require assessment. Some potential responses have been identified [134]. For instance, the increase in vegetation and elements that increase the channel roughness, such as wood or cobles, generate aggradation that propagates upstream while generating degradation downstream; decreasing the slope of the channel and river re-meandering have the same effect.

4. Assessment of Restoration Effectiveness

Limited research exists to evaluate the effectiveness of NFM approaches [135,136]. Therefore, quantitatively determining the effectiveness of restoring river systems and their watersheds to mitigate the impacts generated by floods under different conditions is a topic of current research [137]. It can be performed either with modeling or using real catchments. Current modeling approaches utilize both hydrologic and hydraulic models [138]. Regarding real catchments, the traditional approach to assess the effects of LULC change is the use of experimental catchments, either in pairs or for single catchments [38]. In the first case, the catchments are similar, except in LULC; in the second case, an initial calibration period is characterized with the current LULC at the time. In a second period, the changes in the LULC are made to characterize the response. This approach also can be applied to assess the effects of NFM features. Some evaluation methods and works carried out along these lines are presented next.

4.1. Hydro-Morphological Indexes

Logsdon and Chaubey [101] proposed five methods to evaluate ecosystem services that provide benefits for the communities located in their environment. Among the methods, they suggested an indicator that measures erosion regulation (ER) and flood regulation (FR); the latter is a function of duration, the number of events, and the average flow magnitude during floods. Fuller et al. [139] indicated that natural characteristic indices (NCIs) can be used to identify the effectiveness of restoration actions, where the characteristics of the current state of the river system are contrasted with characteristics of the past, when it had a more pristine state. Examples of such characteristics are the sinuosity of the channels, the width of the floodplain, the active width of the channel, and riparian vegetation. Li et al. [100] highlighted that few studies have evaluated regulating services that consider the effects of climate, vegetation restoration, and dam construction using an ecosystem service-based approach and distributed hydrological models. They proposed an index that measures annual flood regulation (FRI) using an ecosystem service approach and a method based on 27 indices, including precipitation and the simple daily intensity index, which analyzes the impacts of extreme precipitation. Pugliese et al. [14] presented a case study and an evaluation method based on key performance indices (KPIs), and one of them was related to risk reduction. Berg et al. [13] applied the evaluation standard for NBSs proposed by the International Union for the Conservation of Nature and Natural Resources [140], and they highlighted that it has no specific indices for flood risk mitigation. Another approach was presented by Gunnell et al. [93]. They used an index based on the relationship between the generated runoff and the storage capacity provided by green infrastructure. The analysis was performed based on annual water volumes and applied to five cities around the world. Values of less than one indicated that the basin had a higher storage capacity than the precipitation analyzed; values greater than one indicated that the volume of runoff generated was greater than the storage capacity. The history of changes must also be taken into consideration. Wohl et al. [64] indicated that understanding the temporal patterns of human modifications of terrestrial ecosystems and water bodies provides important information for the management and protection of watershed ecosystems and river systems. The spatial scale is relevant as well, since restoration projects generally do not address watershed-scale changes in water and sediment inputs, and changes in the longitudinal connectivity of rivers [141].

4.2. Evaluation of Restoration and NFM Actions

Hydrologic models have been used to characterize the effects of restoration actions [142]. For instance, Dixon et al. [21] analyzed the effects generated by floodplain forest restoration on flood risk reduction. Another case [16] used the SWAT model to predict soil erosion to the Sarrath River basin in northwestern Tunisia; the main benefit of this tool is its effectiveness in assessing the impacts of different land-use management practices on the long-term runoff and sediment load within a watershed and its river system. Dong et al. [115] evaluated the effects of actions such as temporary storage in ponds and restored wetlands, combined with traditional flood control infrastructure (dams and levees), using a scaled hydraulic model. They found that the result was a decrease between 10 and 20 cm of the flood water level. Another example was presented by Federman et al. [117], who used HEC-RAS to evaluate the effect of restoring the reconnection of the channel with the floodplain in the Chesapeake Bay watershed on the east coast of the United States; they found that flow peaks were attenuated by up to 37%. On the other hand, in the case of evaluations with field data, Dixon et al. [21] assessed the effect of placing logs in the channel, and found that it increased the time of concentration by 100% and decreased the peak flows by up to 75%.

