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1 The role of catchment land use planning in flood risk management Dr Richard Johnson, Mountain Environments, UK Paper presented at a Workshop on Flood Management in Local Planning, Austria/Slovenia, 8-10th April 2008 1. Introduction

The EU Directive on the assessment and management of flood risks ("Floods" Directive) states that measures to reduce flood risks should, as far as possible, be coordinated throughout a river basin if they are to be effective. It also notes that floods are natural phenomena which cannot be prevented but it elaborates on this by stating that some human activities and climate change contribute to an increase in the likelihood and adverse impacts of flood events. These human activities are described as including increasing human settlements and economic assets in floodplains and the reduction of the natural water retention by land use. The Directive therefore suggests that catchment land use could potentially have an important role to play in flood risk management but it does not state how it should be planned or implemented or how effective land management can be. Flood risk management involves a suite of management plans which fall into the three categories of prevention, protection and preparedness. The first category can also be subdivided into those actions which address the cause and those which address the effect of flooding. Dealing with the cause is fundamental in a long term solution to any environmental, social or economic issue. In terms of flooding it is identifying where the floodwaters originate, investigating runoff rates in headwater catchments, looking at how river channels convey floods and dealing with the rivers before they become too big to be managed. Dealing with the cause of the floods is therefore looking at the whole catchment and the management technique which can best address the cause is catchment land use planning. In the UK, flood risk management has recently started to look in detail at how land management could be planned on a river catchment scale and how effective it could be in reducing flood risks. Through a series of demonstration sites a range of techniques have been developed aimed at increasing natural water retention by land use. These sites have not only demonstrated the techniques but they have also enabled the estimation of their potential effectiveness by mathematical modelling and their actual effectiveness by intensive hydrological monitoring. Preliminary results from the modelling work are showing that significant reductions in flood risk can be achieved indicating that land use management can have a significant effect in reducing flood risk.

2 2 Land use planning

Land use can be planned on a catchment scale to target the causes of flooding by distributing certain land uses in key areas within the catchment. The land uses to be considered include the catchment's vegetation cover, soil cover, river channels, and related aspects such as ground drainage, access roads and river morphology. For flood management these land uses need to be distributed to control the rate of runoff during storm conditions and the rate of flow down the watercourses To plan the distribution of land uses over a catchment the full range of catchment characteristics need to be linked to the hydrological characteristics of the land use type. Woodland plays an important role in catchment flood management however, as an example, it is not necessary to plan woodland restoration over the whole catchment. Certain key areas exist in each catchment where the woodland will be most effective. These areas could include areas of open hillslope which are steep, with thin soils and situated above watercourses, steep gullies which collect and concentrate overland flows and riparian strips along floodplains which can slow down the rate of flow from the floodplain back into the river following a large flood event (Fig.2). To start the process of identifying flood control areas there is a need to collate spatial data from the catchment and to carry out a survey of flood generation processes. The catchment data should include geology, topography, soils, watercourses, precipitation and land use. This spatial data should be entered into a GIS for later analysis (Fig.1). The survey of flood generation processes should be carried out to include indicators from the catchment of the hillslopes where rapid overland flow could occur, watercourses which respond most quickly, areas of artificial drainage and other features such as areas of hard standing. In addition any evidence from past events should be collated including trash lines along watercourses, sediment deposition features on the channels and landslides. This data then identifies how floods have been generated in the past, where similar floods could be generated in the future and where flood attenuation measures could be developed. These are then the key flood control areas in the catchment where appropriate land use needs to be planned. The identification of key flood control areas within a catchment highlights other areas in the catchment which will have little control on flood generation. These areas could be a significant proportion of the catchment enabling land owners and local communities to use this land for agriculture, forestry, recreation and small scale infrastructure. This integrated approach on the whole catchment scale, involving communities, is therefore land use planning on a sustainable level.


Figure 1

Land use distribution in the Teviot catchment, Scotland


Figure 2

Locations of NFM riparian woodlands and restored meanders in the Upper Teviot catchment, Scotland

5 3 Sustainable flood risk management

Sustainable flood risk management has been incorporated into recent European legislation and planning guidance. The lack of a detailed explanation of what it means however has hindered its development and application. The sustainability of the approach should mean that it integrates a range of flood management requirements using best practices with good planning. Sustainability has to involve the economics of a scheme, good planning, understanding flood generation processes, protecting natural environments and working with communities. This has led to the suggestion that sustainable flood risk management includes the following 8-components: 1. 2. 3. 4. 5. 6. 7. 8. Floods legislation driven by good policy and planning; Flood monitoring networks and warning systems; Flood data for trend analysis and investigating flood generation processes; Protecting, benefiting and involving communities; Engineered flood protection schemes Economics including capital costs, maintenance costs Protecting and enhancing the natural environment; Natural flood management in functional flood control areas involving river restoration and catchment land use planning.

