The Role of Gill Woodland in Relation to Natural Flood Management and Leaky Woody Dams

Natural flood management (NFM) addresses the causes of flooding by restoring natural processes, to deliver ecosystem service and biodiversity benefits alongside controlling flood risk. Interest in NFM is growing due to concerns over the sustainability of traditional flood defences. One widely used NFM technique is using large wood to create leaky barriers or dams in rivers, which occur naturally in unmanaged catchments but are now rare due to human activity. These barriers reduce river flow velocity, and it has been suggested that this could minimise downstream flood risk. There is also increasing evidence that large wood is important for the provision of other ecosystem services and benefits biodiversity. However, further research into catchment-scale impacts and performance of leaky barriers under different flow conditions is needed to confidently conclude that large wood restoration is an effective flood management tool. Mathematical models offer insights to address this knowledge gap by predicting the impacts of adding wood to rivers and streams at different scales and under different scenarios. This approach could be particularly relevant to the gills of the High Weald in south-east England. These are steep-sided, wooded valleys that are ecologically significant for their distinctive assemblages of plant species, yet they receive little conservation or research attention. Large wood is likely important for the functioning of natural gill habitat. Therefore, modelling could be used to assess the extent to which restoring large wood can act as an effective NFM technique for gill streams and contribute to evaluating the significance of flow regulation as an ecosystem service associated with this habitat.

Introduction

The scale of the economic, social and environmental impacts of flooding make it one of the most damaging natural hazards faced by society (Pitt 2008; Jongman et al. 2014). Annual average losses to flooding in the European Union between 2001 and 2012 are estimated at €4.9 billion, and are anticipated to increase given climate change and continued development in at-risk areas (Dankers & Feyen 2008; Hirabayashi et al. 2013; Jongman et al. 2014).

For much of the twentieth century, flood risk management focused protecting sites with hard flood defences (Werritty 2006). However, the long-term effectiveness of these defences is increasingly being questioned, especially considering the increased flood risk under climate change, and recent extreme flood events that have overwhelmed defences in countries such as the UK (Werritty 2006; Johnson & Priest 2008). Moreover, hard engineering projects and river management schemes to control flooding have contributed to biodiversity loss, with floodplain and riparian habitats being destroyed or degraded (Ward et al. 1999; Adams et al. 2004). Therefore, more environmentally sustainable flood management is needed, that minimises flood risk for future generations without compromising biodiversity conservation goals or ecosystem services (Adams et al. 2004; Pitt 2008). Scientific research will be needed to objectively evaluate their effectiveness of potential solutions to support informed choices about the best options for reconciling conservation and flood management.

Natural Flood Management

The need for alternative, sustainable flood management has contributed to a growing policy interest in methods for reducing flooding impacts by working with natural processes in river catchments (Johnson & Priest 2008; Collentine & Futter 2018). These methods are frequently referred to as `natural flood management` (NFM), which can be defined as managing flood risk through protecting, restoring and replicating natural features and processes to enhance the flood-regulating capacity of catchments (Hankin et al. 2017). Increasing the capacity of rivers to cope with extremes should reduce downstream flood risk and minimise the need for hard flood defences (WWF Scotland 2007). NFM differs from traditional flood management in addressing underlying causes of flooding rather than simply constructing defences at vulnerable sites (WWF Scotland 2007). Moreover, NFM measures are expected to deliver a range of ecosystem service benefits alongside flood control, including carbon storage, improved water quality, and increased biodiversity (Dadson et al. 2017; Hankin et al. 2017).

Policy makers and catchment managers are increasingly recognising the appeal of NFM over hard defences, due to their lower initial costs, potential to deliver multiple ecosystem service benefits and the flexibility of the scale at which they can be implemented (Hankin et al. 2017). NFM projects are also more amenable to community participation than hard defences (Howgate & Kenyon 2009; Hankin et al. 2017), and may better facilitate stakeholder engagement in making flood management decisions (Lane et al. 2011). Although use of NFM is spreading, and numerous studies demonstrate NFM success at a local scale, the overall effectiveness of NFM is not always clear (Wingfield et al. 2019). Little research has been done to assess catchment scale impacts of NFM, or NFM performance under extreme conditions (Ghimire et al. 2014; Environment Agency 2017).