A relevant question when assessing the effectiveness of the restoration of enhancement actions for flood mitigation is to determine the degree of the degradation of flood mitigation services the catchment have experienced, since it can allow for the identification of the degree of restoration that can be achieved. Table 4 summarizes the tasks for assessing the potential effects of restoration and enhancement with NFM for flood mitigation. This diagnostic may be the first step when data are available; it uses historical information to determine the cumulative alterations of the analyzed catchment and its fluvial system, and their effect on the streamflow and flood occurrence. In the evaluation stage, hydrological/hydraulic models can be used to quantify the effect of NBS and restoration actions. Field monitoring is implemented once the restoration and NBS actions have been carried out so to assess the temporal evolution of the response of the catchment.

5. Discussion

Multiple causes have progressively increased the risk of flooding in communities and areas with economic activities. For instance, the constant growth of urban areas and economic activities in zones with the pervasive risk of flooding, and the limited resources and capacity to build gray infrastructure for flood risk mitigation of some countries. Another cause is climate change, which can alter precipitation patterns in certain parts of the world and modify hydrological conditions. Climate change, a factor increasing the stress in old, traditional flood control infrastructure, opens the opportunity to rebuild and improve the infrastructure with the understanding of the services provided by riverine systems [143]. However, the cause analyzed herein is related to the reduction and loss of services provided by catchments and rivers, such as the retention and storage of stormwater and sediment, caused by the constant anthropogenic changes in fluvial systems and of their watersheds. As a result, rains with similar characteristics now produce more frequent floods with larger magnitudes and with shorter times of concentration. The degree of observed changes is of such a magnitude that it has been recognized that the traditional approach that assumes that hydrological variables are statistically stationary is no longer valid, and that their non-stationarity should be considered for infrastructure design [144]. LULC change produced by anthropogenic activities consistently increases the volume of runoff. Table 1 presents some of the studies assessing with field data and hydrological models the magnitude of the alteration of the runoff and streamflow. The magnitude of the alteration of runoff ranges from less than 1% to up to 24%, depending on the magnitude of the LULC change and characteristics of the catchment. Also, a consequence of LULC change is the acceleration of soil erosion, with increments of sediment yield up to 22%, according to the studies considered in Table 2. A result of a larger magnitude of sediment eroded and transferred to streams is the sedimentation of river beds, which triggers the alteration of stage–discharge relations, thereby reducing the water conveyance capacity of rivers (see the examples in Table 3). A challenge that needs to be considered is separating the effects on the runoff of LULC change and climate change [145].

The restoration of watershed components can provide ecosystem services, such as flood risk reduction or erosion protection [146]. In recent years, approaches to integrate green infrastructure with gray infrastructure have emerged for flood risk management [138]; NBSs restore and enhance the hydrological processes relevant to water storage and flood mitigation, such as infiltration, interception, and floodplain reconnection [147]. NBSs are progressively being recognized as essential complements to gray infrastructure, since they can provide resilience [148]. Such approaches seek to restore or enhance the storage capacity so to mitigate the flood risk by reducing the runoff volume through increased infiltration, storage in natural or constructed infrastructure, and retention in vegetation canopies, water bodies, and aquifers [93] by means of LULC management, leaky dams, blocking gullies, and ditches, among other features [95]. The restoration of stream–floodplain interaction and wetlands in the upstream region of the zone to be protected is another approach, which is an instance is levee setback [74]. Regarding erosion control, LULC management based on vegetation has been proposed, e.g., [125]. Also, the restauration of morphological processes in streams, such as bank erosion and the reconnection of side channels, allows for the enhancement of their retention capacity, as well as the reconnection with the floodplain for water and sediment storage.

A challenge of restoring fluvial systems and their watersheds for flood mitigation is to quantify the effectiveness of such actions. Currently, there are concerns about the effectiveness of NFM at larger scales, as well as the relation with different geographical settings [95,138]. There is consensus on the lack of field data on the effectiveness of NFM, specifically in the field of flood risk management [135]. The complexity and long-term outcomes of natural infrastructure requires continuous monitoring at different time scales to evaluate their effectiveness and accumulate evidence for best practices [146]. For instance, Bernhardt et al. [20] argued that, of the 37,000 restoration projects that they reviewed, only 10% considered some form of assessment. One method to quantify a priori the effectiveness of this type of measure has been with the help of hydrological or hydraulic models [138]. Another challenge is how to integrate restoration and NFM actions with traditional flood control infrastructure (gray infrastructure); an instance is the management of floodplains, which involves a balance between actions to counteract the long-term trend of increasing floods with gray infrastructure, and, on the other hand, the natural and anthropogenic use of floodplains [149]. Floods are a natural phenomenon; they are part of human civilization, and have both favorable and adverse effects. There is no method or action to avoid flooding; however NBSs and NFM are aiming for the coexistence of ecosystems and human societies, and to help enhance the resiliency of communities.

