A restatement of the natural science evidence concerning catchment-based 'natural' flood management in the UK.

Flooding is a very costly natural hazard in the UK and is expected to increase further under future climate change scenarios. Flood defences are commonly deployed to protect communities and property from flooding, but in recent years flood management policy has looked towards solutions that seek to mitigate flood risk at flood-prone sites through targeted interventions throughout the catchment, sometimes using techniques which involve working with natural processes. This paper describes a project to provide a succinct summary of the natural science evidence base concerning the effectiveness of catchment-based 'natural' flood management in the UK. The evidence summary is designed to be read by an informed but not technically specialist audience. Each evidence statement is placed into one of four categories describing the nature of the underlying information. The evidence summary forms the appendix to this paper and an annotated bibliography is provided in the electronic supplementary material.

Introduction

Flooding is among the most damaging natural hazards globally, with inundation leading to disastrous consequences including the loss of lives and destruction of property. Flooding may be fluvial, pluvial, coastal or groundwater related, or caused by a combination of these processes. Here, we focus on fluvial (river) floods, which occur when the amount of water in a river exceeds the channel's capacity. They are caused primarily by the downstream flow of run-off generated by heavy rainfall on wet ground.

Flooding is a natural process, but floodplains are also ideal for agriculture and urban development close to water resources and navigation. Consequently, development in floodplains has increased the exposure of people, property and infrastructure to floods. In many cases it is not practical, cost effective or politically feasible to relocate communities, property and economic activities away from areas prone to flooding, so measures are put in place to manage flood risk by reducing the probability of inundation and/or the negative consequences when a flood does occur.

In this restatement, we concentrate on the scientific evidence concerning the effectiveness of human interventions in river catchments that are intended to reduce fluvial flood hazard.[1] This hazard is typically associated with high river flows. The hazard is characterized by the depth of water at locations where it may cause harm, and also by the velocity of that water, the rate of rise of water levels, duration of inundation and water quality. Interventions in river channels and floodplains that have been widely used to manage flood risk include the building of flood detention reservoirs and flood defences, channel straightening and dredging.

Recent years have seen increasing interest in management interventions that seek to modify land-use and land management, river channels, floodplains and reservoirs (where present), in order to reduce the frequency and severity of flooding, which we refer to here as ‘Catchment-Based Flood Management’ (CBFM). One subset of CBFM is ‘Natural Flood Management’ (NFM), which seeks to restore or enhance catchment processes that have been affected by human intervention. These activities aim to reduce flood hazard, while also sustaining or enhancing other potentially significant co-benefits including enhanced ecosystem services (aquatic, riparian and terrestrial) such as greater biodiversity, improved soil and water quality, carbon sequestration, reduced soil erosion, greater agricultural productivity and improved public health and well-being.

While it is recognized that implementation of CBFM or NFM can produce multiple co-benefits, it is not easy to establish the precise nature and extent of those benefits. Often a complex set of trade-offs exists between costs and benefits that accrue to different stakeholder groups within and outside the catchment. Also, while the benefits are well understood in principle, uncertainty around the quantitative predictions of the potential for CBFM/NFM interventions to reduce local and downstream flood hazards remains high, especially in large catchments and for major floods. Differences between river catchments make it difficult to transfer empirical evidence from one location to another. The relative importance of the multiple factors that influence flooding varies spatially and with time, which means that even if an intervention may be beneficial locally, a positive impact on flooding downstream cannot be guaranteed for all possible events in all locations.

The aim of this restatement is to review the scientific evidence for the impacts of CBFM and NFM strategies on downstream flood hazard in the UK. Here, we focus on the natural science evidence base; the social sciences and economics also provide important evidence for policy-making but this is not considered here. The objective is to review processes that impact flood frequency[2] and hazard potential, principally with respect to flood volumes and flood levels but also velocity, duration and water depth. These include modifications to land cover and land management to retain water on and within the land before it flows into rivers, and modifications to and protection of channels and rivers to slow the flow of water and reduce water levels in floodplains downstream where there is a flood hazard (table 1).

Table 1.

Catchment-based measures that could contribute to flood management. After [1].

Flood Risk Management theme	specific measure	examples
retaining water in the landscape: water retention through management of infiltration and overland flow	land-use changes	arable to grassland conversion, forestry and woodland planting, restrictions on hillslope cropping (e.g. silage maize), moorland and peatland restoration
	arable land-use practices	spring cropping versus winter cropping, cover crops, extensification, set aside
	livestock land practices	lower stocking rates, restriction of the grazing season
	tillage practices	conservation tillage, contour/cross slope ploughing
	field drainage (to increase storage)	deep cultivations and drainage to reduce impermeability
	buffer strips and buffer zones	contour grass strips, hedges, shelter belts, bunds, riparian buffer strips, controls on bank erosion
	machinery management	low ground pressures, avoiding wet conditions
	urban land-use	increased permeable areas and surface storage
retaining water in the landscape: managing connectivity and conveyance	management of hillslope connectivity	blockage of farm ditches and moorland grips
	buffer strips and buffering zones to reduce connectivity	contour grass strips, hedges, shelter belts, bunds, field margins, riparian buffer strips
	channel maintenance	modifications to maintenance of farm ditches
	drainage and pumping operations	modifications to drainage and pumping regimes
	field and farm structures	modifications to gates, yards, tracks and culverts
	on-farm retention	retention ponds and ditches
	river restoration	restoration of river profile and cross-sections, channel realignment and changes to planform pattern
	upland water retention	farm ponds, ditches, wetlands
making space for water: floodplain conveyance and storage	water storage areas	on- or off-line storage, washlands, polders, impoundment reservoirs
	wetlands	wetland creation, engineered storage scrapes, controlled water levels
	river restoration/retraining	river re-profiling, channel works, riparian works
	river and water course management	vegetation clearance, channel maintenance and riparian works
	floodplain restoration	setback of embankments, reconnecting rivers and floodplains