Wood in Streams and Rivers

Many natural processes operating in streams and rivers have been disrupted by humans, and therefore provide targets for restoration via NFM (Dadson et al. 2017). One example is the accumulation of `large wood` in watercourses, conventionally defined as pieces of wood at least 1 metre long and 10 centimetres in diameter (Gurnell et al. 2002). Observations of undisturbed, forested catchments show that wood is constantly entering streams and rivers, and transported downstream until it catches on an obstruction, where large wood collects to form leaky barriers or dams in the river (Wohl et al. 2019). These barriers can influence the physical and biological characteristics of river systems (Gurnell et al. 1995).

Large Wood and Flow Regulation

Rivers flood when flow levels exceed the channel capacity, and when water from precipitation arrives faster than it can be carried away by drainage (Collentine & Futter 2018). Therefore, flood risk can be reduced by slowing run-off and river flows by increasing upstream water storage, which delays and reduces the magnitude of downstream peak flows (Collentine & Futter 2018). Evidence from a range of studies employing different methodologies show that leaky barriers formed of large wood can perform this function.

Firstly, observations of unmanaged streams indicate that large wood accumulations are associated with increased resistance to flow, with strongest effects under low flow conditions (Environment Agency 1999; Curran & Wohl 2003). Individually, these leaky barriers have minimal effects on downstream flows, but the cumulative effect of a series of barriers can be considerable (Environment Agency 1999).

Experiments provide further evidence for the hydraulic impacts of large wood and consequences for flooding. For example, adding leaky woody barriers to a stream in south east Germany significantly delays the movement of an artificially generated flood wave along the stream (Wenzel et al. 2014). However, laboratory experiments suggest that under certain circumstances, large wood may increase flood risk by raising the water level upstream of blockages at features such as bridges or culverts, or due to the sudden collapse of a debris dam (Gippel 1995).

Further insights come from modelling impacts of large wood on water flow. Modelling studies suggest that leaky woody barriers can decrease flow velocity, delay and reduce the magnitude of the flood peak, but also that many restoration projects occur at too small a scale for these effects to make a material difference to downstream flood risk (Sholtes & Doyle 2010; Thomas & Nisbet 2012).

Existing literature reviews conclude with a moderate level of confidence that large wood in leaky barriers can promote upstream water retention and slow flows at a local scale (Gippel 1995; Krause et al. 2014). However, whether these changes actually reduce downstream flood risk has rarely been tested explicitly, and there is a lack of clarity over catchment scale impacts and leaky barrier performance under high flow conditions (Environment Agency 2017). It is also apparent from the literature that the role of large wood in regulating flow is highly variable, so restoration schemes must be designed carefully to avoid unintended consequences (Gippel 1995; Environment Agency 2017). Effects vary with the characteristics of the stream or river, such as channel dimensions, as well as the properties of the leaky barriers, including their number, spacing and composition, and further research is needed to quantify how these variables affect the magnitude of flow regulation effects (Gippel 1995; Thomas & Nisbet 2012).

Other Large Wood Impacts on Ecosystem Services and Biodiversity

Besides flow regulation, researchers have identified several other impacts of large wood in rivers, with implications for ecosystem services and biodiversity. Woody debris accumulations trap and collect sediment and organic matter (Gurnell et al. 1995). Experimentally removing wood from lowland rivers decreases the amount of organic matter stored in the river bed (Daniels 2006), while adding woody dams to streams increases organic matter storage (Smock et al. 1989). The trapping of sediment can enhance water quality, as chemicals are removed from the water column and deposited, as well as stabilising the river channel and reducing erosion (Dosskey et al. 2010).