Sustainable flood management is therefore an integrated set of procedures linked into a physical catchment. The component of natural flood management has recently been developed as that procedure which involves working with rivers using natural processes with systematic land use planning. This type of management is not new and most of the techniques are part of best practices in farming and forestry. The new part is the design of natural flood management in a catchment enabling most of the current land uses to continue while introducing flood controls in certain parts of the catchment. The term "functional floodplain" has recently been introduced into planning terminology. It is essentially the area of floodplain which is likely to flood and hence where no developments should occur. Definitions of the functional floodplain differ slightly but it is usually determined by the predicted extent of the 1 in 200 year flood. This creates a river corridor where flood inundation can be managed however this is only reacting to the risk of flooding and not dealing with the causes of flooding. Natural flood management deals with the causes of flooding by slowing down runoff rates in the upper catchment and reducing flow rates down watercourses. Fundamental to this is that there are key areas in a catchment where floods are generated and where flood flows should be slowed down. In a catchment there are natural systems such as soils and river meanders which provide this function. If these systems at the key points have been degraded then there is a high risk of increased runoff rates and more rapid flow rates in rivers. These areas are therefore key in the success of natural flood management and can be termed functional flood control areas.

6 4 Natural flood management

Natural flood management (NFM) is a catchment based approach aimed at reducing runoff rates in the uplands and reducing rates of flow down watercourses. It selects the functional flood control areas within the catchment to modify or restore land uses which together reduce downstream flooding. The components of NFM are the techniques which are used, their distribution around the catchment and the quantification of how effective they will be in the short and long term (Fig.3). The techniques which are used in NFM include: · · · · · · · Reforestation of hillslopes Planting dense woodlands in gullies Blocking artificial drains Restoring wetland features Restoring river channel meanders Controlling excessive erosion Management of large woody debris in watercourses

These techniques are varied in form some dealing with runoff rates and some with rates of flow in watercourses. They are all using natural processes and are relatively low cost compared with engineered techniques. Some techniques such as blocking drains have an immediate effect while others such as woodland restoration will take years to become effective. Therefore it is not just about the spatial distribution of NFM techniques but considering their short and long term effectiveness i.e. developing a prescription of measures. Application of NFM techniques can be considered as catchment restoration and as it is aimed at reducing flood peaks it is also river restoration. River restoration is commonly associated with the repair of river banks or improvement of habitats, in NFM it is about applying the techniques to restore the natural flow regime of the rivers. NFM should also work with the rivers so that once the flow regime has been restored the river will undertake the channel remediation works. The prescriptions of measures on the catchment scale needs to be quantified in terms of the changes to the downstream flood hydrograph. The aim is not to reduce the total amount of water in a flood but to reduce the flood peak by extending the length of time when the river is high. Within most catchments there are a number of significant tributaries feeding into the main river. Consideration needs to be given to the magnitude of the flood peaks in each tributary and also the coincidence of peaks. In some catchments it may be beneficial to maintain a high peak flow in one tributary catchment so that this flood water can join the main river several hours before the floods from other tributaries. However this would need to be tested on all rainfall distribution scenarios and it may be safer to treat all of the tributary catchments. The planning and implementation of NFM on a catchment scale has to involve the local community. In most situations the proposed work will be on private land, land owners will have alternative uses of the land, other developments may be planned and situations in the catchment may change. There also needs to be cooperation between neighbouring communities, catchments often cross administration boundaries so there needs to be cooperation between local

7 authorities and on the European scale rivers cross national boundaries so there need to be international cooperation. Therefore NFM should be integrated into other spatial plans so that there are long term social, environmental and economic benefits for communities and natural environment. By taking this partnership approach and by using prescriptions of land use planning the flood management will be effective and sustainable.

30 25

Reduction, %

20 15 10 5 0 0 50 100 150 200 250 Return period, years .

Figure 3

Predicted percentage reduction in flood peaks in the River Teviot resulting from NFM applications

Author: Dr Richard Johnson Mountain Environments Stirling Road Callander FK17 8LE UK Tel: 0044 1877 331080

[email protected]


Catchment management, planning and sustainable flood management

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