NFM based on the restoration and enhancement of the regulation capacity of catchments and their fluvial systems can be considered when there is room to implement the actions in the upstream zone of the area to be protected. Some types of actions are illustrated in Figure 4, and the selection of actions to implement depends on the underlying characteristics of the river corridor (e.g., the pre-existence of wetlands or lakes), the current conditions (e.g., room for the relocation of levees), and available land for actions such as reforestation and afforestation. Hydrological and hydraulic models can be utilized to assess the effectiveness of NFM actions by comparing the characteristic inundations of scenarios with and without NFM actions. Furthermore, the expected annual economic losses associated to the flooded areas can be determined so to assess the mid-to-long-term effects of NFM actions.

6. Conclusions

The growth of population and economic activities is closely related to the modification of watersheds due to LULC change, which alters the hydrological processes as well as the water conveyance capacity of streams. It is also related to the modification of watercourses so to increase their hydraulic capacity through channelization, dredging, and levee construction, thereby changing the dynamics of the flow and limiting the channel–floodplain interaction; the end result is the increase in floods in downstream areas (the magnitude, duration, and frequency). Consequently, floods generated by natural causes can be amplified by the reduction and loss of regulation services provided by watersheds and their fluvial systems (retention, storage, and erosion control), which have been progressively diminished globally. Studies on the response of watersheds and rivers to anthropogenic alterations show moderate-to-significant increments to the runoff, streamflow, and sediment yield, which have important implications for the flood risk, and which is traditionally mitigated by increasing the capacity of gray infrastructure.

NFM proposes actions such reforestation and afforestation to restore the altered hydrological processes and soil erosion mitigation in the upstream areas of zones where flood mitigation is the goal. Also, it seeks to increase the water storage capacity within the catchment and in-channel. The restoration of wetlands and the reconnection of floodplains, with levee retreat, for instance, help to store part of the flow generated in upstream areas during storms so to reduce the floods in the downstream region. On the other hand, restoring watercourses to their original geometry or re-establishing their roughness allows them to regulate the flow and dissipate part of the energy of floods. These processes provide delays in the flood response time and can facilitate overbank flow in the upstream area, thus providing a longer storage period to reduce downstream flood peaks.

Research needs to more effectively address the identification of the type of NFM actions according to the characteristics of catchments and their fluvial systems. As part of this challenge, it is necessary to increase the available observational and modeled data so to assess the effectiveness and viability of these types of solutions at different scales. On the other hand, the definition of strategies for the integration of NFM with gray infrastructure for effective operation is necessary. Also, the continuous assessment from historical data of the degradation of regulation flood services provided by watershed and fluvial systems is an important tool to identify the potential room for improvement of the NFM effectiveness. Furthermore, an integral evaluation of flood control measures requires the assessment of the drawbacks of gray infrastructure, such as the social and economic implications of the degradation of ecosystems that it may produce. Finally, the interaction of hydrologists and hydraulic engineers with other disciplines and with stakeholders is required so to assess the compatibility of these types of solutions with the socioeconomic activities developed within the watersheds.

Author Contributions

Conceptualization, E.J.-D. and A.M.; literature review, E.J.-D. and M.L.A.-J.; writing—original draft, E.J.-D.; formal analysis, E.J.-D. and M.L.A.-J.; writing—review and editing, A.M., M.L.A.-J., and M.B.-V.; figures, E.J.-D. and A.M. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The results of the queries and the Python scripts utilized to generate Figure 1 are available from the corresponding author upon reasonable request.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:

LULC	Land Use and Land Cover
NBSs	Nature-based Solutions
NFM	Natural Flood Management
RDS	Rock Detention Structure

Footnotes

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Figures and Tables

Figure 1 (a) The frequency of the research documents (papers in journals and proceedings of conferences, books, and chapters of books) related to land cover and inundations (Q1), the mitigation of inundations and restoration (Q2), and natural flood management (Q3); (b) a word cloud of the abstracts of the documents that resulted from search Q1, (c) a word cloud of the documents that resulted from search Q2, (d) and a word cloud of the documents that resulted from search Q3.

Figure 2 Interactions between watershed components and processes that determine the runoff, streamflow, and overbank flow characteristics. The different processes occurring within watersheds provide ecosystem services, including storage, erosion regulation, and streamflow regulation.

Figure 3 Stream attributes that can be modified directly or indirectly; the alteration of one attribute triggers the adaptation of the others, and ultimately determines the hydraulic and sediment transport capacity, and thus the capacity to modulate the frequency and magnitude of downstream overbank flow and the occurrence of fluvial inundations.