By controlling organic matter release, woody debris can increase food availability for aquatic invertebrates (Piégay & Gurnell 1997). Using molecular markers to compare invertebrate diets at sites with and without woody debris shows that large wood is associated with increased food quantity and quality, not just from trapped organic matter, but also from vegetation growing on the wood itself (Cashman et al. 2016).

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Further biodiversity benefits could arise from the complexity that large wood adds to the channel structure. The surfaces of the wood, combined with the pools and shallows resulting from debris dams, provide different habitats that could support various species at different life cycle stages (Gurnell & Sweet 1998; Schröder et al. 2013). Some surveys have indeed found higher species richness in rivers with more large wood (Pilotto et al. 2016), or recorded increased biodiversity after experimentally adding wood to streams (Smock et al. 1989; Lester et al. 2007), but at other sites, no significant relationship has been observed (Flores et al. 2011).

Previous overviews of river restoration research were unable to conclude that restoring large wood increases biodiversity, especially when compared to the evidence for other benefits of large wood (Palmer et al. 2010; Feld et al. 2011). One reason for this uncertainty is the lack of adequate monitoring data: few projects using wood in rivers are objectively evaluated after implementation and compared with control sites (Wohl et al. 2015). However, more recently, rigorous before-after control-intervention experiments replicated across several sites returned positive associations between restoring woody debris and biodiversity across multiple trophic levels (Thompson et al. 2018). Moreover, beyond aquatic habitats, the importance of dead wood for supporting ecosystem function and biodiversity has been well established for woodlands (Hodge & Peterken 1998).

Management of Large Wood in Rivers

Although research has identified multiple benefits of large wood in rivers, comparisons with historical records and unmanaged catchments show that human activity has resulted in extensive losses of wood from rivers (Wohl et al. 2019). Accumulations of wood were perceived as unattractive and hazardous, and were regularly removed to improve accessibility and aesthetics (Lassettre & Kondolf 2012; Wohl et al. 2019). Moreover, forest clearance due to agricultural expansion and urbanisation has reduced the amount of wood entering rivers and streams (Elosegi & Johnson 2003; Erskine & Webb 2003). The UK, with its extensively modified catchments, is a typical example, with most watercourses now containing little naturally occurring large wood (Piégay & Gurnell 1997).

The scarcity of large wood in rivers means its importance has been overlooked in the past (Wohl et al. 2019). However, given the growing evidence for the benefits of large wood, and increasing interest in NFM, adding wood in leaky barriers or dams has become a common intervention in river restoration and flood management projects (Krause et al. 2014; Cashman et al. 2018).

Modelling Leaky Barriers

Demonstrating the effectiveness of leaky woody barriers as an NFM measure will depend on addressing key knowledge gaps in NFM research, including understanding effects across catchments and under the high flows typical of flood events (Environment Agency 2017). Experimentally manipulating flow across a catchment to assess leaky barrier performance is logistically challenging, and the rarity of long-term datasets before and after implementation mean there are few observations showing how these measures respond to extremes (Dixon et al. 2016; Environment Agency 2017). Given these problems, mathematical modelling is a useful alternative for studying NFM effects on river flows and catchment flood risk (Dixon et al. 2016).

Mathematical models use sets of variables and functional relationships to generate simplified representations of real-world systems (Clark et al. 2011; Beven 2012). They can be used to run simulations of a system and test hypotheses about how it will respond under different conditions (Beven 2012). Model simulations therefore allow researchers to compare consequences of alternative scenarios, and estimate impacts of low-probability events for which no reliable data is available in reality (Giustarini et al. 2015).

A huge array of models are used to represent environmental processes within rivers and their catchments, and what model is most appropriate for a given system or objective is often debated (Clark et al. 2011). Two broad classes of models are applicable to NFM: hydrological models represent rainfall runoff into streams and rivers across a catchment (Beven 2012), while hydrodynamic or hydraulic models describe fluid motion, to simulate the flow of water in rivers and streams (Hunter et al. 2007; Teng et al. 2017). These two approaches can also be combined, for example, by using the output of one model as input for the other (Teng et al. 2017; Metcalfe et al. 2017).