Material and methods

The restatement is intended to provide a succinct summary of the natural science evidence relevant to policy-making in the UK as of June 2016. The restatement offers a consensus judgement on the strength of the different evidence components using the abbreviated codes established in previous Oxford Martin School restatements:

[Data] a strong evidence base involving experimental studies or field data collection, with appropriate detailed statistical or other quantitative analysis;

[Exp_op] a consensus of expert opinion extrapolating results from relevant studies and well-established principles;

[Supp_ev] some supporting evidence but further work would improve the evidence base substantially; and

[Projns] projections made using well-established models that are based on the available physical principles and/or robust empirical evidence gathered in a wide range of settings.

The categories employed are based on those used in previous restatements [2,3], which were themselves developed from the medical and climate change literature. The statements are qualitative in nature and are not intended to form a ranking. We note that, in many cases, evidence is context- or scale-specific. Moreover, interventions that may be effective in one location and at one scale may have a different effect in another setting. Where further gradation is necessary to reflect the quality of evidence, this is done in the accompanying text.

We note in particular the wide range of models used in hydrological science. Some models are based on well-established physical principles such as conservation of mass, energy and momentum, which are fundamental properties of physical systems but which nonetheless require generalizations about parameter values or model equations in order to be applied. Other models represent generalizations from necessarily limited sets of observations whose conclusions cannot be expected to hold in settings different from those in which they were generated.

Results

The summary of the natural science evidence base relevant to catchment-based ‘natural’ flood management in the UK is given in the appendix, with an annotated bibliography provided as the electronic supplementary material.

Discussion

In this restatement, we have drawn attention to some important evidence gaps. We highlight several immediate priorities:

National monitoring networks provide essential data for estimating flood risk and determining the efficacy of interventions. Maintenance and enhancement of monitoring systems should pay particular attention to the accurate measurement of high water levels and out-of-bank flows. Significant uncertainty about the impacts of different types of intervention both when used individually and in combination arises in part because there has not been sufficient research to establish causal links between CBFM and NFM actions and downstream effects. Long-term monitoring is necessary because major floods are rare events; it is also necessary that prospective studies establish good experimental controls and collect accurate baseline data.

Recent model studies have begun to reproduce field measurements from relatively small monitored catchments. These models could now be used to simulate the impact of changes in land use and management practices in larger catchments. Model studies of large recently flooded catchments (e.g. Yorkshire Ouse, Eden, Parrett, Thames) could help to establish the scale and spatial location of different types of catchment-based intervention that might be required to have a notable effect on flooding. It is important to investigate whether the models' findings can be extrapolated to regions larger than those for which they have been evaluated, given the constraints posed by their formulation and uncertainties in validation data, and to understand whether the benefits of CBFM/NFM measures are more, less or equally predictable than the benefits of hard engineered assets.

The Environment Agency's Catchment Flood Management Plans (CFMPs) assess flood risks across a catchment and can include maintaining or restoring natural processes among the measures that might be taken in the course of flood risk management. Moreover, a large number of catchment-based schemes are currently underway, promoted by Rivers Trusts, Wildlife Trusts and flooded community groups, for example. Many of these local initiatives are neither being planned nor evaluated at larger spatial scales. The lack of monitored baselines and experimental controls creates a risk that the wider and scale-dependent impacts cannot be properly investigated or used to inform decision-making. Research and data available within the water management industry (e.g. water companies, Internal Drainage Boards, land and estate management organizations) would add to the evidence base if it were disseminated more widely.

The performance, longevity and operation and maintenance of CBFM/NFM should be systematically compared with traditional engineering solutions. The risks and uncertainties and benefits associated with each approach need to be more fully understood and communicated. The interactions between fluvial floods and other flood types (e.g. pluvial, coastal and groundwater) and sequences of events also warrant further systematic study. The potential for CBFM/NFM interventions in groundwater-dominated and heavily engineered and drained river systems needs further research. The extent to which these interventions add resilience to the impacts of climate change is also worthy of further investigation.

A practitioner toolkit would help to share practical experience (while noting context-specific issues), paying attention to appropriate design criteria. A practitioner toolkit might comprise a set of documents outlining best practices and the situations in which their effectiveness has been demonstrated, drawing on well-studied examples. This could be accompanied by a protocol for coordinated, high quality, monitoring of the catchment, river corridor and hydro-meteorological conditions, drawing on modern sensor, communications and information technologies.

There would be benefits from improved communication and collaboration between groups undertaking research in river catchments (e.g. water quality, sediment transport, river restoration, biodiversity, agriculture and forestry), which are all relevant to flood risk management. On the basis of current evidence, the cost-effectiveness of NFM at medium-large scales is likely to rely on understanding interactions between flows, debris and sediment management taking into account the range of ecosystem service benefits that accompany NFM.