Figure 4 Examples of NFM actions to restore or enhance the regulation and retention capacity of catchments and rivers. (1) Reforestation/afforestation. (2) Contention barriers in gullies and the surface of the catchment. (3) Wood structures in streams. (4) Relocation of levees to recover river–floodplain interactions. (5) Restoration of wetlands. (6) Restoration of lakes. (7) Re-naturalization of rivers to attenuate floodwaves.

Table 1

Examples of studies related to the assessment of the LULC change to runoff and streamflow.

Source	Method	Findings
Sahin and Hall [38]	Analyzed the data of 145 catchments of a few hectares around the world, considering the land cover from hardwood and conifer forests, Eucalyptus, scrub, grassland, and agriculture.	Deforestation of the conifer-type forest produced a larger increase in water yield in catchments. A 10% reduction in conifer cover increased the water yield by 20–25 mm.
Costa et al. [39]	Analyzed field data in the Tocantins River basin (175,360 km2), Brazil.	Comparing two periods where, in the second period, the agricultural areas increased substantially. The increase in the mean discharge was augmented by 24% in the second period.
Das et al. [27]	Utilized the variable infiltration capacity hydrological model (VIC) [40] to assess the effect of the LULC change in five basins in eastern India.	Between 1985 and 2005, there was an overall 0.032% increase in runoff, while the forested areas diminished by 5.8% and the mangrove areas diminished by 11.5%.
Munoth and Goyal [41]	Utilized SWAT to assess the impact of the LULC change in the Tapi River basin, India.	Between 1976 and 2016, forest land and rangeland decreased by 7% and 10%, respectively. The surface runoff increased by about 36%, while the water yield increased by about 22%.
Horton et al. [42]	Analysis of field data in the Usumacinta catchment, Mexico.	Jungle and forest were removed for agriculture and livestock farming. The 10-year return period discharge increased by 15%.
Astuti et al. [43]	Utilized SWAT to assess the effect of the LULC change in a tropical urbanized watershed in East Java.	The LULC change produced an increase of 8% in runoff and an increase of 0.28% to the water yield, as well as a decrease in groundwater and evaporation by 1.8 and 1.15%.
Sam and Khoi [44]	Utilized SWAT to assess the changes to river discharge caused by LULC changes in the Mekong River basin.	Forest areas were reduced by 2.35% in the period 1997–2010, while river discharge increased by 0.32%.
Gyawali et al. [45]	Utilized SWAT to assess the effect of the LULC change between 1992 and 2016 in Yellow Creek basin, Kentucky, USA.	An increase in surface runoff by 66.85%.
Shuckla et al. [46]	Utilized SWAT to analyze the effect of the LULC from forest to urban settlements, agriculture, or grasslands in the Rur Basin, Germany.	Conversion of forest into urban areas increased the runoff by 41%, to agriculture by 14%, and to grasslands by 4%.
Zhang et al. [47]	Analyzed historical data of the Lhasa River basin, Tibetan plateau.	There was an increase in forested areas and a decrease in grassland and bare land due to ecological projects. Considered that the LULC change caused the decline in runoff.

Table 2

Some studies of how the dynamics of sediment is modified within catchments.

Source	Method	Findings
Munoth and Goyal [41]	Utilized SWAT to assess the effect of the sediment yield.	Comparing the LULC in 1975 and 2016, there was an increase of 22% to the sediment yield.
Sam and Khoi [44]	Utilized SWAT, calibrated with measured data of the Mekong River basin.	Comparing the LULC in 1997 and 2010, the sediment yield increased by 2.86%, with an increase of 2.29% in the agricultural areas.
Gyawali et al. [45]	Utilized SWAT to assess the effect of LULC change.	Between 1992 and 2016, the sediment yield increased by 174.5%.
Zhang et al. [47]	Utilized data from the Lhasa River basin, Tibetan Plateau.	An increase in forested areas caused a decline in the suspended sediment load.

Table 4

Definition of actions that help to quantify the effects of NFM and the restoration of watersheds and river systems.

Task	Addressed Question	Parameters Measured/Characterized
Diagnostic	What extent of flood-related regulation services have been lost in the catchment?	Historical data of streamflow (from gauging stations) and occurrence of inundations, sediment load, and land use. Record of modifications or natural change in channels and constructed infrastructure.
Evaluation	What are the potential effects of restoration and NFM actions?	Changes in streamflow, flood characteristics, erosion patterns, and sediment load characterization.
Monitoring	How do key performance indices evolve in the watershed in response to NFM and restoration?	Continuous measurement of stream flow (gauging stations), sediment transport, and flood occurrence.

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