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Natural Flood Storage and Extreme Flood Events Final Report

9 Generic assessment of natural floodplain storage

9.1 Introduction

The modelling and data analysis discussed in previous sections of this report lead on to a generic methodology to assess the potential for exploiting natural floodplain storage in any Scottish river. The generic methodology is outlined below. A flow chart of the main steps to be followed is shown in Figure 9-1. The methods are suggested as a possible approach to making a broad-scale and generalised assessment of the use of natural flood storage in any catchment. They are not intended as a design or scheme feasibility method, which would require further detailed work to simulate the effects of specific storage options.

9.2 Main steps

The main steps in the suggested assessment of natural storage are as follows:

Calculate storage volume required to achieve reduction in flood risk

This can be done by calculating the volume of the flood hydrograph above a threshold peak flow, as described in Section 3.

Determine areas available as potential 'natural flood storage'

This can be done by generating maximum flood extents that represent the assumed limit of the available floodplain. The interpretation of 'natural' in this case is any area that would currently flood under a particular event scenario. The size or rarity of event that is used to define the 'natural' floodplain is an arbitrary choice, although the resulting outlines could be adjusted to include (or exclude) sub-areas that are thought to be suitable (or not) as additional flood storage.

Determine an average required depth of storage upstream of the risk location

This is done by dividing the required volume of storage by the available potential storage area, working incrementally upstream from the risk location. The result gives an indication (and it is only an indicative figure) of the extent and height of flood banks that may be needed in order to use the natural flood extent to provide the required volume of storage. By restricting the available flood storage area to the 200 year flood extent we are attempting to represent a flood management scenario in which areas that occasionally flood are used for storage, but not areas that lie beyond a reasonable 'standard of protection'. The choice of 200 year threshold is somewhat arbitrary and can be varied.

9.3 Modelling methodology

The modelling of the natural floodplain to simulate downslope conveyance and to determine natural storage can be undertaken in more than one way. The choice of modelling techniques depends on the characteristics of the river network (e.g. river bed gradient, floodplain extent), the presence of riverside embankments and the availability of good quality hydrological and hydraulic models, channel cross-sectional data and topographic data.

9.3.1 Hydrological estimates

The basic hydrological inputs for modelling the floodplain are upstream flow estimates. In all cases, flows for a given return period and including QMED (the median annual maximum flood) will be estimated using the methods set out in the Flood Estimation Handbook, (FEH, Institute of Hydrology, 1999). In most cases, this will require the use of catchment descriptors. If the reaches being modelled are long enough to introduce significant additional catchment area, then 'lateral' flow inputs will be determined by FEH methods using appropriate catchment descriptors.

Flow estimates will be required at locations such as headwaters, main confluences, locations where there are significant steps in river gradient and at intervals for long, continuous reaches in the floodplain.

The simplest method is to generate flow hydrographs using FEH methods at selected points on the river network. A difficulty with this approach is that the FEH estimates can vary spatially in an unrealistic way, depending on exactly how the FEH procedures have been applied (Morris, 2003). To remedy this issue, an automated procedure was developed by the Centre for Ecology and Hydrology (CEH) to provide peak flow estimates that are spatially and logically consistent within any catchment. These estimates, sometimes referred to as 'Auto-FEH', are available for all the main rivers in Scotland and can be obtained from SEPA. However, the Auto-FEH procedure generates a peak flow of specified probability at all the selected points on the river, whereas the probability of observing the same peak flows at all those locations in a single event is likely to be lower.

9.3.2 Flood hydrograph routing

For a catchment-scale analysis, the hydrological 'inputs' to the river network need to be linked to provide a flood hydrograph that varies in a plausible way across the network. The floodplain inundation model used in this study is not fully coupled to the river network. Hence, the requirement for our floodplain modelling method is to generate flow hydrographs, at specified locations in the river network, which represent water spilling out of the channel and onto the floodplain. The 'Auto-FEH' flood estimates provide one such form of linkage, albeit one based on interpolation rules rather than physical principles.

Alternatively, a 1-D flood routing model can be used to represent the movement of the flood hydrograph in the river channel. There are numerous possible options, ranging from a full 1-D hydrodynamic model based on detailed channel survey though to simple 'hydrological' routing with assumed parameters for the attenuation of the hydrograph. For some rivers it may be possible to validate the new routing model by comparison to rating curves produced by other existing models (if any of the model cross-sections coincide) or from existing gauging stations (flow or level) within the modelled area.