However, using models to predict effects of leaky barriers does present its own difficulties. Modelling NFM effects on flood risk can be complex, because NFM measures are often dispersed over large areas and influence multiple catchment processes simultaneously (Hankin et al. 2017). Moreover, no standardised means of modelling large wood in rivers has emerged from the literature, with different studies adopting different approaches by customising existing models (Metcalfe et al. 2017; Pinto et al. 2019).

High Weald Gills and Large Wood

The gills of the High Weald in south-east England may be especially appropriate for NFM via large wood restoration. Gills are steep-sided, wooded, stream-fed valleys or ravines, with a humid and stable microclimate (Sansum 2014). Across Europe, gills are a rare habitat, but they occur at high densities within the High Weald Area of Outstanding Natural Beauty (AONB), and are a key feature underlying AONB designation for this landscape (Rose & Patmore 1997; High Weald Joint Advisory Committee 2019). The gills of the High Weald are distinct from those elsewhere in the UK due to their more continental climate, the presence of groundwater springs, sedimentary bedrock and deeper soil profiles (Rose & Patmore 1997). Consequently, these gills support species communities that are unique in the UK and Europe, making the concentration of gill habitat in the High Weald of both national and international ecological significance (Rose & Patmore 1997; Burnside et al. 2006).

The diverse plant communities of the High Weald gills include several nationally rare species and some gills appear to be refugia for species that are rare elsewhere in lowland England, suggesting that environmental conditions have been stable for long periods (Rose & Patmore 1997; Sansum 2014). This diversity is also thought to reflect high diversity in other taxonomic groups in gill habitat, but this has not yet been studied in depth (Sansum 2014).

However, despite their ecological significance, these gills are poorly represented within the existing network of designated protected areas for conservation (Burnside et al. 2006). Only 9% of gill systems are covered by Sites of Special Scientific Interest and only 4% by Special Areas of Conservation (Burnside et al. 2006; Sansum 2014). Furthermore, effective conservation management of the High Weald’s gills is hindered by an absence of research (Sansum 2014). There is little scientific literature on the extent and quality of the habitat, the distribution of species of conservation concern, and effects of human activities on gill biodiversity and ecosystem services (Sansum 2014).

Large wood is a key feature of natural gill habitat: historically low levels of disturbance and steep wooded valley sides mean large wood collects in gill streams (Sansum 2014). Fallen trees and branches create a complex mesh spanning the channel, providing niches for some of the distinctive plants of gill habitats (Sansum 2014). In gills, flow regulation impacts could be especially significant, because the narrow stream channels mean wood is more likely to get caught and form leaky barriers impeding the flow of water (Piégay & Gurnell 1997). Even here, however, many gills are likely to have lost natural woody debris, reflecting previous management of streams and surrounding woodlands (Piégay & Gurnell 1997; Hodge & Peterken 1998). Therefore, High Weald gills present opportunities for creating artificial leaky woody barriers for both flood risk management and biodiversity conservation.

Conclusion

There is growing support in the literature for localised flow regulation benefits of leaky barriers formed from large wood, alongside other ecosystem service and biodiversity benefits. However, the overall magnitude of any flood risk reduction, and how this varies according to design and distribution of these interventions, remains uncertain. Modelling large wood impacts on flow has emerged as a viable strategy for addressing these gaps in understanding. This could be especially useful for the gills of the High Weald, as large wood in streams is a key feature of this poorly understood but ecologically significant habitat. Using modelling to predict impacts of restoring wood to gill streams in a catchment in the High Weald could develop understanding of the effectiveness of restoring large wood for NFM, while also evaluating the importance of flow regulation as an ecosystem service provided by naturally functioning gill habitat.

Therefore, I will address these issues with a project that aims to assess the potential for leaky woody dam creation in gill streams for NFM, using a modelling approach to achieve the following objectives:

1. To predict the effect of adding leaky woody dams to High Weald gill streams on downstream flows and flood risk
2. To investigate the relationship between the number and distribution of simulated leaky woody dams in High Weald gill streams and downstream flows and flood risk