The routing model can be used to simulate a hydrological 'event' that generates a downstream flow of the required return period. The routing model should, in this case, represent floodplain attenuation either through the use of 'extended' cross sections or a suitable choice of attenuation parameters.

9.3.3 Flood extent model

Flood extents are required to estimate the areas that constitute potential 'natural' storage. Where historical extents are known, these may be used. Another alternative is to make us of flood maps based on modelled water levels projected across a floodplain DEM. This approach takes no account of the basic principle of mass conservation (it assumes that there is always enough water to fill all available space up to the prescribed water level) and relies on interpolation to create a water surface. Perhaps more importantly, it requires a large number of accurate water level estimates for a given flood event, which would generally require a detailed hydraulic model or at least channel survey data.

The 2-D raster flood inundation model we use here is able to determine the flood extents, together with gridded depths and flow velocities, based on a DEM grid and a set of flow inputs. It is designed to conserve mass. A grid with prescribed dimensions is constructed over floodplain area of interest within the model, with the flood risk area at or near the downstream boundary. Flows in excess of the channel capacity are permitted to spill out onto the floodplain for storage and/or conveyance downslope.

Where there has been a river survey, the channel capacity can be estimated using software such as HEC-RAS or the Conveyance Estimation System (CES) to determine rating curves. Where data are lacking, a simpler alternative is to assume that the channel capacity is approximately equal to QMED. This assumption, whilst very broad, has some basis in geomorphological theory (Wolman and Miller, 1960) and has been accepted as a working approach for extreme flood extent modelling in England and Wales.

Whilst there is no suitable mechanism for water to return from the floodplain to the channel in the current version of the inundation model used here, water can still follow the river channel as a steepest line of descent, moving in effect as an uncoupled 'slice' above the channel.

9.3.4 Flood volume analysis

The hydrograph analysis described earlier in this report is a conventional method to get an approximate estimate of flood storage volumes. A volume calculated in this way is not necessarily the appropriate parameter for sizing a flood storage reservoir because the simple analysis does not account for the possible inflow and outflow controls in an engineered storage scheme, nor for its location relative to runoff production in the catchment. Nevertheless, the simple hydrograph analysis provides a useful first estimate of storage volume.

The question that must then be asked is whether the required volume can be found within the catchment. It is generally accepted that flood storage is most effective immediately upstream of a risk location. It is therefore particularly worth knowing whether the required volume of storage can be found close to the risk location.

9.3.5 Feasibility of floodplain storage using simplified average depth approach

By apportioning the required volume (using a simple average flood depth) over the areas identified as possible 'natural' storage, it is hoped to gain a broad indication of the potential for using these areas in a managed way. If there is a large area of floodplain available just upstream of the risk location, then a relatively small average depth of water would be needed to provide the volume of storage needed to reduce flood risk downstream. In such cases enhanced use of the 'natural' floodplain may be a viable option to consider.

The next step would be to examine the economic impacts of assigning land within the assumed flood extent for flood storage. We have used a simplified analysis based on land cover classes to provide an index cost.

Further analysis would involve representing the potential storage area as a reservoir unit within the catchment routing model to check on the predicted reduction in downstream peak flows.

9.4 Environmental and historical assessments

9.4.1 Broad scale ecosystem impact assessment

Consideration of natural floodplain storage as an option for flood management has clear implications for catchment scale ecology, which in turn is embedded within the requirements of environmental planning. There are two consequences: a) that knowledge of how the ecosystem functions can lead to better decisions on the selection and promotion of natural floodplain storage, and b) there is a need to understand how the natural floodplain storage option will impact on the wider environment when assessing the scheme within the Environmental Impact Assessment (EIA) process, as part of any planning submission.

The two issues are generally dealt with at different strategic levels. It is necessary to establish how broader ecosystems function at a catchment or sub-catchment scale, as recognised in the development of the Broad Scale Ecosystem Impact Modelling (BSEIM) toolbox. The BSEIM methodology has been commissioned by the Defra/Environment Agency joint flood management research programme in late 2004, and follows the initial BSEIM scoping study (Cascade Consulting, 2002). Once a scheme has been identified, it is then necessary to establish specific impacts resulting from construction and operation of that scheme, which is generally undertaken within an EIA. For the purposes of this study it is assumed that any natural floodplain storage analysis would be undertaken at a catchment scale, and assessment of any specific scheme or measure would follow later. A broad scale ecosystem impact assessment at the strategic level should be undertaken, and this is commensurate with analysis in data poor catchments.

The recommended BSEIM approach is based on a generic and modular framework. The framework aims to define a set of key inputs to the assessment process that can act as the basis for the methodology and can subsequently be built on as scientific knowledge improves. The key components Figure 9-2 include:

1. Definition of policy drivers and associated scenarios

2. Specification of appropriate ecosystem quality criteria

3. Initial identification of constraints and opportunities

4. Description of baseline (and historic) ecosystem characteristics

5. Identification of a range of input environmental drivers (principally hydrology and geomorphology, including floodplain connectivity)

6. Description of existing driver characteristics (flow envelope, sediment characteristics etc.)

7. Prediction of change in driver characteristics with future policy drivers

8. Prediction of ecosystem response to environmental change (through an evidence based expert system)

9. Definition of ecosystem impacts relative to specified catchment ecosystem quality criteria.

The methodology is predominantly based on existing data sources and uses GIS to define the spatial integration of the catchment. Where temporal data utilising historic information are available they are also included. A key component of the methodology is the definition of the Ecosystem Quality Criteria (EcoQC) that aim to establish the opportunities for ecosystem benefit through biodiversity improvement as part of flood management option appraisal (within Catchment Flood Management Plans in England & Wales). It is through this process that biodiversity benefits from future incorporation of natural floodplain storage should be explored with relevant stakeholders.

Specific data requirements include the hydrology of the existing catchment and the changes from proposed policies or measures, including opportunities or changes to flood extent, depth and duration at a broad scale. The method utilises whatever baseline datasets (e.g. water based SSSIs, river surveys, river habitat scheme surveys, species surveys) are available on the river system in question to build up a (usually incomplete) strategic view of the catchment. An important element of the method is then to talk to key experts with knowledge of the catchment to better understand the key drivers and sensitivities, which help to integrate the baseline data already collected. Prediction of the potential ecosystem consequences from an altered flooding regime uses expert opinion/judgement of the potential dynamic evolution of the ecosystem as a result of the change in system pressures.

At present the methodology is being tested on two catchments in England (Rivers Ribble and Derwent). A guidance document on Broad Scale Impact Assessment is due for release following review in summer 2005 via the DEFRA website (Project Ref. FD2112).

9.4.2 Environmental and historical mapping

Spatial GIS analysis is used to provide a general assessment of whether the flood extent (current or altered) impinges on any environmental or historical sites/areas within the study area floodplain. The flood extent datasets (current and altered) are overlain with GIS datasets on environmental and historic sites within the floodplain, such as SSSIs, SAMs and LBs. If the flood extent impinges on a designated site then, if necessary, further consultation with the relevant authorities (e.g. SNH, HS) would be needed to discuss the implications in more detail. Increased or longer duration flooding on some SSSIs and buried SAMs may improve their longer term conservation or restoration potential, but for other sites/areas the flooding could lead to increased degradation and should therefore be avoided if at all possible. Consultation with the relevant authorities could either reject a potential new site for natural floodplain storage or recommend it for further investigation.

9.5 Floodplain assets

In a similar way to the environmental and historical mapping, GIS analysis is used to provide a general assessment of whether the flood extent (current or altered) impinges on other floodplain assets. The flood extent datasets (current and altered) are overlain with GIS datasets on other floodplain assets, such as land cover, properties, businesses, communication assets and utilities. If the flood extent is shown to impinge of a specific site/area then it may be necessary to consult with the relevant authorities (e.g. Local Councils, utility companies, transportation infrastructure organisations and communication companies) to discuss the implications in more detail. Consultation with the relevant authorities could either reject a potential new site for natural floodplain storage or recommend it for further investigation.

The impact of flood inundation on a particular agricultural or rural land cover is dependant on the timing, extent, frequency and duration of flood events. Arable land and the variety of crops it supports would typically have a greater value than grassland or forest/woodland if these co-exist in a catchment.

An estimation of the potential overall annual cost of actively encouraging floodplain storage could be explored by the use of an appropriate generic compensation value for all agricultural land cover classes. This could be based on recent publications on the subject, or from typical payments currently provided to farmers within agri-environment schemes in England and Wales for certain floodplain areas. An alternative method would be to use the single event based agricultural cost estimates derived from the MDSF flood damage values that have been adjusted for Scottish conditions.

Following the modelling and environmental assessment work any potential sites for enhanced natural storage will initially be targeted at those rural areas where lower value agricultural systems currently exist, such as extensive livestock grazing/grassland production, and where few, if any, farmsteads will be directly impacted by the enhanced flooding. However, enhanced flooding on agricultural land may also provide opportunities for farmers/landowners to receive some compensation through an application for agri-environment scheme monies, such as the Rural Stewardship Scheme (RSS) in Scotland.

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